Birkett Technical Catalogue - Safety Systems UK Ltd

Pressure & Flame Protection 

Safety Relief Valves

Safety Relief Valves 

‘WB’ SERIES SPRING LOADED 

SAFETY RELIEF VALVES 

The WB is designed to safely relieve excess pressure in 

pumps, pipework, tanks, calorifiers, gas and oil 

separators and other process vessels. It is suitable for 

gas, steam, vapour and liquid applications. The WB 

conforms to API 526 pressure/temperature ranges, 

orifice areas and dimensions. 

‘SAFEFLO’ SAFETY AND 

THERMAL RELIEF VALVES 

Safeflo valves are designed for similar duties to the WB 

but for small capacity applications.They safely relieve 

thermal expansion of process fluids in vessels and long 

lengths of pipework, and are suitable for gas and liquid 

applications. 

‘SAFESET’ PILOT OPERATED 


Safeset valves are self contained pilot operated safety 

relief valves which use the system pressure to control 

the valve opening and closing. No other source of 

energy is required. 

A choice of different types of Safeset pilot valves are 

available, including pop and modulating action to suit a 

variety of applications. 

1 

Safeset valves conform to API 526 pilot operated 

pressure/temperature ranges and dimensions, with 

multiple orifice areas contained within each valve 

body size.

CONTENTS 

Page 

Birkett Range 

3 Introduction 

4 International approvals and authorisations 

5 WB Series 

6 Features and benefits 

7 - 10 Valve types and action 

11 - 14 Drawings 

15 - 16 Accessories 

17 Figure numbering system 

18 Material temperature ranges 

19 - 46 Valve selection charts, D to T 

47 - 48 Valve adjustment 

49 - 50 Seat tightness/seat leakage testing 

Page 

Sizing 

81 - 82 Sizing formula 

83 Nomenclature 

84 Back pressure/blowdown limits and 

orifice areas 

85 - 90 Sizing factors 

91 - 96 Capacity charts 

97 Reaction forces 

98 Definitions 

98 Definitions of terms 

99 Operational characteristics 

100 Pressure term relationships 

51 Safeflo 


53 - 54 Drawings 

55 Accessories 


57 - 58 Dimensions 

59 Safeset 


61 Dual Outlet/Full Bore Pilot Valve 

62 Pilot types and basic operation 

63 - 64 Type 2 - Pop Action Pilot 

65 - 68 Type 4 and 8 - Modulating Action Pilot 

69 Technical specification 

69 - 70 Drawings and materials of construction 

71 - 75 Accessories 


77 - 78 Dimensions 

79 - 80 Operating pressure/temperature limits 

2

Safety Relief Valves 

INTRODUCTION 

The effects of exceeding safe pressure levels in an 

unprotected pressure vessel or system can have 

catastrophic effects on both plant and personnel. 

Safety relief valves should be used to protect any 

pressurised system from the effects of exceeding its 

design pressure limit. 

A safety relief valve is designed to automatically 

discharge gas, steam, vapour or liquid from any 

pressure containing system, preventing a 

predetermined safe pressure being exceeded, and 

protecting plant and personnel. 

The Birkett range of safety relief valves 

contains three distinct valve types: 

WB Series – spring loaded safety relief valves. 

Safeflo – safety and thermal relief valves. 

Safeset – pilot operated safety relief valves. 

All types are certified in accordance 

with ASME Code Section VIII. 

All Birkett valves are available through our global 

agent distribution network supported by our own 

regional sales offices around the world. 

3

INTERNATIONAL APPROVALS AND AUTHORISATIONS 

Our approvals and accreditations 

include ISO 9001:2008, ASME, 

Chinese Safety Quality Licence, 

TUV, Bureau Veritas, 

Stoomwezen, PED, ATEX and 

GOST-R. 

APPLICABLE STANDARDS 

ISO 9001:2008 

Quality Standard 

ASME Code Section VIII 

All valves are UV certified 

API 520 : Part 1 

Sizing and selection 

API 526 

Dimensions 

API 527 

Leakage Rates 

ANSI B16.5 

Flange Ratings 

4

WB Series - 

Spring loaded safety relief valve 

1 Insitu testing 

2 Accessories 

3 High performance springs 

4 Bellows back up piston 

(not shown) 

5 Guiding 

6 Bellows 

(not shown) 

7 Trim 

8 Seat integrity 

9 Adjustable blow down 

10 Nozzle design 

11 API 526 face to face 

dimensions 

5

FEATURES AND BENEFITS 

Design verification – all design options and the 

various effects of system conditions, back pressure 

etc. have been verified on Birkett’s 

in-house, extensive mass flow test facility. 

Lighter and more compact construction – 

continuous design improvements have created 

smaller and lighter valves to support current 

industry design trends, especially space and weight 

savings. 

Interchangeable parts – valves can be modified 

from type to type, gas, liquid, conventional and 

bellows simply by changing only a few parts. 

Simplified maintenance and service - 

re-engineering has reduced the number of parts, 

making maintenance easier and more cost 

effective. 

Material selection – a wide range of materials 

are offered including non-ferrous for low 

temperature and oxygen service, as well as exotic 

alloys specifically for the chemical and process 

industries. 

Cryogenic and oxygen service – Birkett’s 

state-of-the-art clean room and vapour degreasing 

facilities ensure compliance with the stringent 

demands of cryogenic and oxygen applications. 

1 In-situ testing – valves can be supplied 

suitable for application of “in-situ” set 

pressure verification devices. 

2 Wide range of accessories – available to 

comply with international codes and to suit 

system requirements. 

3 High performance springs – safety relief 

valve springs are specifically designed to 

guarantee set point repeatability. 

4 Bellows back-up piston – an optional 

auxiliary back-up piston for balanced bellows 

valves ensures fail-safe operation in the event 

of bellows failure (see page 16). 

5 Guiding surfaces – the material selection of 

guiding components, together with a selfaligning 

disc and spindle pivot point, ensures 

correct alignment and no galling of guiding 

surfaces. 

6 Bellows – ensures correct valve 

performance under difficult back pressure 

conditions (see page 12). 

7 Trim – specific gas and liquid trim designs 

give stable operation and eliminate the 

damaging effect of chatter. 

8 Seat leakage integrity – choice of nozzle 

and disc materials (coupled with superior 

lapping techniques) provides seat tightness to 

API 527/ASME VIII. 

9 Adjustable blow down – the valve 

reseating pressure (blow down) can be simply 

adjusted to suit special or specific 

performance requirements. 

10 Nozzle design – the method and location of 

attachment to the body avoids transmission 

of pipe stresses to the nozzle/disc mating 

surfaces. 

11 API 526 dimensions – standardised 

dimensions allows pipework layouts to be 

detailed confidently. 

6

DIFFERENT TYPES 

There are four basic types of WB Series safety 

relief valve: 

WB400 – conventional gas type. 

WB300 – bellows gas type. 

WB200 – conventional liquid type. 

WB100 – bellows liquid type. 

Conventional safety relief valves: 

Can be used on systems where the discharge is 

relatively simple. The pressure in the discharge 

system can be atmospheric, at a constant level or 

where the pressure may build up to a maximum of 

10% of the set pressure. When a constant back 

pressure exists, the valve should be set at the 

differential pressure (refer to page 8). 

Liquid service 

Valves operating on liquid service require a 

modified valve design to cope with the differing 

dynamics of liquid flow. 

A contoured plug disc is used to minimise initial 

flow rate, eliminating any potential inlet pressure 

drops due to excessive valve lift. The valve will 

simmer until sufficient pressure is available to 

generate lift. Once this has occurred, the lift will 

stabilise to suit the flow and pressure conditions 

required, thus avoiding the problem of chatter. 

‘Chatter’ is the rapid opening and closing of the 

valve which can have a damaging effect on the disc 

and nozzle, causing it to leak. 

Bellows safety relief valves: 

The WB Bellows Valves are statically balanced and 

can be used on more complex discharge systems 

such as common discharge manifolds and flares 

where several valves may discharge. This type of 

system creates a variable superimposed back 

pressure. The balanced bellows unit cancels out 

the effects of variable back pressure, on the set 

pressure of the safety relief valve. 

Gas and vapour service: 

The gas/vapour disc can be distinguished by the flat 

underside, unlike the cone profile of the liquid disc. 

Typical liquid relief valve disc 

7

THE EFFECT OF BACK PRESSURE 

The configuration of a closed discharge pipework 

system, typically for toxic or hazardous duty, can 

generate back pressure. Back pressure applied to 

the outlet of the valve will adversely affect its 

performance, unless it is addressed. 

Back pressure may take three forms: 

1. Superimposed constant back pressure 

This exists permanently and a conventional or 

bellows valve can be used. A conventional valve 

can be set at the differential pressure so that the 

spring load is adjusted to take account of the back 

pressure. 

2. Built up back pressure 

Built up back pressure is created by the 

configuration of discharge pipework systems and 

varies from zero when the valve is closed, to a 

maximum, when fully open. Conventional spring 

loaded valves can tolerate up to 10% of set 

pressure as built up back pressure. For back 

pressures in excess of 10%, a balanced bellows 

design is required to maintain valve lift. 

3. Superimposed variable back pressure 

This is caused by other valves discharging into a 

common disposal system, or other circumstances 

that cause the back pressure to be variable. 

Balanced bellows valves should be used for this 

condition, adjusted to the predetermined set 

pressure. 

Constant back pressure - conventional valve 

Variable back pressure - balanced bellows valve 

8

VALVE ACTION 

Principles of operation – spring loaded 

safety and thermal relief valves 

SPRING FORCE 

Safety relief valves use a spring force to hold a disc 

against a nozzle. Under normal system operating 

pressure, the valve will remain closed as the spring 

force is greater than the inlet system pressure 

force. The valve opens when the system pressure 

force becomes greater than the closing force of 

the spring. 

Spring loaded safety and thermal relief valve 

The WB and B/C series are designed to have a 

short simmer, open rapidly to full lift position and 

then re-seat at a controlled shut off pressure. 

This is demonstrated in the graph below, which 

shows the valve action and corresponding pressure 

at the valve inlet. 

SYSTEM PRESSURE FORCE 

POPPING PRESSURE 

SET PRESSURE 

RE-SEAT PRESSURE 

Popping and blowdown 

The opening and closing characteristics of the valve 

can be controlled by the adjustment of a 

blowdown ring, as its position affects the shape 

and volume of the huddling chamber. When the 

blowdown ring is adjusted to its top position, the 

exit area from the huddling chamber is restricted 

to its minimum. The valve will pop distinctly with a 

short simmer and long blowdown. Conversely, 

when the blowdown ring is in its lowest position 

there is a maximum exit area from the huddling 

chamber and the valve will have a longer simmer 

with a shorter blowdown. The blowdown ring can 

be positioned between these two extremes to give 

the required performance, but it is usually factory 

set to achieve re-seating 7-10% below set pressure. 

9

LIFT CYCLE 

Stage 1 – Closed 

Inlet pressure < set pressure 

Inlet pressure is below the set pressure. The valve is 

closed and there is no flow through the valve. 

Stage 2 – Simmering 

Inlet pressure is = > set pressure and < popping pressure 

Inlet pressure increases to set pressure. At this point, the spring 

force and system pressure force are equal; a further rise in inlet 

pressure will then begin to lift the disc slightly. A small amount of 

fluid is released into the huddling chamber (the valve simmers). 

The system fluid is now acting on a larger area inside the 

huddling chamber. 

Stage 3 – Popping and opening 

Inlet pressure = > popping pressure, valve fully open 

The inlet pressure acting on a larger area produces a significant 

force to accelerate the opening. A combination of this pressure 

force, the kinetic energy from the fluid within the nozzle and the 

deflection force of the fluid flow turning through the reaction 

hood, is transformed into disc lifting force. The valve pops open 

at < 5% overpressure and the valve reaches the full open 

position at 110% of set pressure, in accordance with 

international codes. 

Stage 4 – Reseating 

Inlet pressure falls to re-seating pressure 

As system pressure starts to fall, the force from the spring 

begins to close the valve. Typically, the system pressure falls 

between 5-10% below the valve set pressure at which point 

the spring force accelerates the valve disc to re-seat the valve. 

The difference between the set pressure and the re-seating 

pressure is known as blowdown. 

10

WB 400 - CONVENTIONAL GAS TYPE 

(up to and including class 600) 

ITEM PART CARBON STEEL STAINLESS STEEL 

1 Body SA 216-WCB CARB ST SA 351-CF8M ST ST 

2 Casing SA 216-WCB CARB ST SA 351-CF8M ST ST 

3 Cap SA 216-WCB CARB ST SA 351-CF8M ST ST 

4* Nozzle 316 ST ST 316 ST ST 

5* Disc 316 ST ST 316 ST ST 

6* Disc holder ASTM A479-316L ASTM A479-316L 

8 Blowdown ring SA 351-CF8M ST ST SA 351-CF8M ST ST 

9 Guide Assy CARBON ST/17-4 ST ST 316L/17-4 ST ST 

10* Spindle ASTM A479-431 ASTM A479-431 

11 Lower spring plate ASTM A108-1021/Ni PLT ASTM A479-431 

12 Adjusting screw ASTM A479-410 ASTM A479-410 

13 Locking nut ASTM A108-1021 ASTM A479-316L 

14 Setting screw ASTM A479-431 ASTM A479-431 

15 Setting screw rod ASTM A479-316L ASTM A479-316L 

18 Stud SA 193-B7 CR/MOL ST SA 193-B8T ST ST 

19 Nut SA 194-2H CARB ST SA 194-8T ST ST 

22* Spring CARBON STEEL ASTM A313-316 

27* Body gasket ST-706 ST-706 

28* Cap gasket ST-706 ST-706 

29* Set screw gasket ST-706 ST-706 

31* Ball AISI 440C ST ST AISI 440C ST ST 

32 Upper spring plate ASTM A108-1021/Ni PLT ASTM A479-431 

33 Data plate 321 ST ST 321 ST ST 

34 Hammer drive screw ELECTRO BRASSED ST ASTM A479-316L 

35 Grooved pin ASTM A479-431 ASTM A479-431 

42 Drain plug HTS HOLO-KROME ASTM A479-316L 

80* Circlip ASTM A313-316 ASTM A313-316 

* Recommended spares 

11

WB 300 - BALANCED BELLOWS GAS TYPE 

(up to and including class 600) 







6* Disc holder INCLUDED IN ITEM 23 INCLUDED IN ITEM 23 


9 Guide Assy CARBON ST/17-4 ST ST 316 L/17-4 ST ST 


11 Lower spring plate ASTM A108-1021/Ni PLT ASTM A479-431 




15 Set screw rod ASTM A479-316L ASTM A479-316L 

18 Stud SA 193-B7 CR/MOL ST SA 193-B8T ST ST 

19 Nut SA 194-2H CARB ST SA 194-8T ST ST 


23* Bellows assembly ASTM A479-316L/SA240-316L ASTM A479-316L/SA240-316L 



29* Set screw gasket ST-706 ST-706 


32 Upper spring plate ASTM A108-1021/Ni PLT ASTM A479 431 





80* Circlip ASTM A313-316 ASTM A313-316 


12

WB 400 - CONVENTIONAL GAS TYPES 

(class 900 and above) 







6* Disc holder ASTM A479-316L ASTM A479-316L 

7 Reaction hood ASTM A479-431 ASTM A479-431 


9 Guide plate 17-4 ST ST 17-4 ST ST 


11 Lower spring cap ASTM A108-1021/Ni PLT ASTM A479-431 





16* Tabwasher BS 1449-304S15 ST ST BS 1449-304S15 ST ST 

17* Pinning screw ASTM A479-431 ASTM A479-431 

18 Body stud SA 193-B7 CR/MOL ST SA 193-B8T ST ST 

19 Body nut SA 194-2H CARB ST SA 194-8T ST ST 

20 Casing stud SA 193-B7 CR/MOL ST SA 193-B8T ST ST 

21 Casing nut SA 194-2H CARB ST SA 194-8T ST ST 


24* Spindle head ASTM A479-431 ASTM A479-431 



29* Setting screw gasket ST-706 ST-706 


32 Upper spring cap ASTM A108-1021/Ni PLT ASTM A479 431 



35* Grooved pin ASTM A479-431 ASTM A479-431 



13

WB 300 - BALANCED BELLOWS GAS TYPE 

(class 900 and above) 







6* Disc holder base ASTM A479-321 ASTM A479-321 

7 Reaction hood ASTM A479-431 ASTM A479-431 


9 Guide plate 17-4 ST ST 17-4 ST ST 


11 Lower spring cap ASTM A108-1021/Ni PLT ASTM A479-431 





16* Tabwasher BS 1449-304S15 ST ST BS 1449-304S15 ST ST 

17* Pinning screw ASTM A479-431 ASTM A479-431 

18 Body stud SA 193-B7 CR/MOL ST SA 193-B8T ST ST 

19 Body nut SA 194-2H CARB ST SA 194-8T ST ST 

20 Casing stud SA 193-B7 CR/MOL ST SA 193-B8T ST ST 

21 Casing nut SA 194-2H CARB ST SA 194-8T ST ST 


23* Bellows SA240-316L SA240-316L 

24* Spindle head ASTM A479-431 ASTM A479-431 

25 Piston ASTM A479-431 ASTM A479-431 

26 Guide spindle ASTM A479-321 ASTM A479-3431 


28 Cap gasket ST-706 ST-706 

29 Setting screw gasket ST-706 ST-706 

31 Ball AISI 440C ST ST AISI 440C ST ST 

32 Upper spring cap ASTM A108-1021/Ni PLT ASTM A479 431 





50 Grubscrew ASTM A479-321 ASTM A479-321 


14

ACCESSORIES 

Screwed cap 

This is the standard option on all valves. 

Bolted cap 

Option available on the WB Series when required 

by the customer or for critical service where 

fragile gaskets materials may be fitted. 

Open lever* 

The open lever assembly is not pressure tight and 

is therefore only suitable where vapour can safely 

be allowed to escape to atmosphere. 

Packed lever* 

The design of the packed lever assembly ensures 

that leakage does not occur when the valve is open 

or when back pressure is present. 

*A lift lever can be used to test for correct valve operation where corrosion or deposits could prevent the valve from 

opening. They can be used to release foreign particles trapped on the seat and must be fitted when codes dictate. 

GAG SCREW 

SEALING PLUG 

Test gag 

The test gag is used to prevent the safety valve 

from lifting. This is mainly used when carrying out 

a hydrostatic test on the system, during 

commissioning. 

After testing, the test gag must be removed and 

replaced with the sealing plug. 

15

FERRULE 

Ferrule (government ring) 

A ferrule, sometimes known as a government ring, 

is a collar fitted beneath the head of the pressure 

adjusting screw. Some authorities will require a 

ferrule to be fitted to prevent unauthorised interference 

with the set pressure. 

Soft seat 

An O-ring seal offers maximum seat tightness, 

over and above that of the standard metal-tometal 

seats. A wide range of seal materials are 

available including Viton, Nitrile, Kalrez and PTFE. 

For high integrity seat leakage, specify soft seat. 

Steam jacket 

Some process media can solidify or 

form crystals if they cool within 

the system. The medium within 

the valve nozzle is not in the 

flow path and thus cooling can 

occur. Should the medium 

solidify, crystallise, or if 

sublimation of vapour 

was to occur within 

the nozzle, the valve 

may not lift. 

The steam jacket is 

designed to keep the process 

medium hot, helping to 

maximise plant safety. The steam jacket has both 

an inlet and outlet so that low pressure steam can 

be passed through the jacket, keeping the valves 

hot. This allows the valves to stay operational, 

enabling the valve to successfully relieve pressure, 

should an overpressure situation occur. 

The steam jacket is manufactured out of material 

that is compatible with the body of the valve and 

the connections to the jacket can either be flanged 

or screwed. 

Auxiliary back-up piston 

In the event of bellows failure, a potentially 

dangerous situation can arise. The back pressure 

causes an “out-of-balance” situation which may 

cause: 

1 Increase in set pressure. 

2 Decrease in flow capacity. 

3 Increase in re-seat pressure. 

Specifying bellows valves with an auxiliary back-up 

piston ensures that the above does not occur. The 

piston itself has the same effective diameter as the 

failed bellows, so any effect of the back pressure 

increasing the set pressure is counteracted by an 

upward thrust of the piston. This is an added 

safety feature. The WB 300 valve has 

incorporated the auxiliary back-up piston since its 

inception. It is available as standard in all pressure 

classes 900 and above and as an optional feature 

for class 600 and below. 

To ensure absolute safety, specify the auxiliary 

back-up piston. 

16

WB SERIES FIGURE NUMBERING SYSTEM 

/ 

Inlet diameter 

1” - 8” 

API orifice letter 

D - T 

Outlet diameter 

2” - 10” 

Design 

H ANSI 150, 300 and 600 

/ ANSI 900, 1500 and 2500 

Accessories 

B Auxiliary 

back-up piston 

C Bolted cap 

D Screwed cap 

F Ferrule 

(Government ring) 

G Test gag 

H* High Pressure 

M Open lever 

P Packed lever 

R Soft seat 

S Special feature 

3 WB300 Bellows 

Valve type 

1 Liquid bellows 

2 Liquid conventional 

3 Vapour bellows 

4 Vapour conventional 

ANSI flange rating inlet x outlet 

1 150 x 150 

2 300 x 150 

3 600 x 150 

4 900 x 150 

5 900 x 300 

6 1500 x 150 

7 1500 x 300 

8 2500 x 300 

O Special 

Flange face 

1 ANSI RF x RF 

2 ANSI RTJ inlet x RF 

O Special 

Trim material nozzle and disc 

1 Stainless steel PH 17/4 

2 Stainless steel 316 

3 Aluminium bronze / Monel 

4 Hastelloy B 

5 Stainless steel 316 stellited 

6 Monel 


O Special 

Spring material 

1 Carbon steel 


6 Tungsten alloy 

9 Hastelloy B 

A Aluminium Coated CS 

N Stainless steel PH 17/4 

Q Stainless steel PH 17/4 NACE 

T Aluminium Coated Tungsten 

Z Inconel X750 

O Special 

*In some instances when 

both high pressures and 

alloy springs are required, 

the ‘H’ needs adding to 

accessories. See individual 

orifice pages. 

Body material 

1 Carbon steel WCB 

2 Carbon steel WCB NACE 

3 Stainless steel CF8M NACE 

4 Stainless steel CF8M 

5 Carbon steel low temperature LCB 

6 Bronze 

8 Carbon steel WC6 - 0.5% Moly 

9 Hastelloy B 

O Special 

17

RECOMMENDED MATERIAL TEMPERATURE RANGES 

Description Minimum Maximum 

deg F deg C deg F deg C 

BODY 

1 Carbon steel SA 216-WCB –20 –29 800 427 

2 Carbon steel (NACE) SA 216-WCB –20 –29 800 427 

3 Stainless steel (NACE) SA 351-CF8M –450 –267 1000 538 

4 Stainless steel SA 351-CF8M –450 –267 1000 538 

5 Low Temp. CS SA 352-LCB –50 –46 800 427 

6 Bronze (Oxygen spec.) BS 1400 LG2 –450 –267 450 232 

8 0.5% MOLY CS SA 217-WC6 –20 –29 1000 538 

9 Hastelloy B SA 494-N12MV –20 –29 1000 538 

SPRING 

1 Carbon steel –75 –59 450 232 

A Aluminium coated CS –75 –59 450 232 

2 Stainless steel (316) –450 –267 500 260 

6 Tungsten alloy (BH12) –4 –20 1000 427 

T Aluminium coated tungsten –4 –20 1000 427 

9 Hastelloy B –20 –29 800 427 

N Stainless steel (PH17/4) –130 –90 752 400 

Q Stainless steel (PH17/4 NACE) –130 –90 752 400 

Z Inconel X750 –450 –267 1000 538 

TRIM (Nozzle and disc) 

1 Stainless steel (PH 17/4 NACE 29-33 HRC) –130 –90 752 400 

2 Stainless steel (316) –450 –267 1000 538 

3 Al. Bronze/Monel –76 –60 572 300 

4 Hastelloy B –20 –29 1000 538 

5 Stainless steel (316 Stellited 39-43 HRC) –321 –196 1000 538 

6 Monel –321 –196 800 427 

7 Stainless steel 304 –238 –150 1000 538 

GASKETS 

NAF (ST-706) –40 –40 800 427 

Graphite (Supergraf) –328 –200 932 500 

Gylon 3504 –321 –196 500 260 

SOFT SEAT 

Nitrile –40 –40 212 100 

Viton –22 –30 392 200 

Silicon –85 –65 446 230 

Ethylene Propylene –58 –50 275 135 

PTFE –454 –270 428 220 

Kalrez –20 –29 500 260 

BOLTING 

B7 Alloy steel –20 –29 800 427 

B8T Stainless Steel –454 –270 1000 538 

Monel K500 –274 –170 482 250 

Notes 

1 All temperatures are at valve inlet. 

2 Trim items 1 and 5 are recommended for maximum durability. 

3 Alternative materials may be specified if agreed on enquiry. 

18

VALVE SELECTION GUIDE 

D 

– 0.110 in 2 

– 71 mm 2 

TEMPERATURE 

LIMITS API 526 

538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

STAINLESS STEEL 

CARBON STEEL TUNGSTEN 

SS - CF8M CARBON STEEL - WCB 

CS - WC6 

SET PRESSURE - PSIG 

0 

7 

8 9 10 11 

1 

2 3 4 5 6 

12 

13 14 15 16 17 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

100 200 300 400 500 1000 2000 3000 4000 5000 6000 

SET PRESSURE - BAR G. 

0 10 20 30 34.5 50 69 100 200 300 413.8 

ORIFICE D (All dimensions in inches) 

Size Rating A B C* D E F G H† Wt 

(ins) 

lbs (kg) 

1 x 2 150 x 150 4.125 1.437 13.875 4.500 0.582 2 3/8 3/4 40 (18) 

300 x 150 4.125 1.437 13.875 4.500 0.582 2 3/8 3/4 40 (18) 

600 x 150 4.125 1.437 13.875 4.500 0.582 2 3/8 3/4 42 (19) 

1 1 /2 x 2 900 x 300 4.125 1.750 25.000 5.500 0.625 5 3/8 3/4 90 (41) 

1500 x 300 4.125 1.750 25.000 5.500 0.625 5 3/8 3/4 97 (44) 

1 1 /2 x 3 2500 x 300 5.500 2.375 26.875 6.500 0.625 5 3/8 3/4 115 (52) 

* – If a gag is fitted, add 0.5 ins. 

– If a bellows is fitted in the 1 x 2 inch valve add 1.125 inch. 

– If a lever is fitted, add a maximum of 3.5 inch. (Only if flange rating is 600# or less.) 

– For certified height (c), consult factory. 

† Vent hole ‘H’ on bellows valves only. 

19

VALVE SELECTION TABLE 

D 

– 0.110 in 2 

– 71 mm 2 

Flanges ANSI 

Key Valve size 

No. inlet x outlet Inlet Outlet Body 

(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 

Max. Set Pressure (psig) 

and Temperature Limits 

-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 1 x 2 150#RF 150#RF - - 285 185 80 - 285 230 

2 1 x 2 300#RF 150#RF - - 740 615 410 - 285 230 

3 1 x 2 600#RF 150#RF - - 1480 1235 825 - 285 230 

WCB 

4 1 1 /2 x 2 900#RF 300#RF - - 2220 1845 1235 - 600 500 

5 1 1 /2 x 2 1500#RF 300#RF - - 3705 3080 2060 - 600 500 

6 1 1 /2 x 3 2500#RF 300#RF - - 6000 5135 3430 - 740 500 

7 1 x 2 300#RF 150#RF - - - - 510 225 285 230 

8 1 x 2 600#RF 150#RF - - - - 1015 445 285 230 

9 1 1 /2 x 2 900#RF 300#RF WC6 - - - - 1525 670 600 500 

10 1 1 /2 x 2 1500#RF 300#RF - - - - 2540 1115 600 500 

11 1 1 /2 x 3 2500#RF 300#RF - - - - 4230 1860 740 500 

12 1 x 2 150#RF 150#RF 275 275 - - - - 275 230 

13 1 x 2 300#RF 150#RF 720 720 - - - - 275 230 

14 1 x 2 600#RF 150#RF 1440 1440 - - - - 275 230 

15 1 1 CF8M 

/2 x 2 900#RF 300#RF 2160 2160 - - - - 600 500 

16 1 1 /2 x 2 1500#RF 300#RF 3600 3600 - - - - 600 500 

17 1 1 /2 x 3 2500#RF 300#RF 4000 6000 - - - - 720 500 

RF=Raised Face 

Minimum set pressure limits for metal seat trim 

Conventional – 7 psig 

Bellows - Gas – 22 psig 

Bellows - Liquid – 59 psig* 


(Inverted) 

*For liquid bellows valves below this pressure refer to factory 

Note: Soft seated valves require a minimum set pressure of 15 psig. 

High Pressure Version 

There is no requirement to have an high pressure version for this orifice. 

20


E 

– 0.196 in 2 

– 127 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 




CS - WC6 


0 

7 

8 9 10 11 

1 

2 3 4 5 6 

12 

13 14 15 16 17 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

100 200 300 400 500 1000 2000 3000 4000 5000 6000 


0 10 20 30 34.5 50 69 100 200 300 413.8 

ORIFICE E (All dimensions in inches) 


(ins) 

lbs (kg) 

1 x 2 150 x 150 4.125 1.437 13.875 4.500 0.582 2 3/8 3/4 40 (18) 

300 x 150 4.125 1.437 13.875 4.500 0.582 2 3/8 3/4 40 (18) 

600 x 150 4.125 1.437 13.875 4.500 0.582 2 3/8 3/4 42 (19) 

1 1 /2 x 2 900 x 300 4.125 1.750 25.000 5.500 0.625 5 3/8 3/4 90 (41) 

1500 x 300 4.125 1.750 25.000 5.500 0.625 5 3/8 3/4 97 (44) 

1 1 /2 x 3 2500 x 300 5.500 2.375 26.875 6.500 0.625 5 3/8 3/4 115 (52) 


– If a bellows is fitted in the 1 x 2 inch valve add 1.125 inch. 




21


E 

– 0.196 in 2 

– 127 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 1 x 2 150#RF 150#RF - - 285 185 80 - 285 230 

2 1 x 2 300#RF 150#RF - - 740 615 410 - 285 230 

3 1 x 2 600#RF 150#RF - - 1480 1235 825 - 285 230 

WCB 

4 1 1 /2 x 2 900#RF 300#RF - - 2220 1845 1235 - 600 500 

5 1 1 /2 x 2 1500#RF 300#RF - - 3705 3080 2060 - 600 500 

6 1 1 /2 x 3 2500#RF 300#RF - - 6000 5135 3430 - 740 500 

7 1 x 2 300#RF 150#RF - - - - 510 225 285 230 

8 1 x 2 600#RF 150#RF - - - - 1015 445 285 230 

9 1 1 /2 x 2 900#RF 300#RF WC6 - - - - 1525 670 600 500 

10 1 1 /2 x 2 1500#RF 300#RF - - - - 2540 1115 600 500 

11 1 1 /2 x 3 2500#RF 300#RF - - - - 4230 1860 740 500 

12 1 x 2 150#RF 150#RF 275 275 - - - - 275 230 

13 1 x 2 300#RF 150#RF 720 720 - - - - 275 230 

14 1 x 2 600#RF 150#RF 1440 1440 - - - - 275 230 

15 1 1 CF8M 

/2 x 2 900#RF 300#RF 2160 2160 - - - - 600 500 

16 1 1 /2 x 2 1500#RF 300#RF 3600 3600 - - - - 600 500 

17 1 1 /2 x 3 2500#RF 300#RF 4000 6000 - - - - 720 500 







(Inverted) 





22


F 

– 0.307 in 2 

– 198 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

CS - WC6 





0 

7 

8 9 10 11 

1 

2 3 4 5 6 

16 17 

12 

13 14 15 17 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

100 200 300 400 500 600 700 1000 3000 5000 


0 10 20 30 40 48.3 69 200 344.8 

ORIFICE F (All dimensions in inches) 


(ins) 

lbs (kg) 

1 1 /2 x 2 150 x 150 4.875 1.625 14.625 4.750 0.750 2 3/8 3/4 46 (21) 

300 x 150 4.875 1.625 14.625 6.000 0.750 2 3/8 3/4 46 (21) 

600 x 150 4.875 1.625 14.625 6.000 0.750 2 3/8 3/4 46 (21) 

1 1 /2 x 3 900 x 300 4.875 1.750 26.750 6.500 0.500 5 3/8 3/4 101 (46) 

1500 x 300 4.875 1.750 26.750 6.500 0.500 5 3/8 3/4 101 (46) 

2500 x 300 5.500 2.375 26.875 6.500 0.625 5 3/8 3/4 117 (53) 





23


F 

– 0.307 in 2 

– 198 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 1 1 /2 x 2 150#RF 150#RF - - 285 185 80 - 285 230 

2 1 1 /2 x 2 300#RF 150#RF - - 740 615 410 - 285 230 

3 1 1 /2 x 2 600#RF 150#RF - - 1480 1235 825 - 285 230 

WCB 

4 1 1 /2 x 3 900#RF 300#RF - - 2220 1845 1235 - 740 500 

5 1 1 /2 x 3 1500#RF 300#RF - - 3705 3080 2060 - 740 500 

6 1 1 /2 x 3 2500#RF 300#RF - - 5000 5000 3430 - 740 500 

7 1 1 /2 x 2 300#RF 150#RF - - - - 510 225 285 230 

8 1 1 /2 x 2 600#RF 150#RF - - - - 1015 445 285 230 

9 1 1 /2 x 3 900#RF 300#RF WC6 - - - - 1525 670 740 500 

10 1 1 /2 x 3 1500#RF 300#RF - - - - 2540 1115 740 500 

11 1 1 /2 x 3 2500#RF 300#RF - - - - 4230 1860 740 500 

12 1 1 /2 x 2 150#RF 150#RF 275 275 - - - - 275 230 

13 1 1 /2 x 2 300#RF 150#RF 720 720 - - - - 275 230 

14 1 1 /2 x 2 600#RF 150#RF 1440 1440 - - - - 275 230 

15 1 1 /2 x 3 900#RF 300#RF 

CF8M 

2160 2160 - - - - 720 500 

16 1 1 /2 x 3 1500#RF 300#RF 2200 3600 - - - - 720 500 

17 1 1 /2 x 3 2500#RF 300#RF 3400 5000 - - - - 720 500 







(Inverted) 





24


G 

– 0.503 in 2 

– 325 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

CS - WC6 





0 

7 

8 9 10 11 

6 

1 2 3 4 5 

15 16 + 17 

12 13 14 16 17 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

100 200 300 500 1000 2000 3000 4000 


0 10 20 50 69 100 150 200 275.9 

ORIFICE G (All dimensions in inches) 


(ins) 

lbs (kg) 

1 1 /2 x 3 150 x 150 4.875 1.312 18.750 4.750 0.500 2 3/8 3/4 60 (27) 

300 x 150 4.875 1.312 18.750 6.000 0.500 2 3/8 3/4 64 (29) 

600 x 150 4.875 1.437 18.750 6.000 0.500 2 3/8 3/4 66 (30) 

900 x 300 4.875 1.750 27.500 6.500 0.500 5 3/8 3/4 119 (54) 

2 x 3 1500 x 300 6.125 2.125 32.000 6.750 0.500 5 1/2 3/4 126 (57) 

2500 x 300 6.125 2.812 32.000 6.750 0.687 2 1/2 3/4 139 (63) 





25


G 

– 0.503 in 2 

– 325 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 1 1 /2 x 3 150#RF 150#RF - - 285 185 80 - 285 230 

2 1 1 /2 x 3 300#RF 150#RF - - 740 615 410 - 285 230 

3 1 1 /2 x 3 600#RF 150#RF - - 1480 1235 825 - 285 230 

WCB 

4 1 1 /2 x 3 900#RF 300#RF - - 2220 1845 1235 - 740 470 

5 2 x 3 1500#RF 300#RF - - 3705 3080 2060 - 740 470 

6 2 x 3 2500#RF 300#RF - - 3705 3705 3430 - 740 470 

7 1 1 /2 x 3 300#RF 150#RF - - - - 510 225 285 230 

8 1 1 /2 x 3 600#RF 150#RF - - - - 1015 445 285 230 

9 1 1 /2 x 3 900#RF 300#RF WC6 - - - - 1525 670 740 470 

10 2 x 3 1500#RF 300#RF - - - - 2540 1115 740 470 

11 2 x 3 2500#RF 300#RF - - - - 3705 1860 740 470 

12 1 1 /2 x 3 150#RF 150#RF 275 275 - - - - 275 230 

13 1 1 /2 x 3 300#RF 150#RF 720 720 - - - - 275 230 

14 1 1 /2 x 3 600#RF 150#RF 1440 1440 - - - - 275 230 

15 1 1 /2 x 3 900#RF 300#RF 

CF8M 

1600 2160 - - - - 720 470 

16 2 x 3 1500#RF 300#RF 2450 3600 - - - - 720 470 

17 2 x 3 2500#RF 300#RF 2600 3600 - - - - 720 470 







(Inverted) 





26


H 

– 0.785 in 2 

– 506 mm 2 

TEMPERATURE 


SPRING 

BODY 

538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 




CS - WC6 

SET PRESSURE - PSIG 0 

7 

8 9 10 

2 

1 

3 4 5 6 

11 + 12 13 14 

15 

16 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

100 200 300 400 500 1000 2000 3000 


0 10 20 30 34.5 50 69 100 150 206.9 

ORIFICE H (All dimensions in inches) 


(ins) 

lbs (kg) 

1 1 /2 x 3 150 x 150 5.125 1.375 18.750 4.875 0.500 2 3/8 3/4 60 (27) 

300 x 150 5.125 1.375 18.750 4.875 0.500 2 3/8 3/4 60 (27) 

2 x 3 300 x 150 5.125 1.375 19.000 4.875 0.500 2 3/8 3/4 64 (29) 

600 x 150 6.062 1.687 20.000 6.375 0.687 2 3/8 3/4 86 (39) 

900 x 150 6.062 2.312 31.750 6.375 0.687 5 1/2 3/4 176 (80) 

1500 x 300 6.062 2.375 32.000 6.375 0.750 5 1/2 3/4 187 (85) 





27


H 

– 0.785 in 2 

– 506 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 1 1 /2 x 3 150#RF 150#RF - - 285 185 80 - 285 230 

2 1 1 /2 x 3 300#RF 150#RF - - 285 285 285 - 285 230 

3 2 x 3 300#RF 150#RF 

WCB 

- - 740 615 410 - 285 230 

4 2 x 3 600#RF 150#RF - - 1480 1235 825 - 285 230 

5 2 x 3 900#RF 150#RF - - 2220 1845 1235 - 285 230 

6 2 x 3 1500#RF 300#RF - - 2750 2750 2060 - 740 415 

7 2 x 3 300#RF 150#RF - - - - 510 225 285 230 

8 2 x 3 600#RF 150#RF - - - - 815 445 285 230 

9 2 x 3 900#RF 150#RF WC6 - - - - 1225 670 285 230 

10 2 x 3 1500#RF 300#RF - - - - 2040 1115 740 415 

11 1 1 /2 x 3 150#RF 150#RF 275 275 - - - - 275 230 

12 1 1 /2 x 3 300#RF 150#RF 275 275 - - - - 275 230 

13 2 x 3 300#RF 150#RF 720 720 - - - - 275 230 

14 2 x 3 600#RF 150#RF CF8M 1440 1440 - - - - 275 230 

15 2 x 3 900#RF 150#RF 1485 2160 - - - - 275 230 

16 2 x 3 1500#RF 300#RF 1600 2750 - - - - 720 415 







(Inverted) 



High Pressure Version. 

Certain spring materials cannot be used in the low pressure version of the valve, up to the maximum 

pressure. If the required set pressure with your choice of spring material is in excess of the figure shown in 

the table either choose another material or add “H” to the valve accessories to select an high pressure valve. 

Inlet Max. Set Spring Material (pressures in Psig) 

Orifice rating Pressure Psig Carbon st. 316 SS Tungsten 17/4PH 17/4PH NACE Inconel X750 

1.5H 150# 285 n/a n/a n/a n/a n/a n/a 

1.5H 300# 285 n/a n/a n/a n/a n/a n/a 

2H 300# 740 n/a n/a n/a n/a n/a n/a 

2H 600# 1480 n/a 1000 n/a n/a 1000 n/a 

28

VALVE SELECTION GUIDE – 1.287 in 2 

J 

– 830 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 





CS - WC6 

0 

1 

7 

2 

100 200 300 400 500 

8 9 

11 + 12 13 14 15 +16 

10 

3 4 5 6 

1000 

900 

800 

700 

600 

500 

400 

300 

538 

500 

450 

400 

350 

300 

250 

200 

150 

200 

100 

100 

50 

0 

0 

14 15 16 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

1000 2000 3000 


0 10 20 30 34.5 50 69 100 150 206.9 

ORIFICE J (All dimensions in inches) 


(ins) 

lbs (kg) 

2 x 3 150 x 150 5.375 1.625 19.250 4.875 0.750 2 3/8 3/4 64 (29) 

300 x 150 5.375 1.625 19.250 4.875 0.750 2 3/8 3/4 64 (29) 

3 x 4 300 x 150 7.250 2.125 21.750 7.125 0.750 2 3/8 3/4 82 (37) 

600 x 150 7.250 2.125 23.000 7.125 0.750 2 3/8 3/4 99 (45) 

900 x 150 7.250 2.375 33.875 7.125 0.750 5 1/2 3/4 231 (105) 

1500 x 300 7.250 2.750 33.875 7.125 0.750 5 1/2 3/4 253 (115) 





29

VALVE SELECTION TABLE – 1.287 in 2 

J 

– 830 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 2 x 3 150#RF 150#RF - - 285 185 80 - 285 230 

2 2 x 3 300#RF 150#RF - - 285 285 285 - 285 230 

3 3 x 4 300#RF 150#RF 

WCB 

- - 740 615 410 - 285 230 

4 3 x 4 600#RF 150#RF - - 1480 1235 825 - 285 230 

5 3 x 4 900#RF 150#RF - - 2220 1845 1235 - 285 230 

6 3 x 4 1500#RF 300#RF - - 2700 2700 2060 - 600 230 

7 3 x 4 300#RF 150#RF - - - - 510 225 285 230 

8 3 x 4 600#RF 150#RF 

WC6 

- - - - 815 445 285 230 

9 3 x 4 900#RF 150#RF - - - - 1225 670 285 230 

10 3 x 4 1500#RF 300#RF - - - - 2040 1115 600 230 

11 2 x 3 150#RF 150#RF 275 275 - - - - 275 230 

12 2 x 3 300#RF 150#RF 275 275 - - - - 275 230 

13 3 x 4 300#RF 150#RF 500 720 - - - - 275 230 

CF8M 

14 3 x 4 600#RF 150#RF 625 1440 - - - - 275 230 

15 3 x 4 900#RF 150#RF 800 2160 - - - - 275 230 

16 3 x 4 1500#RF 300#RF 800 2700 - - - - 600 230 






Conventional – 1.5 psig 

(Inverted) 









2J 150# 285 n/a n/a n/a n/a n/a n/a 

2J 300# 285 n/a n/a n/a n/a n/a n/a 

3J 300# 740 n/a 500 n/a n/a 430 n/a 

3J 600# 1480 n/a 900 n/a 900 870 900 

30


K 

– 1.838 in 2 

– 1185 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

INCONEL 

CS - WC6 





0 

1 

6 

10 11 

7 8 

12 

+ 

13 

14 

100 200 300 400 500 600 700 

9 

2 3 4 

1000 

900 

800 

700 

600 

500 

400 

300 

538 

500 

450 

400 

350 

300 

250 

200 

150 

200 

100 

100 

50 

0 

0 

14 

-50 

-100 

-100 

-200 

-300 

-150 

-200 

-400 -250 

-450 -268 

1000 2000 3000 

12 13 

5 


0 10 20 30 40 48.3 69 100 150 206.9 

ORIFICE K (All dimensions in inches) 


(ins) 

lbs (kg) 

3 x 4 150 x 150 6.125 2.000 21.750 6.375 0.875 3 3/8 3/4 108 (49) 

300 x 150 6.125 2.000 21.750 6.375 0.875 3 3/8 3/4 108 (49) 

600 x 150 7.250 2.125 23.500 7.125 0.875 3 3/8 3/4 141 (64) 

3 x 6 900 x 150 7.812 2.562 40.000 8.500 0.937 5 3/4 3/4 339 (154) 

1500 x 300 7.750 2.875 40.000 8.500 0.875 5 3/4 3/4 353 (160) 





31


K 

– 1.838 in 2 

– 1185 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 3 x 4 150#RF 150#RF - - 285 185 80 - 285 150 

2 3 x 4 300#RF 150#RF - - 740 615 410 - 285 150 

3 3 x 4 600#RF 150#RF WCB - - 1480 1235 825 - 285 200 

4 3 x 6 900#RF 150#RF - - 2220 1845 1235 - 285 200 

5 3 x 6 1500#RF 300#RF - - 2220 2220 2060 - 600 200 

6 3 x 4 300#RF 150#RF - - - - 510 215 285 150 

7 3 x 4 600#RF 150#RF - - - - 815 445 285 200 

WC6 

8 3 x 6 900#RF 150#RF - - - - 1225 670 285 200 

9 3 x 6 1500#RF 300#RF - - - - 2040 1115 600 200 

10 3 x 4 150#RF 150#RF 275 275 - - - - 275 150 

11 3 x 4 300#RF 150#RF 525 720 - - - - 275 150 

12 3 x 4 600#RF 150#RF CF8M 600 1440 - - - - 275 200 

13 3 x 6 900#RF 150#RF 600 2160 - - - - 275 200 

14 3 x 6 1500#RF 300#RF 750 2220 - - - - 600 200 







(Inverted) 




Certain spring materials can not be used in the low pressure version of the valve, up to the maximum 





K 150# 285 n/a n/a n/a n/a n/a n/a 

K 300# 740 n/a 450 n/a n/a 600 n/a 

K 600# 1480 n/a 750 n/a n/a 570 1070 

32


L 

– 2.853 in 2 

– 1840 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

CS - WC6 





0 

10 

7 

8 9 

6 

2 

1 

3 4 5 

13 

14 15 

11 + 12 13 + 14 15 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

200 400 600 800 1000 1500 


0 10 20 30 40 50 60 69 

103.5 

ORIFICE L (All dimensions in inches) 


(ins) 

lbs (kg) 

3 x 4 150 x 150 6.125 2.000 21.750 6.500 0.875 4 3/8 3/4 108 (49) 

300 x 150 6.125 2.000 21.750 6.500 0.875 4 3/8 3/4 108 (49) 

4 x 6 300 x 150 7.062 2.062 27.000 7.125 0.812 5 1/2 1 234 (106) 

600 x 150 7.062 2.312 28.250 8.000 0.812 5 1/2 1 249 (113) 

900 x 150 7.750 2.687 41.875 8.750 0.750 5 1/2 1 353 (160) 

1500 x 150 7.750 2.937 44.000 8.750 0.812 5 1/2 1 361 (164) 





33


L 

– 2.853 in 2 

– 1840 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 3 x 4 150#RF 150#RF - - 285 185 80 - 285 100 

2 3 x 4 300#RF 150#RF - - 285 285 285 - 285 100 

3 4 x 6 300#RF 150#RF 

WCB 

- - 740 615 410 - 285 170 

4 4 x 6 600#RF 150#RF - - 1000 1000 825 - 285 170 

5 4 x 6 900#RF 150#RF - - 1500 1500 1235 - 285 170 

6 4 x 6 1500#RF 150#RF - - 1500 1500 1500 - 285 170 

7 4 x 6 300#RF 150#RF - - - - 510 225 285 170 

8 4 x 6 600#RF 150#RF 

WC6 

- - - - 1000 445 285 170 

9 4 x 6 900#RF 150#RF - - - - 1500 670 285 170 

10 4 x 6 1500#RF 150#RF - - - - 1500 1115 285 170 

11 3 x 4 150#RF 150#RF 275 275 - - - - 275 100 

12 3 x 4 300#RF 150#RF 275 275 - - - - 275 100 

13 4 x 6 300#RF 150#RF CF8M 535 720 - - - - 275 170 

14 4 x 6 600#RF 150#RF 535 1000 - - - - 275 170 

15 4 x 6 900#RF 150#RF 700 1500 - - - - 275 170 







(Inverted) 









3L 150# 285 n/a n/a n/a n/a 255 n/a 

3L 300# 285 n/a n/a n/a n/a 255 n/a 

4L 300# 740 n/a n/a n/a n/a n/a n/a 

4L 600# 1000 n/a 850 n/a n/a 825 n/a 

34


M 

– 3.60 in 2 

– 2320 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

INCONEL 

CS - WC6 





0 

1 

5 

8 9 10 

6 7 

2 3 

200 400 600 800 

10 

4 

1000 

900 

800 

700 

600 

500 

400 

300 

538 

500 

450 

400 

350 

300 

250 

200 

150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

1000 1500 


0 10 20 30 40 50 60 69 

103.5 

ORIFICE M (All dimensions in inches) 


(ins) 

lbs (kg) 

4 x 6 150 x 150 7.000 2.000 26.500 7.250 0.750 5 1/2 1 234 (106) 

300 x 150 7.000 2.000 26.500 7.250 0.750 5 1/2 1 234 (106) 

600 x 150 7.000 2.250 28.250 8.000 0.750 5 1/2 1 249 (113) 

900 x 150 7.750 2.500 41.875 8.750 0.750 5 1/2 1 377 (171) 





35


M 

– 3.60 in 2 

– 2320 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 4 x 6 150#RF 150#RF - - 285 185 80 - 285 80 

2 4 x 6 300#RF 150#RF - - 740 615 410 - 285 160 

WCB 

3 4 x 6 600#RF 150#RF - - 1100 1100 825 - 285 160 

4 4 x 6 900#RF 150#RF - - 1100 1100 1100 - 285 160 

5 4 x 6 300#RF 150#RF - - - - 510 225 285 160 

6 4 x 6 600#RF 150#RF WC6 - - - - 1015 445 285 160 

7 4 x 6 900#RF 150#RF - - - - 1100 670 285 160 

8 4 x 6 150#RF 150#RF 275 275 - - - - 275 80 

9 4 x 6 300#RF 150#RF CF8M 525 720 - - - - 275 160 

10 4 x 6 600#RF 150#RF 600 1000 - - - - 275 160 







(Inverted) 









M 150# 285 n/a n/a n/a n/a n/a n/a 

M 300# 740 n/a 500 n/a n/a 590 n/a 

M 600# 1100 n/a 825 n/a 1000 900 1000 

36


N 

– 4.34 ins 2 

– 2800 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 





CS - WC6 

0 

1 

5 

8 9 10 

6 7 

2 3 

200 400 600 800 

10 

4 

1000 

900 

800 

700 

600 

500 

400 

300 

538 

500 

450 

400 

350 

300 

250 

200 

150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

1000 


0 10 20 30 40 50 60 69 

ORIFICE N (All dimensions in inches) 


(ins) 

lbs (kg) 

4 x 6 150 x 150 7.750 2.000 29.250 8.250 0.750 5 1/2 1 242 (110) 

300 x 150 7.750 2.000 29.250 8.250 0.750 5 1/2 1 242 (110) 

600 x 150 7.750 2.250 34.250 8.750 0.750 5 1/2 1 258 (117) 

900 x 150 7.750 2.500 41.500 8.750 0.750 5 1/2 1 395 (179) 





37


N 

– 4.34 ins 2 

– 2800 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 4 x 6 150#RF 150#RF - - 285 185 80 - 285 80 

2 4 x 6 300#RF 150#RF - - 740 615 410 - 285 160 

WCB 

3 4 x 6 600#RF 150#RF - - 1000 1000 825 - 285 160 

4 4 x 6 900#RF 150#RF - - 1000 1000 1000 - 285 160 

5 4 x 6 300#RF 150#RF - - - - 510 225 285 160 

6 4 x 6 600#RF 150#RF WC6 - - - - 1000 445 285 160 

7 4 x 6 900#RF 150#RF - - - - 1000 670 285 160 

8 4 x 6 150#RF 150#RF 275 275 - - - - 275 80 

9 4 x 6 300#RF 150#RF CF8M 450 720 - - - - 275 160 

10 4 x 6 600#RF 150#RF 500 1000 - - - - 275 160 







(Inverted) 









N 150# 285 n/a n/a n/a n/a n/a n/a 

N 300# 740 n/a 680 n/a n/a 700 n/a 

N 600# 1000 n/a n/a n/a n/a n/a n/a 

38


P 

– 6.38 ins 2 

– 4120 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

INCONEL 

CS - WC6 





0 

1 

5 

8 9 10 

6 7 

2 3 

10 

200 400 600 800 

4 

1000 

900 

800 

700 

600 

500 

400 

300 

538 

500 

450 

400 

350 

300 

250 

200 

150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

1000 


0 10 20 30 40 50 60 69 

ORIFICE P (All dimensions in inches) 


(ins) 

lbs (kg) 

4 x 6 150 x 150 7.125 2.000 27.750 9.000 0.750 5 1/2 1 254 (115) 

300 x 150 8.875 2.000 29.250 10.000 0.750 5 1/2 1 254 (115) 

600 x 150 8.875 2.250 35.250 10.000 0.750 5 1/2 1 269 (122) 

900 x 150 8.875 2.500 45.000 10.000 0.750 5 1/2 1 412 (187) 





39


P 

– 6.38 ins 2 

– 4120 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 4 x 6 150#RF 150#RF - - 285 185 80 - 285 80 

2 4 x 6 300#RF 150#RF - - 525 525 410 - 285 150 

WCB 

3 4 x 6 600#RF 150#RF - - 1000 1000 825 - 285 150 

4 4 x 6 900#RF 150#RF - - 1000 1000 1000 - 285 150 

5 4 x 6 300#RF 150#RF - - - - 510 225 285 150 

6 4 x 6 600#RF 150#RF WC6 - - - - 1000 445 285 150 

7 4 x 6 900#RF 150#RF - - - - 1000 670 285 150 

8 4 x 6 150#RF 150#RF 175 275 - - - - 275 80 

9 4 x 6 300#RF 150#RF CF8M 300 525 - - - - 275 150 

10 4 x 6 600#RF 150#RF 480 1000 - - - - 275 150 







(Inverted) 









P 150# 285 n/a n/a n/a n/a n/a n/a 

P 300# 525 n/a 350 n/a n/a 390 460 

P 600# 1000 n/a 640 n/a n/a 336 n/a 

40


Q 

– 11.05 in 2 

– 7130 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 

CS - WC6 





0 

4 

5 

1 

6 

7 

2 3 

8 

8 

10 50 100 200 300 400 500 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

600 


0 0.69 2 4 6 6.9 10 15 20 30 41.4 

ORIFICE Q (All dimensions in inches) 


(ins) 

lbs (kg) 

6 x 8 150 x 150 9.437 2.312 31.750 9.500 0.812 3 1/2 1 364 (165) 

300 x 150 9.437 2.312 31.750 9.500 0.812 3 1/2 1 364 (165) 

600 x 150 9.437 2.750 36.750 9.500 0.812 6 1/2 1 399 (181) 





41


Q 

– 11.05 in 2 

– 7130 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 6 x 8 150#RF 150#RF - - 165 165 80 - 115 70 

2 6 x 8 300#RF 150#RF WCB - - 300 300 300 - 115 115 

3 6 x 8 600#RF 150#RF - - 600 600 600 - 115 115 

4 6 x 8 300#RF 150#RF 

WC6 

- - - - 165 165 115 115 

5 6 x 8 600#RF 150#RF - - - - 600 445 115 115 

6 6 x 8 150#RF 150#RF 165 165 - - - - 115 70 

7 6 x 8 300#RF 150#RF CF8M 250 300 - - - - 115 115 

8 6 x 8 600#RF 150#RF 300 600 - - - - 115 115 







(Inverted) 









Q 150# 165 n/a 109 n/a n/a 117 n/a 

Q 300# 300 n/a 109 n/a 165 117 165 

Q 600# 600 n/a 109 n/a 400 117 390 

42


R 

– 16.00 in 2 

– 10300 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 





CS - WC6 

0 

7 + 8 

5 

1 

10 20 30 40 50 100 200 

9 

2 

6 

3 4 

10 

10 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

300 


0 1 2 3 3.45 5 6.9 10 15 20.7 

ORIFICE R (All dimensions in inches) 


(ins) 

lbs (kg) 

6 x 8 150 x 150 9.437 2.312 34.750 9.500 0.812 3 1/2 1 370 (168) 

300 x 150 9.437 2.312 34.750 9.500 0.812 3 1/2 1 370 (168) 

6 x 10 300 x 150 9.437 2.312 34.750 10.500 0.812 3 1/2 1 465 (211) 

600 x 150 9.437 2.750 40.750 10.500 0.812 6 1/2 1 489 (222) 





43


R 

– 16.00 in 2 

– 10300 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 

1 6 x 8 150#RF 150#RF - - 100 100 80 - 60 60 

2 6 x 8 300#RF 150#RF - - 100 100 100 - 60 60 

WCB 

3 6 x 10 300#RF 150#RF - - 230 230 230 - 100 100 

4 6 x 10 600#RF 150#RF - - 300 300 300 - 100 100 

5 6 x 8 300#RF 150#RF - - - - 100 100 60 60 

WC6 

6 6 x 10 600#RF 150#RF - - - - 300 300 100 100 

7 6 x 8 150#RF 150#RF 55 100 - - - - 60 60 

8 6 x 8 300#RF 150#RF 55 100 - - - - 60 60 

CF8M 

9 6 x 10 300#RF 150#RF 150 230 - - - - 100 100 

10 6 x 10 600#RF 150#RF 200 300 - - - - 100 100 







(Inverted) 









6R8 150# 100 n/a 73 n/a n/a 90 n/a 

6R8 300# 100 n/a 73 n/a n/a 90 n/a 

6R10 300# 230 n/a 73 n/a 224 90 147 

6R10 600# 300 n/a 190 n/a n/a 180 n/a 

44


T 

– 26.00 in 2 

– 16770 mm 2 

TEMPERATURE 


538 1000 

427 800 

232 450 

-29 -20 

-59 -75 

-268 -450 

SPRING 

BODY 





CS - WC6 

0 

4 

1 

5 

3 

10 20 30 40 50 60 70 80 90 100 200 

5 

2 

1000 538 

500 

900 

450 

800 

400 

700 

350 

600 

300 

500 250 

400 200 

300 150 

100 

200 

50 

100 

0 

0 

-50 

-100 

-100 

-200 

-150 

-300 

-200 

-400 -250 

-450 -268 

300 


0 1 2 3 4 5 6 6.9 

20.7 

ORIFICE T (All dimensions in inches) 


(ins) 

lbs (kg) 

8 x 10 150 x 150 10.875 2.625 42.250 11.000 1.000 6 1/2 1 661 (300) 

300 x 150 10.875 2.625 42.250 11.000 1.000 6 1/2 1 683 (310) 





45


T 

– 26.00 in 2 

– 16770 mm 2 

Flanges ANSI 



(ins) 

Mat’l 

-76°F 

to 

-450°F 

-60°C 

to 

-268°C 



1 8 x 10 150#RF 150#RF - - 65 65 65 - 30 30 

WCB 

2 8 x 10 300#RF 150#RF - - 300 300 300 - 100 100 

3 8 x 10 300#RF 150#RF WC6 - - - - 300 225 100 100 

4 8 x 10 150#RF 150#RF 50 65 - - - - 30 30 

CF8M 

5 8 x 10 300#RF 150#RF 65 120 - - - - 60 60 

-21°F 

to 

-75°F 

-30°C 

to 

-59°C 

100°F 

to 

-20°F 

38°C 

to 

-29°C 

450°F 800°F 1000°F 

232°C 427°C 538°C 

Max. Back 

Pressure 

(psig) 

Conventional 

Valve 

Balanced 

Bellows Valve 







(Inverted) 









T 150# 65 n/a n/a n/a n/a n/a n/a 

T 300# 300 200 79 200 138 96 83 

46

PRESSURE ADJUSTMENT 

Each valve is factory set and normally would not 

need any pressure adjustment; however, if the 

occasion arises, the following procedures apply: 

ASME 

If the valve is ASME stamped (UV)(NB), repairs 

must be carried out by an ASME authorised repair 

company, i.e. one holding either the (UV) or (VR) 

stamp. 

3 

13 

12 

Test rig 

It is normally desirable to remove the valve from 

the plant and use a suitable test rig, specifically 

designed for safety relief valves. 

Range 

Each spring has a definite range, the limits of which 

should not be exceeded. These should be checked 

with the factory, since if a set pressure change is 

required, it may be necessary to obtain a new 

spring. Should the set pressure be changed it is 

important to ensure that the valve will still have 

sufficient capacity under the new service 

conditions. 

Procedure 

1) Remove the cap (3). This exposes the 

compression screw (12). 

2) Slacken the locknut (13). 

3) Introduce pressure to the valve inlet. The spring 

pressure and consequently the valve set 

pressure may be increased or decreased by 

turning the compression screw. 

Clockwise - increases set pressure 

Anti-clockwise - decreases set pressure 

Check against a calibrated pressure gauge. 

4) Re-tighten the locknut and refit the cap. 

Replace cap gasket if damaged. 

5) Check seat leakage (see page 49/50). 

COLD DIFFERENTIAL TEST PRESSURE 

When setting a valve intended for use at high 

temperature on a test rig using a test fluid at 

ambient temperatures, it is necessary to set 

the valve at a slightly higher pressure, so that 

it will open at the correct set pressure under 

relieving conditions. The necessary allowance 

is shown in the following table. 

Relieving Relieving % Increase in set 

temperature – temperature – pressure at ambient 

Centigrade Fahrenheit temperature 

WB, B + C Series 

Up to 121°C Up to 250°F None 

>121°C to 316°C >250°F to 600°F 1 

>316°C to 427°C >600°F to 800°F 2 

>427°C to 538°C >800°F to 1000°F 3 

D Series 

Up to 100°C Up to 212°F None 

>100°C to 150°C >212°F to 302°F 2 

>150°C to 260°C >302°F to 500°F 3 

47

BLOWDOWN RING SETTING 

The blowdown adjustment is achieved by means of 

a single blowdown ring. 

By reducing the blowdown gap, 

blowdown is increased. 

By increasing the blowdown gap, 

blowdown is decreased. 

Blowdown is defined as the difference between the 

set pressure of the valve and the reseating 

pressure. 

Important 

It is important to reset the blowdown ring to 

the correct position after maintenance has 

been carried out on the valve. 

Method 

Note: It is dangerous to adjust the blowdown ring 

when the valve is under pressure, unless a suitable 

‘test gag’ is fitted to prevent inadvertent valve 

opening 

1) Removing the setting screw in the body permits 

access to the blowdown ring. 

2) Close the blowdown gap until the ring is 

touching the face of the disc (turn the ring from 

left to right). 

3) Consult the chart in the maintenance manual to 

determine the correct blowdown ring setting, 

or consult the factory. 

4) Wind back the blowdown ring the right number 

of notches to establish the correct position 

(count the number of notches or serrations on 

the outside of the blowdown ring). 

5) Relocate the locking pin into the notches 

provided on the outside diameter of the 

blowdown ring. 

6) Tighten up the setting screw, ensuring that an 

undamaged gasket is in place. 

48

SEAT TIGHTNESS / SEAT LEAKAGE TESTING 

(in accordance with API 527) 

Described here are methods of determining the 

seat tightness of metal and soft-seated pressure 

relief valves, including those of conventional, 

bellows and pilot-operated designs. 

The maximum acceptable leakage rates are defined 

for pressure relief valves with set pressures from 

15 Psig to 6,000 Psig. If greater seat tightness is 

required, the purchaser shall specify it in the 

purchase order. 

The test medium for determining the seat tightness 

- air, steam or water - shall be the same as that 

used for determining the set pressure of the valve. 

For dual-service valves, the test medium - air, 

steam or water - shall be the same as the primary 

relieving medium. 

To ensure safety, the procedures outlined shall be 

performed by persons experienced in the use and 

functions of pressure relief valves. 

Test apparatus for 

air seat tightness 

Tube 5 ⁄16" o.d. x 0.035" wall 

1 

⁄2" 

TESTING WITH AIR 

A1 Test apparatus 

A test arrangement for determining seat tightness 

with air is shown opposite. Leakage shall be 

measured using a tube with an outside diameter of 

5/16 inch and a wall thickness of 0.035 inch. The 

tube end shall be cut square and smooth. The tube 

opening shall be 1 ⁄2 inch below the surface of the 

water. The tube shall be perpendicular to the 

surface of the water. 

A2 Test medium 

The test medium shall be air (or nitrogen) near 

ambient temperature. 

A3 Test configuration 

The valve shall be vertically mounted on the test 

stand and the test apparatus shall be attached to 

the valve outlet, as shown opposite. All openings - 

including but not limited to caps, drain holes, vents, 

and outlets – shall be closed. 

A4 Test pressure 

For a valve whose set pressure is greater than 

50 Psig, the leakage rate (in bubbles per minute) 

shall be determined with the test pressure at the 

valve inlet held at 90% of the set pressure. For a 

valve set at 50 Psig or less, the test pressure shall 

be held at 5 Psig less than the set pressure. 

A5 Leakage test 

Before the leakage test, the set pressure shall be 

demonstrated and all valve body joints and fittings 

should be checked with a suitable solution to 

ensure that all joints are tight. 

Air receiver 

API 527 Air Leakage Rates 

Cover plate 

Before the bubble count, the test pressure shall be 

applied for the following times: 

Valve size 

Time 

Up to 2ins 

1 min 

3ins to 4ins 

2 min 

6ins and above 5 min 

The valve shall then be observed for leakage for at 

least 1 minute. 

A6 Acceptance criteria 

For a valve with a metal seat, the leakage rate in 

bubbles per minute shall not exceed the 

appropriate value in chart opposite. For a softseated 

valve, there shall be no leakage for 1 minute 

(0 bubbles per minute). 

49

SEAT TIGHTNESS / SEAT LEAKAGE TESTING 

(in accordance with API 527) 

TESTING WITH STEAM 

S1 Test medium 

The test medium shall be saturated steam. 

S2 Test configuration 

The valve shall be vertically mounted on the steam 

test stand. 

S3 Test pressure 


50 Psig, the seat tightness shall be determined 

with the test pressure at the valve inlet held at 

90% of the set pressure. For a valve set at 50 Psig 

or less, the test pressure shall be held at 

5 Psig less than the set pressure. 

S4 Leakage test 

Before starting the seat tightness test, the set 

pressure shall be demonstrated and the test 

pressure shall be held for at least 3 minutes. Any 

condensate in the body bowl shall be removed 

before the seat tightness test. Air (or nitrogen) 

may be used to dissipate condensate. 

After any condensate has been removed, the inlet 

pressure shall be increased to the test pressure. 

Tightness shall then be checked visually using a 

black background. 

The valve shall then be observed for leakage for at 

least 1 minute. 

S5 Acceptance criteria 

For both metal and soft-seated valves, there shall 

be no audible or visible leakage for 1 minute. 

TESTING WITH WATER 

IMPORTANT– ALL LIQUID TRIM VALVES 

MUST BE TESTED ON WATER. 

OTHERWISE SET PRESSURES AND 

LEAKAGE RATE RESULTS WILL BE FALSE. 

W1 Test medium 

The test medium shall be water near ambient 

temperature. 

W2 Test configuration 

The valve shall be vertically mounted on the water 

test stand. 

W3 Test pressure 


50 Psig, the seat tightness shall be determined 

with the test pressure at the valve inlet held at 

90% of the set pressure. For a valve set at 50 Psig 

or less, the test pressure shall be held at 

5 Psig less than the set pressure. 

W4 Leakage test 

Before starting the seat tightness, the set pressure 

shall be demonstrated and the outlet body bowl 

shall be filled with water. The pressure gauge shall 

be allowed to stabilise with no visible flow from 

the valve outlet. The inlet pressure shall then be 

increased to the test pressure. The valve shall then 

be observed for 1 minute at the test pressure. 

W5 Acceptance criteria 

For a metal-seated valve whose inlet has a nominal 

pipe-size of 1 inch or larger, the leakage rate shall 

not exceed 10 cubic centimetres per hour per inch 

of nominal inlet size. For a metal-seated valve 

whose inlet has a nominal pipe size of less than 

1 inch, the leakage rate shall not exceed 10 cubic 

centimetres per hour. For soft-seated valves, there 

shall be no leakage for 1 minute. 

IMPORTANT: Test rig cleanlines is vital to avoid contamination and damage to the safety relief 

valve seat surfaces. 

50

Safeflo - 

Safety and thermal relief valves 

1 Accessories 

2 High 

performance 

springs 

3 Guiding 

surfaces 

4 Trim 

51


Design verification – The: 

B Series (gas/vapour) 

C Series (liquid) 

D Series (gas, liquid and steam) 

have been developed on Birkett’s in-house, 

extensive mass flow test facility. 

Simplified maintenance – For the flanged 

version, a slip on inlet flange makes it easier to 

realign into existing pipework after servicing. 

Interchangeable parts – Valves can easily be 

modified from gas to liquid or liquid to gas with 

the minimum number of parts. The 

D Series is certified for all media without 

modification. 

Safe and reliable – Proven dependability 

ensures safe and reliable performance. 

Cryogenic and oxygen service – Birkett’s 

state-of-the-art clean room and vapour 

degreasing facilities ensure compliance with the 

stringent demands of cryogenic and oxygen 

applications. (The D Series is not available for 

cryogenic service.) 

1. Wide range of accessories – Available to 

comply with international codes and to suit 

system requirements. 

2. High performance springs – Safety relief 

valve springs are specially designed to guarantee 

set point repeatability. 

3. Guiding surfaces – The material selection of 

guiding components, together with self aligning 

disc pivot points, ensures correct alignment and 

no galling of guiding surfaces. 

4. Trim – B/C Series valves have been designed 

with metal trim to give optimum performance 

at higher pressures. The 7D is available with a 

soft seat or metal seat, while the 6D is metal 

seated only. 

Balanced piston – Available on the 7D Series to 

counter the effects of variable back pressure. 

Material selection – A wide range of 

materials are offered, including non-ferrous for 

low temperature and oxygen service, and 

exotic alloys specified for the chemical and 

process industries. 

52

B/C SERIES THERMAL RELIEF VALVES 

Cryogenic versions of the B and C Series are available 



3 Cap ASTM A108-1021 ASTM A479-316L 



6* Disc holder SA564 17/4 (33HRC) SA564 17/4 (33HRC) 

9 Guide SA351-CF8M ST ST SA351-CF8M ST ST 

11 Lower spring plate ASTM A479-431 ASTM A479-431 


13 Locking nut ASTM A479-316L ASTM A479-316L 

22* Spring C.S. ALUMINIUM COATED ASTM A313-316 



32 Upper spring plate ASTM A479 431 ASTM A479 431 


34 Hammer drive screw ELECTRO BRASSED CS ASTM A479-316L 

76 Capscrew ST ST BS6105 A2-70 ST ST BS6105 A2-70 

85 Flange SA 105 CARB ST SA 182-F316 ST ST 

* Recommended spares. 

53

D SERIES THERMAL RELIEF VALVES 






9 Guide 17/4 PH ST ST 17/4 PH ST ST 

10 Spindle 316 ST ST 316 ST ST 


13 Locking nut ASTM A479-316L ASTM A479-316L 

22* Spring C.S. ALUMINIUM COATED ASTM A313-316 


30 Body gasket ST-706 ST-706 


34 Hammer drive screw ELECTRO BRASSED CS. ASTM A479-316L 

75 Grub screw ASTM A479-316L ASTM A479-316L 

85 Inlet flange SA 105 CARB ST SA 182-F316 ST ST 

147 Flange nut SA564 17/4 (33HRC) SA564 17/4 (33HRC) 

188 Adjusting screw bush VIRGIN PTFE VIRGIN PTFE 

235 Spring end plate ASTM A479-431 ASTM A479-431 

* Recommended spares. 

54

ACCESSORIES 

Packed lever 

The design of the packed lever assembly ensures that 

leakage does not occur when the valve is open or 

when back pressure is present. 

Balanced piston 

This option is only available on the 7D Series 

valves. It is designed to overcome the effects of 

variable back pressure. 

Open lever design is not available on Safeflo valves. 

Gag Screw 

Sealing Plug 

Test gag 

The test gag is used to prevent the 

safety valve from lifting. This is 

mainly used when carrying out a 

hydrostatic test on the system, 

during commissioning. 

The test gag must never be left in 

the valve. The sealing plug should 

be fitted at all other times, allowing 

the valve to operate freely. 

Pressure adjustment: refer to page 47. 

Cold differential test pressure: refer to page 47. 

Seat tightness/Seat leakage testing: refer to page 49/50. 

Minimum pressure setting: B Series, normal attitude 10 Psig. 

C Series, normal attitude 15 Psig. 

7D Series, normal attitude 5 Psig. 

6D Series, normal attitude 740 Psig. 

Lower pressures are NOT available by inverting the valve. 

55

SAFEFLO - SAFETY AND THERMAL RELIEF VALVE 

Different types: There are two different types, both are conventional designs. 

B Series - Safety and thermal relief valve, gas duty. 

C Series - Safety and thermal relief valve, liquid duty. 

D Series - Safety and thermal relief valve, gas, steam and liquid duty. 

All are certified in accordance with ASME Code Section VIII. 

SAFEFLO FIGURE NUMBERING SYSTEM 

/ / 

Orifice 

Area in 2 

1 0.062** 

2 0.110** 

3 0.196** 

4 0.442** 

6 0.070* 

7 0.169* 

Accessories 

C Screwed cap 


G Test gag 

F Ferrule (Government ring) 

S Special feature 

B Balanced piston† 

R Soft seat† 

Valve series 

B Gas vapour 

C Liquid 

D Gas, steam and liquid 

Connection size 

1 1 

⁄2" x 1" orifice 1, 2, 6 or 7 

2 3 

⁄4" x 1" orifice 1, 2, 6 or 7 

3 1" x 1 1 ⁄2" orifice 3 

4 1 1 ⁄2" x 2" orifice 4 – screwed only 

5 1" x 1" orifice 1, 2, 6 or 7 

Inlet connection type 

A BSP Tr male screwed 

B BSP female screwed 

C API male screwed 

D API female screwed 

1 ANSI 300/600 RF flange 

2 ANSI 900/1500 RF flange** 

3 ANSI 2500 RF flange** 

7 ANSI 150 RF flange 

O Special** 

Outlet connection type 

E BSP female screwed 

F API female screwed 

1 ANSI 150 RF flange 

2 ANSI 300 RF flange** 

O Special** 

Trim - nozzle and disc material 

1 Stainless steel PH 17/4 


4 Hastelloy B** 

5 Stainless steel 316 stellited** 

6 Monel 400** 

7 Stainless steel 304** 

O Special** 

Spring material 

A Aluminium Coated CS 


6 Tungsten alloy 

9 Hastelloy B** 

T Aluminium Coated Tungsten 

Z Inconel X750 

O Special** 

Body material 

1 Carbon steel WCB 

2 Carbon steel WCB NACE 

3 Stainless steel CF8M NACE 

4 Stainless steel CF8M 

O Special** 

*Available on D Series only. 

**Not available on D Series. 

†Available on 7D only. 

56

B/C SERIES DIMENSIONS 

Female screwed Male screwed Flanged 

Note: A packed 

lever version and 

gagging facility are 

also available. 

Sizes (ins) 

inlet & outlet 

Inlet & Outlet 

connection 

Orifice 

No. 

Max pressure upto 

Dimensions ins (mm) 

100°F (Psig) 

A B C* D Inlet Outlet 

Weight 

lbs (kg) 

1/2 x 1 Screwed 1 2.500 - 8.625 1.687 5000 

Female 2 (64) (219) (43) 3600 

425 9 (4) 

3/4 x 1 

x 1 2.500 - 8.625 1.687 5000 

Female 2 (64) (219) (43) 3600 

425 9 (4) 

1 x 1 

1 2.500 - 8.625 1.687 5000 

2 (64) (219) (43) 3600 

425 9 (4) 

1 x 11/2 3 

3.250 - 13.750 2.375 

(83) (349) (60) 

5000 425 26 (12) 

11/2 x 2 4 

3.250 - 13.750 2.375 

(83) (349) (60) 

2500 375 26 (12) 

1/2 x 1 

Screwed 1 2.562 0.750 9.375 1.687 3000 

Male 2 (65) (19) (238) (43) 1480 

425 9 (4) 

3/4 x 1 

x 1 2.562 0.750 9.375 1.687 5000 

Female 2 (65) (19) (238) (43) 3600 

425 9 (4) 

1 x 1 

1 2.312 1.000 9.375 1.687 5000 

2 (59) (25) (238) (43) 3600 

425 9 (4) 

1 x 11/2 3 

3.250 1.000 14.750 2.375 

(83) (25) (375) (60) 

3000 425 26 (12) 

11/2 x 2 4 

3.250 1.125 14.875 2.375 

(83) (29) (378) (60) 

2500 375 26 (12) 

3/4 x 1, 

4.625 1.000 10.250 3.750 

150# x 150# 1 + 2 

1 x 1 (117) (25) (260) (95) 

285 285 17 (7.25) 

1/2 x 1, 3 /4 x 1, 

4.625 1.375 10.250 3.750 

300/600# x 150# 1 + 2 

1 x 1 (117) (35) (260) (95) 

1480 285†† 17 (7.25) 

1/2 x 1, 3 /4 x 1, 

4.625 1.187 10.375 3.750 

900/1500# x 300# 1 + 2 

1 x 1 (117) (46) (264) (95) 

3600 425 20 (9) 

3/4 x 1, 1 x 1 2500# x 300# 

1 4.750 2.000 10.500 3.750 5000 

2 (121) (51) (267) (95) 3600 

425 20 (9) 

1 x 11/2 150# x 150# 3 

5.625 1.000 16.750 5.500 

(143) (25) (425) (140) 

285 285 40 (18) 

1 x 11/2 300/600# x 150# 3 

5.625 1.375 16.750 5.500 

(143) (35) (425) (140) 

1480 285†† 40 (18) 

1 x 11/2 900/1500# x 300# 3 

5.625 1.812 16.750 5.500 

(143) (46) (425) (140) 

3600 425 40 (18) 

1 x 11/2 2500# x 300# 3 

5.625 2.000 16.750 5.500 

(143) (51) (425) (140) 

5000 425 40 (18) 

*If a gag screw is fitted, add 2 ins (51mm) to dimension C for orifice nos. 1 & 2 only. 

*If a packed lever is fitted, add 1 3 ⁄4 ins (44mm) to dimension C for orifice nos. 1 & 2 only. 

Orifice sizes: refer to sizing section, page 84. 

Minimum set pressure 

B Series (Gas) = 10 Psig (0.7 Barg) 

C Series (Liquid) = 15 Psig (1.035 Barg) 

Orifice No.1 = 1480 Psig (102 Barg) 

Temperature range (with suitable material selection) 

-320°F to 1,000°F (-196°C to 538°C) 

†425 Psig with 300# outlet 

57

D SERIES DIMENSIONS 

Female screwed Male screwed Flanged 

Note: A packed 

lever version and 

gagging facility are 

also available. 

Sizes (ins) 

inlet & outlet 

Inlet & Outlet 

connection 

Orifice 

No. 

Max pressure upto 

Dimensions ins (mm) 

100°F (Psig) 

A B C† D Inlet Outlet 

†If a packed lever is fitted, add 1.062 ins (27mm) to dimension C for orifice 7 only. 

†If a gag screw is fitted, add 0.395 ins (10mm) to dimension C for orifice 7 only. 

†If a balanced piston is fitted, add 2.125 ins (54mm) to dimension C for orifice 7 only. 

†If a gagged balanced piston is fitted, add 2.520 ins (64mm) to dimension C for orifice 7 only. 

Orifice sizes: refer to sizing section, page 84. 

Minimum set pressure 

7D (Gas, Steam or Liquid) = 5 Psig (0.35 Barg) 

6D (Gas, Steam or Liquid) = 740 Psig (51 Barg) 

Temperature range (with suitable material selection) 

6D and 7D -51°F to 500°F (-46°C to 260°C) 

Weight 

lbs (kg) 

1/2 x 1 Screwed 

6 

1.732 - 7.440 2.165 

3/4 x 1 Female x Female (44) (189) (55) 

1480 285 9 (4) 

1 x 1 

Screwed 

1.687 0.750 8.230 2.165 

6 

Male x Female (43) (19) (209) (55) 

1480 285 9 (4) 

3/4 x 1 

4.610 1.625 10.315 3.750 

ANSI 600# x 150# 6 

1 x 1 (117) (41) (262) (95) 

1480 285 14 (6.5) 

1/2 x 1 Screwed 

7 

1.732 - 7.440 2.165 

3/4 x 1 Female x Female (44) (189) (55) 

740 285 9 (4) 

1 x 1 

Screwed 

1.687 0.750 8.230 2.165 

7 

Male x Female (43) (19) (209) (55) 

740 285 9 (4) 

3/4 x 1 ANSI 150# x 150# 

4.610 1.625 10.315 3.750 285 

7 

1 x 1 ANSI 300# x 150# (117) (41) (262) (95) 740 

285 14 (6.5) 

58

Safeset - 

Pilot operated safety relief valve 

1 Close differential 

pressure setting 

15 State-of-the-art 

piston seals 

2 Close blowdown 

and overpressure 

tolerance 

14 Two part piston 

assembly 

3 Non-flowing 

pilots 

4 Integral 

pilot filter 

5 All stainless 

steel pilots, 

dome, tubes 

and fittings 

6 Adjustable 

orifice areas 

7 Full lift 

capability 

8 Soft seat 

9 Enhanced 

seat 

tightness 

10 Unique full 

nozzle design 

11 API 526 

compliant 

13 Dual outlet option 

12 Integral sensing 

of pilot 

59


Close system operating pressure 

The system operating pressure can be much closer 

to the set pressure on a pilot operated valve than 

with a standard spring loaded safety relief valve. 

System pressures in the region of 95% to 98% of 

set pressure are often accommodated. 

Tolerates high inlet pressure loss 

High inlet pressure losses due to difficult inlet 

piping systems can be accommodated by remote 

sensing the pilots, hence trouble free operation can 

be assured. 

1. Close differential pressure setting – Pop 

acting pilot valves are quick acting and hence 

there is no delay between the pilot and the main 

valve opening pressures. Modulating valves 

inherently have a delay, which is nominally 2%. 

This ensures the pilot is not leaking when the 

system pressure is close to the set pressure. 

2. Close blowdown and overpressure 

tolerance – Pop action pilots can be adjusted 

to give zero overpressure and a blowdown 

typically equal to 3% of set pressure. 

3. Non flowing pilot – Safeset pilots are of the 

non flowing design. The risk of freezing due to 

pressure drops in the flowing medium is 

removed. Additionally, dirt particles are not 

transported to the pilot as the medium is static. 

This improves life in service and ensures trouble 

free operation. 

7. Full lift capability – Pilot valves will maintain full 

lift against high levels of back pressure. Unlike 

spring loaded valves, which would need either 

differential setting or the addition of balanced 

bellows. 

8. Soft seat – Safeset pilots and main valves are soft 

seated to give optimum leak tightness. Materials are 

available for a wide range of duties. Maintenance is 

simplified as seat lapping is reduced. 

9. Enhanced seat tightness – The main valve is 

piston operated. The piston area is in excess of 

30% larger than the seat bore area. This ensures 

the seat closing force is always at least 30% greater 

than the seat opening force for all pressures, right 

up to the set point. This produces more effective 

seat tightness, reducing the possibility of seat 

leakage. 

A spring loaded valve will have maximum seat 

clamping force when there is no system pressure. 

As the system pressure approaches the set point, 

the upward acting valve opening force approaches 

the closing force generated by the spring, hence the 

clamping force approaches zero and seat leakage 

may occur. A pilot valve with a 30% differential 

piston design will maintain a 30% seat clamping 

force right up to the set point, ensuring maximum 

seat tightness. 

4. Integral pilot filter – Safeset pilots are fitted 

with integral filters. 

5. Stainless steel – Safeset pilots, domes, tubes 

and fittings, pistons and nozzles are made from 

stainless steel as a minimum, i.e. all process 

wetted parts. This reduces the risk of oxides 

contaminating seals and hence prolongs their life 

in service. 

6. Adjustable orifice areas – The standard 

range of single outlet main valves consists of 8 

body sizes, which by simple adjustment 

accommodates all 21 standard orifice sizes. This 

allows for easy adjustment should service 

conditions dictate a change in required orifice 

area. 

Seat clamping = 

force 

Valve closing force 

– Valve opening force 

60

10. Unique full nozzle design – The nozzle is 

a patented push-in design, held in position 

with a locking ring. Being of the full nozzle 

design, it reduces mechanical and thermal 

stresses at the seat, reduces inlet pressure 

losses, increases the discharge co-efficient 

and allows for lower specification body 

materials on corrosive duties. 

11. API 526 compliant – Fully conforms to the 

latest edition of API 526 for pressure/ 

temperature ratings and over flange 

dimensions for pilot valves. 

14. Two part piston-disc assembly – This gives a 

reduced guiding geometry, keeping the size of the 

components within the body bowl to a minimum, 

thus increasing the effective discharge area 

through the valve outlet. 

15. State of the art piston seals and bearing 

rings – Reduce friction and galling of materials at 

the guide and pistons surfaces – essential for 

modulating pilots. 

12. Integral sensing – Safeset pilots are sensed 

from the nozzle, giving a compact design and 

accurate pressure sense from within the flow 

stream. Remote sensing for difficult inlet 

piping systems is recommended. 

13. Dual outlet option – The 8 inch valve is 

also available with a dual outlet (see below). 

DUAL OUTLET / FULL BORE PILOT VALVE 

This valve is suitable for extremely high 

capacity duties. It achieves maximum discharge 

capacities by having a full bore 8 inch inlet with 

an available discharge area of 44.178 sq.in. and 

two opposing 10 inch outlets, which can assist 

with reaction force problems. 

Available with all Pilot Types 2, 4, and 8. 

61

MODE OF OPERATION 

Safeset consists of a separate pilot valve 

connected to a main valve via a sensing pipe. 

This pipe senses the inlet pressure within the main 

valve nozzle and provides the pressure signal to 

the pilot. 

Pilot valve 

The pilot valve controls the 

discharge of fluid through the 

main valve by responding 

accurately to the system 

pressure. All Birkett pilots 

are of a non flowing design, 

which means there is an absence 

of system flow through the pilot 

during the relief cycle. 

Main valve 

The main valve conforms to the latest version of 

API 526 Pressure/Temperature ratings and face to 

face dimensions. It has 21 orifice variants in just 8 

inlet x outlet body sizes. Safeset is a differential 

piston operated design, the opening is controlled 

by the pilot valve. 

The benefits of non flowing 

designs are that freezing of 

moisture-containing fluid and 

the carrying of particulate 

matter into the pilot are 

minimised, ensuring correct 

valve performance. 

PILOT VALVE TYPES 

Pop (Type 2) – Controls the main valve in a 

rapid manner, it is either open or closed. 

Specifically for gas applications. 

Modulating (Types 4 and 8) – The main valve is 

opened in a controlled manner, the over pressure is 

constantly monitored and the opening of the main 

valve is proportionate to the overpressure. 

BASIC OPERATION OF SAFESET 

Simply, the pilot valve is a very accurate springloaded 

safety valve, with two seats. At low system 

pressures, pressure from the system is fed from 

the inlet nozzle, through the sense line, past the 

lower pilot seat and into the main valve dome. The 

dome area is in excess of 30% larger than the seat 

bore area. This differential of areas ensures that 

the main valve remains closed. When the system 

pressure reaches the set point of the pilot, the 

lower pilot seat closes and the upper pilot seat 

opens, releasing the dome pressure to the 

atmosphere. With no pressure above the piston, the 

main valve opens. The reverse of this sequence of 

events occurs when system pressure falls and once 

again the main valve dome will be fed with system 

pressure, which in turn closes the main valve. 

62

POP ACTION - TYPE 2 PILOT 

Pilot construction 

The pilot is essentially a spring loaded 

safety valve with blowdown adjustment 

which, during its operation, positions 

itself on one of two seats. The upper 

drain seat is used to determine pilot set 

pressure, the lower feeding seat 

determines pilot blowdown. 

The valve disc is held on the soft upper 

drain seat by the setting spring, which 

also determines the pilot set pressure. 

This valve disc is connected via a spindle 

to the lower feeding seat, which controls 

the system flow into the dome and 

which also controls the pilot blowdown. 

TO AND 

FROM MAIN 

VALVE DOME 

FEEDING 

SETTING 

SPRING 

DISC 

UPPER 

DRAIN 

SEAT 

EXHAUST 

SPINDLE 

The valve disc and lower feeding seat 

move together; this action opens and 

closes each respective seat, thereby 

allowing the pilot to control the 

operation of the main valve during the 

relieving cycle. 

LOWER 

FEEDING 

SEAT 

FILTER 

Pilot operation 

With pop action pilot operation, the main valve is 

either in the fully open or shut position. The 

operation is characterised by a distinct rapid “pop” 

action, which is evident at the opening of the main 

valve, followed by a positive re-seat action when 

the main valve closes. 

The re-seat pressure of the Type 2 pilot valve is 

adjustable externally, independently of the set 

pressure adjustment. 

This graph demonstrates pop action. When set 

pressure is reached, the pilot valve opens rapidly; 

this action de-pressurises the dome volume very 

quickly and the main valve opens. This is shown by 

the vertical (rising) line on the graph, illustrating 

that the main valve achieves its design lift at set 

pressure. 

The re-seating characteristic is equally positive: 

when the system pressure has fallen to the pre-set 

pilot re-seat pressure, the pilot drain seat closes 

rapidly. This action allows the dome to be repressurised 

very quickly, thereby closing the main 

valve completely. This is shown on the graph by 

the vertical (falling) line. 

63

STAGES OF OPERATION - TYPE 2 PILOT 

Stage 1 

System pressure below the pilot 

set pressure, dome pressurised, 

main valve closed. 

Stage 2 

System pressure equal to set 

pressure, drain seat opens, 

dome de-pressurises, main valve 

fully lifts with no overpressure. 

Stage 3 

System pressure equal to or 

greater than set pressure, dome 

pressure is atmospheric, main 

valve is fully open. 

Stage 4 

System pressure falls to equal the 

re-seat pressure, drain seat closes, 

feeding seat opens, dome is 

pressurised, main valve closes. 

Stage 5 

System pressure below the pilot 

set pressure, dome pressurised, 

main valve closed and ready for 

next upset condition. 

Key Points – Pilot Type 2 

• Pilot set pressure = Main valve set pressure. 

• Main valve fully open at 0% overpressure. 

• The pilot is non flowing. 

• Adjustable blow-down feature. 

• Integral filter fitted. 

• Gas duty only. 

64

MODULATING ACTION - TYPE 4 

Pilot construction 

Modulating pilots are essentially 

diaphragm or piston operated safety 

valves with a feedback piston for fine 

control of the pressure in the dome. 

The Type 4 Pilot is used for pressures up 

to 1480Psig 

DETECTOR 

DIAPHRAGM 

SETTING 

SPRING 

When the set pressure is reached the 

main valve opens in proportion to the 

system pressure rise. This is achieved by 

the pilot controlling the dome pressure. 

The main valve will maintain a lift 

necessary to discharge the system flow. 

As the system pressure increases, the 

main valve lift will change to 

accommodate the new flow condition. 

As the system pressure falls, the main 

valve will begin to close, finally closing at 

a pressure just below the set pressure. 

The diaphragm senses the system 

pressure and the feedback piston senses 

the dome pressure. The combination of 

these pressures accurately provide a 

force balanced with the adjusting spring 

to open/close the feeding/drain seats. 

This maintains dome pressure for 

accurate positioning of the main valve. 

FEEDBACK 

PISTON 

FEEDING 

FILTER 

FEEDING 

SEAT 

Type 4/1 Pilot 

(29 to 100 Psig) 

DOME 

RETURN 

SPRING 

DRAIN 

DRAIN 

SEAT 

DETECTOR 

PISTON 

Type 4/2 Pilot 

(100 to 1480 Psig) 

Note: The Pilot Type 4 always drains to the main valve outlet, and a back flow preventer should be fitted (see page 72). 

65

MODULATING ACTION - TYPE 8 

The Type 8 Pilot is similar to the Type 4 

Pilot except that instead of using a 

diaphragm to lift the valve, the Type 8 

Pilot uses a piston. 

SETTING 

SPRING 

The pressure range for the Type 8 Pilot 

is 1480 Psig to 6170 Psig. 

Note: The Type 8 Pilot always drains to 

the main valve outlet, and a back flow 

preventer should be fitted (see page 72). 

FEEDBACK 

PISTON 

DOME 

DETECTER 

PISTON 

RETURN 

SPRING 

FEEDING 

SEAT 

DRAIN 

DRAIN 

SEAT 

FILTER 

FEEDING 

Type 8 Pilot 

PILOT OPERATION 

The action of the main valve may be characterised 

as “modulating”. In order to achieve this the pilot 

accurately controls the pressure in the main valve 

dome which positions the main valve disc to 

match the system upset flow condition; this will 

then control the system pressure. 

Effective pilot operation requires a small 

overpressure above the set pressure, to achieve 

full design lift of the main valve and a small 

pressure drop to re-seat the main valve. 

A typical modulating performance is shown in the 

graph opposite. The overpressure as a 

percentage of set pressure will vary with the 

system flow requirement; this means that an 

infinite number of relieving cycles can occur 

within the limits shown. The valve lift will, 

however, always be in proportion to the rise in 

system pressure, ensuring a safe stable relief cycle. 

66

STAGES OF OPERATION - TYPES 4 AND 8 

Type 4 & 8 Pilots are fully modulating. They are 

intended for use with gas, liquid and mixed phase 

fluids. Both types are non flowing designs. 

STAGE 1 

System pressure below set pressure 

The feeding seat remains open with the drain seat 

closed. This maintains equal pressures in the 

dome and the system. The main valve is closed, 

held tightly against the nozzle. 

Pilot Valve Drain 

STAGE 2 

System pressure approaches set pressure 

When the system pressure approaches the set 

pressure the feeding seat closes. The drain seat 

remains closed and the main valve is closed. 

The pressure in the dome is now controlled by 

the inlet system pressure acting against the pilot 

diaphragm/piston and the dome pressure acting on 

the feed back piston. The combination of these 

two forces controls the opening of the drain seat, 

thereby controlling the dome pressure and hence 

the main valve lift. 

When the drain seat opens it discharges a small 

volume of fluid from the dome. At no time is the 

drain seat continually flowing. It drains in short 

bursts. 

P 

STAGE 3 

System pressure reaches set pressure 

As the system pressure gradually rises to the set 

pressure, the dome pressure gradually falls 

approximately 30%. Due to the differential size, 

top and bottom of the piston, the main valve 

opening and closing forces are now in equilibrium. 

67

STAGE 4 

System pressure above set pressure 

As the system pressure increases above the set 

pressure, modulation occurs with the main valve 

opening an amount sufficient to maintain system 

flow rate. This is brought about by the increased 

system pressure acting against the pilot 

diaphragm/piston to re-open the drain seat. The 

dome pressure is reduced further, allowing the 

main valve to open. The reduced dome pressure 

now acting on the feedback piston produces a 

lower upward force and the adjusting spring closes 

the drain seat. 

The above actions cycle in very small increments 

and hence give the modulating effect. 

STAGE 5 

Main valve fully open 

The main valve will be fully open before the system 

pressure reaches 110% of the set pressure. 

STAGE 6 

Main valve closes 

When the system upset condition has ended, the 

system pressure will begin to fall. The reduced 

system pressure acting on the pilot 

diaphragm/piston will cause the feeding seat to reopen. 

This will increase the dome pressure and 

the main valve will begin to close. The feeding seat 

will close as the dome pressure increases due to 

the feedback piston effect. This cycle will repeat as 

the system pressure is further reduced. The main 

valve will close with a progressive action. When 

the system pressure reaches approximately 97% of 

set pressure the main valve will be closed. 

Key points – Pilot Types 4 and 8 

• Pilot set pressure is when the drain seat first 

opens. 

• Main valve discharge maintains the system 

pressure at its respective flow condition, 

thereby achieving fully modulating action. 

• The pilot valve is firmly closed when the main 

valve closes. 

• Integral filter fitted. 

68

TECHNICAL SPECIFICATION - PILOT VALVE 

Valve type 2 4 8 

Pilot action Pop Modulating Modulating 

Fluid Gas Gas, Liquid Gas, Liquid 

Dual Phase 

Dual Phase 

Overpressure 0%

MATERIALS OF CONSTRUCTION - MAIN VALVE 

BODY MATERIAL CARBON STEEL STAINLESS STEEL 

Item Description † -29 to +260 ºC -46 to +260 ºC -46 to +260 ºC 

1 Body SA 216 WCB SA 352 LCB SA 351 CF8M 

2 Nozzle St. St. 316 St. St. 316 St. St. 316 

3 Guide St. St. 17/4 St. St. 17/4 St. St. 17/4 

4 Disc holder St. St. 316 St. St. 316 St. St. 316 

5 Disc insert Viton * Viton * Viton * 

6 Piston St. St. 316 St. St. 316 St. St. 316 

7 Locknut St. St. 316 St. St. 316 St. St. 316 

8 Lift stop St. St. 316 St. St. 316 St. St. 316 

9 Cover St. St. 316 St. St.316 St. St. 316 

10 Retaining plate St. St. 316 St. St. 316 St. St. 316 

11 Disc holder seal Viton * Viton * Viton * 

12 Piston seal Viton * Viton * Viton * 

13&25 Guide rings Carbon/PTFE Carbon/PTFE Carbon/PTFE 

14 Body stud A193/B7 A193/B8T A193/B8T 

15 Body nut A194/2H A194/8T A194/8T 

16 Lock ring St. St. 316 St. St. 316 St. St. 316 

19 Spring St. St. 316 St. St. 316 St. St. 316 

20 Body gasket Carbon fibre Carbon fibre Carbon fibre * 

21 Guide seal Viton * Viton * Viton * 

22 Nozzle seal Viton * Viton * Viton * 

23 Retaining plate screw Viton * Viton * Viton* 

24 Counter sunk screw St. St. 316 St. St. 316 St. St. 316 

27 Fittings St. St. 316 St. St. 316 St. St. 316 

32 Tubes St. St. 316 St. St. 316 St. St. 316 

42 Drain plug HTS HOLO-KROME HTS HOLO-KROME ASTM A479-316L 

Note: 

*Soft goods materials listed above are standard. For a full listing to cover the temperature ranges of the body materials listed 

above, it is necessary to refer to the seals, gaskets, selection table on the previous page. 

†Maximum temperature is limited by the seal material. 

Alternative materials and accessories are available, for NACE, high temperatures and cryogenic 

applications. 

70

ACCESSORIES 

Back Flow 

Preventer 

Exterior 

Supply Filter 

71

ACCESSORIES 

Back flow preventer 

High back pressures may exist in the outlet for 

various reasons such as common disposal systems. If 

this back pressure can be more than the inlet system 

operating pressure, the main valve could lift allowing 

reverse flow from the outlet to inlet system. 

A back flow preventer is a two-way check valve, 

which is fitted into the dome line. It allows the highest 

pressure from either the inlet system or outlet system 

to enter the dome, ensuring the main valve remains 

closed and prevents the possibility of reverse flow. 

All modulating pilots (Types 4 + 8) should be fitted 

with a back flow preventer. 

A back flow preventer should always be fitted if 

a vacuum can exist in the inlet pipework. 

When using back flow preventers with back pressure 

above 50% of the set pressure, the actual service 

conditions must be reviewed by the factory. 

External supply filter 

This unit protects the pilot valve when working under 

“dirty” flow stream conditions. All Safeset pilots are 

fitted with integral filters as standard; however, under 

conditions where there is likely to be large amounts 

of particulate matter in the flow stream, a supply filter 

should be used. This unit is fitted into the pilot sensing 

pipe, upstream of the pilot, and is suitable for gas and 

liquid duty. It is removed easily for maintenance. 

Remote Pressure Sensing 

Sense Line 

(Max 100ft/30m including 

one 90° bend. Min inside 

diameter 0.254"/6mm) 

Remote pressure sensing 

Many codes and standards restrict the inlet 

pipework pressure loss to 3% of the safety valve 

set pressure. There are occasions however, 

when the pipework loss is greater than this. In 

these instances the safety valve should always be 

connected for remote sensing. The valve is 

normally supplied with integral sensing; this 

means that the pilot senses pressure at the main 

valve nozzle entry. Under conditions of excessive 

system pressure loss, the valve, under flowing 

conditions, may cycle open and closed. This is 

due to the pilot sensing a reduced (artificial) 

pressure. 

In order to overcome this problem the valve 

should be sensed remotely. The pilot in this case 

should have its inlet connected directly to the 

pressure source where the system pressure is 

stable and not flowing. The main valve nozzle will 

not contain a sensing tapping in this case. 

Excessive system pressure losses will also reduce 

the flow rate through the valve. This will be in 

proportion to the absolute system pressure and 

must be taken into account when sizing the valve. 

Remote sensing will ensure that the pilot operated 

safety valve operates without cycling or chatter 

when high inlet pressure losses are encountered. 

72

ACCESSORIES 

Heating or 

Cooling Coils 

Field Test 

Connector 

Remote Pressure Sensing 

Heating or cooling coils 

High or low temperature duties may require the 

addition of coils to act as heat exchangers to either 

warm or cool the medium before it enters the pilot 

valve or main valve dome. This ensures that 

extremeties of temperature do not affect the 

operation of the safety valve. 

The use of such coils allows us to use standard pilots 

on valves with inlet temperatures in the range of - 

196°C (-320°F) to 260°C (500°F). 

Field test connector 

This provides verification of set pressure setting 

during normal system operation. The field test 

connector is a two way check valve which is fitted 

into the sensing pipe and is an integral part of the pilot 

operated valve system. If this facility is required, it 

must be specified on the valve order. 

An external pressure supply needs to be connected 

to the check valve via an isolating valve and pressure 

gauge. The external pressure should be admitted 

slowly through the supply isolating valve. When the 

supply pressure is greater than the system pressure 

the check valve delivery seat will open and the 

system seat will close. 

Pressure can now be applied to the pilot and dome. 

When set pressure is reached the pilot will open. 

N.B. Pop action pilots will ‘pop’ open; this is the 

main valve set pressure. Modulating pilots will start 

to vent from their drain seat; this will occur at 

approximately 2% below the main valve set pressure. 

The nameplate should be referred to in order to 

obtain the main valve set pressure. 

The main valve may open briefly with pop action 

pilots. 

73

ACCESSORIES 

Others 

Similar to the spring loaded safety relief valves 

(see page 15), pilot valves can be fitted with: 

• Packed lift levers. 

• Ferrules (government rings). 

• Test gags (max pressure 1480 Psig). 

Available options 

• Dual pilot assemblies. 

• Dual pilot interlock system. 

• Heated or unheated control cabinets. 

• Dual outlet/full bore design. 

• Differential pressure switch. 

Dual pilots 

If a process plant is running, it is earning a 

profit for its operator. Traditionally, when a 

valve required maintenance the plant had to be 

shut down, resulting in reduced earning time. 

A solution to this is to install a second pilot on 

to the main valve and install a simple system of 

ball valves or an interlocked changeover valve 

system, thus allowing one pilot to be removed 

while the plant is still operating. We can offer 

either solution. 

74

ACCESSORIES 

Remote unloader 

This device enables the main valve to be opened 

remotely. It is a three way spool valve which can 

be operated electronically or pneumatically. 

The valve is mounted into the dome line and 

allows a free flow from the pilot into the dome. 

When remote operation is required, a signal 

(pneumatic or electrical) opens the exhaust vent of 

the spool valve and vents the dome, allowing the 

main valve to open. 

The valve will normally be supplied mounted onto 

the main valve with the dome connected directly 

to the pilot. 

When the spool valve is energised the dome will 

vent directly to atmosphere. 

Remote Unloader 

75

SAFESET PILOT FIGURE NUMBERING SYSTEM 

P 

/ 

Inlet diameter 

1 - 8" 

Orifice designation D-T 

X = 8" x 10" x 10" full bore 

Outlet diameter 2" - 10" 

Pilot description 

2 Pop action, gas 

4 Modulating, LP 

8 Modulating, HP 

ANSI flange rating 

1 150 x 150 

2 300 x 150 

3 600 x 150 

5 900 x 300 

7 1500 x 300 

8 2500 x 300 

A 600 x 300 

B 1500 x 600 

C 2500 x 600 

O Special 

Flange type 

1 ANSI RF x RF 

2 ANSI RTJ x RF 

O Special 

Main valve body code 

1 Carbon steel SA 216-WCB 

2 Carbon Steel SA 216-WCB (NACE) 

3 St steel SA 351-CF8M (NACE) 

4 St steel SA 351-CF8M 

O Special 

Accessories 

A Remote pressure sensing 

B Back flow preventer 

C Cooling/heating coils 

D Screwed cap 

E External filter 

**G Test gag 

*L Liquid duty 


T Field test connector 

U Remote unloader 

S Special 

Pressure range Psig (Barg) 

Range Type 2 Type 4 Type 8 

1 29 - 6170 29 - 100 1480 - 6170 

(2.0 - 425.5) (2.0 - 6.8) (102 - 425.5) 

2 >100 - 1480 

(>6.8 - 102) 

These are min/max pressures of the pilot. 

Several springs are required to cover these ranges 

Main valve trim 

1 St St 316 and PEEK 

2 St St 316 and Viton 

4 St St 316 and Buna N 

8 St St 316 and Polyurethane 

9 St St 316 and PTFE 

O Special 

Main valve spring material 

2 St steel 316 

Z Inconel X750 (NACE) 

O Special 

*Dual phase duties should use the liquid trim versions of the modulating pilot Types 4 and 8 

**Test gag available to a max pressure of 1480 Psig 

76

DIMENSIONS 

Size Orifice Rating A (ins) D (ins) B (ins) C (ins) C (ins) C (ins) Weight 

(ins) Type 2 Type Remote lbs (kg) 

4 and 8 pilot 

1 x 2 D, E, F 150x150 4.13 4.50 1.69 14.00 19.00 9.00 42 (19) 

300x150 4.38 4.50 1.69 14.00 19.00 9.00 45 (20.5) 

600x150 4.38 4.50 1.69 14.00 19.00 9.00 45 (20.5) 

900x300 4.94 4.75 2.25 16.00 21.00 11.50 53 (24) 

1500x300 4.94 4.75 2.25 16.00 21.00 11.50 53 (24) 

2500x300 4.94 4.75 2.25 16.00 21.00 11.50 53 (24) 

1.5 x 2 D, E, F 150x150 4.88 4.75 1.69 14.50 19.50 9.50 46 (21) 

300x150 4.88 4.75 1.69 14.50 19.50 9.50 49 (22) 

600x150 4.88 4.75 1.69 14.50 19.50 9.50 49 (22) 

900x300 5.88 5.50 2.63 17.00 22.00 12.50 57 (26) 

1500x300 5.88 5.50 2.63 17.00 22.00 12.50 57 (26) 

2500x300 5.88 5.50 2.63 17.00 22.00 12.50 57 (26) 

1.5 x 3 G, H 150x150 5.13 4.88 1.69 15.00 20.00 10.25 55 (25) 

300x150 5.13 4.88 1.69 15.00 20.00 10.25 62 (28) 

600x150 5.13 4.88 1.69 15.00 20.00 10.25 62 (28) 

900x300 6.38 6.75 2.62 18.25 23.25 13.50 79 (36) 

1500x300 6.38 6.75 2.62 18.25 23.25 13.50 79 (36) 

2500x300 6.38 6.75 2.62 18.25 23.25 13.50 79 (36) 

2 x 3 G, H, J 150x150 5.38 4.88 1.81 15.25 20.25 10.50 55 (25) 

300x150 5.38 4.88 1.81 15.25 20.25 10.50 60 (27) 

600x150 5.38 4.88 1.81 15.25 20.25 10.50 64 (29) 

900x300 6.56 6.75 2.44 19.25 24.25 14.50 93 (42) 

1500x300 6.56 6.75 2.44 19.25 24.25 14.50 93 (42) 

1500x600 6.56 6.75 2.44 19.25 24.25 14.50 93 (42) 

2500x300 7.00 6.75 2.88 19.25 24.25 15.00 104 (47) 

2500x600 7.00 6.75 2.88 19.25 24.25 15.00 104 (47) 

SINGLE OUTLET 

Notes: 

• Certified dimensions available on request. 

• Dimensions A and B are for RF inlet, sensed 

integrally and remotely. 

• Dimensions A and B are for RTJ inlet, sensed 

remotely. 

• Add 1/2" to dimensions A and B for RTJ inlets 

1" to 3" when sensed integrally. 

• Add 3/4" to dimensions A and B for RTJ inlets 

4" to 6" when sensed integrally. 

• Add 1" to dimensions A and B for RTJ inlet 8" 

when sensed integrally. 

• Height may vary. 

• Weight is approximate for Type 2 Pilot. For 

Types 4 and 8 Pilots add approx. 5kg. 

• Cap withdrawal 40mm (1.6"). 

• Additional filter 3.5kg. 

77

Size Orifice Rating A (ins) D (ins) B (ins) C (ins) C (ins) C (ins) Weight 

(ins) Type 2 Type Remote lbs (kg) 

4 and 8 pilot 

3 x 4 J, K, L 150x150 6.13 6.38 2.00 17.00 22.00 12.00 137 (62) 

300x150 6.13 6.38 2.00 17.00 22.00 12.00 137 (62) 

600x150 6.33 6.38 2.25 17.25 22.25 12.25 154 (70) 

600x300 7.50 7.13 3.00 20.25 25.25 15.50 183 (83) 

900x300 7.50 7.13 3.00 20.25 25.25 15.50 205 (93) 

1500x300 7.50 7.13 3.00 20.25 25.25 15.50 214 (97) 

1500x600 7.50 7.63 3.00 20.25 25.25 15.50 227 (103) 

4 x 6 L, M, 150x150 7.75 8.25 2.31 20.75 25.75 15.50 225 (102) 

N, P 300x150 7.75 8.25 2.31 20.75 25.75 15.50 225 (102) 

600x150 7.75 8.25 2.31 20.75 25.75 15.50 225 (102) 

600x300 9.81 9.19 3.38 25.00 30.00 19.75 370 (168) 

900x300 9.81 9.19 3.38 25.00 30.00 19.75 390 (177) 

1500x300 9.81 9.19 3.38 25.00 30.00 19.75 401 (182) 

1500x600 9.81 10.38 3.57 25.25 30.25 20.25 456 (207) 

6 x 8 Q, R 150x150 9.44 9.50 2.31 25.50 30.50 20.50 403 (183) 

300x150 9.44 9.50 2.31 25.50 30.50 20.50 408 (185) 

600x150 9.69 9.50 2.75 25.50 30.50 20.75 419 (190) 

600x300 9.69 10.44 2.75 26.00 31.00 21.00 556 (252) 

8 x 10 S, T 150x150 10.88 11.00 2.69 29.00 34.00 24.50 595 (270) 

300x150 10.88 11.00 2.69 29.00 34.00 24.50 661 (300) 

600x150 11.69 11.00 3.25 29.75 34.00 24.25 728 (330) 

600x300 11.69 12.00 3.25 30.25 35.25 25.75 948 (430) 

8x10x10 X 150x150 10.88 11.00 1.75 31.75 36.75 31.00 959 (435) 

300x150 10.88 11.00 1.75 31.75 36.75 31.00 959 (435) 

DUAL OUTLET 

78

OPERATING PRESSURES 

Maximum Operating Pressures to API 526 

Size Orifice ANSI Maximum 

(ins) Flange Pressure 

(Psig) 

Inlet Outlet Inlet Outlet 

1x2 D, E, F 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3705 740 

2500 300 6170 740 

1 1 /2x2 D, E, F 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3705 740 

2500 300 6170 740 

1 1 /2x3 G, H 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3705 740 

2500 300 6170 740 

2x3 G, H 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3705 740 

2500 300 6170 740 

2x3 J 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3650 740 

2500 300 3650 740 

3x4 J, K 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3705 740 

Maximum Operating Pressures to API 526 

Size Orifice ANSI Maximum 

(ins) Flange Pressure 

(Psig) 

Inlet Outlet Inlet Outlet 

3x4 L 150 150 285 285 

300 150 740 285 

600 150 1240 285 

600 300 1480 740 

900 300 2220 740 

1500 300 2900 740 

4x6 L, M, N 150 150 285 285 

300 150 740 285 

600 150 1480 285 

900 300 2220 740 

1500 300 3705 740 

4x6 P 150 150 285 285 

300 150 740 285 

600 150 1305 285 

600 300 1480 740 

900 300 2220 740 

1500 300 3080 740 

1500 600 3705 1480 

6x8 Q 150 150 285 285 

300 150 740 285 

600 150 1330 285 

600 300 1480 740 

6x8 R 150 150 285 285 

300 150 740 285 

600 150 915 285 

600 300 1480 740 

8x10 T 150 150 285 285 

300 150 740 285 

600 150 900 285 

600 300 1480 740 

8x10x10 X 150 150 285 285 

300 150 740 285 

Notes: 

• Outlet pressure limits for temperature above 100°F to conform to ANSI/ASME B16.34. 

• Pressure ratings given are for carbon steel bodies at -20°F to 100°F. Austenitic stainless steel and other 

materials suitable for the service may be used within the code limits for pressure and temperature. 

• API Standard 526 specifies lower allowable pressures for service temperatures above and below the 

ranges given in these charts, for both carbon and stainless steel bodies. 

79

FLANGE PRESSURE / TEMPERATURE LIMITS 

Graph 1.0 

ANSI Class 150,300,600 Inlet Flange Valves 

Set Pressure (Barg) 

Graph 2.0 

ANSI Class 900,1500,2500 Inlet Flange Valves 

Set Pressure (Barg) 

Valves can be operated down to -196°C (-320°F) when fitted with appropriate heating coils. 

80

VALVE SIZING 

Overview 

A safety valve is fitted to restrict system 

overpressure to a predetermined level; this is 

normally 110% of the safety valve set pressure. 

In order to ensure that the overpressure is not 

exceeded, the flow rate through the safety valve 

has to be calculated. This calculation uses 

formulae which are derived from ASME VIII and 

API 520 Codes which are recognised throughout 

the world. 

Sizing 

The sizing of the safety valve uses data from the 

physical properties of the fluid, the valve set 

pressure, overpressure limits and effective 

discharge area. 

Formulae are presented for sizing valves on steam, 

gas and liquid. The constants used in the sizing 

formulae may have a different value dependent 

upon the valve type; where this is the case, it is 

clearly illustrated on the graph or table. All 

discharge coefficients are relative to the valve type 

and have been approved to the ASME VIII Code. 

Capacity tables are also shown for sizing on dry 

saturated steam, air and water. When calculating 

the flow rate through the safety valve, it is 

important that the flow rate through the valve is 

greater than the required flow rate generated by 

the system. 

Selection 

The safety valve selected must be suitable for the 

pressure and temperature required in the system; 

the appropriate section of the safety valve 

catalogue should be referred to. 

The selected total discharge area of the safety 

valve must always be greater than the calculated 

discharge area required to relieve the system flow 

rate under all working conditions. 

VALVE SIZING FORMULAE 

Gas and Vapour Flow 

(1) Mass Flow (imperial units) 

A = W TZ 

C P Kd Fb Ff Fp M Kc 

(2) Volumetric Flow (imperial units) 

A = Q GTZ 

1.175 C P Kd Ff Fb Fp Kc 

(3) Constant (C) (imperial units) 

Table 1 

C = 520 k 

2 

k+1 

k+1 

k-1 

Nozzle Gas Constant 

K C C K C C 

Imperial Metric Imperial Metric 

1.00 315 2.40 1.38 354 2.69 

1.02 318 2.41 1.40 356 2.70 

1.04 320 2.43 1.42 358 2.72 

1.06 322 2.45 1.44 359 2.73 

1.08 324 2.46 1.46 361 2.74 

1.10 326 2.48 1.48 363 2.76 

1.12 329 2.50 1.50 364 2.77 

1.14 331 2.51 1.52 366 2.78 

1.16 333 2.53 1.54 368 2.79 

1.18 335 2.55 1.56 369 2.80 

1.20 337 2.56 1.58 371 2.82 

1.22 339 2.58 1.60 372 2.83 

1.24 341 2.59 1.62 374 2.84 

1.26 343 2.61 1.64 376 2.85 

1.28 345 2.62 1.66 377 2.86 

1.30 347 2.63 1.68 378 2.87 

1.32 349 2.65 1.70 380 2.89 

1.34 350 2.66 2.00 400 3.04 

1.36 353 2.68 2.20 412 3.13 

81

(4) Constant, Fb 

Fb = 

2k 

k-1 

Pb 

P 

k 

2/k (k+1)/k 

- Pb 

P 

(k+1)/(k-1) 

2 

k+1 

Liquid viscosity correction (Fv): 

When a relief valve is sized for viscous liquid 

service, it should first be sized as it was for nonviscous 

type application so that a preliminary 

required discharge area, A, can be obtained. The 

next larger orifice size should be used in 

determining the Reynold’s number, R, from either 

of the following relationships: 

Steam Flow (Sonic and Subsonic flow) 

(5) Mass Flow (imperial units) 

or 

R = VL(2800G) 

e A 

(8) 

A = 

W 

51.5 P Kd Fsh Fb Ff Fn Fp Kc 

R = 12,700VL 

u A 

(9) 

Viscosity Correction Factor Fv 

High pressure steam correction factor Fn:- 

Fn = 1.0 when P≤1515 Psia 

Use the following formulae when P>1515 Psia 

(104.5 Bara) and P

NOMENCLATURE 

Symbol Description Imperial Units 

A Orifice discharge area sq. ins 

C 

Gas constant, from the specific heat ratio (k); 

if unknown use 315 (see page 81 equation (3) or Table 1) ......dimensionless...... 

e Liquid absolute viscosity ........Centipoise........ 

Ff 

Back pressure correction factor for gas - takes account 

of subsonic flow Balanced Bellows Valves (Page 87, Graph 2.0) 

Fb 

Back pressure correction factor for gas - takes account 

of subsonic flow Conventional Spring Loaded; WB 400; 

B Series; Pilot Operated Valves; Type 2, 4 and 8 Pilots: 

(use Graph 3.0, page 87 or equation (4) page 82) 

......dimensionless...... 

Fl 

Back pressure correction factor for balanced bellows 

spring loaded valves (WB 100) liquid duty only, 

(use Graph 4.0, page 88) 


Fn 

High temperature steam correction factor. 

Fp 

Subsonic flow factor for low set 

pressure on gas duty only, (use Graph 5.0, page 88) 


Fsh Correction factor for superheated steam (Table 4, page 89) .....dimensionless...... 

Fv Liquid viscosity correction factor (Page 82, Graph 1.0) ......dimensionless...... 

G Specific gravity ......dimensionless...... 

Kc 

Derating factor = 0.9 for use with 

bursting discs; if no bursting disc use 1.0 


Kd 

Certified ASME Code Section VIII discharge coefficient: 

WB100 / 200 = 0.653 (actual) (actuals are used on 

WB 300 / 400 = 0.975 (actual) WB valves, as the 

WB 300B = 0.925 (actual) derating factor has 

B Series = 0.857 (derated) been applied to the 

C Series = 0.509 (derated) the orifice area). 

6D Series (gas/steam) = 0.811 (derated) 

6D Series (liquid) = 0.670 (derated) 

7D Series (gas/steam) = 0.824 (derated) 

7D Series (liquid) = 0.506 (derated) 

Pilots: Types 2, 4, 8 = 0.849 gases (derated) 

Pilots: Types 4, 8 = 0.696 liquids (derated) 

For full bore 8" x 10" x 10" pilot valves refer to page 96 


k Isentropic exponent (ratio of specific heats) ......dimensionless...... 

M Molecular weight ..........kg/kmole.......... 

P 

Set pressure + overpressure + atmospheric pressure 

where:- 

– Overpressure = 10% or 3 Psi whichever is the greater Psia 

– Atmospheric pressure = 14.7 Psia 

Pg Set pressure + Overpressure Psig 

Pb Back pressure at safety valve outlet Psia 

Pbg Back pressure at safety valve outlet Psig 

Q Volumetric flow rate @ 14.7 Psia and 60 ºF SCFM 

R Reynolds number ......dimensionless...... 

T Temperature at valve inlet deg.R = 460 + ºF deg.Rankine 

VL Liquid flow rate us gpm 

W Mass flow rate lb/h 

w Liquid density lb/cu ft 

Z Compressibility factor (if unknown use 1.0) ......dimensionless...... 

83

BACKPRESSURE AND BLOWDOWN LIMITS 

Figures shown are expressed as a percentage of set pressure. 

Valve type Built up Superimposed Constant Blowdown 

back variable back superimposed % 

pressure % pressure % back pressure % 

Safeset Pilot operated safety valves 

Refer to page 69 

Conventional spring loaded safety valves 

WB 200 10 3 80 10 – 15 

WB 400 10 3 80 7 

B 10 3 80 10 

C 10 3 80 10 – 15 

D 10 3 80 15 – 20 

Balanced bellows spring loaded safety valves 

WB 100 50 50 50 10 – 15 

WB 300 20 20 20 7 

WB 300B 70 70 70 7 

7D (piston) Gas 70 70 70 15 - 20 

7D (piston) Liq. 50 50 50 15 - 20 

ORIFICE AREAS 

Orifice WB Series Safeset Pilot 

letter in 2 mm 2 in 2 mm 2 

D 0.110 71 0.164 106 

E 0.196 127 0.256 165 

F 0.307 198 0.338 218 

G 0.503 325 0.616 397 

H 0.785 506 0.871 562 

J 1.287 830 1.427 921 

K 1.838 1185 2.139 1380 

L 2.853 1840 3.167 2043 

M 3.600 2320 4.307 2779 

N 4.340 2800 5.162 3330 

P 6.380 4120 7.068 4560 

Q 11.050 7130 12.864 8299 

R 16.000 10300 17.758 11456 

S – – 22.118 14270 

T 26.000 16770 28.860 18619 

X – – 44.178 28302 

WB Series – Spring Loaded SRV 

WB100 / 200/ 300 / 300B & 400 

The actual orifice area is 11% larger than those 

shown in this table. This ensures that after 

derating the discharge coefficient in accordance 

with industry standards, the full benefits of the 

API 526 orifice area can still be obtained. It is 

important to use the actual coefficient of 

discharge as the areas are already derated. 

Safeset – Pilot Operated SRV 

The areas shown in the table are actual orifice 

areas of the main valves and are larger than the 

standard API 526 dimensions. This ensures that 

after derating the discharge coefficient in 

accordance with industry standards, the full 

benefits of the API 526 orifice area can still be 

obtained. It is important to use the derated 

coefficient of discharge as the areas are actuals. 

Orifice no. 

Safeflo 

in 2 mm 2 

1 0.062 40 

2 0.110 71 

3 0.196 127 

4 0.442 285 

6 0.070 45 

7 0.169 109 

Safeflo – Spring Loaded Types 

The orifice areas shown in the table are in 

accordance with industry standards. API 526 

does not specify requirements for thermal 

relief valves. It is important to use the derated 

coefficient of discharge as the areas are actuals. 

84

Table 2 

Representative data on some vapours and gases useful in sizing 

safety relief valves 

Gas or Vapour k C C M √M G* √G 

imperial metric 

Acetaldehyde 1.14 331 2.51 44 6.633 1.519 1.232 

Acetic Acid 1.15 332 2.52 60 7.746 2.071 1.439 

Acetylene 1.26 343 2.61 26.04 5.103 0.899 0.948 

Air 1.40 356 2.70 28.97 5.382 1 1 

Ammonia 1.31 348 2.64 17.03 4.127 0.587 0.766 

Argon 1.67 377.5 2.87 40 6.325 1.381 1.175 

Benzene 1.12 329 2.50 78.11 8.838 2.70 1.643 

Butadiene 1.3 1.12 329 2.50 54.09 7.355 1.922 1.386 

n-Butane 1.09 325 2.47 58.12 7.63 2.07 1.439 

Iso-Butane 1.1 327 2.49 58.12 7.63 2.07 1.439 

i-Butane 1.11 327 2.49 56.10 7.49 1.937 1.392 

Iso-Butylene 1.12 329 2.49 56.10 7.49 1.998 1.413 

Carbon Dioxide 1.29 346 2.68 44.01 6.634 1.53 1.237 

Carbon Disulphate 1.21 338 2.57 76.13 8.726 2.628 1.621 

Carbon Monoxide 1.40 356 2.70 28.00 5.292 0.967 0.983 

Chloride 1.36 353 2.68 70.91 8.421 2.45 1.565 

Cyclohexane 1.09 325 2.47 84.16 9.174 2.905 1.705 

Decane 1.03 319 2.42 142 11.92 4.91 2.216 

Dowthern A 1.043 320 2.43 165 12.85 5.696 2.386 

Dowthern E — — — 147 12.12 5.074 2.253 

Ethane 1.19 336 2.55 30.07 5.483 1.05 1.025 

Ethene (Ethylene) 1.24 341 2.59 28.05 5.297 0.977 0.988 

Ethyl Alcohol 1.13 330 2.50 46.07 6.787 1.59 1.261 

Ethyl Benzine 1.07 323 2.46 106.16 10.31 3.67 1.916 

Ethyl Chloride 1.19 336 2.55 64.50 8.031 2.226 1.492 

Freon 11 1.14 331 2.51 137.37 11.72 4.742 2.177 

Freon 12 1.14 331 2.51 120.92 10.995 4.174 2.043 

Freon 22 1.18 335 2.55 86.48 9.299 2.985 1.727 

Freon 114 1.09 325 2.47 170.93 13.073 5.90 2.429 

Helium 1.66 377 2.86 4 2 0.138 0.3716 

n-Heptane 1.05 321 2.44 100 10 3.49 1.868 

n-Hexane 1.06 322 2.45 86.17 9.283 2.97 1.723 

Hydrogen Chloride 1.41 357 2.71 36.47 6.039 1.27 1.127 

Hydrogen 1.41 357 2.71 2.02 1.421 0.070 0.265 

Hydrogen Sulphide 1.32 349 2.65 34.08 5.838 1.19 1.091 

Methane 1.31 348 2.64 16.04 4.005 0.555 0.745 

Menthyl Alcochol 1.20 337 2.56 32 5.657 1.11 1.054 

Menthyl Butane 1.08 324 2.46 72.15 8.494 2.49 1.578 

Methyl Chloride 1.20 337 2.56 50.48 7.105 1.742 1.320 

Natural Gas 1.27 344 2.61 19 4.359 0.656 0.8099 

Nitric Oxide 1.40 356 2.70 30 5.477 1.036 1.018 

Nitrogen 1.40 356 2.70 28.02 5.294 0.967 0.9834 

Nitrous Oxide 1.30 347 2.63 44 6.633 1.519 1.233 

Nonane 1.04 320 2.43 128 11.31 4.43 2.105 

n-Octane 1.05 321 2.44 114.22 10.687 3.94 1.985 

Oxygen 1.40 356 2.70 32 5.657 1.10 1.0490 

n-Pentane 1.07 323 2.46 72.15 8.494 2.49 1.578 

Phenol 1.30 347 2.63 94 9.695 3.27 1.808 

Propane 1.13 330 2.50 44.09 6.64 1.55 1.245 

Propylene 1.15 332 2.52 42.08 6.487 1.476 1.214 

Sulphur Dioxide 1.29 346 2.63 64.06 8.004 2.26 1.503 

Steam 1.33 349 2.66 18 4.243 0.622 0.7887 

Styrene 1.07 323 2.46 104.14 10.21 3.60 1.897 

Toluene 1.09 325 2.47 92 9.592 3.18 1.783 

*Air = 1.0 at 14.7 Psia and 60ºF. 

85

Table 3 

Representative data on liquids useful in sizing safety valves 

Liquid G** G 

Acetic Acid 1.05 1.025 

Acetone 0.792 0.890 

Ammonia 0.617 0.786 

Benzene 0.885 0.941 

1, 2 Butadiene 0.658 0.811 

1, 3 Butadiene 0.627 0.792 

Iso-Butane 0.563 0.750 

n-Butane 0.584 0.764 

j-Butane 0.601 0.775 

Carbon Dioxide 0.816 0.903 

Carbon Disulphide 1.26 1.122 

Chlorine 1.423 1.93 

Dowtherm A at 212ºF 0.997 0.999 

Dowtherm E at 212ºF 1.181 1.086 

Ethane 0.377 0.614 

Ethyl Alcohol 0.794 0.891 

Ethyl Benene 0.872 0.934 

Fuel Oil, Bunker C 1.014 (max.) 1.007 

Fuel Oil, No. 3 (60ºF) 0.898 (max.) 0.948 

Fuel Oil, No. 5 (60ºF) 0.993 (max.) 0.997 

Fuel Oil, No. 6 (60ºF) 0.993 (min.) 0.997 

Petrol (Gasoline) 0.75 0.886 

Liquid G** G 

n-Heptane 0.688 0.830 

n-Hexane 0.664 0.815 

Hydrochloric Acid (40%) 1.20 1.095 

Kerosene 0.82 0.906 

Methane 0.248 0.498 

Methyl Alcohol (100%) 0.796 0.892 

Methyl Butane 0.625 0.791 

Naphtha 0.88 0.938 

Nitric Acid (91%) 1.50 1.225 

Nitrogen 0.804 0.897 

Iso-Octane 0.696 0.834 

n-Octane 0.707 0.841 

Oils, Minerals and 

Lubricants 0.910 0.954 

Iso-Pentane 0.625 0.791 

n-Pentane 0.631 0.794 

Phosphoric Acid 1.88 1.371 

Propane 0.508 0.713 

Polythene 0.522 0.723 

Styrene 0.911 0.955 

Sulphuric Acid (87%) 1.80 1.342 

Water 1.00 1.00 

**Water = 1.0 at 70ºF. 

86

Graph 2.0 

Factor Ff – For application sizing and capacity determination of balanced bellows safety 

relief valves against variable and constant back pressure – vapours and gases only. 

Graph 3.0 

Use the curve to evaluate back pressure correction factor Fb, for pilot valves and for 

conventional valves with constant back pressure use. 

87

Graph 4.0 

Factor Fl – For application sizing and capacity determination of balanced bellows safety 

relief valves against variable or constant back pressure – liquids only (10% overpressure). 

Graph 5.0 

Factor Fp – For low set pressure – vapours and gases only. 

88

Table 4 

Superheated Steam Correction Factors – FSH 

For capacity on superheated steam, multiply saturated steam capacity by correction factor below 

Set 

Pressure Saturated 

Total Steam Temperature in Degrees Fahrenheit 

p.s.i.— steam 

gauge Temp.°F 

300 400 500 600 700 800 900 1000 1100 1200 

15 250 1.00 0.98 0.93 0.88 0.84 0.80 0.77 0.74 0.72 0.70 

20 259 1.00 0.98 0.93 0.88 0.84 0.80 0.77 0.74 0.72 0.70 

40 287 1.00 0.99 0.93 0.88 0.84 0.81 0.77 0.74 0.72 0.70 

60 308 1.00 0.99 0.93 0.88 0.84 0.81 0.77 0.75 0.72 0.70 

80 324 1.00 0.99 0.93 0.88 0.84 0.81 0.77 0.75 0.72 0.70 

100 338 1.00 0.99 0.94 0.89 0.84 0.81 0.77 0.75 0.72 0.70 

120 350 1.00 0.99 0.94 0.89 0.84 0.81 0.78 0.75 0.72 0.70 

140 361 1.00 0.99 0.94 0.89 0.85 0.81 0.78 0.75 0.72 0.70 

160 371 1.00 0.99 0.94 0.89 0.85 0.81 0.78 0.75 0.72 0.70 

180 380 1.00 0.99 0.94 0.89 0.85 0.81 0.78 0.75 0.72 0.70 

200 388 1.00 0.99 0.95 0.89 0.85 0.81 0.78 0.75 0.72 0.70 

220 395 1.00 0.99 0.95 0.89 0.85 0.81 0.78 0.75 0.72 0.70 

240 403 - 1.00 0.95 0.90 0.85 0.81 0.78 0.75 0.72 0.70 

260 409 - 1.00 0.95 0.90 0.85 0.81 0.78 0.75 0.72 0.70 

280 416 - 1.00 0.96 0.90 0.85 0.81 0.78 0.75 0.72 0.70 

300 422 - 1.00 0.96 0.90 0.85 0.81 0.78 0.75 0.72 0.70 

350 436 - 1.00 0.96 0.90 0.86 0.82 0.78 0.75 0.72 0.70 

400 448 - 1.00 0.96 0.91 0.86 0.82 0.78 0.75 0.72 0.70 

500 470 - 1.00 0.96 0.92 0.86 0.82 0.78 0.75 0.73 0.70 

600 489 - 1.00 0.97 0.92 0.87 0.82 0.79 0.75 0.73 0.70 

800 520 - - 1.00 0.95 0.88 0.83 0.79 0.76 0.73 0.70 

1000 546 - - 1.00 0.96 0.89 0.84 0.78 0.76 0.73 0.71 

1250 574 - - 1.00 0.97 0.91 0.85 0.80 0.77 0.74 0.71 

1500 597 - - - 1.00 0.93 0.86 0.81 0.77 0.74 0.71 

1750 618 - - - 1.00 0.94 0.86 0.81 0.77 0.73 0.70 

2000 636 - - - 1.00 0.95 0.86 0.80 0.76 0.72 0.69 

2500 670 - - - 1.00 0.95 0.85 0.78 0.73 0.69 0.66 

3000 690 - - - - 1.00 0.82 0.74 0.69 0.65 0.62 

89

Pressure Protection 

90

WATER CAPACITY CHART 

WB 100/200 

Water Capacity U.S.G.P.M. 

10% over pressure or 3 Psig minimum 

ORIFICE SIZE LETTER DESIGNATION 

Set 

Psig 

Gauge 

D E F G H J K L M N P Q R T 

10 10 18 28 45 70 115 164 255 322 388 571 989 1431 2326 

20 13 23 37 60 93 153 219 340 428 516 759 1315 1904 3094 

30 16 28 44 72 112 183 262 407 513 619 909 1575 2281 3706 

40 18 32 51 83 129 212 303 470 593 714 1050 1819 2634 4280 

50 20 36 56 93 144 237 338 525 662 799 1174 2033 2944 4785 

60 22 40 62 101 158 259 371 575 726 875 1286 2228 3225 5241 

70 24 43 67 110 171 280 400 621 784 945 1389 2406 3484 5661 

80 26 46 71 117 183 300 428 664 838 1010 1485 2572 3724 6052 

90 27 48 76 124 194 318 454 704 889 1072 1575 2728 3850 6419 

100 29 51 80 131 204 335 478 742 937 1129 1660 2876 4164 6767 

120 31 56 88 143 224 367 524 813 1026 1237 1819 3150 4561 7412 

140 34 60 95 155 242 396 566 879 1109 1336 1965 3403 4927 8006 

160 36 65 101 166 258 424 605 939 1185 1429 2100 3638 5267 8559 

180 38 68 107 176 274 449 642 996 1257 1515 2228 3858 5587 9078 

200 40 72 113 185 289 474 676 1050 1325 1597 2348 4067 5889 9569 

220 42 76 119 194 303 497 709 1101 1390 1675 2463 4265 6176 10036 

240 44 79 124 203 316 519 741 1150 1451 1750 2572 4455 6451 10483 

260 46 82 129 211 329 540 771 1197 1511 1821 2677 4637 6714 10911 

280 48 85 134 219 342 560 800 1242 1568 1890 2778 4812 6968 11323 

300 50 88 138 227 354 580 829 1286 1623 1956 2876 4981 7212 11720 

320 51 91 143 234 365 599 856 1328 1676 2020 2970 5144 

340 53 94 147 241 377 618 882 1369 1728 2083 3062 5303 

360 54 97 152 248 388 636 908 1409 1778 2143 3150 5456 

380 56 99 156 255 398 653 932 1447 1826 2202 3237 5606 

400 57 102 160 262 409 670 957 1485 1874 2259 3321 5752 

420 59 105 164 268 419 686 980 1522 1920 2315 3403 5894 

440 60 107 168 275 429 703 1003 1557 1965 2369 3483 6032 

460 61 109 171 281 438 718 1026 1592 2009 2422 3561 6168 

480 63 112 175 287 448 734 1048 1627 2053 2475 3638 6301 

500 64 114 179 293 457 749 1070 1660 2095 2526 3713 6430 

520 65 116 182 299 466 764 1091 1693 2136 2576 3786 6558 

540 67 119 186 304 475 778 1112 1725 2177 2625 3858 6683 

560 68 121 189 310 483 793 1132 1757 2217 2673 3929 6805 

580 69 123 192 315 492 807 1152 1788 2256 2720 3999 6926 

600 70 125 196 321 500 820 1172 1819 2295 2767 4067 7044 

620 71 127 199 326 509 834 1191 1849 2333 2812 4134 

640 72 129 202 331 517 847 1210 1878 2370 2857 4201 

660 74 131 205 336 525 860 1229 1908 2407 2902 4266 

680 75 133 208 341 533 873 1247 1936 2443 2945 4330 

700 76 135 211 346 541 886 1266 1964 2479 2988 4393 

720 77 137 214 351 548 899 1284 1992 2514 3031 4455 

740 78 139 217 356 556 911 1301 2020 2549 3073 4517 

760 79 141 220 361 563 923 1319 2047 2583 3114 4577 

780 80 142 223 366 571 935 1336 2074 2617 3154 4637 

800 81 144 226 370 578 947 1353 2100 2650 3195 4696 

850 83 149 233 382 596 977 1395 2165 2732 3293 4841 

900 86 153 240 393 613 1005 1435 2227 2811 3388 4981 

950 88 157 246 403 630 1032 1474 2289 2888 3481 5118 

1000 91 161 253 414 646 1059 1513 2348 2963 3572 5251 

1100 95 169 265 434 678 1111 1586 2463 3107 

1300 103 184 288 472 737 1208 1725 2677 

1500 111 198 309 507 791 1297 1853 2876 

1750 120 213 334 548 855 1401 2001 

2000 128 228 357 585 914 1498 2139 

2250 136 242 379 621 969 1589 

2500 143 255 399 655 1021 1675 

2750 150 267 419 686 1071 

3000 157 279 438 717 

3250 163 291 455 746 

3500 169 302 473 774 

3750 175 312 489 

4000 181 323 505 

4250 187 333 521 

4500 192 342 536 

4750 197 352 551 

5000 202 361 565 

5250 207 370 

5500 212 378 

5750 217 387 

6000 222 395 

Note: This chart should be used as a guideline only. 

91

SATURATED STEAM CAPACITY CHART 

WB 400 

Saturated Steam Capacities lb/hr 



Set 

Psig 

Gauge 


10 142 252 395 648 1011 1658 2367 3675 4637 5590 8217 14232 20607 33487 

20 208 371 581 952 1486 2436 3479 5401 6815 8216 12077 20918 30288 49218 

30 263 469 735 1205 1880 3083 4402 6833 8622 10395 15281 26466 38322 62274 

40 324 578 905 1483 2314 3793 5417 8409 10611 12792 18805 32570 47160 76634 

50 385 686 1074 1760 2747 4504 6433 9985 12599 15189 22329 38673 55997 90995 

60 446 794 1244 2038 3181 5215 7448 11561 14588 17586 25853 44776 64834 105356 

70 506 902 1414 2316 3615 5926 8463 13137 16576 19983 29377 50880 73672 119717 

80 567 1011 1583 2594 4048 6637 9478 14712 18565 22381 32901 56983 82509 134077 

90 628 1119 1753 2872 4482 7348 10493 16288 20553 24778 36242 63086 91347 148438 

100 689 1227 1922 3150 4915 8059 11509 17864 22541 27175 39948 69190 100184 162799 

120 810 1444 2261 3705 5782 9480 13539 21016 26518 31969 46996 81396 117859 191521 

140 932 1660 2601 4261 6650 10902 15569 24167 30495 36763 54044 93603 135534 220242 

160 1053 1877 2940 4816 7517 12324 17600 27319 34472 41558 61092 105810 153208 248964 

180 1175 2093 3279 5372 8384 13745 19630 30471 38449 46352 68140 118016 170883 277685 

200 1296 2310 3618 5928 9251 15167 21661 33622 42426 51146 75185 130223 188558 306407 

220 1418 2526 3957 6483 10118 16589 23691 36774 46402 55941 82235 142430 206233 335128 

240 1539 2743 4296 7039 10985 18011 25721 39926 50379 60735 89283 154636 223908 363850 

260 1661 2959 4635 7595 11853 19432 27752 43077 54356 65529 96331 166843 241582 392571 

280 1782 3176 4974 8150 12720 20854 29782 46229 58333 70324 103379 179049 259257 421193 

300 1940 3392 5314 8706 13587 22276 31813 49380 62310 75118 110427 191256 276932 450014 

320 2025 3609 5653 9262 14454 23697 33843 52532 66287 79912 117474 203463 

340 2147 3825 5992 9817 15321 25119 35873 55684 70263 84706 124522 215669 

360 2268 4042 6331 10373 16188 26541 37904 58835 74240 89501 131570 227876 

380 2390 4258 6670 10929 17056 27963 39934 61987 78217 94295 138618 240083 

400 2511 4475 7009 11484 17923 29384 41965 65139 82194 99089 145666 252289 

420 2633 4692 7348 12040 18790 30806 43995 68290 86171 103884 152714 264496 

440 2755 4908 7688 12596 19657 32228 46025 71442 90148 108678 159761 276703 

460 2876 5125 8027 13151 20524 33649 48056 74594 94124 113472 166809 288909 

480 2998 5341 8366 13707 21392 35071 50086 77745 98101 118266 173857 301116 

500 3119 5558 8705 14263 22259 36493 52116 80897 102078 123061 180905 313323 

520 3241 5774 9044 14818 23126 37915 54147 84048 106055 127855 187953 325529 

540 3362 5991 9383 15374 23993 39336 56177 87200 110032 132649 195001 337736 

560 3484 6207 9722 15930 24860 40758 58208 90352 114008 137444 202048 349943 

580 3605 6424 10062 16485 25727 42180 60238 93503 117985 142238 209096 362149 

600 3727 6640 10401 17041 26595 43601 62268 96655 121962 147032 216144 374356 

620 3848 6857 10740 17596 27462 45023 64299 99807 125939 151826 223192 

640 3970 7073 11079 18152 28329 46445 66329 102958 129916 156621 230240 

660 4091 7290 11418 18708 29196 47867 68360 106110 133893 161415 237288 

680 4213 7506 11757 19263 30063 49288 70390 109262 137869 166209 244335 

700 4334 7723 12096 19819 30930 50710 72420 112413 141846 171004 251383 

720 4456 7939 12435 20375 31798 52132 74451 115565 145823 175798 258431 

740 4577 8156 12775 20930 32665 53553 76481 118716 149800 180592 265479 

760 4699 8372 13114 21486 33532 54975 78512 121868 153777 185386 272527 

780 4820 8589 13453 22042 34399 56397 80542 125020 157754 190181 279574 

800 4942 8805 13792 22597 35266 57819 82572 128171 161730 194975 286622 

850 5246 9347 14640 23986 37434 61373 87648 136050 171673 206961 304242 

900 5549 9888 15488 25376 39602 64927 92724 143930 181615 218946 321861 

950 5853 10429 16336 26765 41770 68482 97800 151809 191557 230932 339481 

1000 6157 10970 17183 28154 43938 72036 102876 159688 201499 242918 357101 

1100 6764 12053 18879 30932 48274 79144 113028 175446 221383 

1300 7980 14218 22270 36489 56945 93362 133332 206962 

1500 9155 16313 25551 41863 65334 107114 152972 237448 

1750 10820 19279 30198 49477 77216 126594 180793 

2000 12561 22382 35058 57440 89643 146969 209891 

2250 14403 25664 40199 65863 102788 168520 240668 

2500 16381 29189 45719 74907 116903 191662 

2750 18549 33052 51770 84821 132375 217028 

2900 19976 35593 55751 91344 


92

AIR CAPACITY CHART 

WB 400 

Air Capacity s.c.f.m. 



Set 

Psig 

Gauge 


10 50 89 140 229 357 586 836 1298 1638 1975 2903 5028 7280 11830 

20 74 131 205 336 525 861 1229 1908 2408 2902 4267 7390 10700 17388 

30 94 167 262 429 670 1098 1568 2434 3071 3703 5443 9427 13650 22181 

40 115 206 322 528 824 1351 1930 2995 3779 4556 6698 11601 16798 27296 

50 137 244 383 627 979 1604 2291 3557 4488 5410 7953 13775 19945 32411 

60 159 283 443 726 1133 1858 2653 4118 5196 6264 9208 15949 23093 37526 

70 180 321 503 825 1287 2111 3014 4679 5904 7118 10464 18123 26241 42642 

80 202 360 564 924 1442 2364 3376 5240 6612 7972 11719 20297 29389 47757 

90 224 399 624 1023 1596 2617 3738 5802 7321 8826 12974 22471 32537 52872 

100 245 437 685 1122 1751 2870 4099 6363 8029 9679 14229 24644 35684 57987 

120 289 514 805 1320 2060 3377 4822 7486 9445 11387 16739 28992 41980 68217 

140 332 591 926 1518 2369 3883 5546 8608 10862 13095 19250 33340 48275 78448 

160 375 668 1047 1716 2677 4390 6269 9731 12278 14802 21760 37688 54571 88678 

180 418 746 1168 1913 2986 4896 6992 10853 13695 16510 24271 42036 60866 98908 

200 462 823 1289 2111 3295 5402 7715 11976 15111 18218 26781 46384 67162 109138 

220 505 900 1409 2309 3604 5909 8438 13098 16528 19925 29291 50732 73458 119369 

240 548 977 1530 2507 3913 6415 9162 14221 17944 21633 31802 55080 79753 129599 

260 592 1054 1651 2705 4222 6922 9885 15344 19361 23341 34312 59427 86049 139829 

280 635 1131 1772 2903 4531 7428 10608 16466 20777 25048 36822 63775 92344 150059 

300 678 1208 1893 3101 4840 7934 11331 17589 22194 26756 39333 68123 98640 160290 

320 721 1285 2013 3299 5148 8441 12054 18711 23610 28464 41843 72471 

340 765 1363 2134 3497 5457 8947 12778 19834 25027 30171 44353 76819 

360 808 1440 2255 3695 5766 9454 13501 20956 26443 31879 46664 81167 

380 851 1517 2376 3893 6075 9960 14224 22079 27860 33587 49374 85515 

400 895 1594 2497 4091 6384 10466 14947 23202 29276 35294 51884 89862 

420 938 1671 2617 4288 6693 10973 15670 24324 30693 37002 54395 94210 

440 981 1748 2738 4486 7002 11479 16394 25447 32109 38710 56905 98558 

460 1024 1825 2859 4684 7311 11986 17117 26569 33526 40417 59415 102906 

480 1068 1902 2980 4882 7619 12492 17840 27692 34942 42125 61926 107254 

500 1111 1980 3101 5080 7928 12998 18563 28814 36359 43833 64436 111602 

520 1154 2057 3221 5278 8237 13505 19286 29937 37775 45540 66946 115950 

540 1198 2134 3342 5476 8546 14011 20010 31060 39192 47248 69457 120297 

560 1241 2211 3463 5674 8855 14518 20733 32182 40608 48956 71967 124645 

580 1284 2288 3584 5872 9164 15024 21456 33305 42025 50663 74478 128993 

600 1327 2365 3705 6070 9473 15530 22179 34427 43441 52371 76988 133341 

620 1371 2442 3825 6268 9782 16037 22902 35550 44858 54079 79498 

640 1414 2519 3946 6466 10090 16543 23626 36672 46274 55786 82009 

660 1457 2597 4067 6663 10399 17050 24349 37795 47691 57494 84519 

680 1501 2674 4188 6861 10708 17556 25072 38918 49107 59202 97029 

700 1544 2751 4309 7059 11017 18062 25795 40040 50524 60909 89540 

720 1587 2828 4429 7257 11326 18569 26518 41163 51940 62617 92050 

740 1630 2905 4550 7455 11635 19075 27242 42285 53357 64325 94560 

760 1674 2982 4671 7653 11944 19581 27965 43408 54773 66032 97071 

780 1717 3059 4792 7851 12253 20088 28688 44530 56190 67740 99581 

800 1760 3136 4913 8049 12561 20594 29411 45653 57606 69448 102091 

850 1868 3329 5215 8544 13334 21860 31219 48460 61148 73717 108367 

900 1977 3522 5517 9038 14106 23126 33027 51266 64689 77986 114643 

950 2085 3715 5819 9533 14878 24392 34835 54072 68230 82255 120919 

1000 2193 3908 6121 10028 15650 25658 36643 56879 71771 86524 127195 

1100 2409 4293 6724 11018 17195 28190 40259 62492 78854 

1300 2842 5064 7932 12997 20283 33254 47491 73717 

1500 3275 5836 9140 14976 23372 38318 54723 84943 

1750 3816 6800 10650 17450 27233 44648 63763 

2000 4357 7764 12160 19924 31094 50978 72803 

2250 4898 8728 13670 22398 34955 57308 81843 

2500 5439 9692 15180 24872 38816 63638 

2750 5980 10656 16690 27346 42677 69968 

3000 6521 11620 18200 29820 

3250 7062 12584 19710 32294 

3500 7603 13548 21220 34768 

3750 8144 14512 22730 

4000 8685 15476 24240 

4250 9226 16440 25750 

4500 9767 17404 27260 

4750 10308 18368 28770 

5000 10849 19332 30280 

5250 11390 20296 

5500 11931 21260 

5750 12472 22224 

6000 13013 23188 


93

AIR CAPACITY CHART 

Pilot:Types 2, 4 & 8 

Main Valve Discharge Capacities s.c.f.m. 

10% over pressure or 3 Psig whichever is the greater. Kd = 0.849 

* 

ORIFICE SIZE LETTER DESIGNATION (DISCHARGE AREA sq in) 

Set 

Psig D E F G H J K L M N P Q R S T 

Gauge (0.164) (0.256) (0.338) (0.616) (0.871) (1.429) (2.139) (3.166) (4.307) (5.162) (7.068) (12.864) (17.758) (22.118) (28.862) 

29 119 186 246 448 634 1039 1556 2303 3133 3755 5141 9357 12917 16088 20994 

40 150 234 309 563 796 1307 1956 2895 3938 4720 6462 11762 16236 20223 26387 

60 206 322 425 774 1095 1796 2689 3980 5414 6488 8884 16170 22321 27802 36276 

80 262 410 541 985 1393 2286 3422 5064 6890 8257 11306 20578 28406 35381 46166 

100 319 497 657 1196 1692 2776 4155 6149 8366 10026 13728 24986 34492 42960 56055 

150 459 717 946 1724 2438 4000 5987 8862 12055 14448 19783 36006 49704 61908 80779 

200 600 936 1236 2252 3184 5224 7819 11574 15745 18871 25838 47026 64917 80856 105502 

250 740 1155 1525 2780 3930 6448 9652 14286 19435 23293 31893 58047 80130 99804 130226 

300 881 1374 1815 3307 4676 7672 11484 16998 23124 27715 37948 69067 95343 118752 154949 

350 1021 1594 2104 3835 5423 8896 13317 19710 26814 32137 44003 80087 110556 137700 179673 

400 1162 1813 2394 4363 6169 10121 15149 22423 30504 36559 50058 91107 125768 156647 204397 

450 1302 2032 2683 4890 6915 11345 16982 25135 34193 40981 56113 102128 140981 175595 229120 

500 1442 2252 2973 5418 7661 12569 18814 27847 37883 45403 62168 113148 156194 194543 253844 

550 1583 2471 3263 5946 8407 13793 20646 30559 41573 49826 68223 124168 171407 213491 278567 

600 1723 2690 3552 6474 9153 15017 22479 33272 45262 54248 74278 135188 186620 232439 303291 

650 1864 2910 3842 7001 9900 16242 24311 35984 48952 58670 80333 146209 201832 251387 328015 

700 2004 3129 4131 7529 10646 17466 26144 38696 52642 63092 86388 157229 217045 270335 352738 

750 2145 3348 4421 8057 11392 18690 27976 41408 56332 67514 92443 168249 232258 289283 377462 

800 2285 3568 4710 8584 12138 19914 29809 44121 60021 71936 98498 179269 247471 308231 402185 

850 2426 3787 5000 9112 12884 21138 31641 46833 63711 76358 104553 190290 262684 327179 426909 

900 2566 4006 5289 9640 13630 22363 33473 49545 67401 80781 110608 201310 277896 346126 451633 

950 2707 4225 5579 10168 14377 23587 35306 52257 71090 85203 116663 212330 293109 365074 476356 

1000 2847 4445 5869 10695 15123 24811 37138 54969 74780 89625 122718 223350 308322 384022 501080 

1200 3409 5322 7027 12806 18107 29708 44468 65818 89539 107313 146938 267431 369173 459814 599974 

1400 3971 6199 8185 14917 21092 34604 51798 76667 104298 125002 171157 311512 430024 535605 698868 

1480 4196 6550 8648 15761 22286 36563 54730 81007 110201 132077 180845 329145 454365 565922 738426 

1600 4533 7076 9343 17028 24077 39501 59127 87516 119056 142691 195377 355593 490876 

1800 5095 7954 10501 19139 27061 44398 66457 98365 133815 160379 219597 399674 551727 

2000 5657 8831 11660 21249 30046 49295 73787 109214 148574 178068 243817 443755 612578 

2200 6219 9708 12818 23360 33031 54191 81116 120063 163333 195756 268037 487836 673429 

2400 6781 10585 13976 25471 36015 59088 88446 130912 178091 213445 292257 531917 734281 

2600 7343 11463 15134 27582 39000 63985 95776 141761 192850 231134 316477 575998 795132 

2800 7905 12340 16293 29693 41985 68882 103106 152610 207609 248822 340696 620079 855983 

3000 8467 13217 17451 31804 44969 73778 110435 163459 222368 266511 364916 664160 916834 

3500 9872 15410 20346 37081 52431 86020 128759 190581 259265 310732 425466 774363 1068962 

3705 10448 16309 21533 39244 55490 91039 136272 201701 274392 328863 450291 819546 1131335 

4000 11277 17603 23242 42358 59892 98262 

4350 12261 19138 25269 46052 65116 106831 

4500 12682 19796 26137 47635 67354 110504 

5000 14087 21989 29033 52912 74816 122746 

5500 15492 24183 31929 58189 82277 134988 

6000 16897 26376 34824 63466 89739 147230 

6170 17375 27121 35809 65261 92276 151392 

*Note: Type 4 Pilot is available to 1480 Psig. 

Type 8 Pilot is available from 1480 Psig. 

For 8" x 10" x 10" full bore capacity chart see page 96. 

94

WATER CAPACITY CHART - U.S.G.P.M. 

Pilot:Types 4 and 8 

Main Valve Discharge Capacities Water u.s.g.p.m. 

10% over pressure or 3 Psig whichever is the greater. Kd = 0.696 

* 

ORIFICE SIZE LETTER DESIGNATION (DISCHARGE AREA sq in) 

Set 

Psig D E F G H J K L M N P Q R S T 

Gauge (0.164) (0.256) (0.338) (0.616) (0.871) (1.429) (2.139) (3.166) (4.307) (5.162) (7.068) (12.864) (17.758) (22.118) (28.862) 

29 25 38 51 92 130 214 320 473 644 771 1056 1922 2654 3305 4313 

40 29 45 59 108 153 251 375 555 756 906 1240 2257 3115 3880 5063 

60 35 55 73 132 187 307 460 680 925 1109 1519 2764 3816 4752 6201 

80 41 64 84 153 216 355 531 785 1069 1281 1754 3192 4406 5488 7161 

100 45 71 94 171 242 396 593 878 1195 1432 1961 3568 4926 6135 8006 

150 56 87 115 209 296 485 727 1076 1463 1754 2401 4370 6033 7514 9805 

200 64 100 133 242 342 561 839 1242 1690 2025 2773 5046 6966 8677 11322 

250 72 112 148 270 382 627 938 1389 1889 2264 3100 5642 7788 9701 12659 

300 79 123 162 296 418 687 1028 1521 2069 2480 3396 6181 8532 10627 13867 

350 85 133 175 320 452 742 1110 1643 2235 2679 3668 6676 9215 11478 14978 

400 91 142 188 342 483 793 1187 1756 2389 2864 3921 7137 9852 12271 16012 

450 97 151 199 362 513 841 1259 1863 2534 3037 4159 7570 10449 13015 16983 

500 102 159 210 382 540 886 1327 1964 2671 3202 4384 7979 11015 13719 17902 

550 107 167 220 401 567 930 1391 2060 2802 3358 4598 8368 11552 14389 18776 

600 111 174 230 419 592 971 1453 2151 2926 3507 4802 8741 12066 15028 19611 

650 116 181 239 436 616 1011 1513 2239 3046 3651 4999 9097 12559 15642 20411 

700 120 188 248 452 639 1049 1570 2324 3161 3788 5187 9441 13033 16232 21182 

750 125 194 257 468 662 1086 1625 2405 3272 3921 5369 9772 13490 16802 21925 

800 129 201 265 483 683 1121 1678 2484 3379 4050 5545 10093 13932 17353 22644 

850 133 207 273 498 704 1156 1730 2560 3483 4175 5716 10403 14361 17887 23341 

900 136 213 281 513 725 1189 1780 2635 3584 4296 5882 10705 14778 18406 24018 

950 140 219 289 527 745 1222 1829 2707 3682 4413 6043 10998 15183 18910 24676 

1000 144 225 269 540 764 1253 1876 2777 3778 4528 6200 11284 15577 19401 25317 

1200 158 246 325 592 837 1373 2055 3042 4139 4960 6792 12361 17064 21253 27734 

1400 170 266 351 639 904 1483 2220 3286 4470 5358 7336 13351 18431 22956 29956 

1480 175 273 361 657 929 1525 2283 3379 4596 5509 7543 13728 18950 23603 30800 

1600 182 284 375 683 966 1586 2373 3513 4779 5728 7842 14273 19703 

1800 193 301 398 725 1025 1682 2517 3726 5069 6075 8318 15139 20899 

2000 203 318 419 764 1080 1773 2653 3927 5343 6404 8768 15958 22029 

2200 213 333 440 801 1133 1859 2783 4119 5604 6716 9196 16737 23104 

2400 223 348 459 837 1184 1942 2907 4302 5853 7015 9605 17481 24132 

2600 232 362 478 871 1232 2021 3025 4478 6092 7301 9997 18195 25117 

2800 241 376 496 904 1278 2097 3140 4647 6322 7577 10374 18882 26065 

3000 249 389 514 936 1323 2171 3250 4810 6544 7843 10739 19545 26980 

3500 269 420 555 1011 1429 2345 3510 5196 7068 8471 11599 21111 29142 

3705 277 432 571 1040 1471 2413 3612 5346 7272 8716 11934 21720 29983 

4000 288 449 593 1081 1528 2507 

4350 300 468 618 1127 1593 2614 

6170 357 558 736 1342 1898 3114 

*Note: Type 4 Pilot is available to 1480 Psig. 

Type 8 Pilot is available from 1480 Psig. 

For 8" x 10" x 10" full bore capacity chart see page 96. 

95

DUAL OUTLET / FULL BORE PILOT VALVE 

This valve is suitable for extremely high 

capacity duties. It achieves maximum discharge 

capacities by having a full bore 8 inch inlet with 

an available discharge area of 44.178 sq.in. and 

two opposing 10 inch outlets, which can assist 

with reaction force problems. 

Available with Pilot Types 2 and 4. 

Capacity chart (full bore 8" x 10" x 10" pilot valve) 

Actual orifice area Air S.C.F.M. Water U.S.G.P.M. 

44.178sq.in. @ 60ºF and 10% over pressure @ 10% over pressure 

Set pressure Pilot types 2 and 4 Pilot type 4 

Psig Derated kd 0.802 Derated kd 0.658 

29 30270 6239 

50 45275 8192 

100 81002 11585 

150 116729 14189 

200 152455 16384 

250 188182 18318 

300 223909 20067 

350 259635 21674 

400 295362 23171 

450 331089 24576 

500 366815 25906 

550 402542 27170 

600 438269 28378 

650 473995 29537 

700 509722 30652 

740 538303 31516 

*Note: Pilot type 2 is available from 29 Psig. Overpressure at 29 Psig is 3 Psig minimum. 

96

REACTION FORCE - VAPOUR AND GASES 

The discharge from a safety relief valve exerts a 

reaction force on the valve or outlet piping. If the 

discharge piping is unsupported, this force is 

transmitted to the valve inlet and associated piping. 

The following formula can be used to determine 

the reaction force, assuming that critical flow of 

the gas or vapour occurs at the valve outlet. 

Discharge piping should be adequately supported. If 

pipework is not supported, it must be 

remembered that the reaction force will act on the 

end of the discharge pipe, and that the discharge 

pipe will act as a lever. The force applied to the 

valve will therefore be determined by the reaction 

force and the geometry of the discharge pipework. 

F = W 

kT 

(k+1) M 

366 

+(Po x Ao) 

F = Reaction force (lbs). 

W = Flow rate of gas or vapour (lb/hr). 

K = Ratio of specific heat (imperial). 

M = Molecular weight of gas or vapour. 

T = Temperature at valve inlet, degrees 

Rankine (equal to degrees F plus 460). 

Po = Outlet pressure (Psig). 

Ao = Discharge connection area (in 2 ). 

97

DEFINITION OF TERMS 

Pressure Relief Devices 

A pressure relief device is actuated by inlet static 

pressure and designed to open during an emergency 

or abnormal conditions to prevent a rise of internal 

fluid pressure in excess of a specified value. The 

device also may be designed to prevent excessive 

internal vacuum. The device may be a pressure relief 

valve, a nonreclosing pressure relief device, or a 

vacuum relief valve. 

A spring-loaded pressure relief valve is a 

pressure relief device designed to automatically 

reclose and prevent the further flow of fluid. 

A relief valve is a spring-loaded pressure relief valve, 

actuated by the static pressure upstream of the valve. 

The valve opens normally in proportion to the 

pressure increase over the opening pressure. A relief 

valve is used primarily with incompressible fluids. 

A safety valve is a spring-loaded pressure relief 

valve, actuated by the static pressure upstream of the 

valve and characterised by rapid opening or pop 

action. A safety valve is normally used with 

compressible fluids. 

A safety relief valve is a spring-loaded pressure 

relief valve that may be used as either a safety or 

relief valve, depending on the application. 

A conventional pressure relief valve is a springloaded 

pressure relief valve whose performance 

characteristics are directly affected by changes in the 

back pressure on the valve. 

A balanced pressure relief valve is a springloaded 

pressure relief valve that incorporates a 

means for minimising the effect of back pressure on 

the performance characteristics. 

Dimensional Characteristics of Pressure 

Relief Devices 

The actual discharge area is the measured 

minimum net area that determines the flow 

through a valve. 

The curtain area is the area of the cylindrical or 

conical discharge opening between the seating 

surfaces above the nozzle seat created by the lift 

of the disc. 

The required discharge area is a nominal or 

computed area of a pressure relief valve used in 

recognised flow formulae to determine the size of 

the valve. It will be less than the actual discharge 

area. 

The nozzle area is the cross-sectional flow area 

of a nozzle at the minimum nozzle diameter. 

A huddling chamber is an annular pressure 

chamber in a pressure relief valve located beyond 

the seat for the purpose of generating a rapid 

opening. 

The inlet size is the nominal pipe size (NPS) of 

the valve at the inlet connection, unless otherwise 

designated. 

The outlet size is the nominal pipe size (NPS) of 

the valve at the discharge connection, unless 

otherwise designated. 

Lift is the actual travel of the disc away form the 

closed position when a valve is relieving. 

A pilot-operated pressure relief valve is a 

pressure relief valve in which the main valve is 

combined with and controlled by an auxiliary 

pressure relief valve. 

A rupture disc device is a nonreclosing differential 

pressure relief device, actuated by inlet static 

pressure and designed to function by bursting the 

pressure-containing rupture disc. A rupture disc 

device includes a rupture disc and a rupture disc 

holder. 

98

OPERATIONAL CHARACTERISTICS 

The maximum operating pressure is the 

maximum pressure expected during system operation. 

The maximum allowable working pressure 

(MAWP) is the maximum gauge pressure permissible 

in a vessel at its designated temperature. The 

maximum allowable working pressure is the basis for 

the pressure setting of the pressure relief devices that 

protect the vessel. 

The design gauge pressure refers to at least the 

most severe conditions of coincident temperature and 

pressure expected during operation. This pressure 

may be used in place of the maximum allowable 

working pressure in all cases where the MAWP has 

not been established. The design pressure is equal to 

or less than the MAWP. 

Accumulation is the pressure increase over the 

maximum allowable working pressure of the vessel 

during discharge through the pressure relief device, 

expressed in pressure units or as a percentage. 

Maximum allowable accumulations are established by 

applicable codes for operating and fire contingencies. 

Overpressure is the pressure increase over the set 

pressure of the relieving device, expressed in pressure 

units or as a percentage. It is the same as 

accumulation when the relieving device is set at the 

maximum allowable working pressure of the vessel. 

Rated relieving capacity is that portion of the 

measured relieving capacity permitted by the 

applicable code or regulation to be used as a basis for 

the application of a pressure relief device. 

Stamped capacity is the rated relieving capacity that 

appears on the device nameplate. The stamped 

capacity is based on the set pressure or burst pressure, 

plus the allowable overpressure for compressible fluids 

and the differential pressure for incompressible fluids. 

The set pressure is the inlet gauge pressure at which 

the pressure relief valve is set to open under service 

conditions. 

The cold differential test pressure is the pressure 

at which the pressure relief valve is adjusted to open 

on the test stand. The cold differential test pressure 

includes corrections for the service conditions of back 

pressure or temperature or both. 

Back Pressure is the pressure that exists at the 

outlet of a pressure relief device as a result of the 

pressure in the discharge system. It is the sum of the 

superimposed and built-up back pressure. 

Built up back pressure is the increase in 

pressure in the discharge header that develops as a 

result of flow after the pressure relief device opens. 

Superimposed back pressure is the static 

pressure that exists at the outlet of a pressure relief 

device at the time the device is required to operate. 

It is the result of pressure in the discharge system 

coming from other sources and may be constant or 

variable. 

Blowdown is the difference between the set 

pressure and the closing pressure of a pressure 

relief valve, expressed as a percent of the set 

pressure or in pressure units. 

Opening pressure is the value of increasing inlet 

static pressure at which there is a measurable lift of 

the disc or at which discharge of the fluid becomes 

continuous. 

Closing pressure is the value of decreasing inlet 

static pressure at which the valve disc re-establishes 

contact with the seat or at which lift becomes zero. 

Simmer is the audible or visible escape of 

compressible fluid between the seat and disc at an 

inlet static pressure above the set pressure and at 

no measurable capacity. 

Leak-test pressure is the specified inlet static 

pressure at which a seat leak test is performed 

(normally 90% of set pressure according to 

AP1 527). 

The term relieving conditions is used to indicate 

the inlet pressure and temperature on a pressure 

relief device at a specific overpressure. The 

relieving pressure is equal to the valve set pressure 

(or rupture disc burst pressure) plus the 

overpressure. (The temperature of the flowing fluid 

at relieving conditions may be higher or lower than 

the operating temperature.) 

Popping pressure is the pressure at which the 

valve disc rapidly moves from a slightly open 

(simmer) position to a practically full open position. 

Discharge capacity is the actual mass flow rate of 

discharge which can also be expressed in volumetric 

terms. 

Equivalent capacity is the mass volumetric flow 

rate of a fluid calculated from the capacity of the 

valve for a test fluid. The fluids commonly used for 

the test purposes are steam, air and water. 

99

PRESSURE TERM RELATIONSHIP 

Pressure Vessel 

Requirements 

Vessel Pressure 

% Design Pressure 

Safety Valve 

Characteristic 

Maximum permited 

accumulated pressure 

110 

Maximum relieving 

pressure 

Accumulation 

Overpressure (Typical) 

Design pressure 

100 

Set pressure 

Blowdown (Typical) 

Reseat pressure 

Operating margin 

Maximum operating 

pressure 

90 

Notes: The system operating pressure must not exceed the reseat pressure of the safety valve. 

Blowdown control is designed to conform to ASME Code Section V111, however liquid 

applications may demand up to 15% blowdown. 

Overpressure and blowdowns shown are typical for spring loaded SRVs and can be 

reduced when using Safeset Pilot Operated SRVs. Refer to the appropriate catalogue 

section for details. 

100

NOTES 

Birkett 

Pressure 

Protection 

101


B/C Series 

D Series 

WB Series 

Safeset 

COMPLEMENTARY PRODUCT LINES 

Bursting Discs 

Pressure Vacuum Valves 

Flame Arresters 

102

Safety Systems UK Ltd. 

Sharp Street,Worsley, Manchester, UK, M28 3NA. 

Tel +44 (0)161 703 1977 

Fax +44 (0)161 703 8451 

Email support@safetysystemsuk.com 

Web site www.safetysystemsuk.com 

BIPR0410 

Registered Office: 

Victoria Road, Leeds, 

LS11 5UG, UK

Birkett Technical Catalogue - Safety Systems UK Ltd

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