fundamentals of centrifugal pumps - Master Pumps and Power

PRODUCT TRAINING 

FUNDAMENTALS OF 

CENTRIFUGAL PUMPS 

1

Fundamentals of Centrifugal 

Pumps 

A centrifugal pump is a mechanical device that 

converts energy to hydraulic work 

Energy is supplied by a driver such as an electric 

motor, turbine, or engine 

Hydraulic work is the movement of a liquid mass 

through a distance 

This presentation is limited to centrifugal pump 

types only 

Sundyne pumps are a special design that will not be 

covered in this presentation 

2



Just Say NO!!!!! 

3



The work takes place at an impeller which 

accelerates the liquid by whirling it through 

the impeller thus adding centrifugal force 

and hence acceleration 

4



When an object is 

spun around in a 

circle it is accelerated 

outward by 

centrifugal force 

5



When liquid is spun 

around in a circle, it 

accelerates outward 

from the center of the 

circle due to 

centrifugal force 

6



An impeller has vanes 

which are blades that 

push the liquid through 

the impeller. The center 

of the impeller where the 

liquid enters the impeller 

is called the eye 

Vanes 

Eye 

7



To obtain useful 

work, the impeller is 

contained in a casing 

which directs the 

accelerated fluid 

along a desired path 

Discharge 

8



Pump Impeller and Shaft 

9



Pump Impeller and Shaft 

with Pressure Casing 

10



Pump Impeller 

and Shaft with 

Pressure Casing 

and Cover 

11



Process 

Fluid 

Adjustment 

Packing 

Stuffingbox 

Cover 

12


Pumps-Impellers 

Pumps Pumps-Impellers Impellers 

Impeller Design 

The impeller is the most important part of the 

pump since it is where the work is taking 

place. Furthermore, the impeller plays an 

important role in the design of other pump 

components. It has a direct effect on the seal 

cavity pressure for example 

13




Many different types of 

impeller styles are used. 

Most, but not all, have vanes 

that curve away from the 

flow path so that the liquid 

is in contact with the 

impeller longer. These are 

referred to as “reverse reverse 

curve” curve vane impellers 

Vanes 

Eye 

14




Impeller vanes may be 

enclosed by “shrouds shrouds”. . 

In general impellers with 

shrouds are slightly less 

efficient due to the drag 

of the liquid on the 

shroud 

15




Therefore many 

impellers have no 

shrouds. They are called 

“open open” impellers. Note 

that the bottom impeller 

is “partially partially shrouded” shrouded 

due to a shroud area 

around the impeller eye 

16




Some impeller designs 

may also have a shroud 

only on one side of the 

impeller. They are also 

said to be partially 

shrouded or semi-open semi open 

or semi-closed semi closed 

17




Shrouds are generally used on larger impellers to 

help support the vanes and maintain the 

impeller shape under extreme pressure and 

temperature conditions. They also have the 

disadvantage of limiting the particle size that can 

pass through the impeller 

18




There are two different impeller 

types used in the process industry. 

They differ by the type of flow 

through the impeller. The most 

common type is a “radial radial flow” flow 

impeller where the liquid makes a 

90 o turn as it passes through the 

impeller 

19




In a turbine type impeller, the liquid 

also makes a turn as it passes through 

the pump, but less than 90 o . These are 

most often found in “diffuser diffuser” type 

pumps which relates to the casing 

design and will be discussed later. 

Since the liquid makes less of a turn, a 

turbine style impeller may be slightly 

more efficient than a similar “radial radial 

flow” flow impeller 

20




The impeller has a direct relationship to 

pump performance. The design of the 

impeller is the single most important factor 

in determining the flow rate and liquid 

pressure that a pump can generate 

21




The Master Pumps & Power catalog is an 

excellent reference resource for most 

pump application problems. It should be a 

part of every engineers library. It is free to 

all of our customers 

22




Flow through an impeller is determined 

primarily by three factors 

Vane width 

Number of vanes 

Impeller speed 

23




A wide vane impeller 

will move more liquid 

per unit time than a 

narrow vane impeller. 

The flow is directly 

proportional to the 

vane width 

24




Flow (Q) through an 

impeller is also 

directly related to the 

impeller speed. The 

more times an 

impeller rotates per 

unit time the more 

fluid is will move 

25




Finally the flow 

through a pump is 

somewhat related to 

the number of vanes 

although it is not 

directly proportional. 

More vanes will move 

more fluid 

26




An impeller creates 

“head head” by 

accelerating the fluid 

to a given velocity. 

As it spins, the fluid is 

accelerated outward 

by centrifugal force 

27




The fluid exits the 

impeller at a given 

velocity. Therefore it 

will rise to a given height 

in a column based on the 

exit speed regardless of 

the weight of the fluid 

28




Therefore a centrifugal pump is said to be a 

“constant constant head” head device. At a given speed it will 

accelerate a liquid to a given velocity regardless 

of the weight of the liquid. 

A heavier liquid would require more 

horsepower and the discharge pressure would 

be higher, but it would rise in a column no 

higher than a light liquid 

29




This phenomenon is based on simple laws of 

physics 

(V 2 = 2 AS) where V is the velocity, A is the 

acceleration of gravity, and S is the height 

Note that this formula makes no 

consideration of weight 

30




If fluids are pushed up a 

column to the same 

height, the pressure at 

the bottom of the 

column would be 

different for fluids of 

different weight. 

The formulae for this 

relationship are as follows 

hd. hd. 

ft. = 

(psi psi X 2.31) / sp. gr. 

psi = 

(hd hd ft. / 2.31) x sp. gr. 

31




To illustrate, a pressure gauge 

at the bottom of a 231 ft. high 

column filled with water 

would read 100 psi. psi. 

If the 

column was filled with butane 

having a specific gravity of 

only .5, the gauge would read 

50 psi 

231 ft. 

32


Pumps-NPSH 

Pumps Pumps-NPSH NPSH 

In order for the pump to move fluid, the system 

must be able to push fluid into the pump as fast 

as the pump can push it out. 

Therefore there must be a certain minimum 

required suction pressure for each pump based on 

the pump flow 

This pressure is expressed in head feet and is 

referred to as NPSH -net net positive suction head 

33




NPSH is expressed in two ways 

NPSHA is the net positive suction head available 

from the system 

NPSHR is the net positive suction head required by 

the pump at a particular flow 

NPSHA must always be greater than NPSHR or 

damage to the pump will occur due to cavitation 

34




Cavitation is the flashing of the liquid at the 

pump impeller eye caused by the pump 

lowering the pressure in the eye area as it 

accelerates fluid across the impeller 

The damage occurs when the flashed gas is 

compressed back to a liquid as it gains 

pressure while traveling through the impeller 

35




Cavitation causes pump problems in two areas 

Severe cavitation can erode the pump 

impeller resulting in decrease performance 

and vibration due to imbalance 

Cavitation normally results in substantially 

higher vibration in the entire pump 

36




NPSH is the total suction head in feet 

of liquid (absolute at the pump 

centerline or impeller eye) less the 

absolute vapor pressure (in feet ) of 

the liquid being pumped 

37




NPSHA is the NPSH available at the pump suction nozzle and 

depends on the suction system design. It must always be equal 

to or greater than the NPSHR 

NPSHR is the NPSH required by the pump for stable operation. 

It is determined by the pump manufacturer and is dependent on 

many factors including the type of impeller inlet, impeller design, design, 

pump flow, rotational speed, nature of the liquid, etc. It is 

usually plotted on the characteristic pump performance curved 

supplied by the pump manufacturer 

38




NPSHA is a difficult calculation. It will require 

the help of a process engineer from the plant 

NPSHA can be determined by direct field 

measurement if the vapor pressure is known. A 

method for this calculation is presented later 

39




*Assume vapor pressure 

of water @ 80 F = .5psia 

or 1.2 feet = hvpa *Assume atmospheric 

pressure @ sea level 

or 34.0 feet = ha Height = hst *Assume pipe losses = 15 feet 

3.5 feet = hfs *NPSHA = ha - hvpa - hst - hfs *NPSHA = 34 - 1.2 - 15 - 3.5 = 

14.3 feet 

NPSHA Example for Suction Lift 

Atmospheric 

Pressure = h a 

40




Atmospheric Pressure = h a 

Height = h st 

15 feet 

*Assume vapor pressure 

of water @ 80 F = .5psia 

or 1.2 feet = h vpa 

*Assume atmospheric 

pressure @ sea level 

or 34.0 feet = h a 

*Assume pipe losses = 

3.5 feet = h fs 

*NPSHA = h a - h vpa + h st - h fs 

*NPSHA = 34 - 1.2 + 15 - 3.5 = 

44.3 feet 

NPSHA Example 

for Flooded Suction 

41




Simple Method to Determine NPSHA 

NPSHA is the total suction head in feet of liquid (absolute at 

the pump centerline or impeller eye) less the absolute vapor 

pressure (in feet) of the liquid being pumped 

Measure the suction pressure and convert to feet of head 

(must be absolute not atmospheric) 

Determine the vapor pressure of the liquid and convert to 

feet of head 

Subtract the vapor pressure from the suction pressure 

42


Pumps-Performance Pumps Pumps-Performance Performance Curves 

A pump manufacturer will supply a curve for 

every pump purchased which graphically 

represents the expected pump performance 

For most applications, a copy of the pump curve 

is required information for properly selecting a 

sealing system and flush plan 

43



Head 

BHP 

Flow, GPM 

NPSHR 

Efficiency 

44



Contents 

What pump curves represent 

How to read pump curves 

45



Head 

BHP 

A Pump Curve 

Flow, GPM 

NPSHR 

Efficiency 

46



Summary of Pump Curve Info 

Graphical representation of performance 

Head Head 

BHP BHP 

Efficiency Efficiency 

NPSHR NPSHR 

47



Summary of Pump Curve Info 

Graphical representation of performance 

Contains more than performance data 

speed 

stages 

may have info about liquid 

impeller, case patterns 

wear ring clearances 

48



Actual Sample Curve 

Pricebook Curve 

49



Actual Sample Curve -Job Curve 

50



P1 

How Pump Curves are Made 

Pump 

P2 

Flow Meter 

51



Test Data 

Flow P2 - P1 BHP 

0 311 20 

400 291 117 

800 234 156 

960 195 163 

Pressure in PSIG 

52



Convert Pressure to Feet of Head 

Head = 2.31 (P -P 2 1) ) / S.G. 

Head is in Feet! 

53



Compute Efficiency 

Efficiency = Theoretical Horsepower 

divided by Actual Horsepower 

Convert to Percent 

54



Test Data 

Flow Head BHP Efficiency 

0 718 20 *** 

400 672 117 58% 

800 540 156 70% 

960 450 163 67% 

Now in feet 

55

TDH Head Portion of Pump Curve 



800 

700 

600 

500 

400 

300 

200 

100 

0 

0 200 400 600 800 1000 1200 

Flow, GPM 

56

BHP 

180 

160 

140 

120 

100 

80 

60 

40 

20 

0 



BHP Portion of Pump Curve 

0 200 400 600 800 1000 1200 

Flow, GPM 

57

Eff 

80 

70 

60 

50 

40 

30 

20 

10 

0 



Efficiency Portion of Pump Curve 

0 200 400 600 800 1000 1200 

Flow, GPM 

58



NPSH 

Net Positive Suction Head 

NPSHR: Required 

NPSHA: Available 

59



NPSH 

Net Positive Suction Head 

NPSHR: Required 

NPSHA: Available 

NPSH = Actual Pressure - Vapor 

Pressure, then convert to feet of head 

60



NPSH Required 

Determined during pump test 

Throttling Throttling suction to pump 

Hot Hot water 

Based on 3% head loss-reduce loss reduce NPSHA 

until 3% loss in produced head is observed 

Based on water 

61



Test Data 

Flow Full Head 3% Loss 

0 718 - 22 = 696 

400 672 - 20 = 652 

800 540 - 16 = 524 

960 450 - 14 = 436 

62

NPSHR 

35 

30 

25 

20 

15 

10 

5 

0 



NPSHR Portion of Pump Curve 

0 200 400 600 800 1000 1200 

Flow, GPM 

63

NPSHR 

NPSH 

TDH 

TDH 

EFF 

Eff 

BHP 

BHP 

35 

30 

25 

20 

15 

10 

5 

0 

800 

700 

600 

500 

400 

300 

200 

100 

0 

80 

70 

60 

50 

40 

30 

20 

10 

0 

180 

160 

140 

120 

100 

80 

60 

40 

20 

0 



0 200 400 600 800 1000 1200 

0 200 400 600 800 1000 1200 

0 200 400 600 800 1000 1200 

0 200 400 600 800 1000 1200 

FLOW IN GPM 

Flow, GPM 

64

Head 

BHP 



A Pump Curve 

Operating Point 

BEP 

Flow, GPM 

NPSHR 

Efficiency 

65



The pump manufacturer will normally show the 

point on the curve where the pump is expected 

to operate 

BEP is the best efficiency point taken at the 

highest point of the efficiency curve 

At BEP the pump normally operates the most stably 

Operation below BEP can result in mechanical and 

hydraulic problems 

66



Summary of Pump Curve Information 

Graphical representation of 

performance 

Contains more than performance data 

67


Pumps-Impeller Pumps Pumps-Impeller Impeller Effect 

Balance Holes 

68



Effect of back wear rings 

and balance holes 

Pressure at O.D. of 

impeller breaks down 

across back wear ring 

Balance holes bleed 

pressure back to suction 

Seal chamber at same 

pressure as balance holes 

69



Impeller with PumpoutVanes 

70



Effect of pump out 

vanes 

Vane O.D. is the same as 

the impeller O.D. and is 

turning at the same speed 

Therefore vane puts up 

same head as impeller 

Therefore back of 

impeller at shaft is at same 

pressure as front 

71



Impeller with no back wear 

rings or pumpout vanes 

72



Effect of no back wear 

rings or pump out vanes 

Pressure at impeller 

O.D. is present 

behind entire impeller 

Seal chamber at 

discharge pressure 

73


Pumps-Pump Pumps Pumps-Pump Pump Case Design 

If a higher pressure 

differential is required 

across the pump the 

designer has several 

options. Two would 

be to: 

Increase the pump 

speed-the speed the flow would 

also increase 

Increase the impeller 

O.D. 

74



In most cases neither is practical 

There is a practical limit to the impeller diameter. 

Beyond that limit it would be difficult to control 

the tolerances to ensure proper fit and balance. 

The hardware would be prohibitively expensive 

There is also a practical limit to the shaft speed. 

Not only would balance be critical, but the 

bearing and lubrication system would be complex 

and expensive 

75



The third option is simpler. To achieve high 

differential head without the expense more than one 

stage or impeller are used. Multistage pumps come 

in many varieties 

Multistage volute 

Split case 

Double case 

Multistage diffuser 

Vertical 

Horizontal 

76



To understand multistage pump design, it is first 

essential to know that there are in general two 

different ways that the pump case directs the 

flow from the impeller to the discharge nozzle 

Volute pattern 

Diffuser pattern 

77



Volute Pump 

One or two passages in 

the pump case guide the 

fluid from the impeller to 

the pump discharge or the 

next stage 

Single volute pumps can 

result in excessive 

hydraulic load on the 

impeller and shaft 

78



Single Volute Double Suction Pump 

79



Diffuser Pump 

A multipassage “diffuser diffuser” 

or “bowl bowl assembly” assembly 

surrounds the entire 

impeller O.D. and guides 

the fluid to the discharge 

nozzle or next stage 

through multiple paths 

80



Diffuser Pumps 

Note that the Byron Jackson design incorporates a 

“mixed mixed flow” flow impeller 

The fluid does not make a full 90 o turn in the impeller. 

Since it is not a radial flow impeller, it is termed a mixed 

flow impeller 

Not all turbine pumps are mixed flow. Some are 

furnished with radial flow impellers such as the 

horizontal diffuser pump shown previously 

81



Multistage Volute Split Case 

82


Pumps-Seal Pumps Pumps-Seal Seal Cavity Pressure 

Most multistage volute type pumps have front and back 

wear rings. 

Therefore the seal cavity pressures would be as 

follows: 

One end-suction end suction 

The other end-discharge end discharge pressure of one of the stages unless 

some measures are taken to reduce the pressure 

Most multistage volute pumps will have several taps 

on the pump case where various pressures are available 

for the flush source 

83



Multistage Volute Double Case 

84



Most multistage double case volute type pumps have 

front and back wear rings. 

Therefore the seal cavity pressures would be as 

follows: 


The other end-discharge end discharge pressure of one of the stages 

unless some measures are taken to reduce the pressure 

Only pump discharge pressure is available for a flush 

source 

85



Many of these pumps as well as other multistage 

designs will have some provision for reducing the 

pressure in the seal chamber at the high pressure end 

Balance line 

Close clearance bushing 

86



A balance line will typically bleed the high pressure seal 

chamber to about suction plus 70% of one stage 

differential 

Depends on the bushing wear 

Also depends on the allowable flow in the 

balance line which represents pump 

inefficiency 

87



Don’t Don t assume 

Check to see if the 

balance line exists 

Measure the seal 

cavity pressure 

The following slide 

illustrates a balance 

line-they line they are not 

always so visible 

Confucius say - “When you 

assume you make a donkey 

out of “u” and “me” 

88



Multistage Volute 

with Balance Line 

89



Multistage Diffuser Horizontal 

90




Since all the impellers face and pump the same 

direction the seal cavity pressures are as follows 


The other end-full end full discharge unless some measure 

is taken 

91




Measure taken to reduce seal cavity pressure on high 

pressure end 

Balance drum (piston) 

Balance disc 

Both are similar to balance lines and close clearance 

bushings 

92




Balance drum (piston) 

Balance disc 

Both have additional function in that they are 

part of the mechanism to reduce the load on the 

pump thrust bearing 

Both are very complex and extremely precise 

mechanical devices 

93



Multistage Diffuser Vertical 

Byron Jackson 

Sumpmaster 

94



Multistage Diffuser Vertical 

Byron Jackson VLT 

(Very Large Turbine) 

Process Pump with Case 

95



Vertical Turbine Pumps 

Note that the sumpmaster is shown without a 

mechanical seal. This is typical for low pressures 

but many of these pump styles do have 

mechanical seals 

Since all the impellers are pumping in the same 

direction and the seal sits in the discharge head of 

the pump, the seal cavity is at discharge pressure 

unless measures are taken 

96




Measures taken depend on the pump seal cavity 

construction 

Internal seal head-the head the seal actually sits in the pump 

discharge flow-it flow it can only be at pump discharge pressure 

External seal or packing head-a head a close clearance bushing 

and balance line arrangement are used to reduce the 

pressure to suction plus 70% of one stage. Again you 

must measure to be sure 

97




Measures taken depend on the pump seal cavity 

construction 

Internal packing head-the head the seal chamber is a 

separate piece but sits in the discharge flow- 

usually some provision is made to reduce the 

pressure in the packing or seal area 

98



Internal 

Seal Head 

99



External 

Seal Head 

100



Internal 

Packing 

Head 

101



There are three typical styles of pump 

construction 

Overhung: an example follows 

Double-ended: Double ended: several examples have been given 

similar to the previous slides and the slide 

following the overhung pump 

Vertical: many variations exist 

Turbine style-an style an example is the previous slide 

Process with and without a bearing bracket-examples 

bracket examples 

will follow 

102



Typical end Suction 

(Overhung) Process Pump 

103



Many pumps use a double suction impeller 

design which is a single impeller with two inlets 

High flow 

Low NPSHA 

Since a double suction impeller must be radial 

flow, they are all in volute type cases almost 

without exception 

104



Double Suction Pumps 

In a single stage double suction pump the seal or 

seals sit in the impeller eye 

The only pressure they can see is suction 

There are limited choices to dealing with 

inadequate vapor suppression margin for these 

pumps 

Close clearance throat bushing with plan 11 or 32 

flush 

Cooling 

Cooling 

105



Double Suction Radially Split 

106



Double Suction Axially Split 

107



Double Suction Overhung 

108



Vertical Process Pumps 

In addition to those previously shown, there are a 

class of pumps that are process pumps mounted in a 

vertical configuration. They are almost always volute 

single stage pumps. There are two styles 

Rigid coupling-no coupling no bearing bracket 

Flexible coupling with bearing bracket 

109



Vertical Inline 

Process Pump with 

Rigid coupling- 

No bearing bracket 

110



Vertical Inline 

Process Pump with 

flexible coupling 

and bearing bracket 

111



Since these pumps are process style pumps, the 

seal cavity pressure can be at anything between 

and including suction and discharge 

The normal rules apply. It is necessary to know 

the impeller and case construction to estimate 

the seal cavity pressure 

112


Pumps-Problem Pumps Pumps-Problem Problem Constructions 

Some pump designs have inherent mechanical 

problems because of their design and resultant 

impeller and shaft loads 

Vertical inline rigid coupling no bearing 

bracket 

Overhung double suction or two stage 

Single volute 

113



Some pump designs have inherent mechanical 

problems because of their design and resultant 

impeller and shaft loads 

Shaft deflection and vibration are common to 

these designs 

Something must be done to address the 

mechanical situation 

114



Cure or eliminate problem pump 

constructions 

L3 /D 4 < or = 40 

Vertical inline with rigid coupling 

Two stage or double suction overhung 

Internal sealed (glandless) pump designs 

Something we haven’t haven t discussed 

No seal chamber or stuffingbox-similar stuffingbox similar to internal seal 

arrangementfor vertical turbine pumps 

115



L3 /D 4 < or = 40 

L = distance in inches from center of radial 

bearing to center of impeller 

D = diameter of shaft in inches under the seal 

sleeve 

116



L3 /D 4 < or = 40 Solutions 

Replace bearing bracket and stuffingbox with 

7th edition upgrade 

Modify existing pump with heavier shaft and 

more robust bearings 

Close clearance non galling wear rings and 

throat bushing 

117



Vertical inline with rigid coupling 

Fine for low horsepower (



Vertical inline with rigid coupling (continued) 

Close clearance non galling wear rings and throat 

bushing 

Add external bearing assembly on top of seal flange 

May require more first obstruction-add obstruction add motor 

spacer ring and lengthen pump shaft 

May be oil mist or grease lubricated 

119



Two stage or double suction overhung, old API pumps with 

large calculated shaft deflection, single volute pumps 

Same problem as L 3 /D 4 < or = 40 

Replace bearing bracket and stuffingbox with 7th 

edition upgrade 

Modify existing pump with heavier shaft and more 

robust bearings 

Close clearance non galling wear rings and throat 

bushing 

120

fundamentals of centrifugal pumps - Master Pumps and Power

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?