Modulating level controls - Filter

Modulating level controls
for steam boilers

2
The background to steam boiler
water level controls
Originally the water level in a steam boiler was
controlled by the boiler attendant regulating the
feedwater inlet valve by hand whilst watching the
water level in the gauge glass.
The number of fatal accidents caused by boiler
explosions due to low water levels was the main
reason for regulations that required automatic
controls and alarms to be fitted. This brochure
concentrates on the automatic controls. Separate
literature is available showing the range of Spirax
Sarco high and low level boiler water level alarms.
The on/off boiler plant
Feedwater on / off
Boiler feed check valve
Feedpump
The modulating boiler plant
Feedwater
flowrate matches
steam demand
Feedwater control valve
Feedpump
Crown valve
Boiler feed check valve
Steam flowrate
Water level control and the
on-off boiler plant
As modern steam boiler design has progressed, the
water volume and water surface area relative to the
evaporation rate have decreased. The result is a highly
efficient, low priced package that will work well if it is
controlled smoothly.
Indeed, Spirax Sarco can provide a range of on-off
level controls which are detailed in separate literature,
but they are not always the ideal mode for boiler
water level control.
In many installations, the on-off feedwater controls
can seriously affect the smooth operation of the boiler,
having effects on boiler pressure, steam flowrate and
burner firing rate.
These effects are explained in more detail on page 3.
Gauge glass
Bottom blowdown valve
Crown valve
Fuel
Steam flowrate
Water level up and down
Firing rate varies
Gauge glass
Bottom blowdown valve
Fuel
Water level
more steady
Steady firing
rate

On-off level controls
The water level is maintained by switching the
feedpump on and off between two levels, typically
50 mm apart. With the pump off, the burner will be
firing to supply the steam load and maintain the boiler
pressure. When the water level drops to the 'pump on'
level the pump delivers a high flowrate of relatively
cold feedwater that douses the boiler, reducing the
steam flowrate and causing a drop in the boiler
pressure. When the feedpump stops, the pressure
and steam flowrate will increase and the burner will
have to reduce its firing rate. The pump on-off control
therefore causes cycling of the steam output. If the
steam flowrate is recorded the effect can be seen
from the saw tooth plot below.
The steam flowrate peaks will tend to increase carryover
of boiler water with the steam. Production processes
can suffer.
The continual cycling of the boiler controls will increase
maintenance, reduce combustion efficiency and
increase the thermal stresses on the boiler.
1 PM
13
1 PM
13
2 PM
14
Steam flowrate from a boiler
with on-off level controls
2 PM
14
3 PM
15
3 PM
15
4 PM
16
4 PM
16
5 PM
17
5 PM
17
Steam flowrate from a boiler
with modulating level controls
Modulating level controls
The water level is maintained by a proportional
controller regulating a valve in the feedwater line with
a continuously running feedpump.
This system works smoothly, is suited to nearly all
boilers, large and small, and matches the feedwater
flowrate to the steam demand.
Boiler lock-outs
The modulating level controls detailed in this pamphlet
are sometimes termed 'single element'. The advantages
of single element modulating level control over on-off
control have been outlined above, and they are better
able to follow sudden load changes.
However, on boilers with very variable loads, even
modulating control can result in boiler lock-outs. This
is due to the well known phenomenon that the water
level in a steam boiler increases due to the increased
quantity of steam bubbles in the boiler below the water
surface. The effect is sometimes referred to as 'swell'.
Thus on a boiler operating with very variable loads, the
increasing water level reduces the feedwater supply
just when it should be increasing to match the increasing
steam demand.
The solution is to use a steam flowmeter signal as a
feed forward input to the level controller to automatically
compensate for the effect of swell. This is called two
element control.
Where the differential pressure across the feedwater
valve varies, due perhaps to several boilers served by
a common feedwater system, it may be necessary to
introduce a third signal from a feedwater flowmeter -
three element control.
Spirax Sarco are able to offer advice and equipment for
two and three element level control systems.
3

4
The optimum level control system
Preamplifier housing probe electronics
Level probe fitted in protection tube in boiler shell
to reduce the effects of turbulence and foaming
How the system works
The controller receives a water level signal from the
level probe. This is compared with the set point in
the controller to produce an 'error signal'. Depending
on the polarity (+ or -) of the error signal one or other
of the control relays in the controller is energised to
open or close the modulating feedwater valve. With
electrically actuated valves a feedback potentiometer
in the modulating valve actuator continuously signals
the valve position and the controller continues to
energise the actuator until the valve position is
proportional to the error signal. In this way
proportional level control is achieved.
In practice the valve will find a steady position and
make small adjustments from time-to-time to maintain
the water level. Internal damping of the water level
signal and a built-in 'dead-band' prevent over frequent
valve movements.
Internally or externally
mounted controls?
The historical development of level controls
determined that float operated units mounted in
external chambers became the standard. It has been
usual to fit a sequencing purge valve to let the steam
and water through the connections to keep them
clear and to test the alarms. However with the advent
of self-monitoring level alarms there is a strong
argument for mounting both the level controls and
alarms directly in the boiler shell to detect the water
where it really matters - above the heated tubes.
The Spirax Sarco level probe is suitable for mounting
in external chambers or directly in the boiler shell
within a protection tube.
Adjustable high
or low level alarm
Adjustable
set point
Transmitter 100 %
Valve closed
Adjustable
proportional
band
Valve open
Transmitter 0 %
Boiler fitted with probes directly mounted in shell

Adjustable
transmitter range
(typically boiler
level gauge glass)
LC2200 controller (LC2300 for
pneumatically actuated valves)
Green LED power on
Red LED
Test button
For remote level signal transmission (optional)
Green LED indicates level is above the set point
Amber LED indicates level is outside the proportional band
Alarm set point adjuster
Level set point adjuster
Proportional band adjuster
Mounting position for probe in Spirax Sarco C2
side chamber with sequencing purge valve
Level transmitter
4 - 20 mA to management system
or remote indicator
Purpose designed boiler feed check valve with
soft seat to provide tight shut-off
Digital
display
unit
Electrically actuated modulating feedwater valve.
Pneumatically actuated valves also available.
User benefits
Power on
Span
Zero
Span adjust
Zero adjust
• Modulating system gives steady feedwater
flowrate matched to steam demand
• Can provide information to management systems
• Can be used as direct replacement for old and
outdated controls (modulating and on-off)
• Three stage level wave filter to maintain
a stable signal.
• Out of range alarm checks for damage to wiring
or probe, or incorrect wiring.
• No need to depressurise boiler to change
probe electronics
5

6
LP20 probe and
PA20 preamplifier
The LP20 probe consists of a stainless steel rod
completely insulated from the water by PTFE. As the
water level rises the capacitance between the rod and
the earthed boiler protection tube or probe chamber
increases. The capacitance is measured by the
preamplifier and is transmitted as a dc signal to the
controller. Control and alarm levels are set in the
controller. The probe, which must not be cut, is
available in the following lengths (dimension L): 370,
470, 600, 750, 900, 1 050, 1 200, 1 350 and 1 500 mm.
It is suitable for boiler pressures up to 32 bar g.
LC2200 controller and
LC2300 controller
Proportional level controllers
LC2200 For electrically actuated valves
LC2300 For pneumatically actuated valves
Features include:-
Adjustable set point
Adjustable proportional band
Adjustable high or low level alarm (LC2200 only)
Multi-voltage selection:-
230 V (198 - 264 V) or
115 V (99 - 121 V)
50 - 60 Hz
Plug-in design with DIN rail clip
Has a dc power supply and is able to accept a
current input (4 - 20 mA) enabling it to power and
be used with a two wire 4 - 20 mA transmitter
(e.g. pressure, differential pressure, temperature).
In-built valve positioner. (LC2200 only)
Several controllers can be connected to a single
probe or 4 - 20 mA source if required.
Three stage level wave filter (input signal damping)
to maintain a stable signal under varying turbulence
conditions.
'Out of range' alarm provides a degree of self
checking on damage to probe or wiring (LC2200
only).
Controllers may be mounted on a 'top hat' DIN rail
using the mounting clip provided or the controller
base may be screwed directly to a chassis plate.
System components
Dimensions (approximate) in millimetres
52
52
Weight 0.5 kg
Depth 120
LC2200
Weight 0.5 kg
Depth 120
LT2010
112
112
145
162
L
8
DS1000
27 A/F
½" BSP taper
Ø 12
25 mm dead length
52
Weight 0.5 kg
Depth 120
LC2300
96
70
93
112
Remote transmission
Where a remote signal representing water level is
required (4 - 20 mA isolated signal) an LT2010 can also
be connected to the level probe. The transmitter range
can be set, for example, to represent the boiler level
gauge glass. It can of course, be set to represent any
range of levels along the probe.
The transmitter output can be used to provide
information to a management system or connected to
a remote display unit such as the DS1000.
48
Panel cutout 92 x 44
Weight 200 g

Control valves
Traditionally most larger shell boilers are supplied with
a DN40 valve. The DN40 valve is available with a range
of reduced seats to match the feedrate. Alternatively,
and particularly for new boiler plant, a smaller valve can
be specified. See separate Spirax Sarco literature for
valve dimensions and selection information.
Electric actuators must include a 1 000 ohm feedback
potentiometer.
Pneumatic actuators must include a positioner (E to P).
Steel enclosure with
chassis plate and two
DIN rails for mounting
two or three level
controllers plus
terminals.
400
300
Electrically actuated
control valve
with feedback
potentiometer
Recommended ancillaries
Enclosures Probe chamber
Plastic enclosure with
DIN mounting rail.
For a single controller.
IP67 rating.
Sequencing purge valve
Type SPV1 with cast iron body, flanged BS 4504
PN16, DN25 for boiler pressures up to 13 bar g.
Type SPV3 with bronze body flanged BS 4504 PN40,
DN25 or BS 10 table F/H, 1" for boiler pressures up to
25 bar g.
83
205 deep
83
288
175
125
150 deep
½" BSP drain
Steel probe chamber
flanged BS 4504
PN40, DN25.
For maximum
boiler pressure
of 32 bar g.
1" BSP
(Reducing
bush
required for
LP20)
Pneumatically
actuated control
valve with
positioner
Disc check valve
Type DCV2/B for fitting between control valve and
boiler to prevent return flow. Will also ensure that
feedtank will not drain into boiler when boiler is
depressurised. See separate literature for full details.
83
65
457
7

Other applications
This brochure deals with the water level controls
normally fitted to shell type steam boilers. They can
also be used on water tube boilers and anywhere a
water level has to be detected.
The following are just a few examples of the many level
control duties possible.
Control valve
Make-up water
LC2200 controller
LP20 probe / PA20 amplifier
Valve modulates to
maintain water level
within this band
Low level alarm
Boiler feedtank make-up with modulating control,
and low level alarm
Valve modulates to
maintain water level
within this band
How to specify
Proportional level controller for electrically actuated
valve with alarm output, three stage wave filter and out
of range alarm. Complete with capacitance probe and
preamplifier.
Optional: Transmitter and
Remote display unit
Control valve
Make-up water
Flow to process
LP20 probe /
PA20 amplifier
High level alarm
LC2200 controller
Valve modulates to
maintain water level
within this band
Process water tank with modulating fill control,
and high level alarm
LP20 probe / PA20 amplifier
Some of the products shown may not be available in certain markets.
LC2300 controller
To drain
Pneumatic control valve
Drainage control of a large process pressure vessel,
modulating control.
How to order
1 - Spirax Sarco modulating level control system
comprising:
LP20 probe 600 mm long.
PA20 Preamplifier
LC2200 Controller
Optional: LT2010 Transmitter
DS1000 Digital display unit
Spirax-Sarco Limited, Charlton House,
Cheltenham, Gloucestershire, GL53 8ER UK.
Tel: +44 (0)1242 521361 Fax: +44 (0)1242 573342
E-mail: Enquiries@SpiraxSarco.com
Internet: www.SpiraxSarco.com
© Copyright 2001 Spirax Sarco is a registered trademark of Spirax-Sarco Limited
SB-P402-02 AB Issue 3
LCM