boiler operation
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Boiler Operation & Control 1
Allah the Most Gracious & Merciful
Supervised by
Mr. Irfan Balouch
Submitted by Mr. Ghulam Sagheer (14A2-210089)
Mr. Ghulam Fareed (14A2-210090)
B. Tech. (Pass) (Mechanical)
Preston University, Islamabad
Boiler Operation & Control 2
INTRODUCTION
A boiler operates using the feed water system, the steam system, the fuel system
and the draft system. The feed water system supplies water to the boiler. The steam system
controls and directs the steam produced in the boiler. The fuel system supplies fuel and
controls combustion to produce heat. The draft system regulates the movement of air for
combustion and evacuates gases of combustion.
Water, steam fittings and accessories are required to supply and control water and
steam in the boiler. Boiler fittings or trim are components such as valves directly attached
to the boiler. Accessories are pieces of equipment not necessarily attached to the boiler,
but required for the operation of the boiler.
Boiler Operation & Control 3
Chapter 1
Common devices used for boiler
operation
Boiler Operation & Control 4
1. Devices used for boiler operation
1.1. Safety Valves are the most important fittings on the boiler. They should open to
re lease pressure when pressure inside the boiler exceeds the maximum allowable
working pressure or MAWP. Safety valves are installed at the highest part of the
steam side of the boiler. No other valve shall be installed between the boiler and the
safety valve. Safety valve capacity is measured in the amount of the steam that can be
discharged per hour. The safety valve will remain open until sufficient steam is released
and there is a specific amount of drop in pressure.
This drop in pressure is the blow down of the safety valve. Safety
valve capacity and blow down is listed on the data plate on the safety valve. Spring loaded
safety valves are the most common safety valves. A spring exerts pressure on the valve
against the valve seat to keep the valve closed. When pressure inside the boiler exceeds
the set popping pressure, the pressure forces the valve open to release. The number of
safety valves required and the frequency and procedures for testing safety valves is also
specified by the ASME Code. Adjustment or repairs to safety valves must be performed
by the manufacturer or an assembler authorized by the manufacturer.
Boiler Operation & Control 5
1.2. Water fittings and accessories control the amount, pressure and temperature of
water supplied to and from the boiler. Water in the boiler must be maintained at the
normal operating water level or NOWL. Low water conditions can damage the boiler and
could cause a boiler explosion. High water conditions can cause carryover. Carryover
occurs when small water droplets are carried in steam lines. Carryover can result in water
hammer. Water hammer is a banging condition caused by hydraulic pressure that can
damage equipment.
1.3. Feed water Valves control the flow of feed water from the feed water pump to the
boiler. Feed water stop valves are globe valves located on the feed water line. They isolate
the boiler from feed water accessories. The feed water stop valve is positioned closest to
the boiler to stop the flow of water out of the boiler for maintenance, or if the check valve
malfunctions. The feed water check valve is located next to the feed water stop valve and
prevents feed water from flowing from the boiler back to the feed water pump. The feed
water check valve opens and closes automatically with a swinging disc. When water is fed
to the boiler it opens. If water flows back from the boiler the valve closes.
1. 4. Water Column minimizes the water turbulence in the gage glass to provide
accurate water level reading. Water columns are located at the NOWL, with the
lowest part of the water column positioned at least 3" above the heating system.
Water columns for high pressure boilers consist of the main column and three
tricocks. High and low water alarms or whistles may be attached to the top and bottom
tricocks.
1.5. The Gage Glass is used to visually monitor the water level in the boiler. Isolation
valves located at the top and bottom permit the changing of gage glasses.
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1.6. Blow down Valve at the bottom of the gage glass is used to remove sludge and
sediment. Tubular gage glasses are used for pressure up to 400 psig. All boilers must have
two methods of determining the boiler water level. The gage glass serves as the primary
method of determining boiler water level. If the water cannot be seen in the gage glass, the
tricocks are used as a secondary method of determining boiler water level. The middle
tricock is located at the NOWL. If water comes out of the middle tricock, the gage glass is
not functioning properly. If water comes out of the top tricock, there is a high water
condition in the boiler. If water comes out of the bottom tricock, water may be safely
added to the boiler. If steam comes out of the bottom tricock, water must not be added to
the boiler. Secure the fuel immediately. Adding water could cause a boiler explosion.
1.7. Makeup Water replaces boiler water lost from leaks or from the lack of
condensate returned in the boiler. Makeup water is fed manually or automatically. Boilers
can have both manual and automatic systems. If the boiler has both, the manual always
bypasses the automatic system. Boiler operators must know how to supply makeup water
quickly to the boiler in the event of a low water condition. Manual systems feed city water
with a hand operated valve. Automatic systems feed city water with a float control valve
mounted slightly below the NOWL. If the float drops from a low water level, the valve in
the city water line is open. As the water level rises, the float rises to close the valve.
Boiler Operation & Control 7
1.8. Low Water Fuel Cut Off shuts off fuel to the burner in the event of a low water
condition in the boiler. The low water fuel cut off is located 2" to 6" below the NOWL.
Low water fuel cut offs are available with or without an integral water column. Low water
fuel cut offs must be tested monthly or more often depending on plant procedures and
requirements. Low water fuel cut offs operate using an electric probe or a float sensor. The
float senses a drop in water level. Switches in the low water fuel cut off are wired to the
burner control to shut off fuel to the burner when the water level drops in the chamber.
1.9. Feed water Regulator maintains the NOWL in the boiler by controlling the
amount of condensate return pumped to the boiler from the condensate return tank. The
correct water level is maintained with a feed water regulator, but boiler water level must
still be checked periodically by the boiler operator.
Feed Water Regulator
1.10. Feed water Pumps are used with feed water regulators to pump feed water to the
boiler. Pressure must be sufficient to overcome boiler water pressure to maintain the
NOWL in the boiler. For maximum safety, plants having one team driven feed water
pump must have a back up feed water pump driven by electricity. Feed water pumps may
be reciprocating, centrifugal.
Boiler Operation & Control 8
Feed water Pump 1.11. Reciprocating feed water pumps are steam driven and use a piston to discharge
water to the feed water line. They are limited in capacity and are used on small boilers.
1.12. Centrifugal feed water pumps are electric motor or steam driven. They are the
most common feed water pump. Centrifugal force moves water to the outside edge of the
rotating impeller. The casing directs water from the impeller to the discharge piping.
Discharge pressure is dependent on impeller speed.
1.13. Turbine feed water pumps are steam drive n and operate similarly to centrifugal
feed water pumps.
1.14. Feed water Heaters heat water before it enters the boiler drum to remove
oxygen and other gases which may cause corrosion. Feed water heaters are either
open or closed. Open feed water heaters allow steam and water to mix as they enter an
enclosed steel chamber. They are located above the feed water pump to produce a positive
pressure on the suction side of the pump. Closed feed water heaters have a large
Boiler Operation & Control 9
number of tubes inside an enclosed steel vessel. Steam and water do not come in
contact, but feed water goes through the tubes and steam is allowed in the vessel to
preheat the feed water. They are located on the discharge side of the feed water pump.
1.15. Bottom Blow down Valves release water from the boiler to reduce water level,
remove sludge and sediment, reduce chemical concentrations or drain the boiler. Two
valves are commonly used, a quick opening and screw valve. During blow down the
quick opening valve is opened first, the screw valve is opened next and takes the wear
and tear from blow down. Water is discharged to the blow down tank. A blow down tank
collects water to protect the sewer from the hot boiler water. After blow down, the screw
valve is closed first and the quick opening valve is closed last.
1.16. Steam Fittings & Accessories remove air, control steam flow, and maintain the
required steam pressure in the boiler. Steam fittings are also used to direct steam to
various locations for heating and process.
1.17. Steam Pressure Gauges and vacuum gages monitor pressure inside the boiler.
The range of these gages should be 1-1/2 to 2 times the MAWP of the boiler. For
example: on a low pressure boiler, a maximum steam pressure on the pressure gage reads
30 psig as the MAWP is 15 psig.
Pressure Gauge
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1.18. Steam Valves commonly used include a gate valve used for the main steam
stop valve and the globe valve. The main steam stop valve cuts the boiler in online
allowing steam to flow from the boiler or takes it off line. This is an outside stem and yoke
or OS&Y valve. The position of the stem indicates whether the valve is open or closed.
The valve is opened with the stem out and closed with the stem in. This provides quick
information to the boiler operator.
1.19. The globe valve controls the flow of steam passing under the valve seat through
the valve. This change in direction causes a decrease in steam pressure.
A globe valve decreases steam flow and can be used to vary the amount of steam
flow. This should never be used as a main steam stop valve.
Globe Valve
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1.20. Steam Traps remove condensate from steam in lines from the boiler. Steam
traps work automatically and increase boiler plant efficiency. They also prevent water
hammer by expelling air and condensate from the steam lines without loss of steam.
Steam traps are located after the main steam header throughout the system. Steam traps
commonly used include the inverted bucket, the thermostatic and the float thermostatic. In
Steam Trap the inverted bucket steam trap steam enters the bottom flowing into the
inverted bucket. The steam holds the bucket up. As condensate fills the steam trap the
bucket loses buoyancy and sinks to open the discharge valve. The thermostatic steam trap
has a bellows filled with a fluid that boils at steam temperature. As the fluid boils
vapors expand the bellow s to push the valve closed. When the temperature drops
below steam temperature, the bellows contract to open the valve and discharge
condensate. A variation of the thermostatic steam trap is the float thermostatic steam trap.
A float opens and closes depending on the amount of condensate in the trap bowl.
Condensate is drawn out by return vacuum.
Steam Trap
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1.21. Steam Strainers remove scale or dirt from the steam and are located in the piping
prior to steam trap inlet. Scale or dirt can clog discharge orifices in the steam trap. Steam
strainers must be cleaned regularly.
Steam Strainer
2. SUMMARY OF DEVICES USED The safety valve is the most important fitting on the boiler. The gage glass is used to
visually monitor the water level in the boiler. Tricocks are used as a secondary device for
determining water level in the boiler.
Makeup water replaces water lost from leaks or lack of condensate return to the
boiler. The low water fuel cut off shuts off fuel to the burner in the event of a low water
condition. Steam pressure gages and vacuum gages are used to indicate the pressure inside
the boiler.
Boiler Operation & Control 13
CHAPTER 2
BOILER OPERATION
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2.1 Process of raising steam from cold in a Scotch boiler If the boiler has been opened up for cleaning or repairs check that all work has been
completed, and carried out in a satisfactory manner. Ensure that all tools, etc. have been
removed. Examine all internal pipes and fittings to see that they are in place, and properly
fitted.
Check that the blow down valve is clear. Then carry out the following procedure:
1. Fit lower manhole door.
2. Check external boiler fittings to see they are in order.
3. See that all blanks are removed from safety valves, blow down line, etc.
4. Fill boiler with water to about one-quarter of the water level gauge glass. If
possible hot water heated by means of a feed heater should be used. The initial
dose of feed treatment chemicals, mixed with water, can be poured in at the top
manhole door at this stage if required. Then fit top manhole door.
5. Make sure air vent is open.
6. Set one fire away at lowest possible rate.
7. Use the smallest burner tip available.
8. By-pass air heater if fitted.
9. Change furnaces over every twenty minutes.
10. After about one hour start to circulate the boiler by means of auxiliary feed pump
and blow down valve connection, or by patent circular if fitted. If no means of
circulation is provided, continue firing at lowest rate until the boiler is well
warmed through especially below the furnaces. Running or blowing out a small
amount of water at this stage will assist in promoting natural circulation if no other
means is available . Continue circulating for about four hours, raising the
temperature of the boiler at a rate of about 6°C per hour. Water draw n off at the
salinometer cock can be used to check water temperature below 100°C. At the end
of this time set fires away in all furnaces, still at the lowest rate.
11. Close the air vent. Nuts on manhole doors and any new joints should be nipped up.
12. Circulating the boiler can now be stopped, and steam pressure slowly raised during
the next 7-8 hours to within about 100 kN/m' of the working pressure.
13. Test the water gauge.
Boiler Operation & Control 15
The boiler is now ready to be put into service. About 12 hours should be allowed
for the complete operation provided some means of circulating the boiler is provided. If
circulation cannot be carried out, the steam raising procedure must be carried out more
slowly, taking about 18-24 hours for the complete operation. This is due to the fact that
water is a very poor conductor of heat, and heat from the furnace will be carried up by
convection currents leaving the water below the furnace cold. This will lead to severe
stresses being set up in the lower sections of the circumferential joints of the boiler shell if
steam raising is carried out too rapidly, and can lead to leakage and 'grooving' of the end
plate flanging . If steam is being raised simultaneously on more than one boiler, use the
feed pump to circulate each boiler in turn, for about ten minutes each.
2.2. General Precautions to be noticed on a working boiler There are various items to be inspected on a running boiler such as all the individual
equipment operating control signals, flow rates, temperatures and general load conditions.
They must be checked regularly so as to become aware quickly of any deviations from the
norm. Rarely do emergency conditions arise without some previous indication, which an
alert should be recognized, investigated, and then taken corrective action before the
situation gets out of hand.
2.3. General precautions for optimum running and safety
regulations
Ensure that all boiler and associated safety shut-down devices are maintained in full
operational condition, and tested at regular intervals so as to be ready for instant operation.
1. All alarm and automatic control systems must be kept within the manufacturer's
recommended operating limits. Do not allow equipment to be taken out of operation for
reasons which could reasonably be rectified.
2. All control room check lists must be kept up to date, with any known deviations from
normal operating procedures noted for immediate reference. Any deviations that are un-
noticed may build up to potentially serious conditions.
Boiler Operation & Control 16
3. Automatic control loops do not think for themselves, and subjected to external
irregularities will still try to perform as normal. This can result in their final control action
being incorrect, or to some other piece of equipment being overworked in an attempt to
compensate.
4. In situations where the automatic control of critical parameters is not dependable, or
where it becomes necessary to use manual control, reduce operating conditions so as to
increase acceptable margins of error.
5. High performance water tube boilers demand high quality feed water, so do not
tolerate any deterioration of feed water conditions; immediately trace the source of any
contamination, and rectify the fault.
6. Do not neglect leakage of high pressure, high temperature steam, as even minor leaks
will rapidly deteriorate.
7. No attempt should be made to approach the site of leakage directly, but the defective
system should be shut down as soon as is practicable and the leakage rectified.
8. Do not allow steam and water leaks to go un-corrected as, apart from reduction in
plant efficiency, they also lead to increased demand for extra feed with an inevitable
increase in boiler water impurities.
9. Always be alert for conditions which increase the potential fire risk within the engine
room: the best method of fire fighting is not to allow one to start. Thus all spaces, tank
tops etc. must be kept clean, dry, and well lit. This not only improves the work
environment, but also makes for the early detection of any leakage and encourages early
repair.
10. Store any necessary stocks of combustibles remote from sources of ignition. Maintain
all oil systems tight and free from leaks and overspills. Follow correct flashing-up
Boiler Operation & Control 17
procedures for the boiler at all times, especially in the case of roof-fired radiant heat
boilers. Be familiar with the fire fighting systems and equipment, and ensure that all under
your direct control are kept a t a full state of readiness at all times.
11. Assess particular risk areas, especially in engine room sp aces, and formulate your
approach in case of emergency; decide in some de tail how you would deal with fires at
various sites in the engine room. Make sure that your are familiar with the quick closing
fuel shut-off valves, the remotely operated steam shut-off valves etc. to enable the boiler
to be put in a safe condition if having to abandon the machinery spaces in the event of a
fire.
2.4. The basic procedure for cleaning a boiler after a period of
service.
The frequency of boiler cleaning depends upon various factors such as the nature of the
service in which the vessel has been engaged, the quality of feed water and fuel with
which the boiler has been supplied.
1. Where possible the boiler should be shut down at least 24 hours prior to cleaning,
with if practicable the soot blowers being operate d just before shut-down. When boiler
pressure has fallen to about 400 kN/m2, open blow down valves on drums and headers to
remove sludge deposits. Finally empty the boiler by running down through suitable drains
etc. Do not attempt to cool the boiler forcibly as this can lead to thermal shock. All fuel,
feed and steam lines must be isolated, and the appropriate valves locked or lashed shut.
Air vents must be left open to prevent a vacuum forming in the boiler as it cools down.
2. Should cleaning prove to be necessary, remove any internal fittings required to
provide access to tubes etc., keeping a record of any items removed. Also note that all
attachment bolts are present and that are accounted for when refitting.
3. Where the boiler design permits, cleaning can 'be carried out by mechanical brushes
with flexible drives; if these are not suitable, chemical cleaning must be used. After
cleaning, flush the boiler through with distilled water.
Boiler Operation & Control 18
4. Upon completion of cleaning, tubes etc. must be proved clear. Where access is
available, search balls or flexible search wires can be used. Where neither is practical,
high pressure water or air jets can be used, the rates of discharge from the outlet end being
used to indicate whether any obstruction is present within the tube. Where necessary,
welded nipples are removed to permit sighting through headers. With welded boilers the
tubes must be carefully searched before welding takes place and suitable precautions then
taken to avoid the entry of any foreign matter into tubes etc.
5. Where work is to be carried out in the drum, rubber or plastic mats can be used, with
flexible wires attached and secured outside the drum so that they are not left inside when
the boiler is closed up.
6. Check all orifices to boiler mountings to prove that they are clear, and ensure that all
tools, cleaning materials etc. have been removed from the boiler. All internal fittings
removed must be re placed. Fit new gaskets to all doors and headers, and close up the
boiler.
7. All personnel working in the boiler must be impressed with the importance of the
avoidance of any objects entering the tubes after the boiler has been searched, but that if a
mishap should occur it must be reported before the boiler is finally closed up.
8. External Cleaning Spaces between tubes can become choked with deposits which are
not re moved by soot blowing. Where sufficiently loose they may be removed by dry
cleaning using brushes or compressed air. But in most cases water washing will be
necessary. Washing will require hot water, preferably fresh, under pressure and delivered
by suitable lances. The water serves two purposes, dissolving the soluble deposits and the
breaking up and flushing away the loosened insoluble residue.
9. Once started. Washing should be continuous and thorough, as any half-dissolved
deposits remaining tend to harden off, baking on hard when the boiler is again fired, then
to prove extremely difficult to remove during any subsequent cleaning operations.
10. Prior to cleaning, bitumastic paint should be applied around tubes where they enter
refractory material, in order to prevent water soaking in to cause external corrosion.
11. Efficient drainage must be provided, with sometimes drains be low the furnace floor
requiring the removal of some furnace refractory. Where only a particular section is to be
washed, hoppers can be rigged beneath the work area, and the water drained off through a
Boiler Operation & Control 19
convenient access door.
12. For stubborn deposits a wetting agent may be sprayed on prior to washing.
13. After washing. Check that no damp deposits remain around tube ends, in crevices etc.
removing any remaining traces found. In a similar manner remove any de posits in double
casings around economizer headers etc., especially if they have become damp due to water
entering during the washing process.
14. Ensure that all cleaning materials, tools. Staging etc. have been removed, and any
refractory removed has been replaced, after which the access doors can be replaced.
15. Run the fans at full power with air registers full open for some minutes to clear any
loose deposits. Then dry the boiler out by flashing up in the normal manner. If this can’t
be done immediately, then hot air from steam air heaters or from portable units must be
blown through to dry the external surfaces.
2.5. Boiler operation from cold start 2.5.1. Preoperational precautions
1. Make sure all maintenance services are finished
2. Make sure all air gates and flue gases gates are closed
3. Make sure no personal are working on site
4. Make sure all electric devices have power
5. Air compressors must be working
6. All air pressures in the system must be at normal
7. Cooling water system must be ready
8. Secondary Steam system must be on
9. Drum must be filled with water
2.5.2. Turning Feed Pump on: 1. Water tank level at normal (0) level
2. Lubricating oil pressure < 1.4 bar
3. Gear box at neutral position
4. Valve for controlling lowest rate of feed water must be open
5. Suction valve must be open
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6. Delivery Valve and bypass must be closed.
7. Cooling water valve must be open
Steps
1. The bypass valve for the delivery pipe is opened
2. The delivery valve is opened
3. The control valve is opened for starting operation
4. The entrance valve to the economizer is opened
The operating range for rate of feed water should be about (200-250 ton/hr)
5. After the drum is filled with water the delivery valve to the drum is closed to start
operation
6. The ammonia (NH3) pump is turned on to increase the water PH
7. Hydrazine (N2H4) is used to remove Oxygen (O2) and increase PH
8. Sodium Phosphate (NA3PO4) is used to re move dissolved salts
2.5.3. Turning the air system and flue gas system on: Precautions before operating
a) Air pressure must be 8 bar
b) Cooling water system must be normal point
c) Inlet and outlet gates for the air must be closed
d) Inlet and outlet gates for the flue gases must be closed
Air pre heaters are to be turned on now
a) Open the inlet and outlet gates for the flue gases
b) Open the inlet and outlet gates for the air
c) The forced air fan is to be turned on now
d) After 15 sec the induced fan is to be turned on.
2.5.4. Turning the Fans on: Precautions for turning fans on:
1. Air Preheater must be turned on
2. Cooling water system must be operational
3. Air suction gates must be closed on both sides
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4. Air de livery gates must be closed on both sides
5. Lubricating oil pump must be operational
6. Hydraulic coupling must be at normal (0) level
Turning air Fans on procedure:
1. Turn the fan on
2. Hydraulic coupling is to be opened 20 %
3. The delivery gate for the fan is to be opened
4. The suction gate for the fan is to be opened
The hydraulic coupling and the fan air suction gates must be set to AUTO setting all gates
must be put to AUTO setting as follows:
Over fire Damper 20 % open
Aux. Dampers 40 % open
Fuel Air Damper 60%
Turning the flue gas fan and flame detector on:
1. Air fans must be turned on
2. Outlet gates for air must be open
3. Circulating Flue gases fans are to be turned on now
2.5.5. Operating Precautions:
1. Air fans must be on
2. Cooling water system must be operational
3. Inlet and outlet flue gases gates must be closed
4. Heater gates must be opened
5. Lubricating oil pump must be on
Secondary steam system must be turned on
Secondary steam must be at 360 C at about 13.5 bar
Secondary steam destinations:
1. Air heaters
2. Gas absorbers
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3. Air dumpers
4. Steam atomizer burners
5. Secondary steam for steam turbines
2.5.6. Fuel System: 1. Fuel level must be normal
2. Leakage preventing pump must be operational
3. Suction valve must be opened
4. Control valve for the lowest level of fuel must be opened
5. The delivery valve must be opened
The main fuel pump can now be turned on minimum pressure for the fuel is 20 bar by
adjusting the control valve
The steam atomizing system is now to be turned on after checking that the steam level is
normal the inlet valve for the secondary steam is to be opened. The atomizing steam
pressure is to be 11 bar.
2.5.7. Purging condition 1. Air flow not much than 30%
2. One or more FDF running
3. Fuel Oil trip valve closed
4. Fuel gas trip valve closed
5. All igniter off
6. All scanner no flame.
7. MFT
8. Igniter gas oil supply pressure must be proper
9. Fuel oil or gas supply pressure must be proper
10. All flue gases and air damper are to be opened
11. All burners valve must be closed
12. BCS power supply normal
13. All Aux. Damper modulating
Boiler Operation & Control 23
2.5.8. Boiler Storage As soon as possible after the end of the heating season, take these steps, where
applicable:
1. Remove all fuses from the burner circuit.
2. Remove soot and ash from the furnace, tubes, and flue surfaces.
3. Remove all fly ash from stack cleanout.
4. Drain the broiler completely after letting the water cool.
5. Flush the boiler to remove all sludge, and loose scale particles.
6. See that defective tubes, nipples, stay bolts, packings, and insulation are repaired or
replaced as required.
7. Clean and overhaul all boiler accessories such as safety valves, gauge glasses, and
firing equipment. Special attention should be given to low-water cutoffs and feed
water regulators to ascertain that float (or electrode) chambers and connections are
free of deposits.
8. Check the condensate return system for tightness of components.
2.6. My Boiler won't start - what to do first! If you notice a change in boiler performance such as new noises, smells, rising stack
temperatures or continually resetting safety devices. Although unexpected mechanical
failures do occur boiler's safety or operational devices is preventing your boiler from
starting. Most safety devices have manual reset buttons that need to be reset before boiler
operation can continue. Continual resetting of safety devices is an indication of unsafe
operating conditions. Prompt attention by your boiler technician is required.
Locate all devices that can prevent your boiler from starting.
2.6.1. Burner controller: The controller is usually located in front of the burner. On a call for heat the controller
starts a sequence of events that ensure safe operation before the burner is allowed to start.
Boiler Operation & Control 24
The controller continues to monitor burner operation while the boiler is running. If for any
reason the controller senses an unsafe operating condition it will shut the burner off.
Pushing the manual reset on the controller will often restart the boiler.
2.6.2. High pressure or temperature switch: This device is a safety backup to the "operator" control. It has a manual reset which when
pressed to start the boiler indicates that the "operator" control has failed.
2.6.3. Gas pressure switches on the fuel train: The natural gas fuel train usually has two pressure switches. The low pressure switch
locks out the boiler when too little gas is available for operation. The high pressure switch
locks out the boiler when the regulator is allowing too high a gas pressure. Both switches
have a manual reset.
2.6.4. Low water cutoff: The low water cutoff may have a manual reset. When reset indicates a low water condition
existed in the boiler.
2.6.5. Other devices that may prevent the boiler from starting: • Time clocks:
Time clocks or other energy management devices may restrict boiler operation during
weekends, evenings or other times of the day. Check their operating schedule.
• Outdoor temperature limits:
These devices sense outdoor temperatures and prevent boiler operation above certain
outdoor temperatures, usually 65 degrees.
Boiler Operation & Control 25
2.7. DANGEROUS CONDITIONS 2.7.1. Low Water A major reason for damages incurred to low pressure steam boilers is the low water within
the boiler. If the condition of low water exists it can seriously weaken the structural
members of the boiler, and result in needless inconvenience and cost. Low pressure
boilers can be protected by installing an automatic water level control device.
Steam boilers are usually equipped with automatic water level control devices. It must be
noted, however, that most failures occur due to low water on boilers equipped with
automatic control de vices. The water control device will activate water supply or feed
water pumps to introduce water at the proper level, interrupt the gas chain and ignition
process when the water reaches the lowest permissible level, or perform both functions
depending on design and interlocking systems. No matter how automatic a water control
device may be, it is unable to operate properly if sediment scale and sludge are
allowed to accumulate in the float chamber.
Accumulations of matter will obstruct and interfere with the proper operation of the float
device, if not properly maintained. To ensure for the reliability of the device, procedures
must be established in your daily preventive maintenance program to allow "blow-down"
the float chamber at least once a day. Simply open the drain for 3 to 5 seconds making
certain that the water drain piping is properly connected to a discharge line in accordance
with City Building Codes. This brief drainage process will remove loose sediment
deposits, and at the same time, test the operation of the water level control device. If the
water level control device does not function properly it must be inspected, repaired and
retested to guarantee proper operation.
2.7.2. Overpressure Safe operation of a boiler is dependent on a vital accessory, the safety valve. Failure to test
the safety valve on a regular basis or to open it manually periodically can result in heavy
accumulations of scale, deposits of sediment or sludge near the valve. These conditions
Boiler Operation & Control 26
can cause the safety valve spring to solidify or the disc to seal, ultimately rendering the
safety valve inoperative. A constantly simmering safety valve is a danger sign and must
not be neglected. Your preventive maintenance program includes the documentation and
inspection of the safety valve. A daily test must be performed when the boiler is in
operation simply raise the hand operating lever quickly to its limit and allow it to snap
closed. Any tendency of a sticking, binding or leaking of the safety valve must be
corrected immediately.
Boiler Operation & Control 27
CHAPTER 3
Boiler control
Boiler Operation & Control 28
3.1 Boiler control overview The determinant that controls all the boiler's operations is called the 'master demand'. In
thermal power-plant the steam is generated by burning fuel, and the master demand sets
the burners firing at a rate that matches the steam production. This in turn requires the
forced draught fans to deliver adequate air for the combustion of the fuel. The air input
requires the products of combustion to be expelled from the combustion chamber by the
induced draught fans, whose flow rate must be related to the steam flow. At the same time,
water must be fed into the boiler to match the production of steam. As stated previously, a
boiler is a complex, multivariable, interactive process. Each of the above parameters
affects and is affected by all of the others.
Funny example for load change these days, the demand for electricity in a developed
nation is also affected quite dramatically by television broadcasts. During a major
sporting event such as an international football match, sudden upsurges in demand will
occur at half-time and full time, when viewers switch on their kettles. In the UK this can
impose a sudden rise in demand of as much as 2 GW, which is the equivalent to
the total output of a reasonably large power station. The master demand in a power-
station application, the response of a boiler/turbine unit in a power station is determined
by the dynamic characteristics of the two major items of plant. These differ quite
significantly from each other. The turbine, in very general terms, is capable of responding
more quickly than the boiler to changes in demand.
The response of the boiler is determined by the thermal inertia of its steam and water
circuits and by the characteristics of the fuel system. For example, a coal-burning, with its
complex fuel-handling plant, will be much slower to respond to changes in demand than a
gas-fired one. Also, the turndown of the plant (the range of steam flows over which it will
be capable of operating under automatic control) will depend on the type of fuel being
burned, with gas-fired units being inherently capable of operating over a wider dynamic
range than their coal-fired equivalents.
Boiler Operation & Control 29
The design of the master system is determined by the role which the plant is expected to
play, and here three options are available. The demand signal can be fed primarily to the
turbine (boiler-following control); or to the boiler (turbine-following control); or it can be
directed to both (coordinated unit control). Each of these results in a different performance
of the unit, in a manner that will now be analyzed.
3.2 Boiler-following operation
Boiler Following Operation
With boiler-following control, the power-demand signal modulates the turbine
throttle-valves to meet the load, while the boiler systems are modulated to keep the steam
pressure constant.
How can we achieve this?
When valve closes, a drop in pressure happens, to regain the pressure to its predetermined
value, we should decrease flow rate to decrease pressure drop across the valve, also when
we decrease flow rate, pump head increases according to performance of the centrifugal
pump. In such a system, the plant operates with the turbine throttle-valves partly closed.
Boiler Operation & Control 30
The action of opening or closing these valves provides the desired response to demand
changes. Sudden load increases are met by opening the valves to release some of the
stored energy within the boiler.
When the demand falls, closing the valves increases the stored energy in the boiler. In
such a system the turbine is the first to respond to the changes. The boiler control system
reacts after these changes have been made, increasing or reducing the firing to restore the
steam pressure to the set value.
3.3 Turbine-following operation In the turbine-following system, the demand is fed directly to the boiler and the turbine
throttle-valves are left to maintain a constant steam pressure. Particularly in the case of
coal-fired plant, this method of operation offers slower response, because the turbine
output is adjusted only after the boiler has reacted to the changed demand and as we know,
the boiler response is much lower than turbine response especially the coal type. However,
the turbine-following system enables the unit to be operated in a more efficient manner
and tuning for optimum performance is easier than with the boiler following system.
We use this for large base-load power plant (where the unit runs at a fixed load, usually a
high one, for most of the time), or with gas-fired plant where the response is
comparatively rapid (as if we make the system boiler following, the boiler may fail to
follow the fast response turbine).
Turbine following system
Boiler Operation & Control 31
3.4 Coordinated unit control However, its design demands considerable knowledge of the characteristics and
limitations of the major plant items. Also, commissioning of this type of system demands
great skill and care if the full extent of the benefits is to be obtained. In particular, the rate
of change of the demand signals, as well as the extent of the is dynamic range, will need to
be constrained to prevent undesirable effects such as the stressing of pipe work because of
excessively steep rates-of-change of temperature.
Co-ordinated unit control’ system
Performance restriction for the control system is very dependent on the rate of
heating the turbine and boiler. Control parameters should always be adjusted as all system
component ages and their performance changes.
Boiler Operation & Control 32
3.5 Brief comparison between plant control modes As stated above, the coordinated unit load controller, w hen properly designed,
commissioned and maintained, will provide the best possible response of the unit within
the constraints of the plant itself. But for practical reasons it is not universally used.
3.5.1 Response of the boiler-following system
Consider what happens when a sudden rise in demand occurs. The first response is for the
throttle valves to be opened.
This increases the power generated by the machine, but it also results in the boiler pressure
falling, and when this happens the boiler control system reacts by increasing the firing
rate. This is all right as far as it goes since, quite correctly, it increases the boiler steaming
rate to meet the increase in demand.
However, as the firing change comes into effect and the steam pressure rises, the amount
of power that is being generated also increases. But as it has already been increased to
meet the demand and in fact may have already done so the power generated can overshoot
the target, causing the throttle valves to start closing again, which raises the boiler
pressure, and so on.
3.5.2 Response of the turbine-following system
In the simplest version of the turbine following system the boiler firing rate, and the rate
of air and feed water admission etc., are all fixed (or, at least, held at a set value, which
may be adjusted from time to time by the boiler operator), and the turbine throttle valves
are to keep the steam pressure constant. However when the fuel, air and water flows of a
boiler are held at a constant value the amount of steam that is generated will not, in
general, remain constant, mainly because of the inevitable variations that will occur in
parameters such as the calorific value of the fuel, the temperature of the feed water etc.
In the simple turbine following system, these variations are corrected by modulation of the
Boiler Operation & Control 33
turbine throttle valve to maintain a constant steam pressure, but this results in variations in
the power generated by the turbine.
Because the steam generation rate of its boiler is not automatically adjusted to meet an
external demand, a plant operating under the control of a simple turbine following system
will generate amounts of power that do not relate to the short term needs of the grid
system. Such a plant is therefore incapable of operating in a frequency support mode,
although this mode of operation may be used where it is not easy, or desirable, to adjust
the fuel input, for instance in industrial waste -incineration plants.
3.6 Boiler components control 3.6.1 Combustion, burner and draught control Naturally, in a fired boiler the control of combustion is extremely critical. In order to
maximize operational efficiency combustion must be accurate, so that the fuel is
consumed at a rate that exactly matches the demand for steam, and it must be executed
safely, so that the energy is released without risk to plant, personnel or environment.
Control of combustion is achieved through controlling air and fuel flow to burner.
Theoretically speaking, burner should keep the ratio between fuel and air constant along
all load range to achieve stoichiometric mixing between them. Unfortunately, when the
realities of practical plant are involved, the situation once again becomes far more
complex than this simple analysis would suggest.
Heat losses in a furnace
Boiler Operation & Control 34
If amount of excess air is increase over a certain limit, it causes loss in efficiency. The
reduction in efficiency is due to losses which are composed of the heat wasted in the
exhaust gases and the heat which is theoretically available in the fuel, but which is not
burned. As the excess air level increases, the heat lost in the exhaust gases increases, while
the losses in unburned fuel reduce (the shortage of oxygen at the lower levels increasing
the degree of incomplete combustion that occurs). The sum of these two losses, plus the
heat lost by radiation from hot surfaces in the boiler and its pipe work, is identified as the
total loss.
The figure above shows that operation of the plant at the point identified at 'A' will
correspond with minimum losses, and from this it may be assumed that this is the point to
which the operation of the combustion control system should be targeted. However, in
practice air is not evenly distributed within the furnace. For example, operational
considerations require that a supply of cooling air is provided for idle burners and flame
monitors, to prevent them being damaged by heat from nearby active burners and by
general radiation from the furnace. Air also enters the combustion chamber through leaks,
observation ports, soot lower entry points and so on. The sum of all this is referred to as
'tramp air' or 'setting leakage'. If this is included in the total being supplied to the furnace,
and if that total is apportioned to the total amount of fuel being fired, the implication is
that some burners (at least) will be deprived of the air they nee d for the combustion of
their fuel.
In other words, the correct amount of air is being provided in total, but it is going to places
where it is not available for the combustion process. Operation of the firing system must
take these factors into account and from then on the system can apportion the fuel and air
flows. If these are maintained in a fixed relationship with each other over the full range of
flows, the amount of excess air will be fixed over the entire range.
Boiler Operation & Control 35
3.6.1.1 Burners control systems
• A simple system: "parallel control" The easiest way of maintaining a relationship between fuel flow and air flow is to use a
single actuator to position a fuel-control valve and an air control damper in parallel with
each other as shown in figure below.
Here, the opening of an air-control damper is mechanically linked to the opening of a fuel
control valve to maintain a defined relationship between fuel flow and air flow. This
system is employed in very small boilers, and we can achieve a non-linear relationship
between valve opening and damper opening to be determined by the shape of a cam, with
a range of cams offering a variety of relationships.
Simple ‘parallel’ control Although this simple system may be quite adequate for very small boilers burning fuels
such as oil or natural gas, its deficiencies become increasingly apparent as the size of the
plant increases.
Boiler Operation & Control 36
• System problems 1. It assumes that for a given opening of fuel valve or air damper we get a certain
amount of flow and this is not true as flow depends also on pressure difference between
valves sides, also flow will depends on properties of fuel and air like density.
2. Another problem is that the response times of the fuel and air systems are never
identical. Therefore, if a sudden load change occurs and the two controlling devices are
moved to pre-determined openings, the flows through them will react at different rates.
With an oil fired boiler, a sudden increase in demand will cause the fuel flow to increase
quickly, but the air system will be slower to react. As a result, if the fuel/air ratio was
correct before the change occurred, the firing conditions after the change will tend to
become fuel rich until the air system has had time to catch up. This causes characteristic
puffs of black smoke to be emitted as unburned fuel is ejected to the chimney.
On a load decrease the reverse happens, and the mixture in the combustion chamber
becomes air rich. The resulting high oxygen content could lead to corrosion damage to the
metalwork of the boiler, and to unacceptable flue gas emissions.
• Flow ratio control
The first approach to overcoming the limitations of a simple 'parallel' system is to measure
the flow of the fuel and the air, and to use closed loop controllers to keep them in track
with each other.
In each of these systems the master demand is used to set the quantity of one parameter
being admitted to the furnace, while a controller maintains an adjustable relationship
between the two flows (fuel and air).
In the system shown in Figure a again block or amplifier in one of the flow signal lines is
used to adjust the ratio between the two flows. As the gain (g) of this block is changed, it
alters the slope of the fuel flow/airflow characteristic changing the amount of excess air
that is present at each flow. Note that when the gain is fixed, the amount of excess air is
Boiler Operation & Control 37
the same for all flows, as shown by the horizontal line.
In practice, this situation would be impossible to achieve, since some air inevitably leaks
into the furnace, with the result that the amount of excess air is proportionally greater at
low flows than high flows.
Fuel/ air ratio control a. Gain adjustment of fuel/air ratio b. Bias adjustment of fuel/air ratio
Boiler Operation & Control 38
The system shown in figure in previous page shows a different control arrangement
working with the same idealized plant (i.e. one with no air leaking into the combustion
chamber). Here, instead of a gain function, a bias is added to one of the signals. The effect
of this is that a fixed surfeit of air is always present and this is proportionally larger at the
smaller flows, with the result that the amount of excess air is largest at small flows, as
show. Changing the bias signal (b) moves the curve bodily as shown.
Each of these control configurations has been used in practical plant, although the version
with bias (Figure 5.3b) exacerbates the effects of tramp air and therefore tends to be
confined to smaller boilers. The arrangement shown in figure (a) therefore forms the basis
of most practical fuel/air ratio control systems.
In these illustrations it has been assumed that the master demand is fed to the fuel valve,
leaving the air flow controller to maintain the fuel/air ratio at the correct desired value.
When this is done, the configuration is known as a 'fuel lead' system since, when the load
demand changes, the fuel flow is adjusted first and the controller then adjusts the air flow
to match the fuel flow, after the latter has changed. It doesn't have to be done this way.
Instead, the master demand can be relayed to the air flow controller, which means that the
task of maintaining the fuel/air ratio is then assigned to the fuel controller. For obvious
reasons this is known as an 'air lead' system.
So, Fuel lead system is the system which manipulates fuel flow according to load and let
the controller adjust the amount of airflow to achieve the predetermined air to fuel ratio.
So, air lead system is the system which manipulates air flow according to load and let the
controller adjust the amount of fuel flow to achieve the predetermined air to fuel ratio.
Comparing the "fuel-lead' and 'air-lead' approaches Of the two alternatives described above, the fuel-lead version will provide better
response to load changes, since its action does not depend on the slower-responding plant
Boiler Operation & Control 39
that supplies combustion air to the furnace. However, because of this, the system suffers
from a tendency to produce fuel rich conditions on load increases and fuel-lean conditions
on decreases in the load.
Disadvantages of working in rich fuel region
Operating in the fuel rich region raises the risk of unburned fuel being ignited in an
uncontrolled manner, possibly causing a furnace explosion.
Disadvantages of working with too much excess air
Whereas operating with too much excess air, while not raising the risk of an uncontrolled
fire or an explosion, does cause a variety of other problems including back end corrosion
of the boiler structure, and undesirable stack emissions.
The air lead system is slow to respond because it requires the draught plant to react before
the fuel is increased. Although this avoids the risk of creating fuel rich conditions as the
load increases, it remains prone to such a risk as the load decreases “as the air takes time
to be reduced, hence the fuel w ill be injected during this period which will make a fuel
rich mixture”. However, the hazard is less than for the fuel lead system.
Disadvantages of both systems
A further limitation of these systems (in either the fuel-lead or air-lead version) is that they
offer no protection against equipment failures, since these cannot be detected and
corrected without special precautions being taken.
For example, in the fuel lead version, if fuel flow transmitter fails in such a way that it
signals a lower flow than the amount that is actually being delivered to the furnace, the
fuel/air ratio controller will attempt to reduce the supply of combustion air to match the
erroneous measurement. This will cause the combustion conditions to become fuel rich,
Boiler Operation & Control 40
with the attendant risk of an explosion. Conversely, if the fuel flow transmitter in the air
lead system fails low, the fuel controller will attempt to compensate for the apparent loss
of fuel by injecting more fuel into the furnace, with similar risks.
3.6.1.2 Cross-limited control
Basic cross-limited control system
Boiler Operation & Control 41
Figure above shows the principles of the cross limited combustion control system.
Individual flow ratio controllers (FRC) (7, 8) are provided for the fuel and air systems,
respectively. The effect of the fuel/air ratio adjustment block (4) is to modify the air flow
signal in accordance with the require d fuel/air relationship. (FT) is a flow transmitter
to give a value for actual flow for fuel and air (2 & 3). Because fuel flow and air flow are
each measured as part of a closed loop, the system compensates for any changes in either
of these flows that may be caused by external factors. For this reason it is sometimes
referred to as a 'fully metered' system. The effect of the fuel/air ratio adjustment block (4)
is to modify the air flow signal in accordance with the required fuel/air relationship.
How this system works?
So far, the configuration performs similarly to the basic systems in previous section. The
difference becomes apparent when the maximum and minimum selectors are brought into
the picture (components 5 & 6). Remembering the problems of the differing response rates
of the fuel and air supply systems consider what happens when the master demand signal
suddenly requests an increase in firing. Assume that, prior to that instant; the fuel and air
controllers have been keeping their respective controlled variable in step with the demand,
so that the fuel flow and modified air flow signals are each equal to the demand signal.
When the master demand signal suddenly increases, it now becomes larger than the
fuel flow signal and it is therefore ignored by the minimum selector block (5) which
instead latches onto the modified air flow signal (from item 4). The fuel controller now
assumes the role of fuel/air ratio controller, maintaining the boiler's fuel input at a
value that is consistent with the air being delivered to the furnace. The air flow is
meanwhile being increased to meet the new demand, since the maximum selector
block (6) has latched onto the rising master signal.
On a decrease in load, the system operates in the reverse manner. The minimum selector
block locks onto the collapsing master and quickly reduces the fuel flow, while the
maximum selector block chooses the fuel flow signal as the demand for the air flow
Boiler Operation & Control 42
controller (8), which therefore starts to operate as the fuel/air ratio controller, keeping the
air flow in step with the fuel flow.
Analysis of the system will show that it is much better able to deal with plant or control
and instrumentation equipment failures. For example, if the fuel valve fails open, the air
controller will maintain adequate combustion air to meet the quantity of fuel being
supplied to the combustion chamber. This may result in over firing but it cannot cause fuel
rich conditions to be created in the furnace. Similarly, if the fuel flow transmitter fails
low, although the fuel controller will still attempt to compensate for the apparent loss of
fuel, the air flow controller will ensure that adequate combustion air is supplied.
3.6.1.3 Multiple-burner systems The systems that have been described so far are based on the adjustment of the total
quantity of fuel and air that is admitted to the combustion chamber. This approach may be
sufficient with smaller boilers, where adjustment of a single fuel valve and air damper is
reasonable, but large r units will have a multiplicity of burners, fuel systems, fans,
dampers and combustion-air supplies. In such cases proper consideration has to be given
to the distribution of air and fuel to each burner or, if this is not practical, to small groups
of burners.
The concept of individually controlling air registers to provide the correct fuel/air ratio to
each burner of a multi burner boiler has been implemented, but in most practical situations
the expense of the instrumentation cannot be justified. Oil and gas burners can be operated
by maintaining a defined relationship between the fuel pressure and the differential
pressure across the burner air register (rather than proper flow measurements), but
even with such economies the capital costs are high and the payback low. The need to
provide a modulating actuator for each air register adds further cost.
A more practical option is to control the ratio of fuel and air that flows to groups of
burners. Figure shown next page shows how the principles of a simple cross limited
system are applied to a multiburner oil fired boiler. The plant in this case comprises
Boiler Operation & Control 43
several rows of burners, and the flow of fuel oil to each row is controlled by means of a
single valve. The combustion air is supplied through a common wind box, and the flow to
the firing burners is controlled by a single set of secondary air dampers.
A control system for multiple burners (one burner group shown)
Boiler Operation & Control 44
In most respects the arrangement closely resembles the basic cross limited system
explained in previous section, with the oil flow inferred from the oil pressure at the row. A
function generator is used to convert the pressure signal to a flow-per-burner signal, which
is then multiplied by a signal representing the number of burners firing in that row, to
yield a signal representing the total amount of oil flowing to the burners in the group.
Working with multiple fuels
The control systems of boilers burning several different types of fuel have to recognize the
heat input contribution being made at any time by each of the fuels, and the arrangements
become more complicated for every additional fuel that is to be considered.
Controlling multiple fuels (one burner group shown)
Boiler Operation & Control 45
Figure above shows a system for a boiler burning oil and gas. The similarities to the
simple cross limited system are very apparent, as are the commonalities with the fuel
control part of the multi burner system (shown within the chain-dotted area of Figure 5.9).
The cross limiting function is performed at the minimum selector block (5) which
continuously compares the master demand with the quantity of combustion air flowing to
the common wind box of the burner group. The gain block (6) translates the air flow into a
signal representing the amount of fuel whose combustion can be supported by the
available secondary air.
The selected signal (the load demand or the available air) ultimately forms the desired
value of both the gas and oil closed-loop controllers. But, before it reaches the relevant
controller a value is subtracted from it, which represents the heat contributed by the other
fuel (converted to the same heat/m s value as the fuel being controlled). The conversion of
oil flow to equivalent gas flow is performed in a function generator (10), while the other
conversion is performed in another such block (14). Each of the two summator units (11
and 13) algebraically subtracts the 'other-fuel’ signal from the demand.
Note that, in the case of this system, the gas pressure signal is compensated against
temperature variations, since the pressure/flow relationship of the gas is temperature
dependent. As before, each fuel flow signal represents the flow per burner and so it has to
be multiplied by the number of burners in service in order to represent the total fuel flow.
These diagrams are highly simplified, and in practice it is necessary to incorporate various
features such as interlocks to prevent over firing and to isolate one or other of the pressure
signals when no burner is firing that fuel. (This is because a pressure signal will exist even
when no firing is taking place.)
3.7 Draught control We will understand draught control via inspecting draught system components, layout and
operation. In the following section we shall see how air is delivered to the furnace at the
right conditions of flow and temperature, starting with the auxiliary plant that warms the
Boiler Operation & Control 46
air and moving on to the types of fan employed in the draught plant. The air heater in a
simple cycle plant, air is delivered to the boiler by one or more forced draught fans and the
products of combustion are extracted from it by induced draught fans as shown in figure
below.
Draught plant arrangement Figure above shows this plant in a simplified form, and illustrates how the heat remaining
in the exhaust gases leaving the furnace is used to warm the air being fed to the
combustion chamber. This function is achieved in an air heater, which can be either
regenerative, where an intermediate medium is used to transfer the heat from the exhaust
gases to the incoming air, or recuperative, where a direct heat transfer is used across a
dividing partition. In the regenerative type, air and exhaust may mix at a certain limit; this
is referred to as ‘air leakage’.
Boiler Operation & Control 47
Leakage happens across the circumferential, radial and axial seals, as well as at the hub.
These leakages are minimized when the plant is first constructed, but become greater as
wear occurs during prolonged usage. When the sheer physical size of the air heater is
considered it will be appreciated that these leakages can become significant.
Air heater leakage 3.7.1 Types of fan according to function
Here, classification is according to function, there are 3 types;
Forced draught fan
Induced draught fan
Booster fan
In addition to the FD and ID fans mentioned above, another application for large fans in a
power-station boiler is where it is necessary to overcome the resistance presented by plant
Boiler Operation & Control 48
in the path of the flue gases to the stack. In some cases, environmental legislation has
enforced the fitting of flue gas desulphurisation equipment to an existing boiler. This
involves the use of absorbers and/or bag filters, plus the attendant ducting, all of which
present additional resistance to the flow of gases. In this case this resistance was not
anticipated when the plant was originally designed, so it is necessary to fit additional fans
to overcome the draught losses. These are called 'booster fans'.
3.7.2 Types of fans according to working principle
In power plant, we use 2 types of fans “according to fan design and working principles”
Centrifugal fans
Axial flow fans
• Centrifugal fans
The blades are set radially on the drive shaft with the air or flue gas directed to the centre
and driven outwards by centrifugal force.
• Axial-flow fans
The air or gas is drawn along the line of the shaft by the screw action of the blades.
Whereas the blades of a centrifugal fan are fixed rigidly to the shaft, the pitch of axial-
flow fan blades can be adjusted. This provides an efficient means of controlling the fan's
throughput, but requires careful design of the associated control system because of a
phenomenon known as 'stall', which will now be described.
• Fan control constrains
There is some constrains for fan operation, this constrains are related to fan theory of
operation and its design, these limitation is explained below:
Boiler Operation & Control 49
• The stall condition
The angular relationship between the air flow impinging on the blade of a fan and the
blade itself is known as the 'angle of attack'. In an axial flow fan, when this angle exceeds
a certain limit, the air flow over the blade separates from the surface and centrifugal force
then throws the air outwards, towards the rim of the blades. This action causes a build-up
of pressure at the blade tip, and this pressure increases until it can be relieved at the
clearance between the tip and the casing. Under this condition the operation of the fan
becomes unstable, vibration sets in and the flow starts to oscillate. The risk of stall
increases if a fan is oversized or if the system resistance increases excessively. For each
setting of the blades there is a point on the fan characteristic beyond which stall will occur.
If these points are linked, a 'stall line' is generate d as shown in figure below and if this is
built into the plant control system (DCS) it can be used to warn the operator that the
condition is imminent and then to actively shift operation away from the danger
region. The actual stall-line data for a given machine should be provided by the fan
manufacturer.
The stall line of an axial flow fan
Boiler Operation & Control 50
• Centrifugal fan surge The stall condition affects only axial flow fans. However, centrifugal fans are subject to
another form of instability. If they are operated near the peak of their pressure/flow curve
a small movement either way can cause the pressure to increase or decrease unpredictably.
The point at which this phenomenon occurs is known as the 'surge limit' and it is the
minimum flow at which the fan operation is stable.
• Air flow control methods
After knowing about fans and their limitation, we will discuss methods of fan control and
characteristics of each control method.
There are 3 methods of fan control;
Damper
Fan speed
Blade angle
1-Fan damper
Boiler Operation & Control 51
The simplest form of damper consists of a hinged plate that is pivoted at the centre so
that it can be opened or closed across the duct. This provides a form of draught control but
it is not very linear and it is most effective only near the closed position. Once such a
damper is more than about 40- 60% open it can provide very little additional control.
Another form of damper comprises a set of linked blades across the duct (like a
Venetian blind). Such multi bladed dampers are naturally more expensive and more
complex to maintain than single bladed versions, but they offer better linearity of control
over a wider range of operation.
• Vane control
Boiler Operation & Control 52
The second form of control is by the adjustment of vanes at the fan inlet.
Such vanes are operated via a complex linkage which rotates all the vanes through the
same angle in response to the command signal from the DCS.
• Variable-speed drives Finally, control of fan throughput can be achieved by the use of variable speed motors (or
drives). These may involve the use of electronic controllers which alter the speed of the
driving motor in response to demand signals from the DCS or they can be
hydraulic couplings or variable-speed gearboxes, either of which allows a fixed speed
motor to drive the fan at the desired speed. Variable speed drives offer significant
advantages in that they allow the fan to operate at the optimum speed for the required
throughput of air or gas, whereas dampers or vanes control the flow by restricting it,
which means that the fan is attempting to deliver more flow than is required.
Boiler Operation & Control 53
Draught profile of a boiler and its auxiliary plant As we know, in a fired boiler, the air required for combustion is provided by one or more
fans and the exhaust gases are drawn out of the combustion chamber by an additional fan
or set of fans. On boilers with retro-fitted flue gas desulphurisation plant, additional
booster fans may also be provided. The control of all these fans must ensure that an
adequate supply of air is available for the combustion of the fuel and that the combustion
chamber operates at the pressure determined by the boiler designer. All of the fans
also have to contribute to the provision of another important function.
Purging of the furnace in all conditions: when a collection of unburned fuel or
combustible gases could otherwise be accidentally ignited. Such operations are required
prior to light off of the first burner when the boiler is being started, or after a trip. The
control systems for the fans have to be designed to meet the requirements of start-up,
normal operation and shut-down, and to do so in the most efficient manner possible,
because the fans may be physically large and require a large amount of power for their
operation (several MW in some cases). In addition, as we know, the performance
constraints of the fans, such as surge and stall, have to be recognized, if necessary by the
provision of special control functions or interlocks.
Boiler Operation & Control 54
3.8 Draught system duties The main duty of draught system is to maintain the furnace draught. Apart from supplying
air to support combustion, the FD fans have to operate in concert with the ID fans to
maintain the furnace pressure at a certain value. The heavy solid line of figure show n
below shows the pressure profile through the various sections of a typical balanced-
draught boiler system. It shows the pressure from the point where air is drawn in, to the
point where the flue gases are exhausted to the chimney, and demonstrates how the
combustion chamber operates at a slightly negative pressure , which is maintained by
keeping the FD and ID fans in balance with each other.
If that balance is disturbed the results can be extremely serious. Such an imbalance can be
brought about by the accidental closure of a damper or by the sudden loss of all flames. It
can also be caused by mal operation of the FD and ID fans. The dashed line on the
diagram shows the pressure profile under such a condition, which known as an
'implosion' .
The results of an implosion are extremely serious because, even though the pressures
involved may be small, the surfaces over which they are applied are very large and the
forces exerted become enormous. Such an event would almost certainly result in major
structural damage to the plant.
Boiler Operation & Control 55
CHAPTER 4
FEED WATER CONTROL SYSTEM
Boiler Operation & Control 56
4.1 Feed water control system Control of feed water is executed via feed water regulator, types of feed water regulators
are presented in the following sections.
4.1.1 Feed water Regulators
A boiler feed water regulator automatically controls the water supply so that the level in
the boiler drum is maintained within desired limits. This automatic regulator adds to the
safety and economy of operation and minimizes the danger of low or high water. Uniform
feeding of water prevents the boiler from being subjected to the expansion strains that
would result from temperature changes produced by irregular water feed. The danger in
the use of a feed water regulator lies in the fact that the operator may be entirely
dependent on it. It is well to remember that the regulator, like any other mechanism, can
fail; continued attention is necessary.
• Oldest feed water regulator
The First commercial feed water regulator
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It consists of a simple float attached to lever to control feed water flow and to keep level
constant as shown above.
Next generation employs the float in a different manner as shown in figure a.
For high capacity boilers and those operating at high pressure, a pneumatic or electrically
operated feed water control system is used.
There are basically three types of feed water control systems:
(1) Single element, (2) two element, and (3) three element.
• Single-element control
Boiler Operation & Control 58
This uses a single control loop that provides regulation of feed water flow in response to
changes in the drum water level from its set point. The measured drum level is compared
to its set point, and any error produces a signal that moves the feed water control valve in
proper response.
Single element control will maintain a constant drum level for slow changes in load, steam
pressure, or feed water pressure. However, because the control signal satisfies the
requirements of drum level only, wider drum-level variation results.
• Two-element control
This uses a control loop that provides regulation of feed water flow in response to changes
in steam flow, with a second control loop correcting the feed water flow to ensure the
correct drum water level. The steam flow control signal anticipates load changes and
begins control action in the proper direction before the drum-level control loop acts in
response to the drum water level. The drum level measurement corrects for any imbalance
between the drum water level and its set point and provides the necessary adjustment to
cope with the “swell and shrink” characteristics of the boiler.
Two element steam-flow-type feed water regulator
Boiler Operation & Control 59
• Three-element control
This uses a predetermined ratio of feed water flow input to steam flow output to provide
regulation of feed water flow in direct response to boiler load. The three element control
regulates the ratio of feed water flow input to steam flow output by establishing the set
point for the drum level controller. Any change in the ratio is used to modify the drum-
level set point in the level controller, which regulates feed water flow in direct response to
boiler load. This is the most widely used feed water control system.
Boiler Operation & Control 60
Three-element feed water-control system: (a) diagram layout of air-operated type; (b) schematic of electronic control system
Boiler Operation & Control 61
4.1.2 Types of feed water regulators
• Thermohydraulic type
A thermohydraulic, or generator diaphragm, type of boiler feedwater regulator is shown in
Figure b. Connected to the radiator is a small tube running to a diaphragm chamber. The
diaphragm in turn operates a balanced valve in the feedwater line. The inner tube is
connected directly to the water column and contains steam and water. The outside
compartment, connecting the tube and valve diaphragm, is filled with water. This water
does not circulate. Heat is radiated from it by means of fins attached to the radiator. Water
in the inner tube of the regulator remains at the same level as that in the boiler. When the
water in the boiler is lowered, more of the regulator tube is filled with steam and less with
water. Since heat is transferred faster from steam to water than from water to water, extra
heat is added to the confined water in the outer compartment. The radiating-fin surface is
not sufficient to re move the heat as rapidly as it is generated, so the temperature and
pressure of the confined water are raised. This pressure is transmitted to the balanced
valve diaphragm to open the valve admitting water to the boiler. When the water level in
the boiler is high, this operation is reversed.
Boiler Operation & Control 62
• Thermostatic expansion-tube-type
The thermostatic expansion tube type feedwater regulator is shown in Figure c. Because of
expansion and contraction, the length of the thermostatic tube changes and positions the
regulating valve with each change in the proportioned amount of steam and water.
Three types of boiler feed water regulators for simple water level control: (a) float-type
regulator; (b) thermohydraulic-type regulator; (c) thermostatic expansion tube regulator.
A two element steam flow type feedwater regulator shown in the above figure combines a
thermostatic expansion tube operated from the change in water level in the drum as one
element with the differential pressure across the super heater as the second element. The
two combined operate the regulating valve.
An air-operated three element feedwater control (Fig. 6.12a) combines three elements to
control the water level. Water flow is proportioned to steam flow, with drum level as the
compensating element; the control is set to be insensitive to the level. In operation, a
change in position of the metering element positions a pilot valve to vary the air loading
pressure to a standatrol self-standardizing relay). The resulting position assumed by the
standatrol provides pressure to operate a pilot valve attached to the feedwater regulator.
The impulse from the standatrol passes through a hand automatic selector valve,
permitting either manual or automatic operation. The hand-wheel jack permits manual
Boiler Operation & Control 63
adjustment of the feedwater valve if remote control is undesirable.
Two-element steam-flow-type feed water regulator
4.2 Steam temperature control 4.2.1 Why steam temperature control is needed: The rate at which heat is transferred to the fluid in the tube banks of a boiler or HRSG will
depend on the rate of heat input from the fuel or exhaust from the gas turbine. This heat
will be used to convert water to steam and then to increase the temperature of the steam in
the superheat stages. In a boiler, the temperature of the steam will also be affected by the
pattern in which the burners are fired, since some banks of tubes pick up heat by direct
radiation from the burners. In both types of plant the temperature of the steam will also be
affected by the flow of fluid within the tubes, and by the way in which the hot gases
circulate within the boiler.
As the steam flow increases, the temperature of the steam in the banks of tubes that are
directly influenced by the radiant heat of combustion starts to decrease as the increasing
flow of fluid takes away more of the heat that falls on the metal. Therefore the steam-
Boiler Operation & Control 64
temperature/steam-flow profile for this bank of tubes shows a decline as the steam flow
increases.
On the other hand, the temperature of the steam in the banks of tubes in the convection
passes tends to increase because of the higher heat transfer brought about by the increased
flow of gases, so that this temperature/ flow profile shows a rise in temperature as the flow
increases. By combining these two characteristics, the one rising, the other falling, the
boiler designer will aim to achieve a fairly flat temperature/flow characteristic over a
wide range of steam flows. No matter how successfully this target is attained, it cannot
yield an absolutely flat temperature/flow characteristic. Without any additional control, the
temperature of the steam leaving the final super heater of the boiler or HRSG would vary
with the rate of steam flow, following what is known as the 'natural characteristic' of the
boiler. The shape of this will depend on the particular design of plant, but in
general, the temperature will rise to a peak as the load increases, after which it will fall.
The steam turbine or the process plant that is to receive the steam usually requires the
temperature to remain at a precise value over the entire load range, and it is mainly for this
reason that some dedicated means of regulating the temperature must be provided. Since
different banks of tubes are affected in different ways by the radiation from the burners
and the flow of hot gases, an additional requirement is to provide some means of adjusting
the temperature of the steam within different parts of the circuit, to prevent any one
section from becoming over heated.
Before looking at the types of steam temperature control systems that are applied, it will
be useful to examine some of the mechanisms which are employed to regulate the
temperature according to the controller's commands. Depending on whether or not the
temperature of the steam is lowered to below the saturation point the controlling devices
are known as attemperators or desuperheaters. (Strictly speaking, the correct term to use
for a device which reduces the steam temperature to a point which is still above the
saturation point is an attemperator, while one that lowers it below the saturation point may
be referred to either as an attemperator or a desuperheater. However, in common
engineering usage both terms are applied somewhat indiscriminately.)
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4.2.2 The spray water attemperators
One way of adjusting the temperature of steam is to pump a fine spray of comparatively
cool water droplets into the vapour. With the resulting intermixing of hot steam and cold
water the coolant eventually evaporates so that the final mixture comprises an increased
volume of steam at a temperature which is lower than that prior to the water injection
point. This cooling function is achieved in the attemperator. The attemperator is an
effective means of lowering the temperature of the steam, though in thermodynamic terms
it results in a reduction in the performance of the plant because the steam temperature has
to be raised to a higher value than is needed, only to be brought down to the correct value
later, by injecting the spray water. Although the inherent design of the attemperation
system may, in theory, permit control to be achieved over a very wide range of steam
flows, it should be understood that the curve of the boiler's natural characteristic will
restrict the load range over which practical temperature control is possible, regardless of
the type of attemperator in use. It is not unusual for the effective temperature control range
of a boiler to be between only 75% and 100% of the boiler's maximum continuous rating
(MCR). This limitation is also the result of the spray water flow being a larger proportion
of the steam flow at low loads.
• The mechanically atomised attemperator
Various forms of spray attemperator are employed. Figure 1 shows a simple design
where the high pressure cooling water is mechanically atomised into small droplets at a
nozzle, there by maximising the area of contact between the steam and the water. With
this type of attemperator the water droplets leave the nozzle at a high velocity and
therefore travel for some distance before they mix with the steam and are absorbed. To
avoid stress inducing impingement of cold droplets on hot pipework, the length of straight
pipe in which this type of attemperator needs to be installed is quite long, typically 6 m or
more. With spray attemperators, the flow of cooling water is relate d to the flow rate and
the temperature of the steam, and this leads to a further limitation of a fixed-nozzle
attemperator. Successful break-up of the water into atomised droplets requires the spray
water to be at a pressure which exceeds the steam pressure at the nozzle by a certain
Boiler Operation & Control 66
amount (typically 4 bar). Because the nozzle presents a fixed-area orifice to the
spray water, the pressure/flow characteristic has a square -law shape, resulting in a
restricted range of flows over which it can be used (this is referred to as limited
turn-down or rangeability). The turn-down of the mechanically atomised type of
attemperator is around 1.5:1.
The temperature of the steam is adjusted by modulating a separate spray-water
control valve to admit more or less coolant into the steam. Because of the limitations of
the single nozzle, the accuracy of control that is possible with this type of
attemperator is no greater than + 8.5 °C.
Boiler Operation & Control 67
• The variable-area attemperator
One way of overcoming the limitations of a fixed nozzle in an attemperator is to use an
arrangement which changes the profile as the throughput of spray water alters.
Figure 2 shows the operating principle of a variable area, multinozzle attemperator. This
employs a sliding plug which is moved by an actuator, allowing the water to be injected
through a greater or smaller number of nozzles. With this type of device, the amount of
water injected is regulated by the position of the sliding plug, a separate spray-water
control valve is therefore not needed. Adequate performance of this type of attemperator
depends on the velocity of the vapour at the nozzles being high enough to ensure that the
coolant droplets remain in suspension for long enough to ensure their absorption by the
steam. For this reason, and also to provide the normal protection for the pipe work in the
vicinity of the nozzles, a thermal liner is often included in the pipe extending from the
plane of the nozzles to a point some distance downstream. The accuracy of control and
the turndown range available from a multi-nozzle attemperator is considerably greater
than that of a single nozzle version, allowing the steam temperature to be controlled to +
5.5°C over a flow range of 40:1.
Principle of a multi nozzle desuperheater
Boiler Operation & Control 68
• The variable-annulus desuperheater
Another way of achieving accurate control of the steam temperature over the
widest possible dynamic range is provided by the variable-annulus desuperheater
(VAD) (produced by Copes-Vulcan Limited, Road Two, Winsford Industrial Estate,
Winsford, Cheshire , CW7 3QL.). Here, the approach contour of the VAD head is such
that when the inlet steam flows through an annular ring betwee n the spray head and the
inner wall of the steam pipe its velocity is increased and the pressure slightly
reduced. The 140 Power-plant control and instrumentation coolant enters at this
point and undergoes an instant increase in velocity and a decrease in pressure,
causing it to vapourise into a micron-thin layer which is stripped off the edge of the
spray head and propelled downstream.
The stripping action acts as a barrier which prevents the coolant from impinging on the
inner wall of the steam pipe. The downstream portion of the VAD head is contoured,
creating a vortex zone into which any unabsorbed coolant is drawn, exposing it to
a zone of low pressure and high turbulence, which therefore cause s additional
evaporation. Due to the Venturi principle, the pressure of the cooled steam is
quickly restored downstream of the vena contract a point, resulting in a very low
overall loss of pressure. An advantage of the VAD is that, due to the coolant
injection occurring at a point where the steam pressure is lowered, the pressure of the
spray water does not have to be significantly higher than that of the steam.
• Other types of attemperator
At least two other designs of attemperator will be encountered in power station
applications. The vapour atomising design mixes steam with the cooling water, thus
ensuring more effective break-up of the water droplets and shrouding the atomised
droplets in a sheath of steam to provide rapid attemperation. Variable-orifice attemperators
include a freely floating plug which is positioned above a fixed seat a design that
generates high turbulence and more efficient attemperation. The coolant velocity increases
Boiler Operation & Control 69
simultaneously with the pressure drop, instantly vaporising the liquid. Because of the
movement of the plug, the pressure drop across the nozzle remains constant (at about 0.2
bar). The design of this type of attemperator is so efficient that complete mixing of
the coolant and the steam is provided within 3 to 4 m of the coolant entry point,
and the temperature can be controlled to + 2.5 °C, theoretically over a turndown range
of 100:1.
Because the floating plug moves against gravity, this type of attemperator must be
installed in a vertical section of pipe with the steam through it traveling in an upward
direction. However, because of the efficient mixing of steam and coolant, it is
permissible to provide a bend almost immediately after the device. Figure 3 shows a
typical installation.
Variable-orifice attemperator installation
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Location of temperature sensors:
Because the steam and water do not mix immediately at the plane of the nozzle or
nozzles, great care must be taken to locate the temperature sensor far enough
downstream of the attemperator for the measurement to accurately represent the actual
temperature of the steam entering the next stage of tube banks. Direct impingement of
spray water on the temperature sensor will result in the final steam temperature being
higher than desired. Figure 4 shows a typical installation, in this case for a variable -
annulus desuperheater.
a typical installation
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4.3 Temperature control with tilting burners The burning fuel in a corner fired boiler forms a large swirling fireball which can be
moved to a higher or lower level in the furnace by tilting the burners upwards or
downwards with respect to a mid position. The repositioning of the fireball changes the
pattern of heat transfer to the various banks of superheater tubes and this provides an
efficient method of controlling the steam temperature, since it enables the use of spray
water to be reserve d for fine tuning purposes and for emergencies. In addition, the tilting
process provides a method of controlling furnace exit temperatures. With such boilers, the
steam temperature control systems become significantly different from those of boilers
with fixed burners. The boiler designer is able to define the optimum angular position of
the burners for all loads, and the control engineer can then use a function generator to
set the angle of tilt over the load range to match this characteristic. A temperature
controller trims the degree of tilt so that the correct steam temperature is attained.
4.3.1 Controlling the temperature of reheated steam In boilers with reheat stages, changes in firing inevitably affect the temperature of both
the reheater and the superheater. If a single control mechanism were to be used for both
temperatures the resulting interactions would make control system tuning difficult, if not
impossible, to optimize. Such boilers therefore use two or more methods of control.
Because of the lower operating pressure of reheat steam systems, the
thermodynamic conditions are significantly different from those of superheaters, and the
injection of spray water into the reheater system has an undue effect on the
efficiency of the plant. For this reason, it is preferable for the reheat stages to be
controlled by tilting burners (if these are available) or by apportioning the flow of hot
combustion gases over the various tube banks. However, if the superheat temperature
is controlled by burner tilting, gas apportioning or spray attemperation must then be used
for the reheat stages. In boilers with fixed burners, steam temperature control may be
achieved by adjusting the opening of dampers that control the flow of the furnace gases
across the various tube banks. In some cases two separate sets of dampers are provided:
one regulating the flow over the superheater banks, the other controlling the flow
Boiler Operation & Control 72
over the reheater banks.
Between the m, these two sets of dampers deal with the entire volume of
combustion gases passing from the furnace to the chimney. If both were to be closed at the
same time, the flow of these gases would be severely restricted, leading to the possibility
of damage to the structure due to over pressurization. For this reason the two sets are
controlled in a so-called 'split-range' fashion, with one set being allowed to close only
when the other has fully opened. These dampers provide the main form of control, but
the response of the system is very slow, particularly with large boilers, where the
temperature response to changes in heat input exhibits a second-order lag of almost
two minutes' duration. For this reason, and also to provide a means of reducing the
temperature of the re heat steam in the event of a failure in the damper systems, spray
attemperation is provided for emergency cooling.
The spray attemperator is shut unless the temperature at the reheater outlet
reaches a predetermined high limit. When this limit is exceeded, the spray valve is
opened. In this condition, the amount of water that is injected is typically controlled in
relation to the temperature at the reheater inlet, to bring the exit temperature back into
the region where gas-apportioning or burner tilting can once again be effective. The
relationship between the cold reheat temperature and the required spray water flow can be
defined by the boiler designer or process engineer. If a turbine trip occurs the reheat flow
will collapse. In this situation the reheat sprays must be shut immediately in order to
prevent serious damage being cause d by the admission of cold spray water to the turbine.
4.3.2 Spray attemperators for reheat applications At first, it may seem that reheat spray-water attemperator systems should be similar to
those of the superheater. This is untrue, because reheat attemperators have to cope with the
lower steam pressure in this section of the boiler, which renders the pressure of the water
at the discharge of the feed pumps too high for satisfactory operation. Although a
pressure reducing valve could be introduced into the spray water line, this would be
an expensive solution w hose long term reliability would not be satisfactory because of the
Boiler Operation & Control 73
severe conditions to which such a valve would be subjected. A better solution would
be to derive the supply from the feed pump inlet. In some cases, even this is ineffective,
and separate pump sets have to be provided for the reheat sprays.
(A) Gas recycling Where boilers are designed for burning oil, or oil and coal in combination, they are
frequently provide d with gas-recirculation systems, where the hot gases exiting the
later stages of the boiler are recirculated to the bottom part of the furnace, close to the
burners. This procedure increases the mass-flow of gas over the tube banks, and therefore
increases the heat transfer to them.
Because the gas exiting the furnace is at a low pressure, fans have to be provided to
ensure that the gas flows in the correct direction. Controlling the flow of recycled
gases provides a method of regulating the temperature of the superheated and
reheated steam, but interlocks have to be provided to protect the fan against high
temperature gases flowing in a reverse direction from the burner area if the fan is
stopped or if it trips.
4.4 Boiler pressure control In a typical generating station will perform the following functions:
To control boiler pressure under normal operating conditions to a specified set point.
To allow warm-up or cool-down of the heat transport system at a controlled rate.
Since, under saturated conditions, steam pressure and temperature are uniquely
related, boiler pressure is used to indicate the balance between reactor heat output
and steam loading conditions. Steam pressure measurement is used since it provides a
faster response than a temperature measurement.
The Boiler Pressure Control is a digital control loop application with a sampling
period every 2 seconds.
Boiler Operation & Control 74
Basic Principles
A steam generator (boiler) is simply a heat exchanger and as such it obeys the
standard heat transfer relationship from one side of the boiler (tubes side) to the
other (shell-side).
Standard Heat transfer relationship can be described as:
Q = U. A. D T
where:
Q = the rate of heat exchange from the HTS to the boiler water (kJ/s).
U = heat transfer coefficient of the tubes (kJ/s/m2)
A = tube area (m2)
D T = temperature difference between HTS and steam generator inventory.
A and U are a function of boiler design and therefore Q is proportional to D T.
If reactor power output increases, then more heat must be transferred to the boiler water. Q
has to rise; therefore DT must also increase. This increase in DT can be achieved by either
allowing the average HTS temperature to increase as reactor power increases (as is the
case for a pressurize installation) or by arranging that the boiler Pressure falls, and
therefore boiler temperature falls, as reactor power increases (as is the case for a Solid
HTS designs with no pressurize). For all units designed with a pressurize, the first
method is employed. Whereas for units without Pressurize, the second method is used.
4.4.1 Boiler pressure control operation for units having a pressurize
Under normal operating conditions, BPC manipulates the reactor power output in order to
control boiler pressure to the set point. The turbine/generator, which is the heat sink for
the boilers, is controlled to an operator specified set point.
"Alternate" or “Reactor Leading” Operation
• If the unit is operating in the reactor leading mode at low power conditions the reactor
power set point is specified by the operator.
• Boiler pressure is then controlled to its set point by manipulation of the steam loads, i.e.,
Boiler Operation & Control 75
turbine and steam discharge valves.
Steam Discharge Valve Control
The Atmospheric Steam Discharge Valves (ASDV) and Condenser Steam Discharge
Valves (CSDV) are, under normal operating conditions, closed due to the introduction of a
bias signal.
If, for any reason, the boiler pressure rises above its set point by 70 kPa the ASDVs will
open. If the rise in boiler pressure is greater than 125 kPa above set point the CSDVs will
start to open. If the positive boiler pressure error is not corrected by the ASDVs and
CSDVs a reactor setback will be initiated to correct the thermal mismatch (i.e. correct
both the demand and the supply).
Boiler Operation & Control 76
4.4.2 Boiler pressure control operation for Units without a
Pressurize
• Units with only feed and bleed systems for Heat Transport pressure control are
normally run as base load, reactor leading, stations.
• The response of the Heat Transport System to transients caused by power maneuvering
is very limited.
• The Boiler Pressure Control System has a role in limiting the potential swell and
shrink of the HTS inventory by maintaining the HTS average temperature
essentially constant over the full operating range.
To control the boiler pressure, (the controlled variable) the following manipulated
variables are used:
(a) Reactor Power
(b) Turbine Steam Flow
(c) Steam Reject Valve (SRV) Steam Flow
• The boiler pressure will be decreased from 5 MPa to 4 MPa as unit power is raised from
0 to 100% full power (this is to minimize HTS temperature changes).
• This is also the turbine operating ramp. The SRV set point is a parallel ramp set 100 kPa
higher than the turbine ramp.
• Should the boiler pressure rise by more than 100 kPa excess pressure will be released by
the small SRVs.
• If the positive pressure transient is not corrected by the small SRVs the large SRVs will
start to open. Opening of the large SRVs will initiate a reactor setback.
• If the boiler pressure falls below the turbine set point the speeder gear will run back to a
point where the decreased turbine power will be matched.
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4.4.3 Boiler Pressure Response to A Requested Increase in
Electrical Output
• request for increased electrical output will create an error signal between the existing
output and the new set point.
• This error signal will cause the speeder gear to run up and thus increase the steam flow
to the turbine.
• This increased steam flow will result in an increased electrical output and eliminate the
electrical error which had been created.
• However, the increased steam flow will inevitably cause boiler pressure to fall.
• The increased governor valve opening results in an increased steam pressure on the
turbine side of the governor valve.
• This pressure increase is used as a feed forward signal which can be used to modify the
reactor power set point in advance of the negative boiler pressure error developing.
• In practice the feed forward signal will limit the size of the negative boiler pressure
transient but is unable to eliminate it completely.
• The resulting drop in boiler pressure is used as a feedback signal to the boiler pressure
control program. This will cause a further adjustment to be made to reactor power output
and thus return the boiler pressure to its set point.
Boiler Operation & Control 78
CHAPTER 5
CONTROL DEVICES
Boiler Operation & Control 79
5.1 Control devices
The purpose of the control system is to start, operate, and shut down the combustion
process and any related auxiliary processes safely, reliably, and efficiently.
A combustion system typically includes a fuel supply, a combustion air supply, and an
ignition system, all of which come together at one or more burners. During system
start-up and at various times during normal operation, the control system will need to
verify or change the status of these systems. During system operation, the control system
will need various items of process information to optimize system efficiency.
Additionally, the control system monitors all safety parameters at all times and will
shut down the combustion system if any of the safety limits are not satisfied.
5.1.1 Control platforms
The control platform is the set of devices that monitors and optimizes the process
conditions, executes the control logic, and controls the status of the combustion system.
• Relay System
A relay consists of an electromagnetic coil and several attached switch contacts that open
or close when the coil is energized or de -energize d. A relay system consists of a number
of relays wired together in such a way that they execute a logical sequence. For example, a
relay system may define a series of steps to start up the combustion process. Relays can
tell only if something is on or off and have no analog capability. They are generally
located in a local control panel.
Advantages of relays
Relays have several advantages. They are simple, easily tested, reliable, and well
understood devices that can be wired together to make surprisingly complex systems.
They are modular, easily replaced, and inexpensive. They can be configured in fail safe
mode so that if the relay itself fails, combustion system safety is not compromised.
Boiler Operation & Control 80
Disadvantages
There are also a few disadvantages of relays. Once a certain complexity level is reached,
relay systems can quickly become massive. Although individual relays are very reliable, a
large control system with hundreds of relays can be very unreliable. Relays also take up a
lot of expensive control panel space. Because relays must be physically rewired to change
the operating sequence, system flexibility is poor.
• Burner Controller
A variety of burner controllers is available from several different vendors. They are
prepackaged, hardwired devices in different configurations to operate different types of
systems. A burner controller will execute a defined sequence and monitor defined safety
parameters. They are generally located in a local control panel. Like relays, they generally
have no analog capability.
Advantages of burner controllers include the fact that they are generally inexpensive,
compact, simple to hook up, require no programming, and are fail safe and very reliable.
They are often approved for combustion service by various safety agencies and insurance
companies.
There are also some disadvantages. Burner controllers cannot control combustion
systems of much complexity. System flexibility is nonexistent. If it becomes necessary to
change the operating sequence, the controller must be rewired or replaced with a different
unit.
5.1.2 Programmable Logic Controller (PLC)
A programmable logic controller (PLC) is a small, modular computer system that consists
of a processing unit and a number of input and output modules that provide the interface to
the combustion components. PLCs are usually rack mounted, and modules can be added
or changed. There are many types of modules available. Unlike the relays and burner
controllers above, they have analog control capability. They are generally located in a
local control panel.
Boiler Operation & Control 81
PLCs have the advantage of being a mature technology. They have been available for
more than 20 years. Simple PLCs are inexpensive and PLC prices are generally very
competitive. They are compact, relatively easy to hook up, and because they are
programmable, they are supremely flexible. They can operate systems of almost any
complexity level. PLC reliability has improved over the years and is now very good.
Disadvantages of PLCs include having to write software for the controller. Coding can be
complex and creates the possibility of making a programming mistake, which can
compromise system safety. The PLC can also freeze up, much like a desktop computer
freezes up, where all inputs and outputs are ignored and the system must be reset in order
to execute logic again. Because of this possibility, standard PLCs should never be used as
a primary safety device. Special types of redundant or fault-tolerant PLCs are available
that are more robust and generally accepted for this service, but they are very
expensive and generally difficult to implement.
5.1.3 Distributed Control System (DCS)
A distributed control system (DCS) is a larger computer system that can consist of a
number of processing units and a wide variety of input and output interface devices.
Unlike the other systems described above, when properly sized, a DCS can also control
multiple systems and even entire plants. The DCS is generally located in a remote control
room, but peripheral elements can be located almost anywhere. DCSs have been around
long enough to be a mature technology and are generally well understood.
They are highly flexible and are used for both analog and discrete (on– off) control. They
can operate systems of almost any level of complexity and their reliability is excellent.
However, DCSs are often difficult to program. Each DCS vendor has a proprietary system
architecture, so the hardware is expensive and the software is often different from any
other vendor’s software. Once a commitment is made to a particular DCS vendor, it is
extremely difficult to change to a different one.
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5.1.4 Hybrid Systems
If you could combine several of the systems listed above and build a hybrid control
system, the advantages of each system could be exploited. In practice, that is what is
usually done. A typical system uses relays to perform the safety monitoring, a PLC to do
the sequencing, and either dedicated controllers or an existing DCS for the analog systems
control. Sometimes, the DCS does both the sequencing and the analog systems
control, and the safety monitoring is done by a fault-tolerant logic system. Most
approval agencies and insurers require the safety monitoring function to be separate from
either of the other functions.
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Simplified flow diagram of a standard burner lightoff sequence
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When we control burners of boilers, we keep 2 bounds in our consideration;
1- If the amount of fuel burned is more than required duty, overheating will occur.
2- If the amount of fuel burned is less than required, drop in power will happen. If we
connect the boiler to turbine, it will make the turbine work in wet region.
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CHAPTER 6
ANALOG DEVICES
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6.1 Analog devices
6.1.1 Control Valves
Control valves are among the most complex and expensive components in any combustion
control system. The type of service and control desired determines the selection of
different flow characteristics and valve sizes. Controls engineers use a series of
calculations to help with this selection process. A typical control valve consists of several
components that are mated together before installation in the piping system:
a) Control Valve Body
The control valve body can be a globe valve, a butterfly valve, or any other type of
adjustable control valve. Usually, special globe valves of the equal percent type are used
for fuel gas control service or liquid service. Control of combustion air and waste gas
flows generally require the use of butterfly valves often the quick opening type. Because
the combustion air or waste line usually has a large diameter, and the cost of globe valves
quickly becomes astronomical after the line size exceeds 3 or 4 inches, butterfly valves are
usually the most economical choice.
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b) Actuator
The actuator supplies the mechanical force to position the valve for the desired flow rate.
For control applications, a diaphragm actuator is preferred because, compared to a piston
type actuator, it has a relatively large pressure sensitive area and a relatively small
frictional area where the stem is touching the packing. This ensures smooth operation,
precision, and good repeatability. Proper selection of the actuator must take into account
valve size, air pressure, desired failure mode, process pressure, and other factors.
Actuators are usually spring loaded and single acting, with control air used on one side of
the diaphragm and the spring on the other. The air pressure forces the actuator to move
against the spring.
If air pressure is lost, the valve fails to the spring position thus, the actuator is chosen
carefully to fail to a safe position (i.e., closed for fuel valves, open for combustion air
valves).
c) Current-to-Pressure Transducer
The current-to-pressure transducer, usually called the I/P converter, takes the 24 VDC
(4 to 20 milliamps) signal from the controller and converts it into a pneumatic signal. The
signal causes the diaphragm of the actuator to move to properly position the
control valve.
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d) Positioner
The positioner is a mechanical feedback device that senses the actual position of the valve
as well as the desired position of the valve. It makes small adjustments to the pneumatic
output to the actuator to ensure that the desired and the actual position are the same.
e) Three-Way Solenoid Valve
When energized, the three-way solenoid valve admits air to the actuator. When
de-energized, it dumps the air from the actuator. Because single-acting actuators are
generally used, the spring in the actuator forces the valve either fully open or fully closed,
depending on the engineer’s choice of failure modes when specifying the valve.
Obviously, a control valve that supplies fuel gas to a combustion system should fail
closed, while the control valve that supplies combustion air to the same system
should fail open.
f) Mechanical Stops
Mechanical stops are used to limit how far open or shut a control valve can travel. If it is
vital that no more than a certain amount of fluid ever enters a downstream system, an “up”
stop is set. If it is necessary to ensure a certain minimum flow, for cooling purposes for
example, a “down” stop is set. In the case of a fuel supply control valve, the “down” stop
is set so that during system lightoff, an amount of fuel ideal for smooth and reliable burner
lighting is supplied. After a defined settling interval, usually 10 seconds, the three-
way solenoid valve is energized and normal control valve operation is enabled.
6.2 Thermocouples
Whenever two dissimilar metals come into contact, current flows between the metals and
the magnitude of that current flow and the voltage driving it, vary with temperature. This
phenomenon is called the Seebeck effect. If both of the metals are carefully chosen and are
of certain known alloy compositions, the voltage will vary in a nearly linear manner with
temperature over some known temperature range. Because the temperature and voltage
ranges vary depending on the materials employed, engineers use different types of
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thermocouples for different situations. In combustion applications, the “K” type
thermocouple (0 to 2400°F or-18 to 1300°C) is usually used. When connecting a
thermocouple to a transmitter, the transmitter should be set up for the type of
thermocouple employed. Installing thermocouples in a protective sheath known as a
thermowell prevents the sensing element from suffering the corrosive or erosive
effects of the process being measured. However, a thermowell also slows the response
of the instrument to changing temperature and should be used with care.
6.2.1 Velocity Thermocouples
Also known as suction pyrometers, the design of velocity thermocouples attempts to
minimize the inaccuracies in temperature measurement caused by radiant heat. Inside a
combustor, the thermocouple measures the gas temperature. However, the large amount
of heat radiated from the hot surroundings significantly affects the measurement. A
velocity thermocouple shields the thermocouple from radiant heat by placing it in one or
more concentric hollow pipes. Hot gas is induced to flow across the thermocouple,
producing a gas temperature reading without a radiant component.
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6.2.2 Resistance Temperature Detectors (RTDs)
Resistance of any conductor increases with temperature. For a specific material of known
resistance, it is possible to infer the temperature. Similar to the thermocouples described
above, the linearity of the result depends on the materials chosen for the detector and
their alloy composition. Engineers sometimes use RTDs instead of thermocouples when
higher precision is desired. Platinum is a popular material for RTDs because it has good
linearity over a wide temperature range. Like thermocouples, installation of RTDs in
thermowells is common.
6.2.3 Pressure Transmitters
A pressure transmitter is usually used to provide an analog pressure signal. These
devices use a diaphragm coupled to a variable resistance, which modifies the 24 VDC
loop current (4 to 20 milliamps) in proportion to the range in which it is calibrated. In
recent years, these devices have become enormously more accurate and sophisticated,
with onboard intelligence and self calibration capabilities. They are available in a wide
variety of configurations and materials and can be used in almost any service. It is possible
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to check and reconfigure these “smart” pressure transmitters remotely with the use of a
handheld communicator.
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CHAPTER 7
FLOW METERS
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7.1 Flow Meters
There are many different types of flow meters and many reasons to use one or another for
given application. The following is a list of several of the more common types of flow
meters, how they work, and where they are used.
7.1.1 Vortex Shedder Flow Meter
A vortex shedder places a bar in the path of the fluid. As the fluid goes by, vortexes
(whirlpools) form and break off constantly. An observation of the water swirling on the
downstream side of bridge pilings in a moving stream reveals this effect. Each time a
vortex breaks away from the bar, it causes a small vibration in the bar. The frequency of
the vibration is proportional to the flow.
Vortex shedders have a wide range, are highly accurate, reasonably priced, highly reliable,
and useful in liquid, steam, or gas service.
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7.1.2 Magnetic Flow Meter
A magnetic field, a current carrying conductor, and relative motion between the both
creates an electrical generator.
In the case of a magnetic flow meter, the meter generates the magnetic field and the
flowing liquid supplies the motion and the conductor. The voltage produced is
proportional to the flow. These meters are highly accurate, very reliable, have a wide
range, but are somewhat expensive. They are useful with highly corrosive or even gummy
fluids as long as the fluids are conductive. Only liquid flow is measured.
7.1.3 Orifice Flow Meter
Historically, almost all flows were measured using this method and it is still quite
popular. Placing the orifice in the fluid flow causes a pressure drop across the orifice. A
pressure transmitter mounted across the orifice calculates the flow from the amount of the
pressure drop. Orifice meters are very accurate but have a narrow range. They are
reasonably priced, highly reliable, and are useful in liquid, steam, or gas service.
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7.1.4 Coriolis Flow Meter
The Coriolis flow meter is easily the most complex type of meter to understand. The fluid
runs through a U-shaped tube that is being vibrated by an attached transducer. The flow of
the fluid will cause the tube to try to twist because of the Coriolis force. The magnitude of
the twisting force is proportional to flow. These meters are highly accurate and have a
wide range. They are generally more expensive than some other types.
7.1.5 Ultrasonic Flow Meter
When waves travel in a medium (fluid), their frequency shifts if the medium is in
motion relative to the wave source. The magnitude of the shift, called the Doppler
effect, is proportional to the relative velocity of the source and the medium. The
ultrasonic meter gene rates ultrasonic waves, sends the m diagonally across the pipe,
and computes the amount of frequency shift. These meters are reasonably accurate,
have a fairly wide range, are reasonably priced, and are highly reliable. Ultrasonic meters
work best when there are bubbles or particulates in the fluid.
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7.1.6 Turbine Flow Meters
A turbine meter is a wheel that is spun by the flow of fluid past the blades. A magnetic
pickup senses the speed of the rotation, which is proportional to the flow. These
meters can be very accurate but have a fairly narrow range. They must be very
carefully selected and sized for specific applications. They are reasonably priced and fairly
reliable. They are used in liquid, steam, or gas service.
7.1.7 Positive Displacement Flow Meters
Positive displacement flow meters generally consist of a set of meshed gears or lobes that
are closely machined and matched to each other. When fluid is forced through the gears, a
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fixed amount of the fluid is allowed past for each revolution. Counting the revolutions
reveals the exact amount of flow. These meters are extremely accurate and have a wide
range. Because the re are moving parts, the meters must be maintained or they can
break down or jam. They also cause a large pressure drop, which can be important for
certain applications.
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