boiler operation and control

8/13/2019 Boiler Operation and Control

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Boiler Operation And Control

1. Operation in a glimpse:

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.

2. A short description of the common boiler devices in

operation:

1. Safety Valves are the most important fittings on the

boiler. They should open to release 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

Safet Valve




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.

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.

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 flowingfrom 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.

4. Water Column minimizes the water turbulence in the gage glass to provide

accurate water level reading.




Ga e Glass

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.

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.

6. A 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.

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.

8. The 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 burnercontrol to shut off fuel to the burner when the water level drops in the

chamber.

9. The 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.

10. Feed water Pumps are used with

feed water regulators to pump feed

water to the boiler. Pressure must be

sufficient to overcome boiler waterpressure to maintain the NOWL in

the boiler. For maximum safety,

plants having one steam driven feed

water pump must have a back up

feed water pump driven by

electricity. Feed water pumps may be

reciprocating, centrifugal or turbine.Feed water Pump

Feed Water Regulator




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.

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.

13. Turbine feed water pumps are steam driven and operate similarly to centrifugal

feed water pumps.

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 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.

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 openednext 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.

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.




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.

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 anoutside 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.

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

Pressure Gauge




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

the inverted bucket steam trap steam enters the bottom flowing into theinverted 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 bellows 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.

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 Trap

Steam Strainer




3. 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 pressureinside the boiler.




4. 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 drawn offat 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.




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.

5. 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.




6. 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.

3.

Automatic control loops do not think for themselves, and subjected to externalirregularities 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 encouragesearly 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

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 at 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 detail 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.




7. 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 operated 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 thatall attachment bolts are present and that a\l 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.

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 replaced. 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 closedup.

8. External Cleaning Spaces between tubes can become choked with deposits which are

not removed 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 then 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 enterrefractory material, in order to prevent water soaking in to cause external corrosion

11. . Efficient drainage must be provided, with sometimes drains below 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 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 deposits

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 thiscan’t be done immediately, then hot air from steam air heaters or from portable

units must be blown through to dry the external surfaces.




8. Boiler operation from cold start

8.1.

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

8.2. 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

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 PH8. Sodium Phosphate (NA3PO4) is used to remove dissolved salts




8.3. 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

Ai r pre heaters are to be tu rn ed on no w

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 nowd) After 15 sec the induced fan is to be turned on.

8.4. 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

4. Air delivery 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

8.5. 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 opened5. 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

3. Air dumpers

4. steam atomizer burners

5. Secondary steam for steam turbines

8.6. 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

8.7. 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

8.8. 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 feedwater regulators to ascertain that float (or electrode) chambers and

connections are free of deposits.

8. Check the condensate return system for tightness of components.




9. 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.

9.1. 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. 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.

9.2. 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.

9.3. 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.




9.4. Low water cutoff:

The low water cutoff may have a manual reset. When reset indicates a low watercondition existed in the boiler.

9.5. Other devices that may prevent the boiler from

starting:

9.5.1. 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.

9.5.2. Outdoor temperature limits:

These devices sense outdoor temperatures and prevent boiler operation above

certain outdoor temperatures, usually 65 degrees.




10.DANGEROUS CONDITIONS

10.1. 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 devices. 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 Ieast 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 thewater level control device. If the water level control device does not function properly

it must be inspected, repaired and retested to guarantee proper operation.




10.2. 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 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.




Section 2

Boiler control

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 beexpelled 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




boiler, 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.

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 (co-ordinated unit control). Each of these results in a

different performance of the unit, in a manner that will now be analyzed.

1-1-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. 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.




1-2-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).




1-3-Co-ordinated 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 their dynamic range, will need to be

constrained to prevent undesirable effects such as the stressing of pipework because

of excessively steep rates-of-change of temperature.

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.




1-4-Brief comparison between plant control modes

As stated above, the co-ordinated unit load controller, when 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.

1-4-1-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 boilerpressure 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.

1-4-2-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 modulated to keep the steam pressure constant. However, whenthe 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 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.




2-Boiler components control

2-1-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.




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-blower 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 need 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 fixedrelationship with each other over the full range of flows, the amount of excess air will

be fixed over the entire range.




2-1-1-Burners control systems

2-1-1-1-A simple system: "parallel control"

The easiest way of maintaining a relationship between fuel flow and air flow is to usea 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.

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 thesize of the plant increases.

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 predetermined 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.

2-1-1-2-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, as shown by the two configurations of figure shownnext page.

In each of these systems the master demand (not shown) 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 a gain 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 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.




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 shown. 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 air flow to achieve the predetermined air to

fuel ratio.

So, air lead system is the system which manipulates air flow according to load and letthe 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 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-richconditions 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 will 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 the 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, 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.

2-1-1-3-Cross-limited control

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 required 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 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.

2-1-1-4-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 larger 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 multi burner oil-fired boiler.

The plant in this case comprises 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 asingle set of secondary-air dampers.




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.

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.)




2-1-2-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

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.

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’.

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.

2-1-2-1-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 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'.

2-1-2-2-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

2-1-2-2-1-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.

2-1-2-2-2-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 becauseof a phenomenon known as 'stall', which will now be described.

2-1-2-3-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




2-1-2-3-1-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 generated 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.




2-1-2-3-2-Centrifugal fan surge

The stall condition affects only axial-flow fans. However, centrifugal fans are subjectto 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.

2-1-2-4-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

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.




2-1-2-4-2-Vane control

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.

2-1-2-4-3-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.




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 mannerpossible, 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.




2-1-2-5-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 inconcert with the ID fans to maintain the furnace pressure at a certain value. The

heavy solid line of figure shown 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.




2-2-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

2-2-1-Feedwater Regulators

A boiler feedwater 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 changesproduced by irregular water feed. The danger in the use of a feedwater 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.

2-2-1-1-Oldest feed water regulator

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 feedwater control system is used.

There are basically three types of feedwater-control systems:

(1) Single element, (2) two element, and (3) three element.

2-2-1-2-Single-element control

This uses a single control loop that provides regulation of feedwater 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 feedwater

control valve in proper response.




Single-element control will maintain a constant drum level for slow changes in load,

steam pressure, or feedwater pressure. However, because the control signal satisfies

the requirements of drum level only, wider drum-level variation results.

2-2-1-3-Two-element control

This uses a control loop that provides regulation of feedwater flow in response to

changes in steam flow, with a second control loop correcting the feedwater 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.




2-2-1-4-Three-element control

This uses a predetermined ratio of feedwater flow input to steam flow output to

provide regulation of feedwater flow in direct response to boiler load. The three-

element control regulates the ratio of feedwater 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 feedwaterflow in direct response to boiler load. This is the most widely used feedwater-control

system.




2-2-2-Types of feed water regulators

2-2-2-1-Thermohydraulic type

A thermohydraulic, or generator-diaphragm, type of boiler feedwater regulator isshown 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 remove

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.




2-2-2-2-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.

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 superheater 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 adjustment of the feedwater valve if

remote control is undesirable.




2-3-Steam temperature control

Why steam temperature control is needed:

The rate at which heat is transferred to the fluid in the tube banks of a boiler orHRSG 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-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 superheater 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 overheated.

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.)

2-3-1-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 evaporatesso 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.

2-3-1-1-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, thereby 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 related 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 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 ina 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

ofattemperator 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.




Figure 1 Mechanically atomised desuperheater

2-3-1-2-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 thermal protection for the pipework 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.

2-3-1-3-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 between the spray head and

the inner wall of the steam pipe its velocity is increased and the pressure slightlyreduced. 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 offthe 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,

Figure 2 Principle of a multi nozzle desuperheater




exposing it to a zone of low pressure and high turbulence, which therefore causes

additional evaporation.

Due to the Venturi principle, the pressure of the cooled steam is quickly restored

downstream of the vena contracta 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.

2-3-1-4-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 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 travelling 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.




Figure 3 Variable-orifice attemperator installation




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.

Figure 4 a typical installation




2-3-2-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 reserved 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.

2-3-3-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 over

the reheater banks.

Between them, 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 reheat 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 reheatsprays must be shut immediately in order to prevent serious damage being caused by

the admission of cold spray water to the turbine.

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 whose long-term reliability would not

be satisfactory because of the 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 provided 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.




2-4-Boiler pressure control

In a typical generating station will perform the following functions:

To control boiler pressure under normal operating conditions to a specified setpoint.

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.

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 D T must also increase.

This increase in D T 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.




2-4-1-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., 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 CSDVswill 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).




2-4-2-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.




2-4-3-Boiler Pressure Response to A Requested Increase in Electrical

Output

• A 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 use d 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.




2-5-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.

2-5-1-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.

2-5-1-1-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-energized. 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.

Disadvantages

There are also a few disadvantages of relays. Once a certain complexity level isreached, 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.

2-5-1-2-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.

2-5-1-3-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 aregenerally located in a local control panel.

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.

2-5-1-4-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.

2-5-1-5-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.




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 omount 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.




2-5-2-Analog devices

2-5-2-1-Control Valves

Control valves are among the most complex and expensive components in any

combustion control system.

As shown in Figure, 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.




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.

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 thevalve. 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.

2-5-2-2-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

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.




Figure 1 thermo couple

2-5-2-3-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.




2-5-2-4-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 thermocouplesdescribed 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.




2-5-2-5-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 to check and reconfigure these “smart” pressure

transmitters remotely with the use of a handheld communicator.

2-5-2-6-Flow Meters

There are many different types of flow meters and many reasons to use one or

another for a 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.




2-5-2-6-1-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.

2-5-2-6-2-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.

2-5-2-6-3-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 narrowrange. They are reasonably priced, highly reliable, and are useful in liquid, steam, or

gas service.




2-5-2-6-4-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.




2-5-2-6-5-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 generates ultrasonic




sound waves, sends them 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.

2-5-2-6-6-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.



2-5-2-6-7-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 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 there are moving parts, the meters must bemaintained or they can break down or jam. They also cause a large pressure drop,

which can be important for certain applications.

boiler operation and control

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