Boiler Operation And Control Page 2
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
Boiler Operation And Control Page 3 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 flowing from the boiler back to the feed water pump. The feed water check valve opens and closes automatically with a swinging disc. When water is fed to the boiler it opens. If water flows back from the boiler the valve closes.
4. Water Column minimizes the water turbulence in the gage glass to provide accurate water level reading.
Boiler Operation And Control Page 4 Gage 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
Boiler Operation And Control Page 5 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 burner control 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 water pressure 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
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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 opened next and takes the wear and tear from blow down. Water is discharged to the blow down tank. A blow down tank collects water to protect the sewer from the hot boiler water. After blow down, the screw valve is closed first and the quick opening valve is closed last.
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.
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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 an outside stem and yoke or OS&Y valve. The position of the stem indicates whether the valve is open or closed. The valve is opened with the stem out and closed with the stem in. This provides quick information to the boiler operator.
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
Boiler Operation And Control Page 8
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 the inverted bucket. The steam holds the bucket up. As condensate fills the steam trap the bucket loses buoyancy and sinks to open the discharge valve. The thermostatic steam trap has a bellows filled with a fluid that boils at steam temperature. As the fluid boils vapors expand the 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
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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 pressure inside the boiler.
Boiler Operation And Control Page 10
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 off at the salinometer cock can be used to check water temperature below 100°C. At the end of this time set fires away in all furnaces, still at the lowest rate.
11. Close the air vent. Nuts on manhole doors and any new joints should be nipped up.
12. Circulating the boiler can now be stopped, and steam pressure slowly raised during the next 7-8 hours to within about 100 kN/m' of the working pressure.
Boiler Operation And Control Page 11 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.
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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 external irregularities will still try to perform as normal. This can result in their final control action being incorrect, or to some other piece of equipment being overworked in an attempt to compensate.
4. In situations where the automatic control of critical parameters is not dependable, or where it becomes necessary to use manual control, reduce operating conditions so as to increase acceptable margins of error.
5. High performance water tube boilers demand high quality feed water, so do not tolerate any deterioration of feed water conditions; immediately trace the source of any contamination, and rectify the fault.
6. Do not neglect leakage of high pressure, high temperature steam, as even minor leaks will rapidly deteriorate.
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7. No attempt should be made to approach the site of leakage directly, but the defective system should be shut down as soon as is practicable and the leakage rectified.
8. Do not allow steam and water leaks to go un-corrected as, apart from reduction in plant efficiency, they also lead to increased demand for extra feed with an inevitable increase in boiler water impurities.
9. Always be alert for conditions which increase the potential fire risk within the engine room: the best method of fire fighting is not to allow one to start. Thus all spaces, tank tops etc. must be kept clean, dry, and well lit. This not only improves the work environment, but also makes for the early detection of any leakage and encourages early repair.
10. Store any necessary stocks of combustibles remote from sources of ignition. Maintain all oil systems tight and free from leaks and overspills. Follow correct flashing-up 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.
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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 that all 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.
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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 closed up.
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 enter refractory 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.
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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 this can’t be done immediately, then hot air from steam air heaters or from portable units must be blown through to dry the external surfaces.
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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 PH
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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
Air preheaters are to be turned on now
a) Open the inlet and outlet gates for the flue gases
b) Open the inlet and outlet gates for the air
c) The forced air fan is to be turned on now
d) After 15 sec the induced fan is to be turned on.
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%
Boiler Operation And Control Page 19 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 opened
5. Lubricating oil pump must be on
Secondary steam system must be turned on
Secondary steam must be at 360 C at about 13.5 bar Secondary steam destinations:
1. Air heaters
2. Gas absorbers
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
Boiler Operation And Control Page 20 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.
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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.
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9.4.
Low water cutoff:
The low water cutoff may have a manual reset. When reset indicates a low water condition 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.
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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 the water level control device. If the water level control device does not function properly it must be inspected, repaired and retested to guarantee proper operation.
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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.
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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 be expelled from the combustion chamber by the induced draught fans, whose flow rate must be related to the steam flow. At the same time, water must be fed into the boiler to match the production of steam.
As stated previously, a boiler is a complex, multivariable, interactive process. Each of the above parameters affects and is affected by all of the others.
Funny example for load change
These days, the demand for electricity in a developed nation is also affected quite dramatically by television broadcasts. During a major sporting event such as an international football match, sudden upsurges in demand will occur at half-time and full time, when viewers switch on their kettles. In the UK this can impose a sudden rise in demand of as much as 2 GW, which is the equivalent to the total output of a reasonably large power station.
The master demand in a power-station application
The response of a boiler/turbine unit in a power station is determined by the dynamic characteristics of the two major items of plant. These differ quite significantly from each other. The turbine, in very general terms, is capable of responding more quickly than the boiler to changes in demand.
The response of the boiler is determined by the thermal inertia of its steam and water circuits and by the characteristics of the fuel system. For example, a coal-burning
Boiler Operation And Control Page 27 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.
Boiler Operation And Control Page 28
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.
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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).
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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.
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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 boiler pressure falling, and when this happens the boiler control system reacts by increasing the firing rate. This is all right as far as it goes since, quite correctly, it increases the boiler steaming rate to meet the increase in demand.
However, as the firing change comes into effect and the steam pressure rises, the amount of power that is being generated also increases. But as it has already been increased to meet the demand--and in fact may have already done so--the power generated can overshoot the target, causing the throttle valves to start closing again, which raises the boiler pressure..., and so on.
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, when the fuel, air and water flows of a boiler are held at a constant value the amount of steam that is generated will not, in general, remain constant, mainly because of the inevitable variations that will occur in parameters such as the calorific value of the fuel, the temperature of the feed water etc. In the simple turbine-following system, these variations are corrected by modulation of the turbine throttle valve to maintain a constant steam pressure, but this results in variations in the power generated by the turbine.
Boiler Operation And Control Page 32 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.
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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.
Boiler Operation And Control Page 34 If amount of excess air is increase over a certain limit, it causes loss in efficiency.
The reduction in efficiency is due to losses which are composed of the heat wasted in the exhaust gases and the heat which is theoretically available in the fuel, but which is not burned. As the excess-air level increases, the heat lost in the exhaust gases increases, while the losses in unburned fuel reduce (the shortage of oxygen at the lower levels increasing the degree of incomplete combustion that occurs). The sum of these two losses, plus the heat lost by radiation from hot surfaces in the boiler and its pipe work, is identified as the total loss.
The figure above shows that operation of the plant at the point identified at 'A' will correspond with minimum losses, and from this it may be assumed that this is the point to which the operation of the combustion-control system should be targeted. However, in practice air is not evenly distributed within the furnace. For example, operational considerations require that a supply of cooling air is provided for idle burners and flame monitors, to prevent them being damaged by heat from nearby active burners and by general radiation from the furnace. Air also enters the combustion chamber through leaks, observation ports, soot-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 fixed relationship with each other over the full range of flows, the amount of excess air will be fixed over the entire range.
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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 use a single actuator to position a fuel-control valve and an air control damper in parallel with each other as shown in figure below.
Here, the opening of an air-control damper is mechanically linked to the opening of a fuel control valve to maintain a defined relationship between fuel flow and air flow. This system is employed in very small boilers, and we can achieve a non-linear relationship between valve opening and damper opening to be determined by the shape of a cam, with a range of cams offering a variety of relationships.
Although this simple system may be quite adequate for very small boilers burning fuels such as oil or natural gas, its deficiencies become increasingly apparent as the size of the plant increases.
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.
Boiler Operation And Control Page 36 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 shown next 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.
Boiler Operation And Control Page 38 The system shown in figure in previous page shows a different control arrangement working with the same idealized plant (i.e. one with no air leaking into the combustion chamber). Here, instead of a gain function, a bias is added to one of the signals. The effect of this is that a fixed surfeit of air is always present and this is proportionally larger at the smaller flows, with the result that the amount of excess air is largest at small flows, as 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 let the controller adjust the amount of fuel flow to achieve the predetermined air to fuel ratio.
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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-rich conditions as the load increases, it remains prone to such a risk as the load decreases “as the air takes time to be reduced, hence the fuel 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
Boiler Operation And Control Page 40 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 air-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
Boiler Operation And Control Page 41 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
Boiler Operation And Control Page 42 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 a single set of secondary-air dampers.
Boiler Operation And Control Page 43 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,
Boiler Operation And Control Page 44 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).
Boiler Operation And Control Page 45 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.)
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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
Boiler Operation And Control Page 47 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
Boiler Operation And Control Page 48 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 fansThe 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 because of 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
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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.
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2-1-2-3-2-Centrifugal fan surge
The stall condition affects only axial-flow fans. However, centrifugal fans are subject to another form of instability. If they are operated near the peak of their pressure/flow curve a small movement either way can cause the pressure to increase or decrease unpredictably. The point at which this phenomenon occurs is known as the 'surge limit' and it is the minimum flow at which the fan operation is stable.
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