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Operation Of Boilers

Operation Of BoilersThe load control range The steady regime of boiler operation The allowable boiler range The unsteady regime of boiler operation

The Load Control RangeThe automatic control system of a boiler responds quickly to the load without the interference of the operating personnel. The lowest limit is from 40-50% of rated load. Smaller boilers not used for power stations have much lower control range

The steady regime of boiler operationThe steam parameters vary insignificantly at any load. The lowest limit is from 3040% of rated load

The Allowable Load Ranges

The allowable load ranges include loads from limit of control range to the lowest load at which the boiler can function steadily

The Unsteady Regime of Boiler OperationLoad variation and fluctuation of steam parameters occur due to internal or external disturbances Internal disturbances are variations in: Flow rate Temperature Fuel consumption rate Combustion air flow rate

The Unsteady Regime of Boiler OperationExternal disturbances are variations in: Steam pressure Load of the turbo-alternator The degree of opening of start-up and shut down device

Steam Generators TestingCold testingHydrostatic Testing Welding testing

Hot testingFlow measurement Temperature measurement Dryness fraction of steam leaving the drum

Flow MeasurementSteam, water, air and gas flow should be measured using orifice, nozzle or venturi tubes so as to check any flow meters connects in the boiler lines. These measuring instruments should be manufactured according to certain standards, say ASME Performance Test Codes.

Temperature measurementsThese are carried out by using different types of thermocouples.

Dryness fraction of steam leaving the drum:this can be measured by a separating and a throttling calorimeter.

Bringing a Boiler on LoadIn bringing a boiler on load the key parameter to be maintained is temperature. In this case it is not maintained constant but is changed according to a predetermined pattern. This a pattern is a compromise between the desire to bring the boiler on-load as quikly as possible and the risk of boiler damage by thermal stresses arising from uneven.

Bringing a Boiler on LoadThere are four stages: Warming up before circulation is established Warming up after circulation is established Stage when significant quantities of steam are being taken Bringing the boiler on load





1.Warming up before circulation is established During this phase the limit on the system is the temperature of boiler tubes. Until circulation is established there is a risk of local overheating in regions of pockets of trapped steam and of serious uneven heating between adjacent tubes. Limited input of energy into the system is needed and light oil burners are used. The provision of boiler circulation pumps removes this stage from procedure.

2.Warming up after circulation is established During this phase the main concern is stresses on the boiler drum arising from uneven heating along its length or through the thickness of its metal. These limitations are met by restricting the permissible rate of rise of drum pressure and hence of drum temperatures. Progressively more energy is taken from the system by steam flow to drains and slightly tighter input is needed. Careful control of the energy input is most important at this stage and this is achieved by varying the number of oil burners if applicable

3.Stage when significant quantities of steam are being takenThis stage is present when boilers used for industrial processes are also used to generate power, and then turbine conditions must match with the steam output conditions. The limit now passes to the maximum permissible value in the superheater tube metal temperatures. The steam and fuel flows are increasing since appreciable energy is being taken from the system, but the steam flows are not yet adequate to ensure superheater cooling. Considerable drainage on the superheater may still be necessary

4.Bringing the boiler on loadHere the limit is still the superheater temperature. During this section there must be smooth change from the light oil used in the initial stages of boiler operation to the heavy fuel oil used under spontaneous operation. The sequential ignition of these burners provides the fine control of the energy needed. As each burner is put into service, care must be taken to see that it ignites properly, and that it burns with bright smokeless flame and does not subsequently go out

The following are the minimum standards for the operation as listed in ASME Section VI, VII

On a daily basis, boiler operation Test performed and the results, Unusual boiler conditions, Changes in equipment status and the reason Steam pressure Stack temperature Flame condition, Confirmation that no smell of gas or evidence of fuel leaks exists Confirmation that no steam or water leaks from the boiler Operators name, initials, and dates of log entry.

On a daily basis, boiler operationConfirmation that pressure and temperatures are within the guidelines set by the manufacturer of the boiler. In no case shall the pressures/temperatures be greater than the manufacturers maximum allowable working pressure/temperatures. The maximum allowable pressure and or temperature are denoted on the nomenclature plate affixed to the boiler (data plate).

On a daily basis, boiler operationObservation of condition of the flame to determine if it is even, and not off color. Gas burner flame should be blue with varying amounts of yellow on the flame ends depending upon the firing rate. The flame should be even around the burner. Oil burner flame should be bright yellow without dark trails off the end of the flame.

On a weekly interval, the boiler operationObserving the boiler during its shutdown and start-up cycle while listening for the fuel solenoids to activate and listening for any unusual noises (such as the fuel valve solenoid chattering, buzzing loudly, etc.) Testing low water cut-off and boiler lowwater alarm for low-pressure boilers while in operation. The test must show the boiler will shut off if a low water condition exists.

On a weekly interval, the boiler operationVisual checks of ignition and flame detection system, looking for any abnormal conditions, such as frayed wires, loose or broken conduit, loose wiring, etc. Checking the firing rate controls and linkage for freedom of movement and linkage connections. Cleaning fuel oil filters and strainers. Unless your boiler is equipped with dual filters, this must be done with the boiler shutdown. Always shutdown the fuel oil pump(s) and valve off to the fuel oil filters prior to cleaning or replacement. Following approved lock out, tag out procedures.

On a monthly interval, the boiler operationVisually checking the boiler during shutdown and start-up. Make sure the burner fan runs through its pre and post-purge ventilation cycles. Watch and listen to the fuel solenoid valves to insure they are closing as prescribed by the manufacturers recommendations.

On a monthly interval, the boiler operationChecking all floor drains in the boiler room for proper operation. First look down the drain to see if water is visible, which will indicate the automatic drain charging system is working properly . Put about one gallon of water down each drain to insure that the drain is free from clog. If water can not be seen in the drain indicate such on the boiler room log and notify the maintenance person to have the situation corrected.

On a monthly interval, the boiler operationInspecting the combustion air louvers and or ventilation screens both inside the boiler room and outside. They must be free from dust and debris, clean if necessary. Visually checking the boiler stack, looking for areas of overheating, blackened area where flue gasses are escaping, areas of rust, free operation of barometric damper (if equipped), and joints coming apart or holes in flue ducting.

On a quarterly basis, boiler operationTesting the flame safeguard control system. This test shall verify proper operation of control devices and cause a safety shut down and lockout. The boiler should not restart without resetting the flame safe guard control.

On a quarterly basis, boiler operationTesting low gas pressure limit control system. Slowly close the gas valve upstream from the low-pressure gas limit control. This should cause the burner to shut off and lock out. For oil fired burners, conduct the same test. without damaging the fuel oil pumps or the system. Checking alternate fuel supply system by switching to the backup fuel source.

Fireside parameter Monitoring and ControlThe preservation of boiler operation and performance includes meeting underlined design assumptions and process control. The boiler design involves the energy balance between the fireside and the steam side parameters.

Fireside parameter Monitoring and ControlIn a typical fossil power plant, there is more steam side instrumentation installed with the original control system than there is flue gas side instrumentation. However, the fireside provides the heat energy input to the boiler system and it is extremely important to control the fireside operating parameters to ensure the boiler performance.

Fireside parameter Monitoring and ControlTypically, the boiler lacks fireside control. The fuel and air are mixed, combustion takes place in the burner system and the next monitoring point in the flue gas path is the boiler exit temperature. Basically, there is nothing in between. There is a simple logic to converting from a two point control, burner and boiler exit temperature, to a three point control; i.e., burner, furnace exit gas temperature (FEGT