Boiler furnace forced air cooling
Boiler furnace forced air cooling
(OP)
When a Boiler is removed from service when are the only times forced cooling by the ID?FD fans should be utilized? What damage can be caused by repeated forced cooling?
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Boiler furnace forced air cooling
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RE: Boiler furnace forced air cooling
Following an MFT ( master fuel trip) and loss of fans, unburned coal or other fuel lies smoldering on the furnace hopper , generating CO. Unpurged NH3 nozzles in the SCR pass may also be leaking NH3 to the convection pass. Persons cannot access the boiler unless one is certain the CO and NH3 is removed.
Another recent need for a fan purge is related to the use of wet FGD systems- they have an effective water seal of about 0.5 in WC ( additive to the gas frictonal pressure drop) whenever the A/R pumps are running, and this prevents "natural draft" from occurring on hot days ( combination of low stack gas temp , low stack, hot ambient temps, and 0.5" WC water seal acrosss the absorber trays). So one needs to use the fan to get a purge.
RE: Boiler furnace forced air cooling
Using FD fans to help cooling downs are normally not allowed because this might cause cooling too fast. Running only ID fan with access doors open and air duct dumpers opened should provide enough cooling speed.
Too fast and frequent heating and cooling can cause thermal stress and thermal fatigue between hot parts and cold parts otherwise material failures or shortening the life of the boiler can happen. The thicker the wall thickness the worse the problem and that's why the steam drum is normally the concern. For supercritical boilers without drums, the headers and other thick wall components are the next ones to monitor.
I remember a case that the state of IOWA still does not allow to use the latest ASME allowable stress values which has lowered material safety factors from 4 to 3.5. Older SF of 4 have to be used in IOWA. Of course this would raise the material cost of the entire boiler. The only exception is that special permission could be given if a calculation can be provided to state that the steam drum of the boiler will not experience more than a certain number of temperature variations during its entire life of service.
Hope these help.
Boilerone
RE: Boiler furnace forced air cooling