Boiler furnace forced air cooling 1

[johno02150]

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?

 

[davefitz]

NFPA rules may require the boiler be purged with air to remove combustibles for 15 minutes .

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.

 

[boilerone]

First of all I should explain that the governing reason why boilers are not allowed to cool down or heat up too fast is because of the need to minimize temperature difference between the steam drum's upper part (steam space) and lower part (water space). The typical limit is 105 deg. F or 40 deg. C and this temperature difference should be closely monitored and never allowed to exceed, no matter it is a planned or an emergency startup/shutdown. If need to gain early access to the boiler after shutdown, typical procedure allows only to turn on ID fan for forced cooling after 12 hrs of natural 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

 

[AdieGenerate]

Well put Boilerone, the only thing that I can add is that we also used to monitor "Through the wall" differential temperatures on boiler drums that we were force cooling in addition to the top to bottom diffs. The actual limits to work to will be dependant on drum steam conditions and structure.