Boiler Venting Issue

[poncho802]

I have a project installed as follows.
(4) 400 BHP steam boilers in boiler room common vented. (3) boilers will only be on line at a time. The stack extends 15 floors on the roof into a plenum with draft inducers. The initial stack read .75" of available draft. We installed barometrics at each boiler to bring down the available draft to .18". with (3) boilers at 1/3 power the system drafts with the barometrics balanced. When one boiler goes to full fire the barometric at that boiler dumps into the room. None of the fans opperate. The stack is still showing a .4" SP with boilers running. Any suggests on how to stop the boilers from dumping and lowering the static pressure?

 

[chicopee]

What do you mean by"...the barometric at that boiler dumps into the room."?

 

[chief]

Your details are confusing , but I read them such that the exhaust from the 4 boilers go to a common collector box. With only one boiler in operation , you are getting a back draft of exhaust fumes back into the boiler room. You have several choices to cure the problem. You are obviously getting back pressure from the collector box and the final length of exhaust pipe.
1. Seal and pressurise your boiler room with a fan supply.
2. Fit flap plate check valves in the in the individual boiler uptakes before the common collector box.
3. Remove the collector box and run separate uptakes.
4. Remove the collector box and swage the exhaust pipes together ,such that the operating exhaust acts as an eductor to the other exhausts.
5. Finally, the most likely source of your problem is an undersize collector box and final exhaust section. You will need some flow calculations to size the correct size collector and final exhaist pipe sizes.

Offshore Engineering&Design

 

[poncho802]

chicopee, I mean that flue gas will push back on the barometric damper causing flue gases to spill into the boiler room.

 

[chicopee]

Ha, that's a better description.
Upon that increase in firing rate of one boiler, why are the induced draft fans not in operation as described in your post?
Obviously there is an increase in back pressure as chief pointed out during that surge of the flue gases for these gases to blow back into the boiler room. What do your back pressure calculations show with the different firing rates?
Chief checklist is a good one; also check for any obstruction in the chimney. Wind could also cause this backpressure.

 

[fjm3188]

There is not enough information to properly diagnois the breach and vent connector conditions. Diameters, eight and design criteria are necessary.

However, one thing to check that is a common error made when barometrics dampers are installed is the sleeve the baromentric is installed in. I know this sounds rediculous but I have seen too many applications on firetube and watertube boilers where the barometric damper is installed too close to the vent connetor. When the draft increases with the firing rate, the barometric does it's job and starts to correct the draft. However, the blade of the damper is drawn into the actual flow of the flue gas causing it to re-direct the gases out the barometric damper.

Check and see if this is happening. If the blade moves into the vent connector, extend the sleeve further out away from the flow of gases. This will allow the flue gases to go up the stack without the damper blade interferring with the path.