Natural gas boiler control seqence start up

[colesa]

I am assisting in the commissioning of a small second hand package boiler at a steam engine museum. The boiler is to be run at 150 psig and at 3,000 lb/ hr. It is equipped with a low natural gas pressure burner and a forced draft fan. I am worried about purge times at start up - particularly after a failed start. I have always had good operating manuals and instrument technitions on previous plant. Please point in right direction. Thanks!

 

[Hipressure]

If you could put your hands on ASME CSD-1 Controls and Safety Devices for Automatically Fired Boilers, could help you. I haven't looked for some time but I think it speaks to fan capacity and furnace volumes.

 

[toothless]

Generally speaking, you will have to purge out X volumes of air with X being mandated by your local codes. Then the time required will be dictated by the number total volume to be displaced and your blower flow rate.

 

[rmw]

In most cases that I have seen, the requirements that "toothless" mentioned are met by a full stroke of the fan damper, full closed to full open and back, if your operator is electrical and slow, e.g. full stroke in 30 sec or less.

But, do some rough calculations on what your fan will put out in that period of time at say 50% volume and compare it to the requirements of the governing authority, which, might be no more than your insurance carrier.

rmw

 

[kbs]

In all locations I've commissioned boilers in, the local code has been the same for everything up to 400,000Mbtu/hr natural gas fired packaged units. The prepurge is calculated at four complete air changes. The volume is the entire flue passage including venting. Also, the timing of the purge does not start until the combustion air gates or dampers reach high fire. Post purge is normally as short as possible to keep the heat in the boiler. Once you know the flow rate of the blower at high fire and the volume of flue passages, multiply by four and add the time to ramp up to high fire. Hope this helps.

 

[toothless]

KBS is correct regarding the damper position vs. when the purge time begins. Since the air flow rate of the blower is based on the damper being open, the purge doesn't "officially" begin until the damper is wide open. When the purge timer ends, then the damper goes back to the minimum position before the pilot gas is activated.

If in doubt, consult the OEM.