ASME Sec I, BFW Pump minimum discharge pressure

[don1980]

I thought there was an ASME Sec I requirement stating that the BFW pump must have enough head to maintain a flow even when the boiler is at its relief pressure. I recall this from a power plant project I did back in the 90's. A question came up today, related to BFW pump head requirements. I went to Sec I, but I don't see this requirement. It's been almost 20 yrs, so maybe I'm not remembering this correctly. I recall that that "requirement" made sense to me because the boiler could be mechanically damaged (overheated) during a relief event if the BFW pumps didn't have enough head to maintain a flow at relief pressure.

Am I overlooking this requirement in Sec I, or does it not exist?

Thanks.

 

[georgeverghese]

Looking at this and applying basic process safety concepts, this provision to have the BFW pumps run at RV set pressure would make sense when there is no instrumented overpressure trip protection to cutoff fuel supply to the boiler at PSH. In this case however, the boiler drum RV would also have to account for full flow liquid (and/or vapor) relief from these pumps at the reduced pumping capacity seen at boiler drum RV set pressure.

If there is a PSH on the boiler drum that reliably cuts out fuel supply, then no need for this pump to have full capacity at RV pressure. It would be a good idea to have some minimal capacity in the pumps at this time for the case when the water mass in the drum at low level is not enough to remove all the residual heat in the tubes. Any residual pumping capacity at RV relieving pressure should be catered for at the drum PSV (if this capacity barely uses the full RV capacity, then add another smaller RV set at 105% of MAWP for this).