Steam Generator Derating 2

[Rajjil]

Hi there,
We are doing de-rating of a steam generator from current 1700 psig design pressure on steam side to 1300 psig due to the lower wall thickness found in the economiser and steam generator tubes during the insepction. This is very old plant and no documentation available on original design. Please refer to the sketch attached. The set up is like this.. BFW pump with PSV on discharge side set at 2000 psig without any control valve, BFW is being preheated in the economiser and steam generated. Two PSVs are set at 1700 psig & 1649 psig respectively to meet the total relief load.
My question is if I derate the design pressure on steam side i.e. from 1700 psig to 1300 psig, do I need to reduce the set pressure of the BFW PSV as well or not?
My understanding is that since there is no control on BFW pump discharge side, the BFW PSV is protecting pump discharge piping up to Economiser inlet side of for blocked outlet(there is a block valve at Economiser inlet). And the Steam PSVs are protecting the Economiser and the Steam Generator with relief capacity equal to max. steam generator's capacity based on the heat transfer area of Economiser and Steam Generator tubes. But then why BFW PSV set 300 psig higher than the steam PSVs?
Would anyone please advise me, Thanks a lot.

 

[TD2K]

The BFW PSVs may have been set at either the piping pressure limit or the pressure limit of the BFW pumps' casing. They may also have been set at 2000 psig to avoid having the online BFW pump being head-headed if the economizer inlet block valve was closed, sort of a crude minimum flow protection if there is no minimum flow loop.

I wouldn't automatically reduce the BFW PSVs just because you reduce the steam PSVs' set pressures.

What's the inlet piping flange rating? 2000 psig is pretty close to the design pressure for class 900 flanges at 250F (I'd expect BFW's design temperature to be at least 250F).

 

[don1980]

There's a specific reason for the BFW PSV to be set higher than the other PSVs on the boiler system. You never want to starve the system of water, even during an overpressure emergency. During such an emergency, when the boiler and superheater PSVs are open, the BFW pumps must continue to supply water to maintain flow through those PSVs. The worst thing that can happen is to run out of flow to the boiler and superheater. The system can overheat and fail, which is a very big safety and economic risk.

That's why ASME Sec I has prescriptive minimum head requirements for the BFW pumps. I don't have my copy of Sec-I in front of me, so I can't cite specific values.You're free to reduce the boiler pressure while leaving the BFW system the same - there's no problem with too much head, except that it's a waste of energy. If you choose to lower the BFW PSV, then read the text in Sec-I on this topic first. You'll understand the constraints.