Superheater and Reheater Safety Valve Requirement(PG-68)

[bitjantuek]

Dear Sir,
At the present we has the problem on main steam safety valve - passing. Now we kill this valve via install gag for SH outlet to stop leaks and protect valve. Then, can someone/specials list explain me for PG-68.2 as " The discharge capacity of the safety valve, or valves on an attached superheater may be included in determiing the number and size of the safety valves for the boiler, provide there no intervening valves between the superheat safety valve and the boiler, and provided the discharge capacity of the safety valve, or valves on the boiler, as distinct from the superheater is at least 75% of the aggregate valve capacity required." For the at least 75%of the aggreate valve capacity required, for example BMCR is 2285 t/hr*75% = 1713 t/hr it means we able to install gag on safety valve and re-calculated the relieve capacity if more than 1713 t/hr is accept by code? please advise

 

[don1980]

ASME Sec I requires you to have a functioning safety valve on the superheater, if the boiler has a superheater. It specifies that all of the PSVs on the boiler system (sum of the capacities for all the PSV) must have a capacity that is greater than or equal to the boiler's nameplate capacity (BMCR). But, it further specifies that you can't put too much PSV capacity on the superheater. During an emergency, there must be some flow through the superheater section to keep it from overheating, but this flowrate (PSV flowrate) can't exceed 25% of the total installed relief capacity on the entire boiler system. In other words, ASME Sec I requires that some flow must pass through the superheater (from memory I think you have to have at least 15% of the relief flow on the superheater, but check that), but at least 75% of the relief flow must pass through the PSVs on the steam drum(s).

If you have gagged the PSV on the superheater, then you have no relief capability on the superheater and that is not in compliance with ASME Sec I.

 

[e43u8]

bitjantuek,

I think your understanding from mentioned ASME statement cannot be correct. Actually it means the relief capacity of the main safety valve(s) on the steam drum plus the relief capacity of the safety valve(s) on the superheater should be equal to the total capacity of the boiler provided that the relief capacity of the main safety valve(s) to be at least 75% of the boiler capacity; then it doesn't mean that there would be a room for gagging the safety valve(s) on the superheater.

 

[bitjantuek]

done1980 and e43u8,
Thank you for your advise and explain krub.
The total relieving capacity is (Drum 1pc + +6pc + SH 2pc + PCV) = 2458.29 t/h. If i kill 1 SH valve, it will be 2458.29 – 177.5 = 2280.79t/h. It can satisfy the BMCR-2280 t/h but normally we operated with TMCR-2191 t/h. It not compliance with ASME Sec I?