Steam relief valves
Steam relief valves
(OP)
We have a boiler generating steam to users (heat exchangers). Our steam system design if as follows:
1) Two PSVs on boiler steam drum set at 109 barg and 110 barg, respectively. PSVs are designed to ASME boiler code. PSVs are sized for 80% of maximum steam generation capacity of boiler.
2) One PSV downstream of boiler superheater at 100 barg. PSV is designed to ASME boiler code. PSV is sized for 20% of maximum steam generation capacity of boiler.
3) Steam users have an MAWP of 100 barg and are designed to ASME Section VIII div. 1. There are no PSVs located near the users. Overpressure protection for the steam users is provided only by the PSVs in the boiler package (PSV's in item 1 and 2 above).
During blocked outlet relief scenario, the PSVs in the boiler package limit the maximum overpressure to less than 116% of MAWP of the steam users (in accordance with ASME Section VIII div. 1 for multiple relief valves). However, the problem we have is that the PSVs on the boiler steam drum have a set pressure higher than 105% of MAWP of steam users (designed to ASME Section VIII div. 1). ASME Section VIII div. 1 does not allow the set pressure to be higher than 105% of the MAWP when multiple PSVs are used for protection.
The question is if the current design is acceptable based on ASME Section VIII div. 1 requirements? My opinion is that the current design does not meet all the ASME Section VIII div. 1 requirements and hence is unacceptable, but wonder if anyone has a different opinion which can be supported by code.
1) Two PSVs on boiler steam drum set at 109 barg and 110 barg, respectively. PSVs are designed to ASME boiler code. PSVs are sized for 80% of maximum steam generation capacity of boiler.
2) One PSV downstream of boiler superheater at 100 barg. PSV is designed to ASME boiler code. PSV is sized for 20% of maximum steam generation capacity of boiler.
3) Steam users have an MAWP of 100 barg and are designed to ASME Section VIII div. 1. There are no PSVs located near the users. Overpressure protection for the steam users is provided only by the PSVs in the boiler package (PSV's in item 1 and 2 above).
During blocked outlet relief scenario, the PSVs in the boiler package limit the maximum overpressure to less than 116% of MAWP of the steam users (in accordance with ASME Section VIII div. 1 for multiple relief valves). However, the problem we have is that the PSVs on the boiler steam drum have a set pressure higher than 105% of MAWP of steam users (designed to ASME Section VIII div. 1). ASME Section VIII div. 1 does not allow the set pressure to be higher than 105% of the MAWP when multiple PSVs are used for protection.
The question is if the current design is acceptable based on ASME Section VIII div. 1 requirements? My opinion is that the current design does not meet all the ASME Section VIII div. 1 requirements and hence is unacceptable, but wonder if anyone has a different opinion which can be supported by code.





RE: Steam relief valves
I2I
RE: Steam relief valves
Do you mean "for relief valves installed on the Section VIII vessel" ?
RE: Steam relief valves
The design is acceptable as far as I can see. Reasons are:
- The PSV after the superheater is set lower than the drum, and is sized for a reasonable flow (cannot remember if 20% flow is correct, but seems reasonable).
- The PSV after the superheater is set at (or below) the design pressure of the main header (ASME 8).
- The only thing that needs to be confirmed, is whether the 9 bar margin (superheater to drum) is correct.
RE: Steam relief valves
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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."
RE: Steam relief valves
The pressure relief device ... need not be installed on a pressure vessel when either of the following conditions apply:
(1) the source of pressure is external to the vessel and is under such positive control that the pressure in the vessel cannot exceed the MAWP at the operating temperature except as permitted in (c) above (NOTE: Mutiple valves & Fire Sizing), or under the conditions set forth in Appendix M.
JAC
RE: Steam relief valves
is there any reference on how to select the ratio of mass flow ?
80 %, 20 % of boiler capacity - how did you decide ?
best regards
Druid