Steam relief valves

[prq123]

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.

 

[insult2injury]

I believe the PSV requirements in Section VIII are for relief valves installed on the Section VII vessel. What may be required by Section VIII is additional PSVs on the those vessels if there is a potential for overpressure but these should have no effect on the set pressures or capacities of the boiler PSVs. You will probably never find a boiler drum PSV set pressure that is less than 5% above the superheater outlet PSVs since the frictional pressure drop through the superheater generally exceeds this.

I2I

 

[DSB123]

insult2injury,
Do you mean "for relief valves installed on the Section VIII vessel" ?

 

[CJKruger]

prq123,

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.

 

[robsalv]

prq123, are their any block valves between the boiler SV's and the user pressure vessels??

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[JAlton]

Refer to ASME Sec. VIII, Div. 1, Para. UG-125(g)(1):
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

 

[TheDruid]

Hi folks,

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