I have a question regarding the use of API 521 3.15:
API 521 3.15 allows to eliminate the fire case for vessel located on structure above 7,6 m. The vessel is a propane receiver in a refrigeration system and has no other source for overpressure other than fire or thermal relief. The vessel can be isolated by automated valves from the PSVs that protects the system upstream from the overpressure during the shutdown of the system. In this case the vessel would not be protected against overpressure but there is also none of overpressure sources except pool fire and thermal expansion. In the shutdown case the vessel and adjacent piping that is isolated is only overpressure protected by system design (OPPSD)
Is it allowed or legal by rules of API 521 to eliminate the fire case for vessel higher than 7,6m, when the application of this 7.6m rule eliminates the only PSV(s) for the propane vessel (LPG!) and switches to OPPSD. In case of a fire there would no overpressure protection at all since the system pressure due to heat input even above 7,6 m (wetted area is currently 9m) would exceed the design and MAWP of the piping and equipment.
I believe the API 521 paragraph that eliminates the fire case for overpressure protection cannot be applied when it removes the only PSV(S) for the system, only when other relief case are exist. Since the vessel can be isolated the PSV covers the fire case based on the heat input for the elevation. Appreciate your thoughts in this matter.