4 replies to this topic

[c04016]

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.

 

 

[fallah]

Hi,

 

Neither the wetted nor the unwetted surface of the vessel higher than 7.6 m above the base of the pool fire is normally included in relief load determination due to a pool fire means if the elevation of the vessel bottom is higher than 7.6 m above the fire base there would be no relief load due to fire case. Of course, there is an exception for spheres in API 521.

 

Nevertheless, even though seems there is no need to PSV for fire case, to satisfy the code requirement a small PSV should be installed on mentioned vessel...

[c04016]

Fallah,

 

Thanks for your prompt response. In my understanding or interpretation of API 521 3.15 the fire case can ignored above 25ft only if you have another relief case like blocked outlet because the heat input above 25ft may significant less less than below and the vessel is covered. But it could also be misunderstood as no PSV is required and that is the dilemma of this particular paragraph of API 521 3.15. Your engineer gut feeling says a PSV is required especially in LPG service and that why add one. Would be nice if API could be a little bit clearer.

[fallah]

Hi,

 

In API or ASME the credibility of a fire case for a vessel has been left to the user's decision and mentioned code/standards just present how a pressure vessel can be protected after initial user's decision. Anyway, as i mentioned, you have to consider a PSV on the vessel for code compliance no matter the fire case is a credible scenario for such vessel or not...

[paulhorth]

c04016,

A pool fire is not the only kind of fire.

Your vessel could be exposed to a jet fire resulting from a pipe flange leak or instrument flange leak impinging on the vessel, even though it is more than 7.6 m above grade. There will be a lot of piping full of propane in the vicinity of the vessel. Jet fires can have a much higher heat flux than pool fires.

It is your responsibility to evaluate the risk of such a jet fire and protect against it.

 

Paul