Fire relief heat input equation in API 521

[ChemEngSquirrel]

I have a vessel with lots of large bore liquid pipework within a fire area.

I want to confirm the correct option for gire heat input as per the API 521 equation. Is it

Option 1: calculate total wetted surface area then do A^0.82

Or

Option 2: calculate A^0.82 for all pipes and vessels individual and sum them.

Option 2 results in significantly larger fire heat inputs but im worried it is overly conservative or even wrong.

Thanks.

 

[EmmanuelTop]

If you are designing a single PSV for fire case that should enable relief of vapor generated within a single fire zone, Option #1 should be used. The reasoning behind is that the total surface area quoted in your post is exposed to fire at the same time, and the generated vapor will be relieved through single PSV device.

Option #2 can be considered, IMHO, only if these piping/vessel areas belong to different fire zones or if there are multiple relief valves for fire relief, and the fire scenario evaluation has confirmed a sequential nature of depressurization.

Dejan IVANOVIC
Process Engineer, MSChE