Fire scenario: heat input for equipment higher than 25 ft from the base of a pool fire?

[QloB]

Dear all,
I would appreciate your insight on this case.
A 20 m3 pressure vessel filled with Methanol is located on the third floor (grated) of a process unit handling flammable liquids.
The elevation is 16 m, hence the fire relief load is nil according to API 521.
But in case of pool fire on ground floor, this vessel is still exposed to thermal radiation.
Even though the heat load is much reduced compared to a true fire case, this eventually can lead to overpressure (especially since the vessel will be isolated by ESD valves).
My questions are:
- Should I include this case among the PSV sizing scenarios?
- If so, how can I estimate the heat load?

 

[RVAmeche]

I'm not sure what the calculations entail but I know others have done them. If you don't want to call it an external fire scenario for whatever reason, I'd label it as abnormal heat input or something to that effect.

You may want to install insulation on the vessel regardless to reduce the risk of a fire on the ground floor. Since it's outside the fire zone, actual fire-rated insulation may not be required.

 

[MortenA]

It is true that heating will lead to increase in pressure in the vessel, but also to increase in heat loss to surroundings.

I cant remember the exact reason for the elevation parameter in 521 but i will guess its either: Too long time to heat to relieving pressure (fire fighting should be established by then) or as i was trying to say, no the pressure wont increase that much because the vessel cool too.

If you wish to include the scenaio either: Include it as a std. calculation (and if its not the design case dont bother) OR try to establish a correction factor 0-1.

Best regards, Morten

 

[QloB]

Thank you RVAmeche and MortenA for sharing your thoughts.

I will try to build a case based on the time it takes to heat up the vessel (especially if insulation is added). Anyhow, in a fire scenario, a PSV can only protect a vessel for a limited time (once the liquid is totally evaporated, the metal temperature rises dramatically and there is high probability of mechanical failure of the vessel).

 

[georgeverghese]

No drip pan below this methanol vessel ? - have that checked out with the plant maintenance folks and have them sign off so they dont change their minds at the 11th hour.