Relief capacity partially underground tank

[evdbroek]

I have a tankfarm with several tanks partially underground. Only the top klopper head with the manhole an nozzles is above ground. Is fire case a credible scenario and how to calculate the relief rate?

In my opinion fire case is a credible scenario an I thought to calculate the effective discharge area according to "4.4.13.2.4.3 Heat Absorption Equations for Vessels Containing Only Gases, Vapors, or Supercritical Fluids" with equation 8, 9 and 10 from API RP521:2014. This applies to unwetted area, but for vessels containing only gas.

All the wetted parts of the vessel are underground, only the head of the vessel is above ground. The design pressure of the vessel is 500 mbarg.

Is this the correct method to calculated the required discharge area/relief capacity of the pressure relief valve?

 

[Latexman]

Is it credible to have a pool of flammable liquid sitting on top of the fill? I.e., does a flammable liquid transfer line run overhead? If so, you may be right. I would not expect mush relief needed for that though. The feeds venting liquid or vapor may be larger.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[evdbroek]

The liquid transfer line are running over the tanks. A pool can be formed adjacent to the tank head. A dozen tanks are connected with a vent header (DN100) with pressure control valve to a cryogenic unit.
I would not expect much relief either, but can I use the equations used for thermal expansion for gas filled tanks?

The tanks contain aceton, ethanol, methanol or other liqht solvents.

 

[chicopee]

Get a copy of NFPA 30 on flammable and combustible liquids code as there are several sections dealing with underground storage tanks. From the description of the OP, it seems that the tanks are fully buried with the exception of the Klopper heads.

 

[don1980]

evdbroek - There are two important considerations. One is what is needed to meet regulatory or legal requirements, and the other is what is needed to provide real risk mitigation. Often, those two are essentislly the same. That is, designing for regulatory compliance results in genuine risk mitigation. But in the case of fire exposure those two considerations are sometimes not the same. That is, designing for regulatory compliance results in a design that provides no meaningful protection from fire exposure. This is true when the fire is contacting little or no wetted surface area. In such cases, the temperature of the vessel wall (contacted by fire) will steadily rise, and the rate of temperature rise is virtually unaffected by the size of the PSV. When the temperature reaches the yield point, the vessel will fail. PSVs are only able to provide some protection from fire when there's a liquid in the vessel and that liquid is boiling at a temperature below the metal yield temperature.

In the case you describe, there is no liquid in contact with the vessel wall which is exposed to fire. Regardless of how you size this PSV, it won't provide any meaningful protection from fire. Provide a PSV (any size) for regulatory compliance, and then consider other protective measures which will at least partially protect from fire (water spray, de-pressurization, insulation).