Propane Blowdown under Fire conditions

[BESAFE]

We have a Blowdown system from storage drums in Propane service in case of fire contingency to
alleviate the concerns of BLEVE (boiling liquid-expanding vapor explosion).
What will happen, if I were to blowdown from the Propane Storage Drums? We have 3 Propane drums close together and one is at 180 psig/100 DegF and the other two are at 300 psig/100 DegF. How will the autorefrigeration effect will come into play under fire conditions? Will it be mixed
vapor/liquid blowdown and at what temperature?

 

[BESAFE]

Continued from earlier post:

If the safety valves on the Propane Storage drums are sized properly for the relief under Fire contingency, is it necessary to have the propane blowdown system? What is the current practice in newly designed Lubes Deasphalting/Propane Dewaxing Units?

 

[chicopee]

I have never heard of propane storage drums eventho I have dealt with bulk storage of propane for a few years.
Under what section of NFPA 58 are you following? The refrigerated container section does not seem to mention blowdown of vessels but there two paragraphs dealing with pressure relieving capacity in the events of loss of refrigeration and of fire exposure. My NFPA 58 edition is 2001.
One question, is the propane at 180psig/100 F refrigerated and compressed to 300psig/100F?

 

[BESAFE]

Hi chicopee,

These are horizontal storage drums with design pressure of 250 and 350 psig resp. Propane is stored in compressed form with both liquid/vapor phases. One drum stores propane at 180psig/100F and the other two store at 300psig/100F.

 

[djack77494]

Given a fluid (propane) and a storage temperature (100F), the vessel's pressure is fixed. If the same pressurized liquid is stored in all three vessels and if the temperatures are the same, then the pressures WILL BE the same. You only have one degree of freedom for a single component saturated fluid. Perhaps, however, you are referring to the design pressures (or the MAWP's) of the two vessels. They could very well be different.

Assuming the PSV's to be located at the tops of the vessels, they will pass only vapor. It doesn't matter how much of the vessels contains liquid, because there will always be vapor spaces at the tops.

If a fire occurred under a vessel (often referred to as a propane "bullet", but don't call it a "tank"), it would heat to the temperature where the saturation pressure equals the PSV setpoint. The PSV would then pop. If an equilibrium condition developed wherein the vapor generated by heat input matched the vapor passing through the PSV, the temperaure would continue to match the fluid's saturation pressure. Auto-refrigeration effects would be overcome by the heat gained due to fire.

I hope that clarifies your situation. If not, write back and I'd be happy to supply additional details.
Doug

 

[MortenA]

Actually the load for the valve equals heat input divided by the heat of evaporation - this means that the temperature is constant at the propane bupple T at the SP+21% overpressure.

If the liquid is a mixture then the temperatire will increase as the lighter fractions boil off.

Best regards

Morten