Relief temperature and Heat of vaporization for fire relief valve

[Shaheryar]

I am evaluating few cases for a relief valve on vessel water (produced water) and one of them is the fire case.

For evaluating latent heat I am assuming that the composition is water and therefore I can use the steam table to find the relief temperature and heat of vaporization.

My problem is something that I have never questioned before and have always done it as a procedure learnt from seniors. We always evaluate the relief temperature and heat of vaporization not at the set pressure of relief valve (set pressure = design pressure of vessel), rather at the reliving pressure which is 21% more (in fire case). Does this mean that we let the vessel be pressurized from design pressure all the way to reliving pressure? If so why is this always acceptable?

I need help to justify this process that is used commonly but I am having a hard time digesting?

 

[Latexman]

[Δ]H_vap decreases as temperature and vapor pressure increases. Not only is using [Δ]H_vap at 1.21 x MAWP correct, but it is conservative at set pressure up to 1.21 x MAWP.

Good luck,
Latexman

 

[MortenA]

I think that i see what your "real" question is:

Why is it that in a fire case we allow a higher pressure that the normally accepted 10% over pressure?

This was also a mystery to me at first - especially since im a chemical engineer and not as many older process engineer - a mechanical engineer.

The reason that i believe is true is that in the fire case the "first line of defence" is the deluge. The deluge is supposed to keep the temperature low - so that overpressuring does NOT occur at all.

Should this fail the vessel will now be engulfed in flames. The assumption is now that he vessel most likely is lost anyway and the main purpose of the PRV is now to prevent escalation in case of a rupture of the vessel. The 21% can therefore be allowed since the PSV does not have a funtion to actually protect the equipment against damage that will impact is future use.

Best regards

Morten

 

[monaco8774]

The relief valve opens at 121% of design pressure then remains open, if you have the design pressure as the opening pressure then it will open at design pressure and close below design giving chattering. The relief sizing is based on a flow of (incompressible/compressible or multiphase) fluid at a certain pressure, the transient opening only lasts for a fraction of a second thereafter the relief capacity is full at the design pressure.

Remember then actually you have no inflow and a heat source underneath you the actual flow of vapours will vary with time, but will always need some obverpressure to keep the PSV open, otherwise you would have it lifting and chattering before you reach the design pressure.

What I have had to come to terms with over some years now is what often is described as an exact science is really a set of guidelines that lead to an acceptable design. Clearly for example if you set the design pressure very much higher than the operating and you have hydrocarbons in the vessel, the vapour temperature at the relieving pressure will actually melt the vessel before you get any relief.

 

[Shaheryar]

MortenA, Thank you but my question was more reacted to why work at 121% for properties versus 100% of design.

Latexman, appreciate your help in presenting your analogy and it is what I had rationalized in my mind. Need to see if someone else it tinking along the same line. It seems to be conservative and therefore the safer thing to do. Also it seems like the only plausible basis for calculating ?Hvap at 121% versus 100%.

monaco8774, I guess from your response that you have been thorough this self challenging process before.

Thank you all.

 

[pleckner]

Shaheryar:

I'm going to try to answer the question as you originally posted it.

Why use the overpressure in the first place rather than the set pressure? Simple, because code allows us to. This criteria was developed by the ASME committee many, many years ago. And you got my curiosity as well as to how or why they came up with 10% overpressure for non-fire scenarios, 21% overpressure for fire and 16% overpressure when utilizing multiple PSVs for the same relief? I've never seen an explanation from ASME as to how these were developed. Perhaps someone on the committe will write in and shed some light. But remember, we test equipment to 150% of design so these overpressure allowances still provide ample protection aginst catastrohic mechanical failure. Also, the higher the relieving pressure for a given scenario, the smaller the relief valve and this translates into lower cost. Perhaps the committee decied to give us an economic break while still providing the necessary protection.