psv fire 1

[gelsi]

hi,
i have read that there are country in which local authority or the law obliges the designer to consider fire like contingency even if there are procedures written that make the event very low probable or impossible. Who knows? and Italy is among these?
tx

 

[don1980]

gelsi - I don't think that's true. The only legal requirement which mandates a fire-sized relief design is in the US, and it (OSHA1910.106) only applies to a subset of storage tanks (those that contain a liquid with a flash point of < 200F).

Regarding fire exposure, typical practices in the US are different from those in Europe. The default practice in the US is to consider including fire exposure in the relief design whenever the vessel is at risk of exposure to a pool fire. That's not true in Europe. The default practice in Europe is to not consider fire exposure in the relief design.

I don't know the explanation why - it's just a difference in the risk management practices which evolved over the past century.

 

[gelsi]

Normally, i design for fire when there is a plan, below equipment, where a pool of flammable liquid can be generated. If the equipment is in elevation major of 8 m from pool, we do not design for fire, with the exception of aircoolers.
I think this practice is good, but now i must comment an engineering study of another engineering society and i don't know how to impose it.

I think it's need a risk evaluation for exclude fire design!

what do you think?

 

[LittleInch]

Gelsi,

Procedures may make something low probability, but they don't make it impossible. Your key questions should be - Can a fire develop in a location which would cause your equipment to be subject to thermal loads beyond the design max temp. If yes then you need to design for fire regardess of the likelehood, especially if a potential consequence is leak, rupture or over pressure. This should normally come out in a HAZOP or HAZID, but much better to design it in at the start rather than having to do mods to the design after a HAZOP.

I've never designed anything like that in Italy, but they have a decent safety culture there and it would certainly be something that needs to be considered and only not allowed for if it is physically impossible or doesn't have an impact on the equipment.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

Procedures alone do not rule out a particular scenario. If the only thing that has to go wrong is someone not following a procedure to create a credible scenario (fire case or any other scenario) then it is credible. If you have to fail a procedure at the same time an automated control fails (for example) then you can say it is not credible for two unrelated things to fail at the same time.

I always consider fire case, but if I can't come up with a pool of flammable liquid then it is not credible. If I can't come up with a scenario (in a vessel which contains liquid with a flashpoint >200 F) that results in the vessel becoming liquid full and blocked in coincident with a pool of flammable liquid then it is not credible. I rarely have a credible fire case in low pressure gas/water field systems--with the minimal liquid inventory boiling all the water doesn't lift the PSV (in most analysis) and once the liquid is gone the metal will flow before the ideal gas law can lift the PSV.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[Latexman]

Usually companies define what is credible and what is not credible for their Engineers' who size pressure relief devices. I have worked for 3 companies. All 3 had this definition/policy. Some defined this well, and some defined this not so well. There were differences. gelsi, you should find your company's policy on this and design accordingly.

Good luck,
Latexman

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[don1980]

David (zdas04) gives good advice. Sizing the relief device for fire exposure isn't an automatic decision. There are cases in which the PRV doesn't provide any meaningful protection. In such cases it's a waste of time and money to size for fire exposure. And worse, it misleads the owner into complacency when they ought to be considering other protective measures - those that are capable of providing some real protection.

All codes and standard intentionally leave it to the user to determine the sizing basis for reief devices. That's true in ASME jurisdictions and in PED jurisdictions such as Italy. As I mentioned above, engineers in Europe don't generally design for fire exposure the same way we do in the US. That doesn't mean they're wrong. It just means that they apply risk management practices differently.

It appears that the OP (gelsi) is being asked to comment on some relief designs which were done by others. Gelsi, my suggestion is that you point out the areas in which fire exposure is a risk, and recommend that the owner assess this risk against their corporate risk tolerance. Maybe this will result in some fire sized PRVs and maybe it won't.

 

[Latexman]

All good advice! I just re-read the OP because I missed the fact gelsi is reviewing someone else's design. It also sounds like there could be local politics involved in the situation too, so gelshi will have to be sensitive to that. Sometimes those battles can be won, and sometimes they cannot. If at the end of the day the local authority says the PSV SHALL be sized for fire, then . . . gelshi should review the fire case whether it is determined credible or not by reasonable people or company policy.

Good luck,
Latexman

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