Piping support during external fire scenerio

[Water_Guy]

Performing pipe stress analysis on relief valve discharge lines during an exterior fire scenario (vaporization), a lot of times results in higher discharge rates than other relieving scenarios (Failure of Automatic controls, Check valve failure, fully opened control valve by-pass, etc) a lot of times results in stresses far exceeding the B31.3 Code allowable stresses. Is it reasonable to base the restraint system to limit the resulting pipe stresses on the "Yield" or "Tensile" stresses?

My thinking is that during an exterior fire scenario, it may be practical, and reasonable, to make sure the piping system does not either fail or become permanently deformed. Rather than making sure the resulting stresses are below the allowables.

I have not found where the Code addresses this subject, and was curious what suggestions and advice the 'community' has to offer.

Many thanks for your comments.

 

[XL83NL]

No way you can go to yield or UTS. However have you checked the options of appendix V?

Huub

 

[RVAmeche]

I'm a little confused. You begin by talking about stress analysis but you're also talking about relief valve discharge rates.

By code the relief valve piping is required to handle the capacity of the relief valve installed at it's set pressure. Whoever has done the relief valve calculations should have considered an external fire scenario and the relief valve's capacity needs to be higher than the required relief for an external fire scenario.

What is the system you're looking at? I assume you're talking about making sure the pipe stresses are acceptable while the relief valve is going off due to external fire (but at it's rated capacity) and the pipe temperature is elevated due to said fire?

 

[Water_Guy]

The relief valve calculations have considered the case for an exterior fire around the vessel that it is protecting and the resulting discharge rate. We are now at the stage of evaluating the discharge piping under the resulting forces and the piping restraint requirements. The question was raised as to what to use as acceptable pipe stresses. On the surface, designing the restraints to make sure the resulting stresses are below the allowables, under the fire scenario seems kind of an over design. So that is why I was reaching out for some opinions.

In many cases there is auto-refrigeration as the discharged vapor exits the relief valve. So, I'm not sure that Appendix V applies, but thank you 'XL83NL" for the reference.

Again, Many thanks!

 

[TGS4]

It seems that you are discussed what the nuclear industry refers to as a "Beyond Design Basis Event". In general, there are no rules that cover such events other than what other posters have described. My preference would that the design scenarios would encompass these events.

 

[LittleInch]

I'm not quite getting this either.

You have an external fire case which results in a fire case relieving rate to stop your vessel exploding. Your relief valves are rated for this duty. Therefore you need to ensure that your relief pipework is also rated for this duty.

Sounds pretty straightforward to me.
Like another question here, I don't get the "over design" bit for something that is a valid design case. It's part of design to ensure that your system can withstand all design cases. If they are short term and transitory, then there is some allowance usually in the design code for exceeding "normal" values of pressure or stress, but not to the point where it yields or breaks. B31.3 allows for a lot of stuff "with the approval of the owner". Best to run this idea past him first to avoid any wasted design work.

31.3 allows up to 130% of pressure and maybe the same in stress for short term events. I don't quite get the "stresses far exceeding the B31.3 Code allowable" - err how far does it exceed it?

IMHO there is little point in doing the calculations, generating an external fire case, sizing your relief valves accordingly and then letting your relief valve piping fail and feed the fire or cause an additional back pressure and cause your vessel to fail.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.