10 times operating no relief?

[StoneCold]

I remember hearing that if your design pressure is ten times your operating pressure you do not need relief on that vessel. Is that true. I have some small portable filters that are normally connected to other equipment that have relief but sometimes they are sitting valved in with liquid in them. I am just concerned about the fire case.
Anyone have a reference on the 10x statement? Or am I all wet?

Regards
StoneCold

 

[dcasto]

Not correct. What I believe the ten times operating most likely refers to is the expanding gas in a vessel. To make a gas expand 10x, it takes a 10x change in temperature, so if you started with a 520 K temp, then to get 10x pressure the temperature would be 5200K. At which poit, you just say the vessel will fail when it gets to 1500K and go on.

I guess a similar thing might happen with a boiling liquid fire case. For example a propane tank that operates at 200 psia and 565 K and the vessel is rated at 1000 psia. As heat is added, the propane boils and the pressure goes up to 616 psia and 205 F. The tank has not started to see a significant loss in strenght and it is still below the set pressure of 1000 psia and it is now a supercriticle fluid. The pressure must increase to 1000 or 1.5 times the pressure. Then at 1000 psia, the temperature will be 1.5 * (205 +460)-460 = 537 F, somewhere between the time the pressure is increasing from 616 to 1000, the temperature goes from 205 to 537 F, the metal will weaken enough that it may fail at a lower pressure than the relief valve would open. (all you hardcore guys, don't jump on the above for not correcting for compressibility, especially above the criticle point, this only an simple example).

This is an extreme cases and one could argue the need for a psv with this system. Aw, but what about blocked flow, tube failures, control valve failure cases?

 

[Latexman]

That sounds somewhat like the language in NFPA's standard on containing an internal deflagration. I don't recall the multiple they quote, but I think it's higher than 10X. But that standard would only apply to an internal deflagration scenario and would not give one justification to omit a relief for all scenarios.

Good luck,
Latexman

 

[Ashereng]

I haven't heard of 10x.

The only time you do not need a PSV is when the vessel pressure rating is more than the pressure can ever get to in the vessel (absolutely, the pressure will not rise to meet the vessel's pressure rating). In this scenario, you do not need a PSV.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[StoneCold]

Ok, well it turns out that the design pressure is acutally only about 2x operating anyway. I wish I could remember the situation where this was applied, of course it could have been wrong even then.

Thanks

StoneCold

 

[EGT01]

Maybe this isn't what you are remembering but along the lines of what Latexman makes reference, API 521 has a section "Design of disposal system components" that discusses "Design Details" for seal drums and knockout drums...

"Most knockout drums and seal drums are operating at relatively low pressures. To ensure sound construction, a minimum design gauge pressure of 345 kPa (50 psi) is suggested for knockout drums in subsonic flare or other low-pressure applications. A vessel with a design gauge pressure of 345 kPa (50 psi) should not rupture if a deflagration occurs. Stoichiometric hydrocarbon-air mixtures can produce peak explosion pressures on the order of seven to eight times the absolute operating pressure. Most subsonic-flare seal drums operate in the range of gauge pressure from 0 kPa to 34 kPa (0 psi to 5 psi)."