Undersized relief valves
Undersized relief valves
(OP)
Here's an interesting question that I've been thinking about:
What would happen to the pressure in a vessel which is protected by an undersized PSV? Obviously, it would go up but to what value?
For compressible choked flow, as pressure increases mass flow would increase also so, for some cases, it would seem logical that the vessel pressure would increase to a pressure above 10%/21% overpressure. The PSV would then either pass the required flow or a flange/the vessel would fail. Do you agree?
How about in a fire case? As pressure increases, the heat of vaporization decreases and hence the relief load increases. Would this situation be a positive feedback loop where pressure goes up -> relief load goes up -> pressure goes up etc. ?
What would happen to the pressure in a vessel which is protected by an undersized PSV? Obviously, it would go up but to what value?
For compressible choked flow, as pressure increases mass flow would increase also so, for some cases, it would seem logical that the vessel pressure would increase to a pressure above 10%/21% overpressure. The PSV would then either pass the required flow or a flange/the vessel would fail. Do you agree?
How about in a fire case? As pressure increases, the heat of vaporization decreases and hence the relief load increases. Would this situation be a positive feedback loop where pressure goes up -> relief load goes up -> pressure goes up etc. ?





RE: Undersized relief valves
RE: Undersized relief valves
Good luck,
Latexman
RE: Undersized relief valves
RE: Undersized relief valves
Any tips for calculating what pressure the vessel would rise to? The case I'm struggling with is a fire case. This is tough since the relief load increases with falling heat of vaporization as the vessel heats up.
RE: Undersized relief valves
You can purchase the code from CGA for about $109.
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RE: Undersized relief valves
http://www
RE: Undersized relief valves
Good luck,
Latexman
RE: Undersized relief valves
For a non fire case, failure of the vessel is unlikely if the vessel is in as original condition and the system pressure stays within the original hydrotest envelope (most likely up to 150%DP). Failure is still not very likely even if the system pressure goes outside this envelope, but now we're in uncharted territory - any previously blunted undetected defects buried in a weld seam may become active.
Depending on the exact scenario, there is a good chance flange joints will leak before the vessel fails.
If your vessel is in poor shape, with pits, some defects and/or areas of corrosion losses, then a significant overpressure could cause failure.
This is all supposition though.
It's a different story in a fire case. A vessel will fail if the metal temperature gets hot enough for the metal to flow - irrespective of DP or system pressure.
Interesting stuff.
RE: Undersized relief valves
Most states recognize the ASME Boiler and pressure code as law. Since you apparently know, or even suspect , that the relief system is undersized as defined by the code, you are prohibited from operating the unit. If you are caught operating it by the boiler inspector, your company gets a BIIIG fine and you get to apply for unemployment. If, God forbid, there is an accident related to this, you go to jail. The phrase "Criminally negligent" comes to mind.