Relief valve required between two closed valves?

[jgibbs22]

Hi Everyone:

Someone once told me that ASME B&PV Code had an exception stating that UV stamped relief valves were NOT required if you are protecting against thermal releif or liquid expansion between two closed valves. Is this true?? Does B31.3 say anythign like this?

Thanks

 

[racookpe1978]

Well, like all things, the physical "need" for a relief valve in an isolated pipe depends on the circumstances of the isolation. (Regulatory need for the PRV should go back to the fundamental design basis.)

A relief valve is needed going all the way back to the original steam engines in 1795: Continuous Additional Energy going in = more heat in the pipe = more steam (or water pressure = busted pipe, boiled off steam boiler = fire, flooding, feast (not!) and famine.

but note the difference: In an isolated pipe, the "extra" energy going in is going to come from what? Ambient energy into the pipe is going to be limited to solar heat on the pipe, room heat through the insulation from other nearby operating lines or burners, and not much else. If anything, your pipe is going to cool off, the liquid or gas or vapor inside cool down, possibly condense, and the new pressure actually become a vacuum until leakage builds up through the isolation valves. So a classic PRV can't (won't) protect your system against vacuum. And, if you have a vacuum relief valve, then you can't refill the system until that valve is isolated.

If leakage is the source of the pressure between the two valves, the only source of leakage is from the (otherwise isolated) operating system. Which is protected by the PRV's in the two operating loops against maximum operating pressure.

 

[jgibbs22]

Thank you for the response. Yes I understand your points, let me be more specific. I have a LN2 delivery system. obviusly if the valves are closed that section of pipe could heat, thus increasing the pressure inside. My question is not whether they should be a relieving dvice, but if the Code will allow you to use a NON-UV stamped relief valve for this area of pipe? For example, could you use a check valve with a spcific opening pressure and it be accepted by B31.3?

 

[racookpe1978]

I don't know that detail of the code, and will not distract you by guessing.

The extreme nature of your "liquid nitrogen heating to room temperature" problem (again) exceeds my experience in power plant systems, so (again) I will not guess. 8<)

 

[zeusfaber]

What would you choose to do, having looked at the application, if there were no code and you had to think it through from scratch? I'd try answering that first, then seeing if the result is code-compliant.

A.

 

[pennpiper]

It seems to me that it also depends on the commodity in the line.
Years ago I worked on a Chlorine Plant and we had to install relief valves everyplace a section of line could be "Blocked in". I think the requirement for this came from the Chlorine Institute Standards.

prognosis: Lead or Lag

 

[nmiller127]

There are many spring loaded check valves that are non-code compliant that could be used in this type of application for vacuum or pressure relief. As to whether or not it would be allowed for your particular application is beyond my knowledge unfortunately.