Thermal relief on LPG lines

[GarethChem]

Hi everyone

I read the following in a FEED:
"LPG - any section more than 500L of LPG that can be blocked in shall be provided with a TRV."

Do you have any idea why there is a limit to the smallest volume that requires a TRV? I'd imagine a rupture in the line or flange or a failing gasket is still possible even with small volumes. I would have said that ANY volume that can be blocked in needs a TRV.

Thanks!
Gareth

 

[e43u8]

Gareth,

Yes, for blocked in LPG if ambient temperature conditions and/or sun radiation may lead to a prevailing pressure exceeding piping design pressure, no limitation can be applied to the blocked in volume...

 

[LittleInch]

I can only think someone is trying to stop hundreds of trvs appearing, but they are incorrect as you point out.

My motto: Learn something new every day

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

 

[BigInch]

Maybe they're trying to avoid excessive numbers of block valves. Tha'd work for me!

you must get smarter than the software you're using.

 

[GarethChem]

I agree, an excessive amount of TRVs is the only thing I can think of. Thanks everyone, glad to see I'm not the only one that thought that it was necessary for small volumes too

BigInch - you said it'd work for you, do you mean that you agree with the FEED (i.e. not required on smaller volume lines)?

 

[SNORGY]

I would have thought that part of the rationalization for spacing would include:

* available surface area for heat input
* amount of voume that could be expected to spill from each PSV (where do the PSV outlets go?)

I have not vetted this article for correctness, but it may be of some benefit here:

http://webwormcpt.blogspot.ca/2008/07/thermal-relief-of-non-flashing-liquid.html

 

[BigInch]

If anything I'd imagine that the feed had too many valves.

you must get smarter than the software you're using.

 

[GarethChem]

Thanks for the article Snorgy.

BigInch - Why's that? A 15m (50ft) length of 200NB pipe (8") is just less than 0.5m[sup]3[/sup]. To me, if a line that size is liquid full, it'd be a serious problem, especially if it is one of the lines just after the outlet nozzle of the LPG sphere... Or am I missing something?

Snorgy - LPG on our site is vented to atmosphere

Thanks again for the help guys!

 

[LittleInch]

Vented or flared?

My motto: Learn something new every day

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

 

[GarethChem]

Vented....

 

[chicopee]

NFPA 58 is the code that you should use. The proper term for such valve is hydrostatic valve and is required in liquid lines that can be isolated on both ends. You can review the NFPA codes, free of charge, at this link

http://www.nfpa.org/codes-and-standards/free-access.

To support the NFPA, you should buy a copy of the code that I mentioned.