4 replies to this topic

[michael liu]

Dears,

I studied the relative previous post raised on Mar 2007 in this forum, still I have some queries on the TSV calculation and selection as follow,

1. In API STD 521 2007 SEC. 5.14.1, it state the temperature increase is also caused by FIRE. I don't know how to distinguish this case against the PSV used in FIRE CASE?

2. In the LNG tank farm, the fluid state in pipe is liquid, but the outlet of the TSV will be gas, do i need calculate the relief load? or just select a common TSV recommend in API STD?(It is not a long run pipline)

Regards.

[JoeWong]

michael liu,

5.14.1 is dedicated to hydraulic expansion. Hydraulic expansion can be caused by solar, ambient heating, fire, etc. General Fire case can be hydraulic expansion, liquid boiling and gas expansion...

I suppose your line contain LNG. Ambient heating may cause hydraulic expansion follow by liquid discharge. However, once the vapor space present, liquid boiling will occur. So... it is hard to say a common PSV (DN15x20 type) recommended in API is sufficient...If you have not done this type of calculation before, why not take this opportunity to do it once ??

[michael liu]

JoeWong,

Thank you for your reply.

I think I should do this calculation based on the scenario of safety analysis, but if there is a liquid boil or pool fire(due to the leak of another pipe) scenario, I guess it would be treated as PSV but not thermal relief valve. Am I correct?
looking forward to your reply.

Regards.

[CMA010]

API STD 5.14.1 also states that fire is usually not considered for piping.

Personally, I have never seen piping protected against pool fires and I wonder what it would mean for a petro/chemical plant if you would have to protect every single piece of piping that can be blocked-in against fire.

[JoeWong]

Michael Liu,

Not all the piping contains liquid will be protected with PSV from thermal expansion. A pressure-relief device might not be required to protect piping from thermal expansion if :

• the piping is always contains a pocket of non-condensing vapour such that it can never become liquid-full (even it is heated and compressed); OR
• the piping is in continuous use (i.e., not batch or semi-continuous use) and drained after being blocked-in using well supervised procedures or permits; OR
• the fluid temperature is greater than the maximum temperature expected from solar heating (the pipe temperature direct under solar heating can reached approximately 60 °C to 70 °C, it can be as high as 85 °C for certain area i.e. Middle east) and there are no other heat sources such as heat tracing; OR
• the estimated pressure rise from thermal expansion is within the design limits of the equipment or piping.