2 replies to this topic

[ChemEng01]

I have a thermal relief valve on an LPG line. The tailpipe is routed to the flare.

 

It seems very conservative to check outlet line sizing based on the rated flow of the thermal relief valve as the pressure will drop instantaneously when the RV opens.

 

Maybe considering built-up back pressure with required relief rate would be better to use. However this still seems unrealistic as with thermal relief the relieving fluid is probably not even going to reach the flare header before the pressure has dropped and RV closed.  

 

What's the normal approach for thermal relief outlet line sizing?

 

cheers

 

 

 

 

 

[fallah]

ChemEng01,

 

In most cases in which a TSV has been applied to relieve the load of a theremal expansion the relief capacity is very low, then the relief valves are normally selected as a normal minimum size of 3/4" *1" without relief rate or sizing calculations. In such cases the TSV outlet line size would be same as TSV outlet nozzle size or one size larger. 

 

Obviously where it might there would be a very high heat input rate to the blocked in liquid, such as long pipelines of large diameter in uninsulated aboveground situation and liquid filled vessels both are subject to solar radiation or any other heat source, a rquired relieving capacity can be calculated. If the outlet fluid from the TSV would be still liquid the required relief load and if would be two phase flow or vapor the rated one should be considered as a base for TSV outlet line sizing. 

Edited by fallah, 19 May 2014 - 05:50 AM.

[ChemEng01]

Thanks Fallah

 

I'll determine the conditions during relief and go from there. Pretty sure it will be liquid during relief due to high JT, so will probably use required relief rate for tailpipe check.

 

Thanks for your advice