Piping for PSV on thermal relief

[carol2005]

I have a question addition to the discussions on thread1203-147200. I agree a norminal D size PSV is more than adequate to handle liquid thermal expansion.
Once the PSV spec is determined, can the piping pressure loss calculation be exempted due to the small quantity of the relief load? Does the inlet/outlet piping pressure loss have to be compliant with the principles of <3% and <10% of set pressure ?
If piping calculation is needed, should the required relief load or PSV relief capacity be used for delta P calculation?

 

[SeanB]

You are right - typically with the small relieving rates you do not have to worry about inlet / outlet losses. An exception would be on long shipping lines where you may have multiple thermal PSV's cascading back to a source tank. In such a situation the built up pressure losses are still not a concern, but the superimposed back pressure is the concern. In this case you would have to ensure that your set pressures were "set" properly. So I guess my point is, the built up losses are usually negligible, but don't forget to consider any superimposed back pressure.

 

[Ashereng]

And also, a normal C size PSV (NPT conn) are usually more than adequate.

Even on long shipping lines, it is probably rare that the flow back to a source tank will be an issue - most relief lines do not go that far (they try to find the nearest low pressure relief) since it is cost that really doesn't do much for the process (strictly protection).

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Albert Einstein
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[dcasto]

Remember there are options other than PSV for thermal expansion. 3/8" Check valves around block valves can be used.