Pressure Safety Valves

[lopen]

Could anybody tell me if there is a temperature rise during opening of a pressure safety valve?

Situation
Pressure safety valve is set at 895 bar(g) the outlet is atmosheric there is at max. a back pressure of 0.2 bar(g).
The liquid has a cP value of 25cP and density of 850kg/m3.
The flow is 954 litre/hour.

 

[MortenA]

If this is a normal spring loaded PSV then therse the obvious: The pressure must rise for the valve to lift. Its not the lifting that causes the pressure increse though. In order for the valve to reach max. capacity then the pressure must rise a certain % e.g. 10% (or 21% or whatever). Again the valve does not cause the increase but the valve will not increase flow if the pressure does not rise.

If you are worried about a surge then this could happen when the valve re-seats. Reseating is equivalent to the flow stopping and this is how surge is generated.

Best regards

Morten

 

[lopen]

I AM TALKING ABOUT A TEMPERATURE RISE WHEN THE VALVE IS FULLY OPEN.

 

[quark]

That is a bad manners to type in all capital letters and it seems you did it deliberately. You can use bold letters instead when you want to point out something specifically.

Coming to your question, it is very basic and you should know the pressure temperature relationship yourself.

Good luck,

 

[lopen]

Mortena,

I am sorry I did not mean to be rude I did not notice that I was writing in capitals.

Quark,

Beside that this is basic for you, the only thing I come up with is that there would be a temperature rise due to wriction inside the PSV.

 

[quark]

OK, before that you just check whether there can be a chance of flashing (because you didn't specify the liquid and its boiling point). If there is flashing due to the pressure reduction then you have cooling of the fluid.

Friction can't cause significant change in the temperature unless the liquid is circulating.

 

[MortenA]

I was considering upstream - but yes: The work performed when letting down pressure over a valve will cause liquid temperature to go up. The same is also true for a gas - but normally the cooling from expansion is higher than the heating from the work. However, reverse "JT effect" can be observed at very high pressure (+400 barg). The JT effect is acutually isenthaplic expansion - compared to isentropic expansion. Thats also why gas out of an expander is colder than gas out of a valve - the work is "removed# from the system when using an expander.

Best regards

Morten