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Set Pressure Of Thermal Relief Valves

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#1 J_Leo


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Posted 13 February 2018 - 12:13 PM



For other types of PSVs, the set pressure is generally the design pressure of the equipment protected. I have been working on two projects with thermal relief valves with set pressure 25 to 50 psig lower than the cold side design pressure of heat exchangers. What is the basis for this practice?




Edited by J_Leo, 14 February 2018 - 12:51 PM.

#2 Technical Bard

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Posted 13 February 2018 - 08:09 PM

I have not seen that done.

#3 christopherchoa


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Posted 15 February 2018 - 01:34 AM

It may be done when PSV is located high. 



But this is really uncommon because PSV are usually at the same elevation as the protected equipment.

#4 latexman


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Posted 15 February 2018 - 07:45 AM

More data please.  What is the normal operating pressure?  What is the maximum anticipated operating pressure?  What is the MAWP?  Maybe it is reasonable without going up to the MAWP.

#5 J_Leo


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Posted 25 February 2018 - 09:29 PM

Hi Latexman,


Sorry I was away for a while. Those thermal relief valves are all on the cooling water side of the exchangers. The cooling water header design pressure is about 120 psig. The operating is around 50 psig on the supply header. The cooling water side design pressure for the heat exchangers are all greater than 120 psig, I guess to meet the 10/13 rule.




#6 jorgerrera


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Posted 09 June 2018 - 11:38 AM

thermal relief valves also goes in lines of tanks

#7 ChemEng01


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Posted 30 August 2018 - 02:27 AM

If you have a cascading relief and the valve is conventional it may be that the CDTP (cold diff test pressure) is being shown instead of the set pressure on the P&IDs. I've seen this mistake in the past. 


Does the thermal PSV go to atmosphere or into another line with another thermal PSV? 


Example: if your PSV relieved into a line with another PSV set t 50psig,then the CDTP of the PSV should be set 50psig less than the required set pressure, but P&ID should show the set pressure i.e. CDTP + the 50psig. 

#8 AndyChemEng


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Posted 31 August 2018 - 09:56 AM

I've seen Pressure Safety Valves set at lower pressures than the design pressure with the basis being that it is still well above the normally attained operating pressure and relief of two phase materials into the discharge systems would not result in static build-up pressure greater than the vessel design pressure.


I've not seen it for thermal relief valves before but the same principle can be applied. As ChemEng01 has mentioned, does the valve go to atmosphere or into a line that could generate static head upon relief?

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