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Too Much Allowable Overpressure!

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#26 Pan Nainggolan

Pan Nainggolan

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Posted 10 January 2017 - 03:55 AM

//quoted: PSV is designed for protecting overpressure, of course if the PSV is failed, the system won't be protected......//


In this case off course the system very well protected. The set pressure is 3.5 barg only much lower than the system design pressure (20 barg). Hence, in the occurrence the psv failed at indicated set pressure the system very well safe. Then the system is still  safe with or without psv.


//quoted: But that doesn't mean your PSV is not needed. In deed it's code mandated for pressure equipment per ASME VIII//


That's quite funny, so the design engineer have to install something because the local or international regulation and code said so without trying to figure it out why is it mandatory?


I believe before start to do advance things like to determine the govern scenario in a psv whatever it is, block outlet, control valve failure, check valve failure. Every design engineer must understand the fundamental function of psv.


I didn't say what you told before is wrong. Yes, if we have psv's set pressure higher than the MAWP of protected system then the psv useless and in the case describe by the OP, the psv useless also (on basis the function of psv).


I don't know exactly what it is wrong here but there are three possibilities. First the OP doesn't bring enough data to describe the system, the second very bad documentation, and the third the engineer who was designed the psv didn't know what he/she did.


And absolutely the problem is not about "too much allowable overpressure".





#27 mirandomka


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Posted 10 January 2017 - 03:31 PM

Hi PN,

The person who asked the question did not say there is no overpressure scenario. So presumably, the PSV has been designed for applicable overpressure scenarios,ie, there are some scenarios drive the system pressure over 20 bar(g).

Now, the PSV for protecting this 20bar(g) system is only set at 3.5bar(g).

Yes, this is not normal. However, like I said earlier, PSV is still protecting the system from overpressure 20bar(g) against the applicable scenarios because it will be opened much earlier and provide sufficient relief capacity at relieving pressure which is 20barg +allowed accumulation.

Therefore, the PSV is not useless.

The reason behind the low set pressure may be because it's adjusted for static or superimposed back pressure.
Or maybe it's protecting components which DP is 3.5 bar(g).

Edited by mirandomka, 11 January 2017 - 03:54 PM.

#28 sgkim


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Posted 11 January 2017 - 04:02 AM

Hi Shahidu, 


Let's go back to square one - you found a 3.5-barg-liquid-relief-valve installed in the pipeline of which the design pressure is 20 barg.  From that information you received, I guess that a certain equipment, full of liquid, is connected to the pipeline and is to be protected from thermal expansion. We do not know the detail story of this, but I think the piping must be based on the flange rating of class 300 or 20 barg for which you interpret as 'design pressure', though.  


Usually, relief valves for thermal expansions is not complicated in upset scenarios if ever.  And the design pressure of the piping is not critically related to the design pressure of the equipment connected.  So it does not seem to be meaningful to establish 'grudgingly' a link between <the set pressure of the thermal relief valve> with <the design pressure of the piping>.  


So far the thermal relief valve is concerned, it has only to protect anything connected to itself up to 3.5 barg of MAWP.  Therefore, the design pressure of the piping needs no less than the set pressure 3.5 barg of the relief valve.   And the maximum overpressure of the relief valve would be allowed to typically 25% of the thermal relief set pressure.  But this issue is related to the size of the valve and may not be critical due to the ambiguous scenario of the system.  


Ultimately, you'll have to check either the set pressure(SP) of the relief valve or the design pressure(DP) of the piping: DP  20 barg presently >>  SP 3.5 barg.  If the former SP is correct then DP would be too high, and if the latter DP is taken, then the relief valve would become useless.   


Stefano G. Kim

Edited by sgkim, 11 January 2017 - 04:18 AM.

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