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Psv Set Pressure & Inlet Pressure Loss

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#1 SawsanAli311


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Posted 18 January 2019 - 04:55 AM

Dear All, 


I am thinking about whether in case someone finds the inlet pressure loss of an existing PSV slightly exceeded beyond the conventional 3 % criteria as well in comparison to the existing PSV blowdown, would reducing the PSV set pressure compensate for the lower inlet pressure at the PSV inlet and accordingly acts earlier to cover up for this case. I presume that the operating pressure profile envelope of the system would have to accordingly shrink. Also the vessel would have to be allowed to experience higher overpressure. 


I am doubting that reducing the set pressure would actually help, however, I am also looking at the case of compensating for the CDTP of conventional PSVs for the constant superimposed backpressure and reducing the PSV set point accordingly. 



#2 latexman


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Posted 18 January 2019 - 08:15 AM

The opposite.  Raising set pressure, if possible, will lower inlet dP as a % of set pressure.  Assuming gas/vapor, density increases at higher pressure and dP decreases.  Also the denominator in % of set pressure gets larger.


And adjusting set pressure doesn't affect dP as a % of set pressure very strongly.  It is quite insensitive.  Other variables have a stronger effect.

Edited by latexman, 18 January 2019 - 08:35 AM.

#3 SawsanAli311


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Posted 18 January 2019 - 11:33 AM

Thanks a lot Latexman... 


I have read in other forums that we can say that the pressure drop would decrease upon increasing the set pressure if and only if the relieving rate would stay the same in Kg/hr and accordingly velocity decrease would govern reducing the pressure drop more than the effect of increase in density on the pressure drop (due to the increase in set pressure). Kindly please correct me...


As another different scenario, I was actually thinking that if I have an existing PSV for example with a fixed area and suddenly PSV at the inlet decreases due to improper inlet line size, the valve lift capacity will decrease changing in this way that maximum amount which can be relieved (less mass rate) through the valve, in this way with the reduction in gas density due to more inlet pressure loss coupled with the less relieving rate (as per API 520 PI orifice sizing equation) would lead to more overpressure inside the vessel.. so in order to restore the relieving capacity through the valve it may be needed to increase the set pressure of the PSV such that the valve lift would be increased..


In simple words,  the API 520 critical flow equation for vapor PSV area calculation accounts for the P1 at the inlet of the PSV  as the upstream relieving pressure (overpressure+ PSV set pressure). This equation doesnt account for the effects of the PSV inlet pressure losses and assumes that they are negligible. Hence, in order to account for the impact on the PSV relieving capacity if in reality the inlet pressure loss is not negligible would P1 in the equation have to be considered other than the upstream relieving pressure?  

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