Dear all,

I have query regarding set pressure & Spring load of PSV. As you all know, for balanced bellow type PSV, back pressure allowed is 30-50% of set pressure based on design philosophy of company.

Scenario:
Consider MAWP of the equipment is 20 kgf/cm2g and consider back pressure as 50 % of set pressure i.e. 10 kgf/cm2g. Set pressure is 20 Kgf/cm2g since it is balanced bellow we can neglect influence of back pressure on set pressure in closed condition. When pressure in vessel reaches 20 kgf/cm2g, PSV will pop up. My question is, once psv opens, the total resistance pressure will be 20 Kg/cm2g of spring + 10 kgf/cm2g of backpressure i.e. 30 kgf/cm2g. So this will make PSV to close. So, how can we say that for balanced bellow, back pressure is not taken in account for calculation of spring load ? If we account the back pressure for calculating spring load, it will resembles conventional type PSV.

I know this is basic question. However, I am not convince.

Regards,

Gunjan

Gunjan,
Please open the API RP 520 Part 1, section 2.2.1.3 and figures in page 29, you will find the answer...

Grab some of the manufacturer's description of conventional and Balanced PSV construction listed in "Useful Documents Related to Pressure Relief Valve (PRV) - Part 1", it will further enhanced your understanding.


Good luck.

Thanks Joewong.