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?