Can back pressure be totally transfered through a pilot PSV? At my plant we have a pilot PSV set at 700 psig with a Rupture disc set at 360 psig on the inlet. The normal operating pressure is 625psig, which is too high for the rupture disc, but there is a 330psig back pressure on the outlet of the PSV. If the back pressure is transfered that would help explain the rupture disc setting.

Unfortunately this change was done on the fly many years ago, and there is no documentation why. The set up seems to work, but I struggle with why. I realize that pilot PSV's are not effected by back pressure, but I did not think that they transferred it.

Thanks in advance