We have open discharge and closed discharge PSVs in plants.

For open discharge piping the PSV outlet first leg is of very short length and then piping is vertical at the end of which we put downward PSV relieving force multiplied with DLF in static analysis.

For closed discharge piping the PSV outlet first leg is of short length; here we put full PSV relieving force multiplied with DLF in static analysis in direction opposite to fluid flow.

Why in larger legs in PSV inlet and outlet the PSV relieving force multiplied with DLF in static analysis will not be applied is not understood!

It is as if of steam hammer load discussed in article in Coade MEN Jun 94, the first leg from TTV valve is considered for steam hammer load, not other legs! In page 401 of Peng book on piping stress analysis, it is written that the longer the leg, the greater the net force with maximum for Length sonic velocity*closing time figure.

Relatively, for 660-800MW STG unit TTV of closing time of 10mSec is rare, but PSV with 10mSec in process plants are commonnplace.

Are we underdesigning our PSV piping currently by ignoring occasional loads on other legs ?
reg,
sam