Free-Draining, No Pockets, Slope - PSV Discharges/Headers

What is the difference between these terms on a P&ID:

-Free draining
-No pockets
-Slope

All of these refer to the concept of preventing (via piping configuration) liquid build-up (possibly due to condensation) downstream of the safety valve in order to avoid backpressure on the PSV spring, possible freezing of liquids or waterhammer effects. Typically the discharge piping is fairly short, so "powerlining" is not usually an issue.

Of the three, sloping the discharge line is the best process design solution, but it is also the most labour-intensive for fabrication/construction due to elbow cut-backs/non-parallel butt welds and the sometimes need to trim pipe shoes progressively to maintain the slope.

"No pockets" is pretty self-explanatory.

"Free draining" is the ambiguous one. Does this imply that a slope is necessary or does it refer to no internal obstruction (say, a FOT reducer, flow orifice, etc.).

Paul Received on Fri Jan 26 00:50:00 2007