PSV Drain Connections

Because it is a P & ID drawing it is not necessarily a drain. It could simply be a pressure "take-off". I've seen (once) an absolute pressure transmitter on such a port to tell when a PSV has become un-seated (or if it's passing).

Yes, this drain valves are used for drains as well as purging , while taking the PSV for maintainance, as the discharge of PSV should have been to routed to flare system.

While raising such type of queries, one should be very specific about the name of fluid which PSV discharges, whether it is a Compressed liquid like propylene , Ammonia and the I/L & O/L header connection of PSV, if possible the main sizing case of PSV should be mentioned, like fire scenario/ tube rupture case etc.

Rupalkumar Buch

In my previous life at a chemical facility, we used block valves downstream (and sometimes upstream) of PSVs to isolate certain equipment for maintenance without having to potentially expose ourselves to the common flare lines or take the entire facility down to change out a PSV. The bleed valves on either side of the relief valve were for verifying a zero energy state. Our PSVs were on a 3 year inspection and repair cycle and in a facility with over 100 PSVs, we had a lot of changes going on.

Now the terms which have come up are :

Bleed valves

Purging

When do we use these valves for bleeding, purging and draining??