PSV Discharge Piping

[Gator]

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. Probably I've missed some other considerations. Typically the
discharge piping is fairly short, so "powerlining" (line sag between supports) is not usually an issue.

Of the three, sloping the discharge line is the best process design
solution, but it is also probably 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 obstructions?

Thanks for any insightful comments, guys.

Paul

 

[BigInch]

In my book,

Free draining means there should be no high point or any increase in elevation downstream.

Slope is normally accompanied by a slope value or an indicated drain point and elevations, so the slope from the notation point to the drain or low point is thereby defined.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Gator]

Understood. I'm writing from a "CAD document deliverables" (read: Isogen) perspective, where the software tends to do odd things graphically with sloped lines.

In the board days we'd draw the discharge line and indicate "slope to header" and leave it at that. These days the software wants to know exact angles (like 0.00234) and stuff.

Does "no high point or any increase in elevation downstream" imply that flat piping is OK?

I'm so confused now!

Paul
Piping Design Central

 

[BigInch]

Flat piping is OK. Anywhere you might have a riser and/or a downcomer somewhere in the line (entering and exiting pipe racks for example) the line would not necessarily require sloping. The trick is to know when it is not OK. That would usually be in a PSV line or vapor (example steam) lines where liquids may condense out of the gas stream in large quantities and collect and it is of dire necessity to slope them so that the condensate can get to a steam trap or similar drain point, or not cause any additional backpressure when moving a collected liquid along in the mostly vapor stream, or it is necessary to guarantee that no liquid can enter a downstream process or compressor (for example).

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com