Dear All,

Morning!

I am little confused with line pocketing. Earlier I know that pump suction should be free of any pockets, as that will create cavitation; and to avoid that futher we put eccentric reducers on the suction lines.

Now I came to know: For vessel overhead line to an air cooler; Do not pocket/The line should free drain. <==== The question is Why? and What if the flow is from a veseel toward overhead cooler?

Also if somebody can pls summarize different conditions/lines where pockets are to be avoided; in process plants.

like what about the line coming from vertical column to an aircooler? what about lines going toward exchangers...etc.

Regards

For vessel overhead line to an air cooler; Do not pocket/The line should free drain. <==== The question is Why? and What if the flow is from a vessel toward overhead cooler?
This overhead stream starts out as vapor from the top of the vessel and proceeds through some sort of condenser (heat exchanger) which turns it into a liquid and then terminates in the "Overhead Accumulator". The line from the fractionation vessel to the condenser (exchanger) must be free drain and the line from the condenser to the accumulator vessel must also be free drain. The Overhead Accumulator is normally a horizontal vessel (Drum) supported 7 to 8 meters above grade. The reason the overhead line from the vessel to the condensed (air-cooled or water cooled) is designated as a "Free Drain" line is so that as the process stream cools the fluid/liquid will free drain down hill and not cause a liquid plug in the line and prevent the free flow of vapor from the vessel.

Please summarize different conditions/lines where pockets are to be avoided; in process plants.
Pockets must or should be avoided in all line in a process plant. That being said it must be recognized that you cannot design the plant so ALL lines are without a pocket somewhere. The process Engineer is responsible for designating those lines that must absolutely not have pockets. He or she does so by placing the words "Do Not Pocket" on the P&ID under the line in question.
Lines that should not be pocketed:

- Overhead Vapor Lines(from a fractionation Column to the condenser)
- The line from the condenser to the accumulator
- All lines up-stream of a vapor Relief valve
- All lines down stream of any relief valve in vapor or liquid service
- All Flare headers inside a unit
- All Flare headers from the unit Battery Limits to the Flare Knock-Out Drum
- All Flare system lines after the Flare KO Drum to the flare Stack

What about the line coming from vertical column to an air-cooled heat exchanger?
If the line is the overhead vapor line then it must not be pocketed.

What about lines going toward shell and tube exchangers used as condensers?
If the line is the overhead vapor line then it must not be pocketed.

In P&ids it should be clearly indicated whether the line requires NO POCKET. Otherwise their a big chance everybody will miss it.

Generally the following rules are followed for good piping arrangement:-
1) For vapour lines avoid high pockets.
2) For liquid lines avoid low pockets.

nlbalaji,
Please forgive me but I do think you have these reversed.

"1) For vapour lines avoid high pockets."
"2) For liquid lines avoid low pockets."

Please give it some thought.

"NO POCKET" P&ids should be indicated with, if the line requires it..!

I would add high pressure steam lines to no pocket lines unless the pocket goes to a drip leg and trap. It is very important in long runs of steam lines that the condensate is adequately drained as conditions can arise to cause water hammer and severe shocks in the system which could lead to failure.