"Do not pocket" means that the vapor can condense and perhaps fill the vapor line with liquid if it is not level or sloping toward the next piece of equipment. An example would be if the line drops to a lower level then rises thus forming a loop that could fill with liquid; like the surge control return lines on the HP compressors at the last offshore platform where I once worked. ;-)
I think the question was more what is the difference between a loop and a pocket.
We've got what a pocket is.
Adding an expansion loop does not necessarily mean creating a pocket, but if you can't flat turn a line out of the rack there will be a pocket (i.e. if you're trying to loop a line sandwiched between other lines, you got to go up and over more than likely).
Why would a P&ID declare "no loop"? I have to guess... maybe because the line is very sensitive to pressure drop or a turn might separate a mixed flow?
I've only seen "no loop" on P&IDs produced by a ChemE "process engineer." I haven't seen such a note by a M.E.
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Perhaps it was the mechanical engineers ;-) that prepared the P&ID's for my 26,000 hp gas turbine driven multiple stage compressor packages and pocketed the outlet of the surge valves returning to the upstream suction scrubber.
This is extremely important in air separation plants in the oxygen (and O2-rich) streams. Hydrocarbons can collect over time and make things nastily go boom due to various factors.
No loop applies on floating platforms where an expansion loop at trim can create a pocket. It's unusual for it to be a problem but when it is trying to prove that you can still vent through the line is a major head-ache!
In Jet A1 systems water can separate out. In some aircraft with aluminium fuel tanks bacteria living in this water excrete acids that cause pin hole leaks. Thus it is common for all piping to the filter coalescers to totally drain. Hence no pockets, sloping lines etc. Even changes between sch 5s and 10s piping have the internal welds ground out to prevent pockets of water.
