Purge a gas main 2

[PIPEMECH]

How can I purge air from a gas main (the gas is lighter than air) I assume I need 2 holes in the main, but is the position and size of these in/out holes important.

 

[MTPipeliner]

AGA XK0104-1N1 "Purging principals and practice" has quite a bit of information about how to purge a pipeline with natural gas including some tables on how long to purge.

The fact that the gas is lighter than air is overcome by introducing gas into the line at a rapid enough rate so that there is minimal mixing and the gas pushes the air out rather than combining with it.

As far as the location of "holes" ideally they are at the extreme ends of the line so that the gas can fully push out all the air. Ieally each "hole" has a valving arangement so that you can control the amount of gas during the purge as well as close the outlet valve so that no air can get in once you quit the purge.

 

[rconner]

This is probably a stupid question (as I have no operational experience), but I was just curious if pipeline "pigs" can also be used in any fashion to facilitate purging of a product from a pipeline? [I realize pigging would probably not entirely eliminate "mixing" as well].

 

[MTPipeliner]

We don't normally use pigs as part of a purge to fill the line with gas. Quite often we do not have launching/recieving facilities at each end so there is no way to get the pigs in or out.

We sometimes do use pigs in a purge to remove the gas from a line that is to be abandon, this is partially done to remove any liquids that may be present.

If the line has pig launching and recieving facilities at each end then pigs could be used as part of a purging process. As you mention they probably won't completely eliminate mixing (but it should help), so the purge should continue for an additional time after the pig is recieved.