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rpmmillwright
02-26-2009, 07:01 PM
I need a little advice.
We are installing a new valve into an existing natural gas line.
We have to cut the pipe (6inch) and weld on new flanges to acomidate the new valve. Our plan is to evacuate the pipe double purge the pipe with an inert gas(fill,drain and refill) Cut the existing pipe (cold cutting porta-band or sawzall) Prep the pipe install flanges and weld into place. I have heard stories that the pipe can absorb gas fumes. Is this true? Does this plan sound good? Anyone have any recomendations?
Thank you for your advice.
RPM:D

hankj
02-26-2009, 08:32 PM
I watched the workers at New Jersey Power and Light as a child. They welded hot NMG pipes wit OA every day.

Years later, I saw a gas crew in Californis (yeah, the land of fruits and nuts, where some are actually on trees) transition an old steel line to a fitting for the new plastic stuff, and it was still O/A!

This was done on pressurized pipe, so I don't know how it would relate to a dead line, but the byproducts of natural gas DO coat the inside of a pipe. If you [lan to weld on the pipe with atmosphere present, I'd be thinking of using a purging gas.

Hank

stevinator
02-26-2009, 10:57 PM
I am assuming you are cutting out a cylinder of the pipe the same length as your gaskets ,flanges and new valve will take up.My question is what is either direction from where you are welding and how long is the run of 6''? If you can leave something open at each end, do that while welding and grinding.Purging with air or nitrogen like you mentioned should be fine.We use an air mover sometimes to evacuate the pipe and then leave everything open.We can check the pipe with a combustible gas indicator also after moving the air.In the old days you blew the pipe awhile and flashed the ends with a propane torch if you were worried.:D After you purge it you could have some residual in the pipe but if purged long enough it shouldn't be a problem.We have once in a great while got a belch or small flame that burned itself out when flashed.I have worked for a nat gas co for 28 yrs and many welders still flash everything just out of habit.

rpmmillwright
02-27-2009, 11:39 AM
Thank You guys.
Yes i will be removing a cylinder of pipe equal to flanges valve and gaskets.
cutting the pipe about 4-5 feet from the regulator and installing an earthquake valve. The rest of the system is about 50 feet of pipe feeding several taps for the plant. I can open the system up and move some air thru it.
Thanks again for the advice.
RPM;):D:)

stevinator
02-27-2009, 10:01 PM
For the repair your doing,purging and leaving ends open and blowing out with air all work.For external branches like Hankj mentioned we make sure we weld on the live pipe with 100% gas on it.The systems might not have valves for miles or blocks so shutdowns are not options usually.We have some hot weld procedures where we induce gas and weld with a fire control setup and keep a flame burning in the air but I don't like to type well enough to try to explain that.:D That flame draws some lookers,and they say isn't that the gas main :eek:and we say yeah wer'e welding on it and you turn around and they're gone.