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I'm doing a bathroom/closet reno on the second floor of my home. In the process I discovered an old gas line (black steel pipe) that I believe is still active. I traced it back near the gas meter where this line is the 3rd of 3 coming off a main trunk line. I know how to turn off the gas at the meter. However because of the way the pipes are joined I think I'm going to have to cut out a section before I can unthread the line with wrenches.

How do I make sure that there is no gas in the line before I start cutting out a section? Any other tips?

Tracing backward from the line toward the meter, the connections I need to engage consist of line->elbow->double threaded male pipe ->female coupler. I want to get the line and elbow off and cap there.

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2 Answers 2

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You could evacuate the line by blowing compressed nitrogen through it -- but it would be obnoxious to rent or buy a cylinder of nitrogen just for this. I'd expect most people would use compressed air instead. In either case you'll need to disconnect all the appliances and the meter for two reasons:

  1. provide paths for air to enter and exit the plumbing through all its branches
  2. avoid damaging components that are typically designed for 0.25 PSI (4 ounce) gas pressure (assuming natural gas; pressure is marginally higher in propane systems)

In all honesty, though, I expect most plumbers would not do even that. Just close off the supply, open the system somewhere to vent the pressure, and go to work cutting with a tube cutter. The type pictured below are rotated around the pipe to be cut, with the knob tightened a little every turn or two until the cut is complete. Note that some cutters are intended only for soft tubing like copper. Read the package before purchase.

small tube cutter large tube cutter

(photos: grainger.com and harborfreight.com)

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Gases want to equalise (in this case with the global atmosphere), so all you have to do is (while keeping the mains closed!) open up that section to the atmosphere. The volume is relatively limited, so even if the gas has a high caloric value (I don't know what your gas supplier uses), all of it should be able to escape in hours.

Give it a day (a lot less should be required, but just in case). Then you should be able to work on it safely.

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