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No.
I assume you mean black iron pipe, which is used to run the gas line to your furnace.
NO!! Black iron pipe has a special coating to resist the highly corrosive effect of natural gas. Galvenized pipe CANNOT be used for gas service.
Because that is what is used for gas.
You should use black pipe -- Galv. pipe will flake off inside of pipe
Absolutely Not!
natural gas line
It is Natural Gas Pipe Line Regulator
No.
It is Natural Gas Pipe Line Regulator
Yellow
Yellow
Use only black pipe
I assume you mean black iron pipe, which is used to run the gas line to your furnace.
Either shut the gas off at the meter or install a valve on the pipe. As long as you don't have an open flame somewhere close to the gas line and have everything ready, you can take the current flex line off and install the valve with little gas excaping. Use pipe dope on the threads of the pipe instead of Teflon tape. This lets you cap the pipe with your thumb while you apply the pipe dope. Then just install the valve and tighten, making sure to hold the pipe so that you don't disturb the next joint further down the line.
Fill pipe?
It costs money to turn natural gas into a liquid. It would cost a whole lot more money to insulate the pipe line to keep natural gas a liquid along the length of the pipeline. Thus, it is not sent as a liquid due to cost considerations. If some one can figure out a way to create inexpensive strong insulated pipe, it will be sent as a liquid.