Shielded metal arc welding is the standard way to connect two natural gas pipes together. You first need to tack weld the two pipes together to hold them in place when welding. Then, you can apply butt welding techniques to create the main weld. If you do not have experience with pipe welding, consider consulting a professional pipe welder, who has already developed the skills unique to pipe welding and is equipped to handle the hazardous materials that many pipelines contain.
Clean all weld surfaces of loose slag, rust, oil and foreign matter. Make sure the weld surfaces are smooth and uniform.
Align the piping in the proper configuration. Secure the two pipes together with a clamp.
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Use your welding tool to create small welds of a uniform size, spaced at even intervals, around the pipe joint. The small welds, called tack welds, will hold your pipes in place when you make the main weld to hold the joint in place.
Weld around the entire circumference of the joint. Use a curved, zigzag motion when welding metal less than 2 mm in diameter. Use a straight weld when welding thicker metal.
Do not weld onto a live natural gas pipe unless you are experienced with such welding and have the appropriate equipment. When welding live natural gas piping, make sure the piping is thick enough and your welding parameters are low enough that you can prevent burning through to the inside. Simultaneously, you must ensure that the welding parameters are "high enough" that the weld is not excessively hard. Specialized computer software is often necessary to determine the appropriate piping and welding specifications to ensure safety.