Steel pipelines – whether gas supply or galvanized water supply – are installed section by section, starting at the source and ending at the appliance. Consequently, if a pipe section in the middle of the line has to be removed, all other pipe sections between it and the end of the line have to be removed first. The installation of a union in the pipeline eliminates the need to remove other pipes. The union consists of two female threaded ends, with a central nut holding the two ends together.
Measure the length of the union joint with all three parts threaded together, subtracting from this measurement the width of the threads on each end of the union. Choose the pipe section in which the union will be installed. Mark two points on this pipe section the same distance apart as the length of the union.
Cut through the pipe at one of the marks. Place a tubing cutter around the pipe, resting the pipe against the two small rollers on one of the cutter's jaws. Tighten the cutter's circular blade on its other jaw against the pipe, at one of the marks. Rotate the cutter once, then tighten the blade again. Rotate again. Repeat this process until the pipe is cut through. Follow the same procedure to cut through the pipe at the second mark. Remove the cutout pipe section.
Remove the existing pipes on both sides of the cut. Place a pipe wrench around each pipe next to the nearest coupling, and turn the wrench counter-clockwise to loosen and remove the pipe from the coupling. Take each pipe section to a major home-improvement store, and have the cut ends of both pipes threaded.
Wrap yellow sealing tape twice clockwise around the threaded ends of both gas pipes. (Use white sealing tape for galvanized water pipes). Screw the pipes by hand into the couplings from which they were removed. Tighten each of the pipes to the couplings using the pipe wrench.
Screw the union's end nut, with the larger central nut attached, by hand onto one pipe end. Tighten it in place with a crescent wrench. Screw the union's other end nut onto the other pipe end, again tightening it with the wrench. Wrap yellow sealing tape twice clockwise around the union's central thread for gas lines (white sealing tape for galvanized lines). Screw the union's central nut onto the central thread, then tighten it in place with the wrench.