Things You'll Need
Union joints are installed in steel pipelines so that pipe sections near each side of the joint can be removed without having to first take out every pipe section from the end of the line to the location of the union. The union joint itself consists of two female threaded end nuts, with a central threaded holding nut which when tightened in place holds the end nuts together. Repairing a leaking union requires removing the union and applying more sealing tape to the threads before refitting the joint back into the pipeline.
Turn off the water at the main shutoff valve. Place a pipe wrench around the union's central holding nut. Turn the wrench handle counter-clockwise to loosen and remove the central holding nut from the thread that it attaches to, on one of the union's end nuts. Slide the central holding nut six inches along the pipe, resting it temporarily in that position.
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Place the wrench around the end nut closest to the central holding nut. Turn the wrench counter-clockwise to loosen and remove the nut from the end of the pipe. Clean around the threaded pipe end with a steel brush to remove all sealing tape. Wrap new white sealing tape twice clockwise around the threaded pipe end. Screw the end nut by hand back onto the pipe end, and tighten in place with the wrench.
Remove the other end nut from the pipe with the wrench, brush off the existing sealing tape from the pipe thread with the steel brush, and wrap new sealing tape twice clockwise around the threaded pipe end. Screw on the end nut by hand and tighten in place with the wrench.
Brush off existing sealing tape from the thread that the central holding nut attaches to, using the steel brush. Wrap sealing tape twice clockwise around the thread, and screw the central holding nut back onto the thread. Use the pipe wrench to tighten the central holding nut in place. Turn on the water supply.
Union joints come in different diameters to fit different diameter pipes.