Pipe unions are installed in pipelines so that sections along the line are easily removed without having the dismantle the entire line. The union itself comprises a male end and a female end, with the female end having a central radial nut. It is this nut that must be loosened to separate the two halves of the union joint.
Using a Pipe Wrench
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Open the jaws of a pipe wrench, and insert the union's central radial nut. Tighten the pipe wrench around the nut. Turn the wrench's handle counter-clockwise to loosen the nut from the thread on the end of the male section.
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Tap the Nut With a Hammer
Tap lightly several times around the sides of the central nut using a hammer. This will help dislodge any rust attached to the nut's threads. Place the wrench around the central nut, tighten it and try to turn the wrench handle counter-clockwise to loosen the nut.
Apply Lubricating Spray
Spray an ample amount of lubricating spray around both sides of the central nut, so that the spray penetrates to the threads inside the nut. Wait 15 minutes for the spray to seep into the threads. Place the wrench around the central nut and turn the wrench handle counter-clockwise.
Use Larger Wrench
Tap around all sides of the nut with the hammer as before, apply more lubricating spray and wait 15 minutes. Place a larger pipe wrench around the central nut, and turn its handle counter-clockwise -- the wrench's longer handle will create greater leverage. Take extra care fitting the nut into the larger wrench's jaws, as it is easy to damage the nut when using larger pipe wrenches.