Two ends screw onto the pipes and a large nut draws them together on a ground or gasketed joint. Unions go next to threaded valves, traps and equipment so they can be removed without unscrewing a whole run. Dielectric unions add an insulating sleeve and washer to separate copper from steel and stop galvanic corrosion.
Put a union on the outlet side of that ball valve, or the next guy has to cut the line to change it.
Never put dope on the union seat itself; dope goes on the pipe threads, and the ground faces seal metal to metal.
Also heard as
- ground joint union
Related terms
Nipple
A short length of pipe, threaded or plain on its ends, used to connect two fittings or a valve to a fitting.
Make up
To screw a threaded joint together and tighten it, or to assemble and torque a flanged joint.
NPT
National Pipe Taper: the tapered pipe thread used for threaded valves and fittings in North America.
Pipe dope
Paste thread sealant brushed on male pipe threads before makeup to seal and lubricate the joint.