A union, similar to a coupling, allows the convenient future
disconnection of pipes for maintenance or fixture replacement. In
contrast to a coupling requiring solvent welding, soldering, or
rotation (for threaded couplings), a union allows easy connection
and disconnection, multiple times if needed. It consists of three
parts: a nut, a female end and a male end. When the female and male
ends are joined, the nut seals the joint by pressing the two ends
tightly together. Unions are a type of very compact flange
Dielectric unions, with dielectric insulation, separate dissimilar
metals (such as copper and galvanized steel) to prevent galvanic
corrosion. When two dissimilar metals are in contact with an
electrically conductive solution (ordinary tap water is
conductive), they form an electrochemical couple which generates a
voltage by electrolysis.
When the metals are in direct contact with each other, the electric
current from one to the other also moves metallic ions from one to
the other; this dissolves one metal, depositing it on the other. A
dielectric union breaks the electrical path with a plastic liner
between its halves, limiting galvanic corrosion.
Rotary unions allow mechanical rotation of one of the joined parts,
while resisting leakage.