What Is a Union? Threaded Pipe Connection

A pipe union is a three-piece fitting that connects two pipes while allowing easy disconnection without cutting or unthreading the entire line. The three components are the male end, the female end, and a central nut that draws the two ends together against a seat (ground joint, flat face, or O-ring). Unions are manufactured per ASME B16.11 in sizes NPS 1/8 to NPS 4 and are required for maintenance-accessible piping connections.

Property	Detail
Standard	ASME B16.11
Design	Three-piece (male end, female end, nut)
Size range	NPS 1/8 to NPS 4
End types	Threaded (NPT), socket weld
Pressure classes	2000#, 3000#, 6000#
Seat types	Ground joint (metal-to-metal), flat face, O-ring
Materials	ASTM A105 (CS), A182 F304/F316 (SS), A182 F11/F22 (alloy)

How a Union Works

1. The male end threads onto (or socket welds to) one pipe
2. The female end threads onto (or socket welds to) the other pipe
3. The central nut slides over the male end before installation
4. When the nut is tightened, it draws the male and female ends together, compressing the joint seat
5. To disconnect, the nut is loosened and the two pipe sections separate without disturbing either pipe connection

This mechanism is the key advantage: a coupling requires unthreading the pipe itself (which may rotate the entire attached piping), while a union disconnects at the union seat without rotating either pipe.

Union vs Coupling vs Flange

Feature	Union	Coupling	Flange
Disconnection	Easy (loosen nut)	Difficult (unthread pipe)	Easy (unbolt)
Pipe rotation	Not required	Required	Not required
Piece count	3	1	2 flanges + gasket + bolts
Size range	NPS 1/8 to NPS 4	NPS 1/8 to NPS 4	NPS 1/2 to NPS 60+
Seal type	Metal seat or O-ring	Thread sealant	Gasket
Cost	Moderate	Low	Higher
Weight	Light	Lightest	Heavy
Leak risk	Low (when properly seated)	Low	Very low (bolted joint)

Seat Types

Seat Type	Description	Best For
Ground joint (conical)	Metal-to-metal conical seat	General service, steam, hydrocarbons
Flat face	Flat metal-to-metal contact	Low-pressure utility service
O-ring	Elastomer seal in groove	Clean fluids, air, water (no high temp)

Ground joint unions are the industry standard for process piping. The conical metal seat provides a reliable seal without elastomers, suitable for temperatures from cryogenic to approximately 500 degrees C depending on material grade.

Applications

Unions are installed wherever a piping connection must be periodically broken for maintenance:

• Instrument isolation: Before and after control valves, flow meters, pressure transmitters
• Equipment connections: Small pumps, filters, strainers that require regular servicing
• Sample points: Chemical sample line connections
• Steam tracing: Connections that allow tracing line removal during equipment maintenance
• Utility piping: Compressed air, nitrogen, water connections to portable equipment

Installation Best Practice

Unions should be installed in accessible locations, oriented so the nut can be reached with standard wrenches. Pipe supports must be positioned to support both sides of the union when disconnected, preventing the unsupported pipe from sagging or falling.

For pipe sizes above NPS 4, flanged connections replace unions as the standard disconnectable joint.

Read the full guide to pipe fittings