Introduction to Union Pipe Fittings
A pipe union is a fitting designed to connect two pipes while allowing easy disconnection without cutting or damaging the pipeline. Unlike a coupling, which may require cutting during dismantling, a union can be unscrewed and reassembled.
Pipe unions are essential in pipelines that need frequent maintenance, inspection, or replacement. They are widely used in oil & gas, firefighting, water supply, irrigation, and HVAC industries.
This article covers:
- Types of pipe unions
- Materials used
- Connection methods
- Applications across industries
- Global standards (IS, ASTM, DIN, JIS)
- Advantages over couplings
👉 For a full overview of fittings, check our Pipe Fittings – Complete Guide.
1. Types of Union Pipe Fittings
Threaded Union
- Both ends have internal threads.
- Easy to install and dismantle.
- Suitable for low and medium-pressure pipelines.
Socket Weld Union
- Ends designed for welding into small bore pipes.
- Leak-proof, used in high-pressure applications.
- Standard in Class 3000 and above.
Flanged Union
- Union with flange ends.
- Used in heavy-duty, large diameter pipelines.
- Easy dismantling during maintenance.
Class 3000 Union
- Designed for high-pressure pipelines.
- Common in oil & gas and chemical industries.
- Conforms to ASME B16.11 standard.
Materials Used in Union Fittings
Metallic Unions
- Mild Steel (MS): Affordable, welding use.
- Stainless Steel (SS 304/316): Corrosion-resistant, used in food, chemical, pharma industries.
- Galvanized Iron (GI): Plumbing, firefighting.
- Brass & Copper: HVAC and thermal applications.
- Cast Iron/Ductile Iron: Sewage and heavy-duty industrial pipelines.
Non-Metallic Unions
- PVC & CPVC: Domestic plumbing.
- HDPE: Irrigation and agriculture.
- PPR/PP: Hot & cold water lines.
Connection Types of Union Fittings
Unions are connected using:
- Threaded Union – Screwed connection, easy removal.
- Socket Weld Union – High pressure, industrial pipelines.
- Butt Weld Union – Permanent, strong weld.
- Flanged Union – Large pipelines, bolted connections.
Applications of Union Pipe Fittings
Unions are highly useful in pipelines requiring regular dismantling and inspection.
- Plumbing & Water Supply – GI threaded unions.
- Oil & Gas Pipelines – SS socket weld unions.
- Firefighting Systems – GI unions for hydrant lines.
- Irrigation & Agriculture – HDPE unions.
- HVAC Systems – Brass/copper unions.
Standards for Union Pipe Fittings
- IS (India): IS 1239, IS 1538 for MS/GI unions.
- ASTM/ASME (USA): ASME B16.11, ASTM A182 for forged steel unions.
- DIN/EN (Europe): DIN 2950, EN 1092.
- JIS (Japan): JIS B2311, B2312.
Advantages of Unions over Couplings
| Feature | Union | Coupling |
|---|---|---|
| Dismantling | Easy, unscrewed | Requires cutting |
| Maintenance | Reusable | Single-use |
| Cost | Higher | Lower |
| Applications | High maintenance pipelines | Permanent pipelines |
How to Select the Right Union
- Choose threaded for small pipelines.
- Select socket weld for high-pressure systems.
- Use flanged unions for large diameters.
- Match standards with project requirements.
- Select material based on fluid & environment.
Conclusion
Pipe unions are a vital fitting for systems that require frequent dismantling and inspection. From threaded GI unions in plumbing to socket weld SS unions in high-pressure oil pipelines, they provide flexibility and durability.