A union is a type of pipe fitting that allows for easy disassembly. It consists of three components: a male end, a female end, and a gasket.
Its structural features include an external thread on the male end, an internal thread on the female end, and a threaded connection between the two, with a gasket placed in between to ensure sealing. Union fittings are available in various materials, including copper, stainless steel, cast iron, and plastic. Selection should be based on factors such as the pipeline medium, pressure, and temperature.
MSS SP-83 Pipe Union Specification
| Size: | 1/8″ to 3″ / DN6 to DN80 |
| Pressure: | 3000 LB, 6000 LB, 9000 LB |
| Connection: | Socket Weld and Threaded Union |
| Thread Type: | Female and Male NPT, BSPP and BSPT |
| Alloy Steel: | ASTM A182 F1, F5, F9, F11, F12, F22, F91 |
| Carbon Steel: | ASTM A105, A350 LF2/LF3, A694 F52, F60, F65, F70 |
| Stainless Steel: | ASTM A182 F304, F316, F317, F310, F321 |
| Duplex Stainless Steel: | ASTM A182 F51, F53, F55 |
Different Types of Pipe Unions
A pipe union is a type of detachable pipe fitting connection consisting of two end joints and a locking nut in the middle, enabling quick assembly and disassembly without rotating the pipe body. Depending on the connection method, pipe unions are mainly divided into two categories: socket weld unions and threaded unions. These two types differ significantly in terms of structural design, installation method, sealing performance, and applicable scenarios.
Socket Weld Unions
Socket weld unions are primarily used in piping systems that require a more secure and reliable connection. Unlike threaded unions, the ends of socket weld unions feature sockets, into which the pipes are inserted and then welded along the socket edges. The two ends of the unions are secured together using nuts or clamps, and metal or non-metal seals are installed at the connection surfaces to ensure reliable sealing. Similar to threaded unions, they also include a nut for connection and sealing.
Advantages of Socket Weld Unions:
Reliable and secure connection: Welded connections have significantly higher strength than threaded connections, capable of withstanding higher pressures and vibrations.
Excellent leak prevention: Welding provides a permanent leak-proof seal, suitable for applications with extremely high sealing requirements.
Easy installation: Socket weld unions do not require precise alignment during installation, making the process relatively simple.
Disadvantages of Socket Weld Unions:
Welding construction is required, resulting in longer installation and disassembly times.
The heat-affected zone from welding may alter metal properties, necessitating strict welding procedures.
Class 3000 Socket Weld Union Dimension
| Nominal Size | Pipe End | Socket Bore Dia. |
Socket Wall | Water Way Bore |
Laying Length |
THRD Per Inch | Depth of Socket |
Length Assem |
Clear Assem. Nut | |
| DN | NPS | A | B | C min | D | E | H | Kmin | L nom | N |
| 6 | 1/8 | 21.8 | 10.9 | 3.2 | 6.1 | 20.7 | 16 | 9.7 | 41.4 | 51 |
| 8 | 1/4 | 21.8 | 14.3 | 3.3 | 8.5 | 20.7 | 16 | 9.7 | 41.4 | 51 |
| 10 | 3/8 | 25.9 | 17.7 | 3.5 | 11.8 | 23.8 | 14 | 9.7 | 46 | 56 |
| 15 | 1/2 | 31.2 | 21.9 | 4.1 | 15.0 | 23.8 | 14 | 9.7 | 49 | 59 |
| 20 | 3/4 | 37.1 | 27.3 | 4.3 | 20.2 | 28.6 | 11 | 12.7 | 56.9 | 66 |
| 25 | 1 | 45.5 | 34.0 | 5 | 25.9 | 30.2 | 11 | 12.7 | 62 | 79 |
| 32 | 11/4 | 54.9 | 42.8 | 5.3 | 34.3 | 36.6 | 10 | 12.7 | 71.1 | 94 |
| 40 | 11/2 | 61.5 | 48.9 | 5.5 | 40.1 | 38.1 | 10 | 12.7 | 76.5 | 112 |
| 50 | 2 | 75.2 | 61.2 | 6 | 51.7 | 41.4 | 10 | 15.7 | 86.1 | 132 |
| 65 | 21/2 | 91.7 | 73.9 | 7.7 | 61.2 | 56.9 | 8 | 15.7 | 102.4 | 150 |
| 80 | 3 | 109.2 | 89.9 | 8.3 | 76.4 | 58.7 | 8 | 15.7 | 109 | 176 |
Threaded Union
The threaded union is the most widely used type of union, characterized by its use of threaded connections for piping. It consists of three main components: a male end with external threads, a female end with internal threads, and a nut. By rotating the nut, the sealing surfaces of the male and female ends can be tightly aligned to achieve connection and sealing. The thread forms can be BSP, NPT, G, etc., and the connection is further secured by a locking nut and seal ring.
Advantages of Threaded Unions
Easy to install, no welding required, suitable for quick on-site construction.
Facilitates disassembly, maintenance, and replacement of pipeline systems.
Suitable for applications where welding is not feasible.
Disadvantages of Threaded Unions
Threaded connections are prone to loosening due to thermal expansion and contraction and vibration, resulting in slightly lower sealing performance compared to welded types.
Not suitable for long-term use in high-pressure, high-temperature, or highly corrosive environments.
CLASS 3000 Threaded Union Dimensions
| Nominal Size | Pipe End | Wall | Bone | Male Flange | Nut | THRD Per Inch | Bearing | Length Assem | Clear Assem Nut | |
| DN | NPS | Amin | Cmin | D | Fmin | Gmin | H | Jmin | Lnom | N |
| 6 | 1/8 | 14.7 | 2.4 | 8.4 6.4 |
3.2 | 3.2 | 16 | 1.2 | 41.4 | 51 |
| 8 | 1/4 | 19.1 | 3.0 | 11.1 9.4 |
3.2 | 3.2 | 16 | 1.2 | 41.4 | 51 |
| 10 | 3/8 | 22.9 | 3.2 | 14.3 13.5 |
3.4 | 3.4 | 14 | 1.4 | 46.0 | 56 |
| 15 | 1/2 | 27.7 | 3.7 | 17.9 17.1 |
3.7 | 3.7 | 14 | 1.5 | 49.0 | 59 |
| 20 | 3/4 | 33.5 | 3.9 | 23.0 21.4 |
4.1 | 4.1 | 11 | 1.7 | 56.9 | 66 |
| 25 | 1 | 41.4 | 4.5 | 29.0 27.7 |
4.6 | 4.4 | 11 | 1.9 | 62.0 | 79 |
| 32 | 11/4 | 50.5 | 4.9 | 37.7 35.4 |
5.3 | 5.2 | 10 | 2.1 | 71.1 | 94 |
| 40 | 11/2 | 57.2 | 5.1 | 43.5 41.2 |
5.8 | 5.6 | 10 | 2.3 | 76.5 | 112 |
| 50 | 2 | 70.1 | 5.5 | 55.6 52.1 |
6.6 | 6.4 | 10 | 2.7 | 86.1 | 132 |
| 65 | 21/2 | 85.3 | 7.0 | 66.3 64.3 |
7.5 | 7.1 | 8 | 3.1 | 102.4 | 150 |
| 80 | 3 | 102.4 | 7.6 | 82.6 77.3 |
8.3 | 8.0 | 8 | 3.5 | 109.0 | 176 |
Material & Grades of Forged Unions:
Carbon Steel Threaded Flanges :
Carcon Steel: ASTM A105 / A105N,ASTM A350 LF2 / LF3, A694 F42 / 46 / 52 / 56 / 65 / 70
Alloy Steel Threaded / Screwed Flanges :
ASTM / ASME A/SA 182 & A 387 F1, F5, F9, F11, F12, F22, F91
Stainless Steel Threaded Flanges :
ASTM A 182, A 240 F 304, 304L, 304H, 316, 316L, 316Ti, 310, 310S, 321, 321H, 317, 347, 347H, 904L
Duplex & Super Duplex Steel Threaded Flanges :
ASTM / ASME A/SA 182 F 44, F 45, F51, F 53, F 55, F 60, F 61
Copper Alloy Steel Threaded Flanges :
ASTM SB 61 , SB62 , SB151 , SB152 UNS No. C 70600 (Cu-Ni 90/10), C 71500 (Cu-Ni 70/30), UNS No. C 10100, 10200, 10300, 10800, 12000, 12200
Nickel Alloy Threaded Flanges :
ASTM SB564, SB160, SB472, SB162 Nickel 200 (UNS No. N02200), Nickel 201 (UNS No. N02201), Monel 400 (UNS No. N04400), Monel 500 (UNS No. N05500), Inconel 800 (UNS No. N08800), Inconel 825 (UNS No. N08825), Inconel 600 (UNS No. N06600), Inconel 625 (UNS No. N06625), Inconel 601 (UNS No. N06601), Hastelloy C 276 (UNS No.N10276), Alloy 20 (UNS No. N08020)
Socket Weld Union VS Threaded Union
| Comparison Item | Socket Weld Union | Threaded Union |
| Connection Method | Pipe end inserted into socket and fillet welded | Direct threaded connection |
| Sealing Performance | High, suitable for high-pressure and high-temperature service | Moderate, suitable for low-pressure and ambient temperature service |
| Installation Ease | Requires welding, longer installation time | No welding required, quick installation |
| Disassembly & Maintenance | Relatively difficult | Very convenient |
| Suitable Environment | High pressure, high temperature, and leak-prone media | Low pressure, ambient temperature, general industrial media |
Threaded union are mainly used in low-pressure, normal-temperature pipeline systems that require frequent disassembly and maintenance. Their core advantage lies in their ease of installation and disassembly.
Socket weld union are suitable for high-pressure, high-temperature pipeline systems with strict sealing requirements. Their core advantage lies in their secure and reliable connection.
Applications of Pipe Unions
Civil and building water supply and drainage systems
Industrial piping systems
Food and pharmaceutical industries
Oil, gas, and energy sectors