What Is a Socket Weld Union Tube Fitting?

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A socket weld union tube fitting combines two distinct engineering features in a single assembly: socket weld (SW) end connections on both ports, and a separable union body secured by a threaded union nut. The tube slides into the socket bore and is permanently attached by a full circumferential fillet weld; the union nut then allows the two welded halves to be disconnected and reconnected for maintenance or component replacement — without cutting the tube or disturbing the weld.

Unlike compression tube unions — which rely on ferrule grip and face-seal pressure to hold and seal the tube — socket weld unions create a metallurgical bond between the tube and the fitting body. This makes them the preferred choice for service conditions that exceed the performance envelope of compression fittings: high-temperature steam lines, high-pressure wellhead piping, vibration-intensive process loops, and ASME B31.3 piping systems where welded joints are specified by the piping class.

TES-LOK Socket Weld Union Tube Fittings are forged or machined to ASME B16.11 socket bore dimensions, available in Class 3000 and Class 6000, and certified to ASTM A182 (stainless and alloy steel) or ASTM A105 (carbon steel). Equal and reducing configurations are available. All materials are supplied with full Mill Test Certificates.

How a Socket Weld Union Works

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The socket weld union consists of three components:

• Body Half A — machined socket on one end; shoulder/seat on the other end for the union nut to bear against
• Body Half B — machined socket on one end; male thread or mating seat on the other end for the union nut
• Union Nut — threaded collar that draws the two body halves together face-to-face, creating a metal-to-metal seal at the union joint

After both tube ends are welded, the union nut can be loosened to separate the two body halves — allowing the line to be broken for valve replacement, instrument removal, or pipe section maintenance — then retightened to restore the sealed connection.

Port Layout — Socket Weld Union

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Component	Connection Type	Function
Body Half A — Tube End	Socket weld (SW)	Tube inserted and fillet-welded — permanent attachment
Body Half B — Tube End	Socket weld (SW)	Tube inserted and fillet-welded — permanent attachment
Union Nut (centre)	Threaded union collar	Draws both body halves together; loosened to separate line for maintenance
Union Joint Face	Metal-to-metal face seal	Primary pressure seal when union nut is tightened

Specifications

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Parameter	Details
Product name	Socket Weld Union Tube Fitting (SW Union)
Brand	TES-LOK (Tesco Steel & Engineering)
End connection type	Socket weld (SW) both ends per ASME B16.11
Union joint type	Threaded union nut — metal-to-metal face seal
Pressure class	Class 3000 · Class 6000 (per ASME B16.11)
Tube OD range	1/4" to 1" (imperial) · 6 mm to 25 mm (metric)
Socket bore standard	ASME B16.11 — bore diameter + 1/16" expansion gap
Configuration	Equal (same bore both ends) · Reducing (different bore each end) on request
Materials (SS)	SS 304 / ASTM A182 F304 · SS 316 / A182 F316 · SS 316L / A182 F316L
Materials (CS / Alloy)	Carbon Steel ASTM A105 · LTCS A350 LF2 · Alloy A182 F11 / F22 / F91
Materials (Special Alloy)	Monel 400 · Inconel 600 · Hastelloy C-276 · Duplex 2205 · Super Duplex 2507
Temperature range	−100 °F to +1500 °F (material and class dependent)
Dimensional standard	ASME B16.11
Piping code	ASME B31.3 Process Piping · ASME B31.1 Power Piping
Quality certification	ISO 9001:2015 · EN 10204 3.1 MTCs · 3.2 MTCs on request
Weld process	GTAW / TIG (SS & alloy) · SMAW (carbon steel)

Tube OD × Pressure Class Reference

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Tube OD	NPS Equivalent	Class 3000 Rating (SS 316)	Class 6000 Rating (SS 316)
1/4"	1/8" NPS	Up to 15,000 PSI*	Up to 20,000 PSI*
3/8"	1/4" NPS	Up to 10,000 PSI*	Up to 15,000 PSI*
1/2"	3/8" NPS	Up to 7,500 PSI*	Up to 12,000 PSI*
3/4"	1/2" NPS	Up to 6,000 PSI*	Up to 9,000 PSI*
1"	3/4" NPS	Up to 5,000 PSI*	Up to 7,500 PSI*
6 mm	—	Consult P-T table	Consult P-T table
12 mm	—	Consult P-T table	Consult P-T table
25 mm	—	Consult P-T table	Consult P-T table

* Indicative values at ambient temperature for SS 316 per ASME B16.11 P-T ratings. Actual ratings depend on material, wall schedule, operating temperature, and applicable code. Always refer to the published ASME B16.11 pressure-temperature tables for design purposes.

6 Design Advantages of TES-LOK Socket Weld Union Fittings

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Socket Weld Union vs Other Union Connection Types

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Fitting	End Connection	Separable?	Max Temp	When to Choose
Socket Weld Union	Socket weld (welded)	Yes — union nut	Metal rating (~1500 °F)	High-temp / high-pressure / ASME B31.3 code compliance required
Compression Union	Double-ferrule compression	Yes — ferrule nut	~450 °F (elastomer limited)	General instrumentation; frequent disconnection; no welding available
Butt Weld Union	Butt weld (pipe end prep)	Yes — union nut	Metal rating	Large bore or schedule pipe; full-penetration weld required
Threaded Union	NPT / BSP threads	Yes — union nut	~750 °F (PTFE limited)	Medium pressure threaded piping; no welding required
Weld Adapter	Socket weld one end, compression other	No — permanent	Metal rating	Transition point from welded pipe to compression tube system

Socket Weld Installation & Welding Guide

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Step 1 — Prepare the tube. Cut the tube square with a tube cutter or cold saw (not a hacksaw). Deburr the OD and ID of the cut end. Clean the tube surface and the inside of the socket bore with acetone or a suitable solvent — weld contamination from oil, grease, or scale causes porosity.

Step 2 — Insert the tube and set the gap. Push the tube fully into the socket until it bottoms. Then withdraw it exactly 1/16" (1.6 mm). This mandatory expansion gap per ASME B16.11 accommodates differential thermal expansion during the weld thermal cycle and prevents restraint cracking. Mark the tube at the socket face as a reference before tack-welding.

Step 3 — Tack weld in position. With the fitting assembly positioned correctly (union nut and body half assembled loosely), apply two or three tack welds 120° apart to hold the gap and prevent distortion during the root pass.

Step 4 — Full fillet weld. Apply a full circumferential fillet weld at the socket face. Weld throat size should equal the socket wall thickness. For SS 304/316 and alloy steels, use GTAW (TIG) with matching filler wire (ER316L for SS 316 applications); for carbon steel, SMAW with E7018 electrodes. Maintain inter-pass temperatures per the applicable WPS.

Step 5 — Post-weld treatment. For SS materials, brush the weld area with a passivating acid gel (citric or nitric) and rinse with clean water to restore corrosion resistance. For P91, F22, or other Cr-Mo alloy steels, post-weld heat treatment (PWHT) per ASME B31.3 table 331.1.1 is required — do not pressurise until PWHT is complete and documented.

Step 6 — Tighten union nut. Once both welded halves have cooled to ambient temperature, engage the union nut and tighten to the specified torque for the fitting size and material. Leak-test per the applicable pressure test procedure before commissioning.

Industries & Applications

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Industry	Typical Application
Oil & Gas	High-pressure wellhead instrument piping, subsea control module SW connections, offshore topside process lines
Refinery / Petrochemical	High-temperature process lines above compression fitting limits, catalytic cracker instrument loops, fired heater piping
Power Generation	High-pressure steam header unions, boiler feed instrument piping, turbine extraction line branches
Chemical / Petrochemical	Corrosive service with special alloys, high-temperature reaction piping, NACE MR0175 sour service lines
Nuclear & Defence	Safety-class instrumentation piping, ASME B31.1 Code Class 1/2/3 systems, seismically qualified loops
Marine & Offshore	Deck piping requiring welded connections, vibration-intensive machinery spaces, high-pressure hydraulics

Frequently Asked Questions

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