ASME B16.5 is the most widely used international standard for pipe flanges and flanged fittings. It covers nominal pipe sizes from NPS 1/2 to NPS 24, pressure classes including Class 150, 300, 600, 900, 1500, and 2500, as well as dimensions, materials, marking, pressure-temperature ratings, and testing requirements.

What is ASME B16.5 Socket Weld Flange

A socket weld flange (SW flange) is a type of forged flange with a recessed socket end. The pipe is inserted into this socket, and a fillet weld is applied around the outside of the flange hub.

Socket weld flanges are designed for small-diameter, high-pressure lines where strong, leak-resistant connections are required. They provide good alignment and reduce welding difficulty compared with other flange types.

ASME B16.5 Class 150 Socket Weld Flange Dimensions

Sealing Face Types of SW Flange

ASME B16.5 socket weld flanges use several standard sealing faces:

• Raised Face (RF)Most common type. A small raised surface provides gasket seating. Used in general industrial service.
• Flat Face (FF)Full flat face, used with flat gaskets, mostly for low-pressure applications.
• Ring Type Joint (RTJ)For high-pressure and high-temperature conditions, using metallic gaskets.

RF is the standard for Class 150 socket weld flanges.

Material Selection & Grades

Ø Carbon Steel

ASTM A105 – Standard material for Class 150 flanges

ASTM A350 LF2 – For low-temperature service

Ø Stainless Steel

ASTM A182 F304 / F304L

ASTM A182 F316 / F316L

Ø Alloy Steel (Chrome-Moly)

ASTM A182 F11

ASTM A182 F22

Ø Duplex Stainless Steel

ASTM A182 F51 (2205)

Materials are selected based on design pressure, temperature, medium, and application environment.

Marking Requirements

Flanges must be permanently marked according to ASME B16.5:

• Manufacturer’s trademark (Optional)
• Flange Name
• Nominal size
• Pressure class
• Hub Thickness (for weld neck flange)
• Sealing Face
• Material
• Standard
• Heat number (mandatory for cast steel flanges)

Clear marking ensures traceability and correct installation.

Applications

Socket weld flanges are recommended for:

• Small-bore piping (NPS 1/2 to NPS 4)
• High-pressure systems
• Clean, non-corrosive or mildly corrosive fluids
• Oil, gas, refining, power, and process piping
• Applications requiring good alignment and easy installation

They are not recommended for highly corrosive services where crevice corrosion may occur.

Installation Steps & Key Notes

Installation Steps

1. Clean the pipe end and flange socket.
2. Insert the pipe into the socket until it reaches the bottom.
3. Pull the pipe back slightly to leave a small expansion gap.
4. Perform fillet welding around the hub.
5. Finish welding and inspect for defects.

Key Notes

1. Maintain proper gap to avoid thermal stress.
2. Ensure full penetration of the fillet weld.
3. Use correct gaskets and fasteners matching flange rating.

Why Use Socket Weld Flange?

• Strong connection suitable for high pressure
• Good alignment, easy to install
• Smooth inner flow path, low turbulence
• Good fatigue resistance
• Compact design for limited space
• Ideal for small-diameter piping systems

These benefits make socket weld flanges a reliable choice for many industrial systems.

Socket Weld Flange vs Slip On Flange

Conclusion

ASME B16.5 Class 150 socket weld flanges provide a strong, reliable, and compact connection for small-bore piping systems. Understanding dimensions, materials, sealing faces, installation, and differences from slip on flanges helps engineers and buyers make correct selections for safe and efficient operation.

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