Flange Types and Their Applications – Weld Neck, Slip-On, Blind, Socket Weld, Threaded, Lap Joint

 

Flanges may look simple, but each type has a specific role in how a piping system handles pressure, temperature, and vibration. Using the wrong flange type can lead to leaks, misalignment, or even catastrophic failure.

This guide explains the most common flange types used in process, chemical, and power plants — what they’re for, where to use them, and what to avoid.

Flange Types and Their Applications

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1. Weld Neck Flange (WN)

Weld neck flange – ideal for high-pressure, high-temperature service.

Description:
A weld neck flange has a long tapered hub that transfers stress to the pipe, reducing high-stress concentration at the base of the weld.
It’s welded to the pipe using a butt weld, which gives excellent mechanical strength and flow integrity.

Best For:
High-pressure, high-temperature, or cyclic service.

Advantages:

• Excellent fatigue and stress resistance
• Smooth bore transition, minimal turbulence
• Strongest connection among all flange types

Common Applications:
Pressure vessels, steam lines, chemical reactors, and process piping.

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2. Slip-On Flange (SO)

Slip-on flange – simple, economical, and easy to install.

Description:
The slip-on flange simply “slips over” the pipe and is fillet welded on both sides.
It’s easy to install and align, making it popular in non-critical or low-pressure systems.

Best For:
Low-pressure or non-cyclic service.

Advantages:

• Easy alignment and low fabrication cost
• Quick installation in large-diameter lines

Limitations:

• Weaker fatigue resistance due to fillet welds
• Not suitable for high pressure or cyclic stress

Common Applications:
Cooling water systems, air lines, and general utilities.

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3. Blind Flange (BL)

Blind flange – used to close or isolate a piping section.

Description:
A blind flange has no bore and is used to close the end of a pipeline or vessel nozzle.
It allows access to the line for inspection, maintenance, or pressure testing.

Best For:
Sealing or terminating a line or nozzle.

Advantages:

• Easy to remove and reinstall
• Provides full isolation for hydrotest or maintenance

Common Applications:
Pressure vessel ends, piping terminals, and test closures.

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4. Socket Weld Flange (SW)

Socket weld flange – compact design for small-diameter, high-pressure lines.

Description:
The pipe is inserted into the socket end of the flange and fillet welded around the outer edge.
This design ensures smooth bore alignment and good flow characteristics.

Best For:
Small-diameter, high-pressure systems.

Advantages:

• Strong, leak-resistant joint
• Better internal surface finish compared to slip-on

Limitations:

• Not suitable for larger diameters or cyclic vibration
• Fillet welds require precise workmanship

Common Applications:
Instrumentation lines, hydraulic systems, and small-bore process piping.

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5. Threaded Flange (TH)

Threaded flange – simple connection without welding.

Description:
A threaded flange connects to a pipe with matching external threads.
No welding is required, which makes it ideal for systems where welding is not feasible.

Best For:
Low-pressure, non-welded systems such as gas or air lines.

Advantages:

• No welding needed
• Easy to assemble and disassemble

Limitations:

• Not suitable for high temperature or pressure
• Threads can leak under vibration or stress

Common Applications:
Compressed air lines, gas systems, fire protection lines.

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6. Lap Joint Flange (LJ)

Lap joint flange – often used with a stub end for easy alignment.

Description:
A lap joint flange is used with a stub end, allowing the flange to rotate freely around the pipe.
This makes alignment during bolt-up much easier, especially for large-diameter systems.

Best For:
Corrosive systems or frequent disassembly.

Advantages:

• Easy alignment and reassembly
• Flange material can differ from pipe material (cost saving)

Common Applications:
Corrosive process lines, stainless-to-carbon transitions, and removable spool connections.

7. Comparison Table

 

Type	Welding	Method	Pressure Range	Typical MaterialCommon Use
Weld Neck	Butt Weld	High	CS, SS, Alloy	Process piping, vessels
Slip-On	Fillet Weld	Low–Medium	CS, SS	Utilities, water lines
Blind	None	High	CS, SS	Line isolation, testing
Socket Weld	Fillet Weld	High (small dia)	CS, SS	Instrument lines
Threaded	Threaded	Low	CS, Brass	Gas or air lines
Lap Joint	Stub End + Flange	Medium	CS + SS	Corrosive or removable joints

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Field Notes

• For high pressure or cyclic stress, Weld Neck is always the safest option.
• Avoid Slip-On in vibrating systems — fillet welds tend to crack over time.
• Socket Weld is great for small-bore lines but must be precisely welded to avoid pocket corrosion.
• Lap Joint flanges save cost when the flange material can differ from the pipe (e.g., CS flange + SS stub end).

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Conclusion

Selecting the right flange type is about more than just size or pressure rating.
Each flange design exists for a reason — to balance strength, cost, and maintenance.
Choose based on your process conditions, not just what’s on the shelf.

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