Difference Between Lap Joint Flange and Stub End

A lap joint flange (LJF) is a two-piece assembly consisting of a stub end welded to the pipe and a separate backing flange that slides over it.

A stub end is just the short pipe section that is welded to the main pipeline and provides the sealing surface for the flange connection.

The key difference is that a lap joint flange allows for rotation and easier alignment, whereas a stub end is a fixed component that must be welded to the pipe.

Function and Components

A lap joint flange is designed for flexibility and cost savings in piping systems that require frequent maintenance or disassembly.

It consists of two parts:

Stub end: Welded to the pipe, it provides the sealing face for the flange connection.
Backing flange: A loose flange that slides over the stub end and allows for easy bolt alignment.

A stub end alone does not serve as a flange—it needs a lap joint flange to function as a complete connection.

A Closer Comparison

Lap joint flanges and stub ends have different roles in a piping system.

A stub end is welded to the pipe and provides the sealing surface.

A lap joint flange includes a stub end and a loose backing flange that helps with alignment.

This table shows the main differences between them:

Feature	Lap Joint Flange (LJF)	Stub End
Function	A complete flange system with a rotating backing flange.	A short pipe section welded to the main pipe.
Rotation	The backing flange rotates for easy alignment.	Does not rotate—fixed in place.
Sealing Face	No sealing face—relies on the stub end for sealing.	Provides the sealing surface for the connection.
Maintenance	Easy to remove without cutting the pipe.	Must be welded to the pipe, making removal harder.
Material Flexibility	Backing flange can be a lower-cost material.	Stub end must match pipe material.
Best Used For	Systems requiring frequent disassembly or realignment.	Welded piping systems where flexibility isn’t needed.

When to Use a Stub End vs. a Lap Joint Flange

Deciding when to use a stub end vs. a lap joint can be confusing. To help, we’ve shared some ideal scenarios where one of options works better than the other.

Use a Stub End

In permanent piping systems where the connection does not need frequent disassembly.
When a welded connection is required for structural integrity.
If a standard flange type (weld neck, slip-on, or socket weld) is being used instead of a lap joint flange.

Use a Lap Joint Flange

In systems that require frequent maintenance or cleaning.
When bolt-hole alignment is difficult, and a rotating flange is needed.
In cost-sensitive applications, where the backing flange can be made from a lower-cost material.
For large pipes or high-precision installations, where rotation helps ensure an accurate fit.

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Conclusion

Now that you know the difference, you can choose the right fitting for your system.

A stub end provides the sealing face, whereas a lap joint flange makes installation easier and helps reduce costs.

If your system needs frequent maintenance or easier alignment, a lap joint flange is the better option.

At Longan Flange, we provide high-quality lap joint flanges designed for durability and efficiency.

Get in touch with us today to learn more or visit our website to see a full range of flange products with details.

Reference

Lap Joint Flange (LJF) Explained – saVRee