What Is a Lap Joint Flange?

Quick Answer

A lap joint flange is a two-piece assembly: a flat backing flange (the ring) that slips over a stub end. The stub end is butt-welded to the pipe and provides the sealing surface. The backing flange, which never contacts the process fluid, can be made from cheaper carbon steel even when the piping is stainless or alloy.

How It Works

The stub end is welded to the pipe. The lap joint flange ring slides freely behind the stub end’s flared lip. When bolted, the flange compresses the stub end face against the gasket, creating the seal. Because the ring does not touch the process fluid, it can be a lower-cost material.

Property	Detail
Standard	ASME B16.5 (NPS 1/2-24)
Pressure classes	150 and 300 (most common)
Face type	Flat face (FF) only
Flange material	Typically A105 carbon steel
Stub end material	Matches pipe (A403 WP316L, A403 WP304L, etc.)
Stub end standard	ASME B16.9 (Type A long, Type B short)
Rotation	Flange ring rotates freely for bolt-hole alignment

When to Use Lap Joint Flanges

Lap joint flanges are specified in specific situations:

• Exotic piping materials (stainless steel, duplex, nickel alloys): Use a carbon steel ring with an alloy stub end to cut material costs by 30-50%
• Frequent disassembly: The free-rotating ring makes bolt-hole alignment easy, even in tight spaces
• Galvanized or lined piping: Avoids welding directly to the flange, preserving the lining
• Low-pressure systems (Class 150): Most common application range

Pro Tip

Lap joint flanges are rarely used above Class 300. At higher pressure classes, the flat-face design and lack of a hub limit their structural integrity. For high-pressure alloy piping, weld neck flanges in the matching alloy material are standard—despite the higher cost.

Cost Comparison

The cost advantage depends on the piping material. Here is a typical comparison for NPS 6 Class 150:

Configuration	Flange Material	Approx. Cost (USD)
WN flange, A182 F316L	Full stainless	$280-350
LJ ring A105 + stub end A403 WP316L	Carbon ring + SS stub	$170-220
WN flange, A105	Full carbon steel	$60-80

The savings are significant in stainless and alloy systems. For carbon steel piping, there is no cost advantage—use a standard weld neck or slip-on flange.

Common Mistake

Specifying Type B (short) stub ends with raised-face mating flanges. Type A (long pattern) stub ends have the correct lap length and are recommended for most piping applications. Type B stub ends are shorter and may not seat properly against the backing ring.

Limitations

Lap joint assemblies have lower fatigue resistance than weld neck flanges. ASME B31.3 assigns lap joint connections an SRF of 0.85. They are also limited in face type—only flat face (FF) is standard. RTJ and RF facing are not available for lap joint configurations.

Read the full guide to flanges