Lap Joint Flanges

Free-Rotating Flange Used with Stub End Fittings for Alignment Flexibility

What Is a Lap Joint Flange?

A lap joint flange is a two-piece assembly consisting of a flat flange ring that slides over the pipe and a stub end fitting that is butt-welded to the pipe. The flange is free to rotate around the stub end, making bolt hole alignment easy during installation.

The flange itself never contacts the process fluid — only the stub end does. This allows the flange ring to be made from a lower-grade (and less expensive) material while the stub end is matched to the process requirements. Lap joint flanges are designed per ASME B16.5.

Key Design Features

  • Two-piece assembly (flange ring + stub end)
  • Flange rotates freely for easy bolt hole alignment
  • Only the stub end contacts the process fluid
  • Flange has a flat face with a radius at the bore to seat over the stub end
  • Lower-grade material can be used for the flange ring since it does not contact the fluid

Common Applications

  • Piping systems requiring frequent disassembly (for inspection or cleaning)
  • Lined piping systems (PTFE, rubber, glass-lined)
  • Exotic alloy piping (use carbon steel flange with alloy stub end to reduce cost)
  • Systems where precise bolt hole alignment is difficult
  • Large-diameter piping where rotation aids installation

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Available Sizes and Pressure Classes

Per ASME B16.5, lap joint flanges are available in NPS 1/2 through 24, in pressure classes 150 through 2500. They are primarily used in Class 150 and Class 300, where the cost savings and alignment benefits are most advantageous.

Common Materials

Flange ring: Carbon Steel (A105) is the most common choice since the ring does not contact the process fluid. This is the primary cost advantage of the lap joint design.

Stub end: The stub end material is matched to the process requirements — Stainless Steel, Alloy, or exotic materials as needed to resist corrosion and meet temperature demands.

Cost Advantages

The main advantage of lap joint flanges is cost savings on exotic material piping. Instead of a solid stainless or alloy flange, you can use a carbon steel lap joint ring with a small alloy stub end. Since the flange ring is the heavier component, the material savings can be significant — especially on large-diameter flanges where the weight difference is substantial.

Related References