Flange Types and How to Choose One

Weld neck, slip-on, socket weld, threaded, lap joint and blind flanges, plus facings and pressure classes.

Flanges join pipe to pipe, pipe to valve and pipe to equipment, while letting you take the joint apart later. The type you pick affects strength, cost, fit-up time and how well the joint copes with heat and vibration.

The main types

  • Weld neck. Tapered hub, butt-welded. Spreads stress smoothly into the pipe. Preferred for severe, high-pressure or temperature-cycling service.
  • Slip-on. Slides over the pipe and is welded twice. Simple to align. Lower fatigue strength than a weld neck.
  • Socket weld. Pipe sits in a socket and is fillet-welded. Common on small-bore lines. Leave a small gap at the bottom of the socket for weld shrinkage.
  • Threaded. No welding needed. Suits small, low-pressure utility lines. Avoid where thermal cycling could loosen the threads.
  • Lap joint. A loose backing flange over a stub end. The flange can rotate, which makes bolt-hole alignment easy and saves alloy on the wetted part.
  • Blind. A solid closure for line ends, test points and future tie-ins.

Facings

The face is where the gasket sits. Raised face (RF) concentrates gasket stress on a small band and is the most common. Flat face (FF) is used against cast iron or other brittle flanges. Ring-type joint (RTJ) faces carry a groove for a metal ring and turn up in high-pressure service. Read raised face or flat face for the trade-offs.

Pressure classes

ASME B16.5 covers common steel pipe flanges and groups them into classes such as 150, 300 and 600. A class is not a fixed pressure. The allowed pressure falls as temperature rises, and it depends on the material group. Always check the rating table for your material and design temperature.

Choosing in four questions

  • What are the design pressure and temperature, including upsets?
  • What will it bolt to? Match the mating face and class.
  • Is there vibration, thermal cycling or bending load? Lean towards weld neck.
  • Will the joint be opened often? Think about access, lap joints and blinds.

Then pick a gasket to suit: see how to choose a flange gasket. This page is general guidance, not a design calculation.

Bolt-up checklist