Flanges look simple, but a flange enquiry carries a lot of information: type, size, pressure class, facing, material and standard. Knowing the main flange types — and when each one is used — helps you quote faster, avoid wrong deliveries and speak confidently with engineers and purchase teams anywhere in the world.

What a flange does

A flange joins pipes, valves, pumps and equipment with a bolted connection that can be opened for inspection, cleaning or replacement. Two flanges are bolted together with a gasket between them to make a leak-tight seal. The most widely used dimensional standard is ASME B16.5 (NPS 1/2" to 24"), with ASME B16.47 for larger sizes. In Europe, buyers often specify EN 1092-1.

The six main flange types

1. Weld neck (WN)

A weld neck flange has a long tapered hub that is butt-welded to the pipe. The taper spreads stress away from the joint, which makes it the best choice for high pressure, high temperature and cyclic loads. It is the standard flange in oil & gas, refineries and power plants.

  • Strong, reliable and easy to radiograph
  • Must match the pipe bore (schedule) exactly
  • Heavier and more expensive than slip-on

2. Slip-on (SO)

The pipe slides into the flange and is fillet-welded inside and outside. Slip-on flanges are cheaper and easier to align, so they are popular for low-pressure water, cooling and utility lines.

  • Lower cost and simpler fit-up
  • Lower strength and fatigue life than weld neck

3. Blind (BL)

A blind flange has no bore. It closes the end of a pipe, valve or vessel opening and allows future extension or inspection access. Blind flanges carry high bolt and pressure loads, so they are often thicker than other types in the same class.

4. Socket weld (SW)

The pipe fits into a socket and is fillet-welded on the outside. Socket weld flanges are used on small-bore, high-pressure lines, usually NPS 2" and below. A small gap is left at the bottom of the socket to allow for thermal expansion during welding.

5. Threaded (screwed)

Threaded flanges screw onto a threaded pipe with no welding. They suit areas where welding is unsafe or not allowed, and small low-pressure lines. They are not recommended for high temperature or heavy cyclic service.

6. Lap joint (LJ)

A lap joint flange is used with a stub end welded to the pipe. The flange itself rotates freely, which makes bolt-hole alignment easy. Because the flange does not touch the fluid, it can be made from cheaper carbon steel while the stub end is stainless — a cost saving in corrosive systems that need frequent dismantling.

Speciality flanges

  • Orifice flanges — weld neck flanges with tapped holes for flow measurement.
  • Spectacle blinds, spacers and paddle blinds — for isolating sections of a line.
  • Reducing flanges — connect pipes of different sizes.
  • Long weld neck (LWN) — an extended neck used for vessel and tank nozzles.

Pressure classes

ASME B16.5 flanges come in classes 150, 300, 400, 600, 900, 1500 and 2500. The class is not a pressure in psi. The allowable pressure depends on the material group and temperature — for example, a Class 150 carbon steel flange is rated much lower at 300 °C than at ambient temperature. Always check the pressure–temperature tables for the material.

Facing types

  • Raised face (RF) — the most common; the gasket sits on a small raised ring.
  • Flat face (FF) — used with cast iron or low-pressure equipment to avoid cracking.
  • Ring type joint (RTJ) — a machined groove for a metal ring gasket in high-pressure service.
  • Tongue & groove / male & female — for precise gasket location.

How to read a flange specification

A complete flange description looks like this:

WNRF, 4", Class 300, SCH 40, ASME B16.5, ASTM A105

  • WNRF = weld neck, raised face
  • 4" = nominal size
  • Class 300 = pressure class
  • SCH 40 = bore to match the pipe
  • ASME B16.5 = dimensional standard
  • ASTM A105 = material (forged carbon steel). For stainless, A182 F304/F316L; for low-temperature carbon steel, A350 LF2.

If any of these is missing in an enquiry, ask before quoting. Wrong bore or facing is one of the most common reasons flanges get rejected.

Presenting flanges to global buyers

Overseas buyers search for exact terms such as "ASTM A182 F316L weld neck flange" or "Class 600 RTJ blind flange". Product pages with clear tables of types, sizes, classes, facings and materials attract these searches. Our guide on trust signals every manufacturer website needs shows what else those pages need.

To turn flange enquiries into professional quotations in minutes, QuoteMitra lets you paste an enquiry and build a priced quote with weights and terms.

Frequently asked questions

Which flange type is the strongest?

The weld neck flange is generally the strongest because its tapered hub transfers stress smoothly into the pipe. It is the standard choice for high-pressure and high-temperature service.

What does WNRF mean?

WNRF stands for weld neck, raised face — one of the most commonly ordered flange combinations.

Can Class 150 and Class 300 flanges be bolted together?

No. They have different bolt circle diameters, bolt sizes and thicknesses, so they do not match. Both mating flanges must be the same size and class.

What material is used for carbon steel flanges?

Forged carbon steel flanges are usually ASTM A105. For low-temperature service, ASTM A350 LF2 is common, and stainless flanges are normally ASTM A182 F304/F304L or F316/F316L.

Key takeaways

  • Weld neck for critical service, slip-on for economy, blind to close lines.
  • Socket weld and threaded suit small bores; lap joint suits frequent dismantling.
  • Class, facing, bore, standard and material complete the specification.

A supplier who reads a flange spec correctly the first time saves the buyer time — and that is what earns repeat orders.