What is a flange

Flange Overview

A flange is a method of connecting pipes, valves, pumps and other equipment to form a piping system. It also facilitates cleaning, inspection or modification. Flanges are usually welded or threaded. A flange connection is made by bolting two flanges together with a gasket in between to provide a seal.

Types of Flanges

The most used flange types in Petro and chemical industry are:
Welding Neck Flange
Slip On Flange
Socket Weld Flange
Lap Joint Flange
Threaded Flange
Blind Flange
All types except the Lap Joint flange are provided with a raised flange face or flat face or Ring type joint face.

Except the most used standard flanges, there are still a number of special flanges such as:

  • Orifice Flanges
  • Long Welding Neck Flanges
  • Weldoflange / Nipoflange
  • Expander Flange
  • Reducing Flange

Materials of Flanges

Pipe flanges are made from a variety of different materials, such as stainless steel, cast iron, aluminum, brass, bronze, and plastics, but the most commonly used material is forged carbon steel with a machined surface.

In addition, flanges, like fittings and pipes, are sometimes equipped with a layer of material on the inside for a specific purpose that is of a completely different quality than the flange itself, which is called a “liner flange”.

The material of the flange is essentially determined at the time the pipe is selected, and in most cases the flange is made of the same material as the pipe.

Unless otherwise noted, all flanges discussed on this site conform to ASME and ASTM standards; ASME B16.5 describes dimensions, dimensional tolerances, etc., and ASTM describes the different material qualities.

Flanged vs. welded connections

There is no standard that specifies whether or not flanged connections can be used.

In new plants, flanged connections are usually minimized because only one weld is required to connect two pipes. This saves two flanges, gaskets, bolts, a second weld, the cost of non-destructive testing of the second weld, and so on.

Some other disadvantages of flanged connections:

Every flanged connection can leak (some argue that it is impossible for a flanged connection to be 100% leak free).
Flanged piping systems require more space (just think of pipe racks).
Flanged piping systems are more expensive to insulate (special flange covers).
Of course, there are great advantages to flanged connections, such as

A new production line can contain multiple pipe spools and can be manufactured in the shop.
These pipe spools can be assembled in the factory without welding.
There is no need for NDO (X-rays, hydrotesting, etc.) in the factory, as this is already done in the shop.
There is no need for sandblasting and painting in the factory as these are already done in the shop.
(Only paint damaged during installation needs to be repaired).

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