Fittings for Pipe Joints

How Fittings are connected to Pipes

BSP fittings are a family of fittings used to connect up threaded pipe and equipment. They are manufactured from pipe, bar, hollow bar, castings or forgings. The fittings are used in non-critical, low pressure applications where welding is not possible or required. They therefore provide a relatively low cost method of connection. BSP fittings are usually fitted with a sealant (paste or tape such as PTFE) and are considered to be permanent pipe-work. Butt weld or socket weld fittings are a type of fittings used for forming circumferential butt weld joints in pipework systems. They are used for critical systems and in areas where pipe-work is permanent and are designed to provide good flow characteristics.

Screwed Unions

A union is similar to a coupling, except it is designed to allow quick and convenient disconnection of pipes for maintenance or fixture replacement. While a coupling is usually a permanent joint or requires the ability of being able to rotate all the pipe to one side of it to unscrew it, a union provides a simple nut transition, allowing easy release at any time. When using unions with dissimilar metals (such as copper and galvanized steel) a dielectric union should be used. This breaks the electric current with a plastic liner between two halves of the union, thus limiting galvanic corrosion.


There are many different flange standards to be found worldwide. To allow easy functionality and inter-changeability, these are designed to have standardized dimensions. Common world standards include ANSI (USA), PN/DIN (European) and BS10, (British/Australian). ANSI designations such as ANSI 150, ANSI 300 and so on are often followed by a # (hash symbol). The ANSI number does not directly relate to a pressure rating, but to a class of flange. For example, the hash (#) or ‘pound’ reference; e.g. 300 pound, can be misleading in that an ANSI 300 flange is actually rated for a test pressure of 740 psi (~5100 kPa), and only within a certain working temperature range (-20 to 100 deg F.) In most cases these are not interchangeable (e.g. an ANSI flange will not mate against a PN flange). Further many of the flanges in each standard are divided into “pressure classes”, allowing flanges to be capable of taking different pressure ratings. Again these are not generally interchangeable (e.g. an ANSI 150 will not mate with an ANSI 300). These “pressure classes” also have differing pressure and temperature ratings for different materials. The flange faces are made to standardized dimensions and are typically “flat face”, “raised face”, “tongue and groove”, or “ring joint” styles, although other obscure styles are possible.

Flange designs are available as:

  • Weld neck
  • Slip-on
  • Socket Weld
  • Threaded
  • Stub-end or Lap flange
  • Blind Flange

Weld Neck Flange

Weld neck flanges are used in critical applications. These are circumstantially welded onto the system at their necks which means that the integrity of the butt-welded area can easily be examined by X-ray radiography. The bores of both pipe and flange match thus reducing turbulence and erosion.

Socket Flange

A socket flange is counter-bored to accept the pipe, which is then fillet welded. The bore of both the pipe and the flange are the same to ensure good flows.

Slip-On Flange

A slip-on flange is slipped over the pipe and then fillet welded. Easy to use in fabricated applications.

Screwed Flange

A screwed or threaded flange requires no welding and is used to connect other Threaded components in low pressure non-critical applications.

Lap Flange

Lap flanges (or backing flanges) are used with a stub end which is butt-welded to the pipe with the lap flange acting as a loose collar behind it. Thus the stub end always provides the sealing face. This type of joint is easily assembled and aligned, and it is favored in low pressure applications. To reduce costs the ‘Backing’ flanges can be made from a lower grade of material such as stainless steel in Hastelloy systems.

Blind Flange

A blind flange or sometimes called a blanking flange, this is used for blanking off pipelines, valves and pumps and as an inspection cover. (adsbygoogle = window.adsbygoogle || []).push({});

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