Flanges: Types, Flange Facing


Flanges are available in a range of standard fittings, either made of cast iron or carbon steel.  The number of bolt holes in a flange will vary from 4 to 20 or more, depending upon the size and pressure rating.  With few exceptions, the bolt holes in flanges of different pressure ratings will not match, so a low pressure flange can not be mated with a high pressure flange.  The size of flanges will depend on the size of the piping to which they are attached and the inside pressure that the flange must contain.
There are seven basic pressure ratings or classes for each pipe size.  When the API alloy steel flange with ring groove was standardised, it had been used extensively at high pressures and had proven to be adequate.  The API rating for an alloy steel flange having the dimensions of an ANSI 600-pound flange became an API 2,000 psi working pressure flange, an ANSI 900 became an API 3,000, and an ANSI 1,500 became an API 5,000 psi (i.e. divide ANSI Class by 0.3 to obtain the API flange which will mate to it).
Because the API 2,000, 3,000 and 5,000 psi working pressure flanges had the same dimensions as ANSI 600, 900 and 1,500 pound flanges, API at that time adopted the description “Series 600, 900, and 1500 to coincide with ANSI even though the pressure ratings were different.  API now describes API flanges in accordance with their working pressures.  The API pressure rating is valid for temperatures of -20 – 250°F (-28 – 121 °C) which covers most conditions encountered.  Corrections to pressure ratings due to temperatures in excess of this can be found in API Spec 6A.
Flange types and dimensions are stamped on the outer edge of the flange.  This information should be checked on each pair of mating flanges prior to make-up to see that both have the same pressure rating and pipe wall thickness.  Other information that will be stamped on the outer edge of the flange includes manufacturer, nominal size, pressure rating, face designation, wall thickness of matching pipe, material designation and heat code (see Figure 8-13)..

Figure 8-13
7.1       Flange Types
In pipe systems of more than 2 inch diameter, the flange connection is more practical and dependable in assembling than screwed connections.  This is because flanges allow easy disassembling of piping, are stronger and less likely to leak than threaded fittings.
Flanges are manufactured in compatible material to suit all of the nominal pipe sizes.  There are various types of flanges such as welding neck (WN), slip-on (SO), socket-weld (SW) and threaded.  The most common is the weld neck flange with the type of facing dependent on the service it is in.
Weld neck flanges are forged with a re-inforced hub to give greater strength and can be welded directly to butt-weld fittings.  Socket-weld weld flanges are attached in the same way.
Slip-on flanges are usually machined from plate.  They are used for lower pressure.  Two fillet welds are required to attach this type of flange to pipe work.  This type of flange should not be used for severe conditions of pressure, temperature and cycling.
A lap-joint flange is used to reduce the cost of pipe work.  A stub end of the same material as the line pipe is butt-welded to the pipe with a loose backing flange of carbon steel or stainless steel having been slipped over the stub end.  There are five types of flange connections in use. They are:

  • Weld Neck Flange.
  • Lap Joint Flange.
  • Slip on Flange.
  • Screwed or Threaded Flange.
  • Socket Weld Flange.

These type of flange are as follows:

  • Welding neck. (see Figure 8-1).

Figure 8-1.

  • Lap joint. (see Figure 8-2).

Figure 8-2.

  • Slip‑on. (see Figure 8-3).

Figure 8-3.

  • (see Figure 8-4).

Figure 8-4.

  • (see Figure 8-5).

Figure 8-5.

  • (see Figure 8-6).

Figure 8-6.
7.2       Flange Facings
Depending on the operating conditions a pipe system is designed for, five different flange facings can be used.  They are shaped according to the way the gasket will be fitted.  The flange facing must be suitable for the intended service and for the gasket and bolting being used.
The flat face is used with gaskets whose outside diameter equals the outside diameter of the flange and where one or both flanges are of brittle material and low mechanical strength.  Flat face flanges will in general require soft gaskets such as rubber.
The raised face is the most commonly used facing for process plants.  The facing is machined to “bite” into the particular gasket that is to be used.  This surface finish also helps to prevent the gasket from being sheared by the pressure load acting on the inside diameter and being partially ejected.
Too smooth a finish can reduce the ability of the gasket to seal.  When replacing a gasket, face cleaning or preparation is required to give a good leak free seal.
The ring joint facing is commonly used in production and uses a metal ring instead of a gasket for sealing the flange faces.  The ring type, when properly installed, is less likely to leak and is often preferred in high pressure applications or in large diameter piping.
The shape of the ring and the groove is such that the ring makes a line of contact on each side of the groove when the joint is assembled.  Ring joints are used for severe duties, high pressure, high temperature and to guarantee against blow-out on wellhead installations.  The most commonly used are the flat face, raised face and the ring joint.

  • Raised Face
  • Tongue &groove
  • Male to Female
  • Flat Face
  • Lap Joint
  • Ring Joint

Common flanged faces are as follows:

  • Raised face. (see Figure 8-7).

Figure 8-7.

  • Tongue and grooved. (see Figure 8-8).


Figure 8-8.

  • Large male and female. (see Figure 8-9).


Figure 8-9.

  • Flat face. (see Figure 8-10).

Figure 8-10.

  • Lapped face. (see Figure 8-11).

Figure 8-11.

  • Ring joint. (see Figure 8-12).

Figure 8-12.