Pipe Flanges


A flange is a method of connecting pipes, valves, pumps and other equipment to form a piping system. Usually welded or screwed, it provides easy access for cleaning, inspection or modification. Steel Flange joints are made by bolting together two flanges with a gasket between them to provide a seal.

Types of Flanges

The most used steel flange types are:


  • Welding Neck Flange

  • Slip on Flange

  • Socket Weld Flange

  • Lap Joint Flange

  • Threaded Flange

  • Blind Flange

*All types except the Lap Joint steel flange are provided with a raised flange face.

Special Flanges

Besides 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

*Special Flanges are currently not provided on this website. 

Materials for Flanges

Pipe flanges are manufactured in all the different materials like stainless steel, cast iron, aluminum, brass, bronze, plastic etc. But the most used material is forged carbon steel that include machined surfaces.

In addition, flanges, like fittings and pipes, for specific purposes sometimes internally equipped with layers of materials of a completely different quality as the flanges themselves, which are "lined flanges".

The material of a flange, is basically set during the choice of the pipe, in most cases, a flange is of the same material as the pipe.

All flanges, discussed on this website fall under the ASME and ASTM standards, unless otherwise indicated. ASME B16.5 describes dimensions, dimensional tolerances etc. and ASTM the different material qualities.

  • A105 = This specification is the standard for forged carbon steel piping components for ambient- and higher-temperature service in pressure systems.

  • 304 = This specification is a common standard for stainless steel due to its malleability, high corrosive resistance weld-ability. 

  • 316 = This specification is a molybdenum steel that provide excellent tensile, creep and stress-rupture strengths at elevated temperatures. 

  • Alloy 20This specification is a "Super" stainless steels that was designed for maximum resistance to acid corrosion, cracking and pitting attack.

  • Alloy 200This specification is an unalloyed wrought nickel with excellent corrosion resistance, mechanical, magnetic and magnetostrictive properties.

  • Alloy 400This specification is used for its excellent combination of corrosion resistance, strength, ductility and weld-ability. 

  • Alloy 600 = This specification is a nickel-chromium-iron alloy used for applications which require resistance to corrosion and heat. 

  • Duplex 2205 = This specification is a ferritic-austenitic stainless steel with very good pitting and uniform corrosion resistance, as well as high mechanical strength.

Material Description for Flanges     

Dimensions of Flanges

Each flange according to ASME B16.5 has a number of standard dimensions. If a draftsman in Japan or a work preparer in Canada or a pipe-fitter in Australia is speaking about a Weld Neck Flange NPS 6, Class 150, Schedule 40 ASME B16.5, a common identified Flange must be understood. 

The availability of a certain flange size or dimension is dependent on what material the flange is made from.

Size & Dimension table for Flanges

Pressure Classes of Flanges

Forged steel flanges, according to ASME B16.5, are made in seven primary ratings:

Class 150 - 300 - 400 - 600 - 900 - 1500 - 2500

*Only class 150, 300, and 600 are currently provided on this website. 

The concept of flange ratings is relatively simple. A Class 300 flange can handle more pressure than a Class 150 flange, because a Class 300 flange are constructed with more metal and can withstand more pressure. However, there are a number of factors that can impact the pressure capability of a flange.

The Pressure Class or Rating for flanges will be given in pounds. Different names are used to indicate a Pressure Class.

For example: 150 Lb or 150 Lbs or 150# or Class 150

Flange faces

Different types of flange faces are used as the contact surfaces to seat the sealing gasket material. ASME B16.5 and B16.47 define various types of flange facings, including the Raised Face, Flat Face, Ring-Type Joint, Tongue-and-Groove and Male-and-Female.

Raised Face (RF)
The Raised Face flange is the most common type used in process plant applications, and is easily to identify. It is referred to as a raised face because the gasket surfaces are raised above the bolting circle face. This face type allows the use of a wide combination of gasket designs, including flat ring sheet types and metallic composites such as spiral wound and double jacketed types.
The purpose of a RF flange is to concentrate more pressure on a smaller gasket area and thereby increase the pressure containment capability of the joint. Diameter and height are in ASME B16.5 defined, by pressure class and diameter. Pressure rating of the flange determines the height of the raised face.
The typical flange face finish for ASME B16.5 RF flanges is 125 to 250 µin Ra (3 to 6 µm Ra).

Raised Face height
For the height measures H and B of all described dimensions of flanges on this website, with exception of the Lap Joint flange, it is important to understand and remember the following:
In pressure classes 150 and 300, the height of raised face is approximately 1.6 mm (1/16 inch). 
In pressure classes 400, 600, 900, 1500 & 2500, the height of raised face are approximately 6.4 mm (1/4 inch)

 Flat Face (FF)

The Flat Face flange has a gasket surface in the same plane as the bolting circle face. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting.

Flat face flanges are never to be bolted to a raised face flange. ASME B31.1 says that when connecting flat face cast iron flanges to carbon steel flanges, the raised face on the carbon steel flange must be removed, and that a full face gasket is required. This is to keep the thin, brittle cast iron flange from being sprung into the gap caused by the raised face of the carbon steel flange.

Ring-Type Joint (RTJ)

The Ring Type Joint flanges are typically used in high pressure (Class 600 and higher rating) and/or high temperature services above 800°F (427°C). They have grooves cut into their faces with steel ring gaskets. The flanges seal when tightened bolts compress the gasket between the flanges into the grooves, deforming (or Coining) the gasket to make intimate contact inside the grooves, creating a metal to metal seal.

An RTJ flange may have a raised face with a ring groove machined into it. This raised face does not serve as any part of the sealing means. For RTJ flanges that seal with ring gaskets, the raised faces of the connected and tightened flanges may contact each other. In this case the compressed gasket will not bear additional load beyond the bolt tension, vibration and movement cannot further crush the gasket and lessen the connecting tension.

Ring Type Joint gaskets
Ring Type Joint gaskets are metallic sealing rings, suitable for high-pressure and high-temperature applications. They are always applied to special, accompanying flanges which ensure good, reliable sealing with the correct choice of profiles and material.
Ring Type Joint gaskets are designed to seal by "initial line contact" or wedging action between the mating flange and the gasket. By applying pressure on the seal interface through bolt force, the "softer" metal of the gasket flows into the microfine structure of the harder flange material, and creating a very tight and efficient seal.
Most applied type is style R ring that is manufactured in accordance with ASME B16.20 used with ASME B16.5 flanges, class 150 to 2500. Style "R" ring type joints are manufactured in both oval and octagonal configurations.
The octagonal cross section has a higher sealing efficiency than the oval and would be the preferred gasket. However, only the oval cross section can be used in the old type round bottom groove. The newer flat bottom groove design will accept either the oval or the octagonal cross section.
The sealing surfaces on the ring joint grooves must be smoothly finished to 63 Microinches and be free of objectionable ridges, tool or chatter marks. They seal by an initial line contact or a wedging action as the compressive forces are applied. The hardness of the ring should always be less than the hardness of the flanges.
Style R ring type joints are designed to seal pressure up to 6,250 psi in accordance with ASME B16.5 pressure ratings and up to 5,000 psi.

Tongue-and-Groove (T&G)

The Tongue and Groove faces of this flanges must be matched. One flange face has a raised ring (Tongue) machined onto the flange face while the mating flange has a matching depression (Groove) machined into its face.

Tongue-and-groove facings are standardized in both large and small types. They differ from male-and-female in that the inside diameters of the tongue-and-groove do not extend into the flange base, thus retaining the gasket on its inner and outer diameter. These are commonly found on pump covers and Valve Bonnets.

Tongue-and-groove joints also have an advantage in that they are self-aligning and act as a reservoir for the adhesive. The scarf joint keeps the axis of loading in line with the joint and does not require a major machining operation.

Male-and-Female (M&F)

With this type the flanges also must be matched. One flange face has an area that extends beyond the normal flange face (Male). The other flange or mating flange has a matching depression (Female) machined into its face.

The female face is 3/16-inch deep, the male face is1/4-inch high, and both are smooth finished. The outer diameter of the female face acts to locate and retain the gasket. Custom male and female facings are commonly found on the Heat Exchanger shell to channel and cover flanges.

Flange Face Finish

The ASME B16.5 code requires that the flange face (raised face and flat face) has a specific roughness to ensure that this surface be compatible with the gasket and provide a high quality seal. A serrated finish, either concentric or spiral, is required with 30 to 55 grooves per inch and a resultant roughness between 125 and 500 micro  inches. This allows for various grades of surface finish to be made available by flange manufactures for the gasket contact surface of metal flanges. The picture on the right shows a serrated finish on a Raised Face.

Detailed Description of Flanges

Welding Neck flange

Welding Neck Flanges are easy to recognize. They include a long tapered hub that gradually meets the wall thickness from the pipe or fitting.

The long tapered hub provides an important reinforcement for use in several applications involving high pressure, sub-zero and / or elevated temperatures. In conditions of repeated bending, caused by line expansion or other variable forces, the smooth transition from flange thickness to pipe or fitting wall thickness is extremely beneficial.

These flanges are bored to match the inside diameter of the mating pipe or fitting so there will be no restriction of product flow. This prevents turbulence at the joint and reduces erosion. They also provide excellent stress distribution through the tapered hub and are easily radiographed for flaw detection.

This flange type will be welded to a pipe or fitting with a single full penetration, V weld (Buttweld).

Details of Welding Neck flange

1. Weld Neck flange   2. Butt Weld  3. Pipe or Fitting

Slip On flange

The calculated strength from Slip On flanges under internal pressure is two-thirds that of Welding Neck Flanges, and their life under fatigue is about one-third that of Welding Neck Flanges.

The connection with the pipe is done with two fillet welds, one on the outside and the other on the inside of the flange.

After sliding the flange on the pipe, the distance inside the flange between the end of pipe and the flange face should be: Wall thickness of pipe + 3 mm (1/64 inch). This space is necessary to prevent damage to flange face during the welding process.

Details of Slip On flange

1. Slip On flange   2. Filled weld outside  3. Filled weld inside   4. Pipe

Socket Weld flange

Socket Weld flanges were initially developed for use on small-size high pressure piping. Their static strength is equal to Slip On flanges, but their fatigue strength 50% greater than Slip On flanges.

The connection with the pipe is done with one fillet weld at the outside of the flange. But before welding, a space must be created between flange and the end pipe.

ASME B31.1 1998 127.3 Preparation for Welding (E) Socket Weld Assembly says:
In assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16" (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.

The purpose for this bottoming clearance in a Socket Weld is to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal. The image shows you the X measure for the expansion gap.

The disadvantage of this flange is due to this gap. In piping system using mainly stainless steel pipe and corrosive products, the crack between pipe and flange can give corrosion problems.

Details of Socket Weld Flange

1. Socket Weld flange   2. Filled weld   3. Pipe  X = Expansion gap

Lap Joint flange

Lap Joint Flanges have all the same common dimensions as any other flange named on this website however it does not have a raised face. Lap Joint Flanges are used in conjunction with "Lap Joint Stub Ends". These flanges are nearly identical to Slip On flanges with the exception of the radius at the intersection of the flange face and the bore in order to accommodate the flanged portion of the Stub End. Their pressure-holding ability is little, if any, better than that of Slip On flanges and the fatigue life for the assembly is only one tenth that of Welding Neck flanges. Lap Joint Flanges may be used at all pressures and are available in a full size range. These flanges slip over the pipe, and are not welded or otherwise fastened to it. Bolting pressure is transmitted to the gasket by the pressure of the flange against the back of the pipe lap (Stub End).

Lap Joint flanges have certain special advantages including:

  • Freedom to swivel around the pipe facilitates the lining up of opposing flange bolt holes.
  • Lack of contact with the fluid in the pipe often permits the use of inexpensive carbon steel flanges with corrosion resistant pipe.
  • In systems which erode or corrode quickly, the flanges may be salvaged for re-use.

Details of Lap Joint Flange

1. Lap Joint flange   2. Stub End  3. Butt weld   4. Pipe or Fitting

Stub End
A Stub End always will be used with a Lap Joint flange, as a backing flange.
This flange connections are applied, in low-pressure and non-critical applications, and is a cheap method of flanging.
In a stainless steel pipe system, for example, a carbon steel flange can be applied, because they are not come in contact with the product in the pipe.
Stub Ends are available in almost all pipe diameters. Dimensions and dimensional tolerances are defined in the ASME B.16.9 standard. Light-weight corrosion resistant Stub Ends (fittings) are defined in MSS SP43.

Lap Joint Flange with a Stub End


Threaded flange

Threaded Flanges are used for special circumstances with their main advantage being that they can be attached to the pipe without welding. However, a seal weld can also be used in conjunction with the threaded connection.

Although available in most sizes and pressure ratings, screwed fittings today are used almost exclusively in smaller pipe sizes.

A threaded flange or fitting is not suitable for a pipe system with thin wall thickness, because cutting thread on a pipe is not possible. Thus, thicker wall thickness must be chosen.

ASME B31.3 Piping Guide says:                                         
Where steel pipe is threaded and used for steam service above 250 psi or for water service above 100 psi with water temperatures above 220° F, the pipe shall be seamless and have a thickness at least equal to schedule 80 of ASME B36.10.

Details of Threaded flange

1. Threaded flange 2. Thread 3. Pipe or Fitting

Blind flange

Blind Flanges are manufactured without a bore and used to blank off the ends of pipes, valves and pressure vessel openings. From a standpoint of internal pressure and bolt loading, blind flanges are of the most highly stressed flange types, especially in the larger sizes. Because most of these stresses are of bending types near the center and there are no standard inside diameters, these flanges are suitable for higher pressure/temperature applications.

Details of Blind flange

1. Blind flange   2.Stud Bolt   3. Gasket   4. Other f lange


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