The pipe flanges are the second most used joining method after welding. They are used when joints need dismantling. It provides flexibility for maintenance. Flange Connects the pipe with various equipment and valves. Breakup flanges are added in the pipeline system if regular maintenance is required during plant operation.

In this article, the following types of flanges are covered.

  • Threaded Flanges
  • Socket-Weld Flanges
  • Slip-On Flanges
  • Lap Joint Flanges
  • Weld Neck Flanges
  • Blind Flanges

A flanged joint is composed of three components; flanges, gaskets, and bolting; which are assembled by pipefitter. Special controls are required in the selection and application of all these elements to attain a joint, which has acceptable leak tightness.

However, it is not advisable to use a flange connection in underground piping when it supposed to be buried. The flange is also the most common source of leak and fire in a process plant. There are a variety of flanges available to suit the requirements. A flange can be classified in several alternate ways based on the following:

  1. Types of Connection
  2. Flange facing Types
  3. Pressure-Temperature Ratings
  4. Material Types

Threaded Flanges

Threaded Flanges are also known as screwed flange, and it is having a thread inside the flange bore which fits on the pipe with matching male thread on the pipe. This type of joint connection is speedy and simple but not suitable for high presser and temperature applications. Threaded Flanges are mostly used in utility services such as air and water.

  • Threaded Flange mostly used in utility services such as air and water
  • It is not suitable for high presser and temperature applications
  • It also available in limited Size of NPS 4” and below
  • Threaded Flanges can be FF or RF
  • It is a Low-cost flange. 

Socket-Weld Flanges

Socket-Weld Flanges has a female socket in which the pipe is fitted. Fillet welding is done from outside on the pipe. Generally, it is used in small bore piping and only suitable for low pressure and temperature application.

  • Socket-Welded Flanges can be FF or RF
  • The cost of flange and fabrication is moderate.
socket weld flange crossection

 Slip-On Flanges

Slip-On flange has a hole with matching outside diameter of pipe from which pipe can pass. The flange is placed on pipe and fillet welded from both inside and outside. Slip-On Flange is suitable for low pressure and temperature application.

  • This type of flange available in large size also
  • It can be FF or RF
  • The cost of flange and fabrication is moderate.
Slipon flange crosssection

Lap Joint Flanges

Lap flange is having two components, a stub end, and a loose backing flange. Stub end is butt-welded to the pipe and Backing flange freely moves over the pipe. The backing flange can be of a different material than stub material and normally of the carbon steel to save the cost. Lap flange is used where frequent dismantling is required, and space is constrained.

  • The backing flange can be of a different material than stub material and normally of the carbon steel to save the cost.
  • This flange provides better joining due to butt-weld as compared to a socket and threaded type flanges
  • Not suitable for small size, Costly components, and fabrication as compared to threaded, slip-on, and socket flange.
  • Lap flange provides connection flexibility as you can freely rotate the flange on the pipe. It is used where frequent dismantling is required, and space is constrained.  
Lap Flange
Lap Flange Crossection

Weld Neck Flanges

Weld neck flange is the most widely used type in process piping. It gives the highest level of joint integrity due to Butt-welded with a pipe. These types of flanges are used in high pressure and temperature application. Weld neck flanges are Bulky & costly with respect to other types of the flange.

  • High skill is required for fabrication and also required more space to accommodate in the piping system due to the long hub.
  • Weld Neck Flange is available in all sizes & it can be FF, RF or RTJ type
  • You can see the Weld Neck flange welded with pipe
Weld Neck Flange
crossection of weldneck flange

Blind Flanges

The blind flange is a blank disc with a bolt hole. These types of flanges are used with another type of flange to isolate the piping system or to terminate the piping as an end. Blind flanges are also used as a manhole cover in the vessel.

Types of Flange Based on the Face

Depending on the flange facing Types, it can be further classified as

  1. Flat Face (FF)
  2. Raised Face (RF)
  3. Ring Joint (RTJ)
  4. Tongue and groove (T&G)
  5. And Male and Female type
Flange Facing Types

Flat Face

As the name suggests, the flat face flange has a flat face. Flat face flanges are used when the counter-flanges are flat face. This condition occurs mainly in connection to Cast Iron equipment, valves, and specialties. Full face gasket is used when a flat face flange is used.

Raised Face

Raised face flange has a small portion around the bore is raised from the face. The gasket seat on this raised face. The height of the raised face depends on the flange pressure-temperature rating that is known as a class of the flange. For 150# & 300# height of the raised face is 1/6” and above 300# it is 1/4”. The inside bore circle type of gasket is used with a raised face flange.

RTJ Face

Ring joint type face flange has a specially designed grove in which metal gasket seat. This type of flange is used in high pressure and temperature services.


Flange Pressure-Temperature Class (Service Rating)

The flanges are classified as per their pressure-temperature ratings which are designated as 150#, 300#, 400#, 600#, 900#, 1500#, and 2500#. Large diameter flanges that are 24” to 60” are available up to 900# class. Pressure-temperature ratings are maximum allowable working gauge pressures in the bar & the temperatures in degrees Celsius.

Higher the rating, heavier the flange, and can withstand higher pressure and temperature. When the temperature goes up, the pressure goes down, and vice versa. Please note that different material has different pressure ratings. Learn more about rating and class.

Flange Materials

Flanges are manufactured from following materials

  • Carbon steel
  • Low alloy steel
  • Stainless steel
  • Combination of Exotic materials (Stub) and other backing materials

The list of materials used in manufacturing are covered in ASME B16.5 & B16.47.

  • ASME B16.5 -Pipe Flanges and Flanged Fittings NPS ½” to 24”
  • ASME B16.47 -Large Diameter Steel Flanges NPS 26” to 60”

Commonly used Forged material grads are

  • Carbon Steel: – ASTM A105, ASTM A350 LF1/2, ASTM A181
  • Alloy Steel: – ASTM A182F1 /F2 /F5 /F7 /F9 /F11 /F12 /F22
  • Stainless Steel: – ASTM A182F6 /F304 /F304L /F316 /F316L/ F321/F347/F348

Flange Inspection

During an inspection you have to check the following;

  • Outer & Inner Diameter of body
  • Bolt Circle & Bolt hole Diameter
  • Hub Diameter & thickness of weld end
  • Length of the Hub
  • Straightness and alignment of the bolt hole

ASME B16.5 and B16.47 standards covers permissible tolerances for inspection.

Flange inspection

Marking on the Flange Body

Marking of flange must includes the following;

  • Manufacturer logo
  • ASTM material code
  • Material Grade
  • Service rating (Pressure-temperature Class))
  • Size
  • Thickness (Schedule)
  • Heat No
  • Special marking if any QT (Quenched and tempered) or W (Repair by welding)
Flange Marking

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