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Pipe Joints Method : Pipe Flanges and Classification

Introduction to flange joint:

Flange joint is one of the methods of pipe joints. Pipe flanges are used at places where piping, instruments, valves, equipment etc. are required to be dismantle for maintenance. Flange joint is an assembly of three different components: Flanges, Gaskets & Bolting brought together in a proper method by some external source. Special measures shall be considered in selection and application of each component of the joint to achieve acceptable leak tightness.

The flanges are normally made of forging only in few cases it can be fabricated from plates. In ASME B16.5, only Blind flange and reducing flange can be fabricated from a weldable quality plates.

The generally used flange materials are as below:

ASTM A105- Forged Carbon Steel

ASTM A181- Forged Carbon Steel for General Purpose

ASTM A182- Forged Alloy Steel and Stainless Steel

ASTM A350- Forged Alloy Steel for Low Temperature Services.

Different ways of classification of pipe flanges are:

1.1) Pipe flanges based on Pipe Attachment:

One way to classify flanges is their method of attachment to the pipe.

Slip-on Flanges

Socket-Weld Flanges

Screwed/Threaded Flanges

Lap-joint Flanges

Welding neck Flanges

Blind Flanges

Reducing Flanges



1.2) Pipe flanges based on Pressure-Temperature (PT) rating:

Based on the PT rating the flanges are classified as:

150#, 300#, 400#, 600#, 900#, 1500#, 2500#

The PT rating charts as specified in ASME B16.5 specify the working pressure which a flange can withstand at a particular temperature. The indicated pressure class of 150#, 300#, etc is the basic ratings and the flanges can withstand higher pressure at lower temperatures.

ASME B16.5 indicates the allowable pressure for different materials of construction at different temperature.

1.3) Pipe flanges based on Facing:

There are different flange facing used in the pipe system depending upon the fluid, PT rating, material of construction, connecting equipment etc.

Flat Face (FF) Flanges

Raised Face (RF) Flanges

Male-Female (M/F) Face Flanges

Tongue-Groove (T/G) Face Flanges

Ring Type Joint (RTJ) Flanges

1.4) Pipe flanges based on Face Finish:

Stock Finish:

These flanges are used with non-metal gaskets. The allowable roughness for these type of Flanges is 250-500 or 500-1000 micro inch AARH.

Smooth/Serrated Finish:

These flanges are used with metal or spiral wound gaskets. The allowable roughness for these type of Flanges is 125-250 micro inch AARH.

Extra Smooth Finish:

These flanges are used with metal RTJ and T/G flanges. The maximum allowable roughness for these type of Flanges is 63 micro inch AARH.




1.5) Pipe flanges based on Serrations:

Concentric Serrations:

These serrations are used where the conveying fluid is highly flammable or toxic or lower molecular weight gas.

Spiral Serrations:

These are generally used type serrations.

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