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What is a Flange Joint?

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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 10 September 2016
  • Copyright Protected:
    2003-2016
    Conjecture Corporation
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A flange joint is a non-permanent, removable method of joining or blanking off steel and plastic pipes. Flange joints consist of a pair of identical discs with a row of attachment holes close to their outer circumference. When two pipes are to be joined, the two discs are placed against each other in such a way that the holes line up. Bolts are then passed through the holes and tensioned, thus pulling the flanges against each other firmly. Flanges are available in a range of designs including slip-on flanges, raised face flanges, and lap joint flanges all of which suit differing pipe system requirements.

The flange joint is one of the most convenient and efficient removable pipe joining and blanking systems in common use. Flange joints are suitable for a range of pipe designs, materials, and sizes and are commonplace on systems as diverse as large municipal water mains and small high pressure steam lines. Flange joints are quick to assemble and disassemble and may be included on stock pipe lengths or installed in-situ on existing pipe circuits. The flange joint allows for quick and effective connecting of pipe sections and for joining sections of piping to ancillary equipment such as pumps, valves, and metering devices. The wide range of types also means that there is a flange joint suitable for most transported mediums.

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A typical flange joint consists of two identical discs or plates which feature similarly placed sets of mounting holes arranged around the periphery. Some flanges are permanently welded or epoxied onto the tube or even cast as part of the pipe or equipment. Other flange designs feature a slip-on disc which pulls up against a collar or shoulder permanently attached to the pipe. When the joint is made, the two plates or discs are placed against one another and bolts are passed through the holes. When the bolts are tensioned, the two plates pull up hard against one another, thus forming a secure joint.

Depending on the application and the particular flange design, a packing or gasket may be placed between the discs prior to tensioning. The tensioning force then compresses the gasket to form a good hermetic seal. Flanges often have corresponding slots and protrusions in their faces or sets of concentric serrations which aid in seating the gasket. These seals are typically used on pipe systems carrying high pressure fluids or gases.

There are a wide range of flange joint designs on the market which cater to an equally diverse selection of pipe types and applications. These include threaded flanges which screw onto the pipe and slip on or lap joint flanges with free rotating discs. These free flanges joints are the easiest to install on existing pipe systems because the discs can be rotated to line up the holes when the pipes can't. Other flange joint types include blind flanges used to blank off unused pipe ends and insulated flanges which prevent cathodic corrosion by electrically insulating pipe sections. Flanges are all made according to sets of standard dimensions which ensure cross application compatibility and ease of use.

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