What Is a Plastic Flange?

Paul Scott
Paul Scott
Woman holding a book
Woman holding a book

A plastic flange is a piping component used to join two lengths of plastic pipe. The flange typically consists of a flat disc of a larger diameter than the pipe, with each disc featuring an equally-spaced row of holes arranged around its surface a short way in from its outside edge. To join the pipes, a flange is attached to one end of each length of pipe. The two flanges are then placed flush against one another and bolted together through the holes effectively joining the two pipes together. In high pressure systems, additional packings or O-rings are placed between the flanges to enhance the integrity of the seal that they form.

Plastic pipes are used in a wide range of applications from industrial installations to domestic irrigation and potable water supplies. They are inert and sanitary, lightweight, and, through the use of plastic flange systems, easy-to-join. The flange joint is cost-effective, simple, and easy to assemble and disassemble when the need arises. The basic principle on which the plastic flange joint, and. in fact, most flange joints, are based is the bolting or screwing together of two flat discs attached to the ends of the individual pipe lengths.

There are two basic types of plastic flange, namely free and captive disc types, with the captive disc type being the simpler of the two and most commonly used for small gauge plastic pipes. These flanges consist of a sleeve with a flat disc of a larger diameter on its one end. A row of equally-spaced holes is located around the disc surface a short distance from its outer edge. Two sleeves are either screwed or epoxied onto the ends of the two pipe sections with the discs towards the outside. The discs are then placed flush against each other and bolted together through the holes pulling the pipes together and forming a fluid tight joint.

This type of plastic flange is easy and quick to use, but does require at least one pipe section to be free to rotate so that the holes in the individual flange discs can be aligned with each other. In cases where the pipes cannot be rotated, as is often the case in large, high-pressure systems, a free disc plastic flange is more suitable. These flanges consist of a simple flat plastic disc similar to those found on the captive disc flange, but without the integral sleeve. The disc slips around the pipe and is stopped from exiting by a small lip at the end of the pipe. The disc has a countersunk groove along its inner edge into which the lip fits, preventing it from obstructing the joint.

This arrangement sees the flange disc rotate freely around the pipe, so, when the time comes to bolt the two flanges together, there is no need to rotate the pipes to align the holes in the discs. These high-pressure flanges often feature additional grooves cut into the disc surfaces into which a packing or O-ring is inserted prior to tightening the bolts. This serves to further enhance the quality of the seal between the two flange units.

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