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Expansion joints are often included in industrial piping systems to accommodate movement due to thermal and mechanical changes in the system. When the process requires large changes in temperature, metal components change size. Expansion joints with metal bellows are designed to accommodate certain movements while minimizing the transfer of forces to sensitive components in the system.

Pressure created by pumps or gravity is used to move fluids through the piping system. Fluids under pressure occupy the volume of their container. The unique concept of PRESSURE BALANCED EXPANSION JOINTS (PBEXJ) is they are designed to maintain a constant volume by having balancing bellows compensate for volume changes in the bellows (line bellows) which is moved by the pipe. An early name for these devices was "pressure-volumetric compensator".

Two general configurations of constant volume expansion joints are those which are installed where the pipe changes direction and those installed where the pipe does not change direction (often called "in-line"). The figure shown on the left is an example of a PBEXJ used where the pipe changes direction. The two bellows are mechanically linked so any change in volume in the line bellows due to change in its length, either compression or extension, is compensated by an opposite change in the balancing bellows. In this PBEXJ the two bellows have the same diameter so the volume does not change during compression or extension of the line bellows. Pressure thrusts are transferred to the members which tie the bellows together rather than in the piping system.

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