BASIC PRINCIPLES: Metal Bellows Expansion Joints

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WARNING: Bellows cannot absorb torsional rotation (the twisting of one end with respect to the other end about its longitudal axis).

We have just noted how placing a flexible metal bellows at selected location in a piping system can accommodate its thermal movement. There are some side effects which occur when a pipe is cut in two and a bellows (flexible seal) inserted to take the movement of the pipe.

1. Pressure Thrust
   By the nature of its being flexible (to adsorb movement) a bellows will extend (straighten out) due to the line pressure. The pressure thrust must then be adsorbed by some means or the line will tear itself apart. The force may be accommodated by anchoring the pipe or by using an expansion joint which incorporates tie rods, hinges, gimbals or pressure balancing bellows.
2. Spring Rate
   In very low pressure applications the more significant force may be the spring rate which is expressed in pounds per inch of motion. Thus, as the pipe grows due to increasing temperature the bellows will resist compression by the force noted in the spring rate.

A comparison of pressure and force data to spring rate will show that it does not require very much line pressure for pressure thrust to be the dominant factor of the two in expansion joint applications.

*From: Hyspan Precision Product's “Technical Notes for Metal Bellows Expansion Joints and Packed Expansion Joints”, © 1995 Hyspan Precision Products, Inc.