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Large Axial Movements

Sat, 07/06/2014 - 11:12 -- Tim Robinson
Axial Movement

The problem of how best to design for large axial movements in long pipe runs can be approached in a number of ways, all with different advantages.

Lateral

Probably the simplest approach is to deal with all the axial movement at the end of the pipe run using a tied lateral joint set at 90 degrees to the direction of axial movement. This requires an elbow to be installed but provides the simplest expansion solution, lowest spring rate and elimination of pressure thrust. The lateral joint centre spool should be designed to be long enough to keep angulation in the bellows ideally under 5 degrees. Depending on the application the lateral joint may have simple tie rods, hinges or gimbals.

Pressure balance

If no change in direction of the line is available the expansion joint must provide axial movement and pressure thrust must be considered. The only solution to eliminate the thrust force is a pressure balanced joint. This is a solution for pressure thrust but add 2-3 times the spring force of a simple axial joint. Care must also be taken if the media is corrosive or can easily condense as the pressure balance portion of the joint is liable to become relatively cool.

Externally Pressurised

If pressure thrust can be designed for but low spring rate and large movements are required an externally pressurised expansion joint may be the best solution. The external pressurisation allows for many convolutions to be used without squirm and large movements achieved within a few joints.

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