3.3 Expansion Joint Modeling And Evaluation
Expansion joints are used when it is necessary to provide a large amount of flexibility in a small space. Expansion joints are constructed out of sheet metal, which, after rolling and welding to for a cylinder, has convolutions (also called corrugations) formed in it via either hydraulic pressure or rolling. Expansion joints may vary in terms of the number and type of convolutions, the material, the number of plies, all of which effect the pressure capacity, the stiffness, and the allowable movement.
For the most part, these details are taken care of by the expansion joint manufacturer. A typical expansion joint piping design proceeds:
The decision is made to use an expansion joint in the piping system. (In many design problems the joint is used to protect a sensitive piece of equipment from excessive nozzle loads.)
Based upon the design temperature and pressure, a standard expansion joint is selected from a manufacturer's catalog. The properties of that bellows are then inserted into the piping model.
If the bellows reduces loads and stresses as intended then the range of expansion movements on the bellows must be checked. For each bellows there is a limit to the cumulative axial, bending and lateral displacement that can be absorbed by the joint without excessively deforming the convolutions or causing fatigue failure. These limits are presented in different ways in different manufacturer's catalogs, but are always functions of the number of applied cycles, bellows material properties and convolution shape. Where excessive displacement is a problem, increasing the number of convolutions can be the solution.
Once the bellows movement is within the allowable range of movements, the design is completed. A competent expansion joint manufacturer should be able to provide assistance throughout the design stage as required.
Modeling And Analysis Of The Piping System