How to Specify Slip Joints in Piping Systems

Figure 1: Specifying the longitudinal stiffness for a slip joint.

Slip joints are connectors in piping systems that allow for significant axial expansion. A typical application for a slip joint is a pressurized piping system such as a boiler, a wind tunnel or an oil pipeline.

In the PipePak piping design and analysis system, a slip joint is a specifically defined expansion joint, which allows translational movement in the direction of the longitudinal axis but all other directions are rigid. Such an expansion joint can be modeled (in either PipePlus or Superdraw III) by using a bellows element as follows:

• In the PipePlus spreadsheet, press "b" when the cursor is in the "Extra data" column to specify the "Bellows" option for the point. The "Expansion Joint Definition" screen will appear. Specify the value for the "Longitudinal stiffness" field (see Figure 1) and then click on the "OK" button.
• In Superdraw III, use the "FEA Add:PipePak Analysis:Bellows..." command sequence. The "Bellows" pop-up menu will appear. Click on the "Values..." button to specify the translational stiffness value in the longitudinal axial direction. Then, click on a line of the model to add a bellows.

A bellows element is a generalized structure element, which can be used to simulate almost any connector including expansion joints. In addition to a slip joint, the following kinds of expansion joints can be simulated:

• Gimbal expansion joint - allows rotation in both the horizontal and vertical axes; no translational movement is possible.
• Universal joint - allows deflection in all but the torsional direction.

For more information about slip joints, expansion joints and the bellows element, see the PipePak User's Guide.