Tied Bellows - Simple vs. Complex Model - CAESAR II - Reference Data

CAESAR II Applications Guide

Reference Data

Although there are no strict rules that dictate when to use a simple model or when to use a complex model, there are some basic guidelines that you can follow.

Complex Model Guidelines

Simple Model Guidelines

Use a complex model when any the following conditions exist:

Use a simple model when any of the following conditions exist:

  • You are investigating a failure.

  • The pipe diameter and number of convolutions become large.

  • The nuts are only on the outside of the flange, allowing the tie bars to only carry tension.

  • You have good values for the load distribution in the tie bars. Simple models give no indication of the load distribution. In cases where the tie bars combine to resist relative bending of the joint ends, one pair of tie bars can be in compression while the other pair is in tension. This effective redistribution of load in the tie bars is never observed in a simple model. When this does occur, and if the tie bars are very long, buckling of the rods in the complex model should be investigated (evaluate whether the rods can withstand the compressive forces reported in the output report).

  • The single tied bellows is designed to absorb movement by lateral deflection only. There is no axial deflection or relative bending rotations at the joint ends.

  • For bellows with only two tie rods, there can only be rotation in one direction.

  • The tie bars are either guaranteed to be carrying tension or have nuts on either side of the flange and can carry compression, if needed.

  • There is no relative rotation of the ends.

Because of the uncertainty of the application, enter the lateral instead of the bending spring rate from the manufacturer’s catalog.

Add the weights of the bellow and associated hardware to the flange weights on either side of the bellow. This is particularly true if the expansion joint is between a hanger to be sized and an anchor.

When using expansion joints, verify that the displacement limits for the expansion joint after the protected equipment loads are within the allowables. In CAESAR II, you can use the Analysis > Expansion Joint Rating command to help compute relative bellow movements for evaluating the bellow distortion.

You can build simple models of single tied bellows by entering a large axial stiffness. This axial stiffness simulates the tie bars, preventing relative axial movement of the bellows. You can model tie rods with a single rigid element along the centerline of the bellow. With zero weight and rotational restraints, this prevents the ends of the joint from rotating relative to one another. In reality, the tie bars being offset from the centerline prevent this rotation.

You can build complex models of tied bellows by running pipe elements whose diameters are equal to the diameter of the tie bars, and whose wall thicknesses are equal to half of the tie bar diameter, between rigid elements that extend normal to the pipe axis and from the centerline and to their intersection with the tie bar centerline. For more information on building complex models, see the Tied Bellows Expansion Joint - Complex Model.

Some manufacturers believe that friction at the tie bar ends, plus other effects, serve to limit the overall lateral flexibility of this joint. A 30% increase in lateral stiffness is sometimes used to compensate for these frictional effects. Field situations, such as loose nuts on tie bars, can be modeled using the complex expansion joint model.