Universal Expansion Joints - Simple Model - CAESAR II - Reference Data

CAESAR II Applications Guide

PPMProduct
CAESAR II
PPMCategory_custom
Reference Data
Version_CAESAR
12

The tied universal bellows is designed to absorb movement by lateral deflection only. There is no axial deflection or relative bending rotations at the joint ends, assuming three or more tie rods exist.

Instead of lateral spring rates, enter bending spring rates from the manufacturer’s catalog. For more information about bellows stiffnesses, see Simple Bellows with Pressure Thrust.

Manufacturers publish a wide variety of data for universal expansion joints. In most cases, the published spring rates are for the universal joint as an assembly. When the lateral stiffness is given for the whole assembly, the simple or complex models of single bellows can be used. In this case, the manufacturer must also provide a cumulative assembly displacement limit so that the piping designer can verify that neither of the bellows are over-extended.

Many universal expansion joint assemblies have stops along the tie bars that are connected to the center spool-piece. These stops are designed to prevent over-extension of the bellows and can be modeled in the complex universal joint model. For the simple universal joint models, you must check the results to verify that the stops are not engaged. Stops should typically be considered a safety feature and should not be included as a working part of the design unless particular attention is paid to the design surrounding the stop components.

Check the displacement limits for each of the expansion joints after the protected equipment loads are within the allowables. You can use the Analysis > Expansion Joint Rating command to calculate relative bellows movements for evaluating the strength of the convolution. The Expansion Joint Rating analysis module works only on single bellows, which requires that you first model and then check each bellows in the universal assembly.

Some manufacturers believe that friction at the tie bar ends, plus other effects, serve to limit the overall lateral flexibility of this joint. A 10% increase in overall lateral stiffness is sometimes used to compensate for these frictional effects.

The complex models are built 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.

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

Field situations, such as loose nuts on tie bars, can be modeled using the complex expansion joint model.

Descriptions of various universal models are shown below. Each model also includes example inputs. Only use simple models when you know that both ends of the tie bars are fixed to the flanges, that is, when there are nuts on both sides of the flange.

The top drawing shows nuts on only one side of the flange at the left end. Model this configuration with a complex joint model unless you are certain that all tie bars will remain in tension.

The top model is used when you are given global assembly data for the universal, such as the assembly lateral stiffness. The second model is used when you are given angular spring rates for each of the two bellows used in the model.

When provided equivalent single bellows lateral stiffness for the whole assembly:

The model below does not show the addition of any extra
hardware or bellows weights, which could affect load distribution
and spring hanger design in the area.

When provided individual bellows angular stiffness:

Tie rods and center spool pieces should normally be modeled at ambient temperature.

Pressure thrust is contained by double-nutted tie rods. Effective ID and axial restraints can be eliminated.

This model does not show the addition of any extra hardware or bellows weights, which could affect weight load distribution and spring hanger design in the area.