Where it is impractical to increase flexibility sufficiently, to reduce the piping stresses or equipment loads to acceptable limits, use of expansion joints or bellows should be considered. Expansion joints are often installed in piping and duct systems and can serve the following purpose:
- Compensate thermal expansion or contraction
- Reduce forces and moments on equipment connections
- Isolate vibration from connected systems
- Compensate misalignment
- Compensate foundation settlement
Hinged Expansion Joints
IMAGE COURTESY: SPIROFLEX
These expansion joints contain hinges which cause the unit to bend in one plane.
If hinged expansion joints are used, no loads are transferred to the pipe anchors by the pressure force; the load is carried instead by the hinges. A hinged joint cannot compensate for axial movements, it can compensate only lateral movements. A straight pipe section should be broken down into a L-shaped, Z-shaped or U-shaped system to use a hinged joint.
In some cases, the expansion joints are provided with slotted hinged pins which permit axial movement. In this case loads are transferred to the anchors.
Gimbal Expansion Joints
These joints consist of two sets of hinges which are set perpendicular to each other. They can accept bending in any plane. No forces are transferred to the anchor.
Universal Expansion Joints
IMAGE COURTESY: BELLOWS MFG.
It normally comprises of two bellows connected via an intermediate pipe and a system of tie rods to withstand the internal pressure thrust. Universal joints can take care of axial, lateral and angular movements. Universal expansion joints can permit small axial movements but are capable of absorbing large amounts of lateral deflection. These joints are used when large lateral deflections are anticipated in piping systems. The amount of lateral deflection capability can be increased or decreased by changing the length of the spool pipe between the two bellows.
When the expansion Joint is provided with 2 tie rods at 180 degrees, the expansion joint is free to deflect angularly and laterally. With three or more tie rods only lateral deflections are permitted. These joints are use in steam and hot water distribution systems because of the large amounts of movement they can absorb with a minimum of guiding and anchoring. When using a universal expansion joint, anchors must be sufficiently designed to withstand the full thrust forces .
Pressure Balanced Elbow Expansion Joints
IMAGE COURTESY: SPIROFLEX
In this type of expansion joint, the pressure thrust is balanced such that no anchoring is required. A pressure balanced expansion joint accommodates axial and lateral movements and counteracts the bellows pressure thrust. An additional bellow is included into the assembly which is subject to the line pressure and generates a force equal and opposite to that on the main bellows. Tying these bellows in one assembly balances the pressure load on the unit. This feature is important when connecting pipes to turbines, pumps, compressors or other equipment which can accept low external loads.
Inner sleeves
Inner sleeves are specified on bellows if deposits or abrasion are likely. The sleeves extend the life of joint by protecting the convolutions from direct flow impingement. It is important to ensure that the internal sleeve does not impair lateral or angular movement. In some designs the inner sleeve is reduced in diameter to allow lateral movement.
Internal sleeves should not be used when high viscosity fluids are transmitted through the piping system.
Protective Cover
Sometimes external protective covers are specified on the expansion joints. They are specified when environmental conditions are abrasive such as in desert conditions.
Tie-rods
Tie-rods are used to restrain the pressure thrust of expansion joint. Expansion joints with two tie-rods can be used to only allow lateral movements. In some cases three tie-rods are used which serve as limit rods. Limit rods are used to restrain over compression or extension in expansion joints.
Additional resources
A number of resources can be found on expansion joints. Listed below are links to some useful information.
Expansion Joint Guide from US bellows Inc.
Design and draw a custom expansion joint from Senior Flexonics Pathway.