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Metal Bellow
Restrained Assemblies
Definition: Assemblies capable of restraining the pressure thrust of the system. Intermediate anchors are required to withstand the spring force generated when the expansion joint is deflected. The need for main anchors is eliminated.
Tied Single Bellows Assembly:
The addition of tie rods to a Single Bellows Assembly adds design flexibility to a piping system. The tie rods are attached to the pipe or flange with lugs that carry the pressure thrust of the system, eliminating the need for main anchors. With the assembly tied, the ability to absorb axial growth is lost. Only lateral and angular movement can be absorbed with the tied expansion joint. The addition of tie rods does not eliminate the need for a well-planned guide system for the adjacent piping.
Tied Universal Assembly:
Similar in construction to a Universal Assembly except that tie rods absorb pressure thrust and limit movements to lateral offset and angulation. Large offset movements are possible in a Universal Assembly by increasing the distance between the two bellows.
Hinged Bellows Assembly:
When a Hinged Expansion Joint is used, movement is limited to angulation in one plane. Hinged Assemblies are normally used in sets of two or three to absorb large amounts of expansion in high pressure piping systems. Only low spring forces are transmitted to the equipment. The hinge hardware is designed to carry the pressure thrust of the system, and often times, used to combat torsional movement in a piping system. Slotted Hinged Expansion Joints are a variant of the standard Hinged Expansion Joints that allow axial and angular movement. Be careful! Once a Slotted Hinge is introduced, torsion in the piping system is still resisted but the hinge no longer carries pressure thrust.
Gimbal Bellows Assembly:
The gimbal restraint is designed to absorb system pressure thrust and torsional twist while allowing angulation in any plane. Gimbal Assemblies, when used in pairs or with a Single Hinged unit, have the advantage of absorbing movements in multi-planer piping systems. The gimbal works the same as an automobile's universal drive shaft.
Basic Types of Expansion Joints
Unrestrained Assemblies:
Definition: Assemblies not capable of restraining the pressure thrust of the system. The pressure thrust must be contained using main anchors or the connecting equipment.
Single Bellows Assembly:
The simplest type of expansion joint consists of a single bellows element welded to end fittings, normally flange or pipe ends. The single bellows can absorb small amounts of axial, lateral and angular movement with ease, but adequate anchors and guides must be provided.
Universal Expansion Joint Assembly:
This assembly consists of two bellows connected by a center spool piece with flange or pipe ends. The universal arrangement allows greater axial, lateral and angular movements than a Single Bellows Assembly. Increasing the center spool length produces increased movement capability. Like the single, adequate anchors and guides must be provided.
Externally Pressurized Assembly:
Line pressure acts externally on the bellows by means of a pressure chamber. This allows a greater number of convolutions to be used for large axial movements, without fear of bellows instability. Externally Pressurized Expansion Joints have the added benefit of self-draining convolutions if standing media is a concern. Anchors and guides are an essential part of a good installation.
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