The flexible element of an expansion joint consisting of one or more convolutions.
Bellows expansion joint
Any device containing one or more bellows used to absorb directional changes, such as those caused by thermal expansion or contraction of a pipeline, duct, or vessel
Bellows side effects
We have noted how placing a flexible metal bellows at selected locations in a piping system can accomodate its thermal growth. There are some side effects which occur when a pipe is cut in two and a bellows (flexible seal) is inserted to take up the movement of the pipe:
Pressure Thrust: By its very nature of being flexible (to absorb movement) a bellows will extend (straighten out) due to the line pressure. This pressure thrust must then be absorbed by some means or the line will tear itself apart. This force may be accommodated by anchoring the pipe or by using an expansion joint which incorporates tie rods, hinges, gimbals, or pressure balancing bellows.
Spring Rate: In very low pressure applications the more significant force may be the spring rate which is expressed in pounds per inch of motion. Thus, as the pipe grows due to increasing temperature, the bellows will resist compression by the force noted as the spring rate.
A comparison of pressure and force data to spring rate will show that it does not require very much line pressure for pressure thrust to be the dominant factor of the two in expansion joint applications.
Control rods are devices attached to the expansion joint with the primary function of distributing movement between the two bellows of a universal expansion joint. Control rods are not designed to restrain bellows pressure thrust. (These devices do not serve the purposes defined by the Fluid Sealing Association as applied to rubber expansion joints.)
The smallest flexible unit of a bellows with total movement of a bellows being proportional to the number of convolutions.
Expansion joints require careful handling and must be protected from any impact, weld spatter, etc. Before insulating an expansion joint, care must be taken that foreign material is not trapped in the convolutions. It is suitable to install a metal cover over the flanges and then wrap the insulation around it.
A directional anchor, or sliding anchor, is one which is designed to absorb loadings in one direction while permitting motion in another. It may be either a main or intermediate anchor, depending upon the application considered. When designing a directional anchor, an effort should be made to minimize the friction between its moving or sliding parts, since this will reduce the loading on the pipe and equipment, and will ensure proper function of the anchor.
Installation of expansion joints require special attention to ensure proper function.
Intermediate anchors are designed to withstand differences in spring forces and friction forces. In the case of different pipe diameters, the anchor must be designed to withstand the difference in pressure thrust of the two pipe sections. Anchors at the ends of pipe sections are usually called main anchors and must be designed to withstand the full pressure thrust of the bellows in addition to other forces.
*A pipe section in which an expansion joint is installed must be firmly fixed with anchors at both ends.
*Expansion joints should be installed adjacent to an anchor; otherwise, a pipe guide must be installed on each side of the extension joint.
*Pressure Thrust is the result of the bellows effective area multiplied by the medium pressure. If the pressure changes, pressure thrust changes.
*Forces transmitted to the pipe anchors are the sum of the spring rate, friction forces of pipe guides, and pressure thrust of the bellows. (Provisions for pressure thrust are not required if the expansion joint has tie rods.)
When pipe movement requires the use of several axial expansion joints, intermediate anchors are necessary. Only one expansion joint can be installed between two anchors.
An intermediate anchor is one which divides a pipeline into individual expanding pipe sections. Such an anchor must be designed to withstand the forces and moments imposed upon it by each of the pipe sections to which it is attached. In the case of a pipe section containing one or more bellows units, these forces will consist of forces and/or moments required to deflect the bellows unit plus the frictional forces due to the pipe moving over its guides. The pressure thrust is absorbed by the other anchors or devices on the bellows unit such as limit rods, tie rods, hinged restraints, etc.