Expansion Joint Glossary of Terms

Finished Products

  • Angular Rotation: Bending about the longitudinal centerline of the expansion joint.
  • Axial Extension: Extension of the bellows length due to pipe contraction when piping system is anchored properly.
  • Axial Compression: Compression of the bellows length due to the pipe expansion when piping system is anchored properly.
  • Bellows: 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.
  • Concurrent Movement: Simultaneous movement axially, laterally and angularly.
  • Control Rod: Devices attached to the expansion joint with the primary function of distributing movement between the two bellows of a universal joint.
  • Convolution or Corrugation: The smallest flexible unit of a bellows, with total movement of a bellows being proportional to the number of convolutions.
  • Cycle: A cycle is one complete movement from initial position to operating position and back.
  • Cycle Life: Also known as fatigue life expectancy, is affected by various factors including (but most limited to): operating pressure, operating temperature, bellows material, and bellows design/profile. Change to any of these factors will change cycle life.
  • Directional Anchor: A directional anchor, or sliding anchor, is one that is designed to absorb loads 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.
  • External Cover/Shroud: Expansion Joints require careful handling; they must be protected from any impact, weld spatter, etc. Before installation of an expansion joint, care must be taken that foreign material is not trapped in the corrugations. It is suitable to install a metal cover over the flanges to protect the convolutions and then wrap the insulation around it.
  • Flow Liner: Installed in the inlet bore of the expansion joint to protect the bellows from erosion damage due to abrasive media or resonant vibration due to turbulent flow or excessive velocities. Directional flow arrow should be placed on the exterior of the expansion joint to indicate the direction of flow.
  • Intermediate Anchor: An intermediate anchor is one which divides a pipeline into individual expanding pipe sections containing multiple expansion devices of the same pipe size. 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.
  • Lateral Offset: Motion, which is perpendicular to the plane of the pipe with the expansion joint fittings remaining parallel.
  • Limit Rod: Devices with the primary function of restricting the bellows movement range. The limit rods are designed to prevent bellows over-extension or over-compression while restraining the full pressure thrust in the event of a main anchor failure.
  • Main Anchor: A main anchor is one installed at any of the following locations in a pipe system containing one or more bellows:
  1. At a change in directions of flow
  2. Between two bellows units of different size installed in the same straight run 
  3. At the entrance of a side branch onto the main line
  4. Where a shut-off or pressure-reducing valve is installed in a pipe run between two bellows units at a capped end of pipe.
  • A main 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 an unrestrained bellows, these will consist of the pressure thrust, the force required to deflect the bellows unit, and the frictional force due to the pipe moving over its guides.
  • Pipe Alignment Guide: A pipe alignment guide is a form of sleeve fastened to some rigid part of the installation, which permits the pipeline to move freely in only one direction, i.e. along the axis of the pipe. Pipe alignment guides are designed primarily for use in applications involving axial movement only.
  • Pipe Support: A pipe support is any device, which permits free movement of the piping and carries the total weight of in line equipment such as valves, meters, expansion joints, and the weight of the contained fluid. Pipe supports cannot be substituted for pipe alignment guides. Pipe rings, U-bolts, roller supports, and spring hangers are some examples of conventional pipe supports.
  • Planar Guide: A directional pipe planar guide is a pipe alignment guide modified to permit limited movement and/or bending of the pipe in one plane. It is used only in applications involved lateral deflection or angular rotation resulting from 2- or 3-hinge piping configurations.
  • Pressure Thrust: Extension of the bellows due to line pressure. This pressure thrust must then be absorbed by some means or the line pressure will cause the bellows to over extend and tear itself apart. 
  • Spring Rate: In very low pressure application the more significant force, transmitted to the piping system, 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 in the spring rate.
  • Squirm: Structural instability caused by internal pressure on the bellows.
  • Thermal Movement: Expansion and/or contraction due to temperature changes. In a piping or ducting system, these thermal changes can produce stress on the system at fixed points such as vessels, rotating equipment as well as the piping or duct work system itself.
  • Tie Rod: Devices with the primary function to restrain the bellows pressure thrust.
  • Torsion: Twisting about the longitudinal axis of a metal expansion joint when it is located at any point in a piping system that would impose torque as a result of thermal change. Structure settling, or a seismic event will impose torque.