Understanding Expansion Joint Industry Terminology
This glossary defines eleven terms used in expansion joint selection: six related to pressure ratings and five related to movement and spring rate. Definitions follow standards established by the Fluid Sealing Association.
Pressure Terminology
Operating Pressure
The actual pressure in which the system performs under normal working conditions. This value may be positive or negative (vacuum).
System Design Pressure
The maximum pressure expected during operation. Typically, the calculated operating pressure plus safety allowance.
Expansion Joint Design Pressure
The maximum pressure the expansion joint can handle.
Surge Pressure
Operating pressure with added increase the joint will be subjected to for short increments due to pump start up, valve closings, etc.
Maximum Allowable Pressure
Defines the maximum pressure recommended for a specific expansion joint.
Hydrostatic Test Pressure
This is used to demonstrate system or expansion joint capability. The standard for this test is 1-1/2 times the maximum allowable pressure held for ten minutes without leaks.
Movement & Spring Terminology
Axial Compression
Movement along the central axis reducing the face-to-face measurement.
Axial Elongation
Movement along the central axis resulting in an increase of the face-to-face measurement.
Lateral or Transverse
Movement is a shear force perpendicular to the central axis.
Angular
A bending force where the faces of the joint are no longer parallel.
Torsional
A twisting motion not recommended for most expansion joints.
