All piping systems must be designed to withstand sustained loadings. Sustained loads are classified as those caused by mechanical forces which are present throughout the normal operation of the piping system. Therefore these loads:
Are force driven, as opposed to displacement driven, and
Are present for relatively extended periods of time, as opposed to those which change dynamically.
Typical sustained loads consist of:
Pressure — loads due to operating (or design) pressure,
Weight — uniform loads due to the weight of the pipe, fluid, and insulation, and concentrated loads due to the weight of in-line components (such as valves, flanges, etc.), and
Spring hanger pre-loads and other applied forces.