Every piping system requires some type of support system to function properly. The piping system can be supported from a building or other structure on traditional pipe hangers or from the ground on piers or bents. It even can be supported from another piping system. As long as all the piping system’s support points remain motionless relative to the piping system and relative to one another, the system is unaffected. However, if some of the piping system’s supports move relative to the pipe or relative to one another, the piping system will attempt to follow that motion and will experience a change in its state of stress. This condition is called relative anchor movement.
Relative movements of a piping system’s supports can be caused by a number of phenomena. Some of the more common causes include
Thermal expansion-related movement of the connection point on a larger piping system, where the subject system is attached
Earthquake-induced relative movements of the various points on a building’s structure where the subject piping system is supported
Thermal expansion or mechanically induced movements of a piping connection (nozzle) on a machine, pressure vessel, or heat exchanger
The amount of stress, or more properly stated, the change in stress, that a piping system experiences from relative anchor movements is a function of two variables: the magnitude of the anchor movement and the stiffness of the system. As one might expect, larger movements will result in greater changes in stress. Moreover, for a given magnitude of movement, stiffer piping systems will experience greater changes in stress than those that are less stiff. In general, systems that are shorter, have fewer changes in direction, and are made up of larger-diameter pipe are stiffer than those for which the opposite conditions are true.
Certain phenomena result in a loading case in which both the magnitude and direction of the piping system’s terminal movements are known, such as movements resulting from the thermal expansion of a pipe. In such a case, the known magnitudes and directions of the anchor movement are input to the piping system stress analysis, and the attendant stress levels are predicted.
There are other cases, however, in which only the magnitude of the anchor movement is known. Examples include earthquake-induced anchor movements or the movements of a building due to wind loading. In this case, the magnitude of the movement is input into analysis as known, but the direction is assumed such that the worst-case change in the state of stress of the piping system under study results. This approach ensures that the piping system stress analysis is conducted in the most conservative manner.
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