As seen in Section 2.2 located at (Piping Design for Loading Types), as more restraint is provided to a piping system, weight stresses decrease. Conversely, Section 2.3 located at (Piping Design for Loading Types) demonstrated that as restraint is removed from a system, expansion stresses decrease. This contradiction must be resolved through some type of compromise.
Likewise, in systems supported only with rigid supports, it is possible that the pipe might lift off of some supports and lock up against others once it thermally expands. This is demonstrated through a review of the restraint loads during the operating load case for the system shown in Figure 2-12, which shows that the pipe lifted off of the supports at node points 36,44, and 72 (rendering them inactive) and had a partial lock up at node point 55, overloading the support.
It would be preferable if the pipe could move to its new position and, at the same time, onto its supports; or even, if the support could move with the pipe. One solution is a weight and pulley assembly:
The drawback to this is that the assembly is bulky, and requires restraint design for twice the pipe load.
The mechanical implementation of this concept is the constant spring, or constant effort support (Figure 2-29). This support has an internal pivot arm attached to a spring; as the pipe moves up or down, the moment arm about the pivot and the spring force vary inversely, creating a constant moment about the pivot, and therefore a constant force acting against the pipe weight. The drawback of constant springs is that they are often too expensive for the application; therefore they are usually used only when pipe movements are very large.