DESCRIPTION OF A VARIABLE SUPPORT HANGER
Variable support hangers are used to support components and structures (e.g. pipes, vessels, etc.) that experience vertical movement. Unlike constant support hangers, the variable support hangers are used where the supporting components have the capability to withstand load variations of up to 25%.
The vertical movement is accommodated by a compression spring coil (or stack of coils in series) which is pre-compressed to an initial load setting. During operation, the supported component will travel through its specified movement and the coil(s) will be compressed/decompressed to the final load setting. This difference, or variation, in loads between the initial setting and the final setting gives rise to the term “variable” spring support.
A maximum variability factor of 25% has been established by MSS to limit the load differential which may be prescribed in a variable spring support. The variability is a ratio of the change in load compared to the final load. When prescribing variable spring supports, attention must be given to the spring rate and the desired amount of travel to insure the variability factor remains below 25%.
The movement of a variable spring hanger rod supporting a load is animated in the video clip. The initial load position is designated as “cold position” in the animation. As the load moves upward, the spring is decompressed to the final load position designated as “hot position”. The load difference (variation) is produced by the coil’s change in compression multiplied by the spring rate. The change in compression of the coil causes a direct change in the supported load as seen at the hanger rod.