A less expensive alternative is a variable spring hanger, in which the spring load varies somewhat as the pipe moves. From the analysis of the system shown in Figure 2-12 it was evident that some change in the support loads as the pipe goes from the cold to the hot condition is tolerable from a stress point of view. The trick is to design the system with an acceptable load range.
A variable spring hanger (Figure 2-30), pre-set to some load, provides support throughout the range of pipe movement; as noted above, there is some change in load as the pipe moves from cold to hot position. As the pipe moves up, the load plate on the spring moves up, allowing the spring to decompress, thus decreasing the load which the spring puts on the pipe. As the pipe moves down, the load plate on the spring also moves down, further compressing the spring, and thus increasing the load of the spring on the pipe.
The objectives of spring hanger design are to choose a spring which:
will provide the weight support load necessary to balance the piping system after the pipe has moved from its cold (installed to its hot (operating) position,
permits the total movement of the pipe from its cold to hot position, and
does not cause an excessive expansion stress range in the pipe as the spring load ranges from its cold to hot load.
Since the variable spring hanger load changes as the pipe moves from its cold to its hot position, and one objective of hanger design is usually to provide the weight support load necessary to balance the piping system in its hot position, it is necessary to install the spring with an unbalanced "cold load". This unbalanced load can be determined by: