Under certain circumstances, specifications may recommend that the relative load change be minimized, by limiting the Load Variation to a value such as 10% or 25%. Load Variation is defined as:
Since the hot load and thermal movement are dictated by the piping system configuration, the variability of an individual spring can be controlled only by varying the spring rate. Most manufacturers provide springs with three (or more) different spring rates per load size, recommended for short-range (0 to 1/2 inch), mid-range (0.5 to 1 inch), long-range (1 to 2 inch) displacements. Since all springs in a given load size support the same range of loads over their total travel, the spring rate (and therefore the variation) of a long range spring is typically one-half of that of a mid-range spring, which in turn is one-half that of a short range spring.
The use of a spring load variation criteria is normally a holdover from an earlier era, when it was used as justification for not including spring stiffnesses in expansion loading cases. If the spring stiffnesses are included in the analysis (as is normally the case with CAESAR II), load variation criteria may probably be considered to be an unduly restrictive requirement.