The procedures described above assume that the hot load and thermal movement required for spring selection are already known. How does the engineer calculate the hot loads and thermal movements? The procedure for the entire hanger design process is as follows:
Pick out support locations using standard span criteria, and do a weight analysis, assuming that there are rigid Y-restraints at each location. This analysis is called the "restrained-weight" analysis. The weight loads distributed to each of the restraint during this analysis are used as the hot loads during spring selection.
Next, remove the restraints from the support locations, and do a thermal expansion analysis. This analysis is called the "free-thermal" analysis. The thermal movements at each of the support locations are used as the thermal travels for selecting the springs. (Note that due to the technical effects of possible non-linear effects in the system, CAESAR II performs not a true "free-thermal" load case, but rather a load case called "operating for hanger travel", which includes the effects of thermal loads, weight loads, and the spring hot loads calculated in the restrained weight case. Since the piping weight loads and the spring hot loads essentially cancel each other out, this effectively results in a thermal only load case, but with non linear effects considered.)
Using the hot loads calculated from the restrained-weight case and the travels calculated from the free-thermal case, select a spring for each location from the spring selection table as described above. Use the spring constant to determine the required cold load (pre-set hanger load) for installation.
Alter the model to reflect the presence of the springs by adding a restraint at each support location with a stiffness equal to the spring constant of the spring, and by adding the pre-set spring load (cold load) as a force active during the sustained load case. Then re-analyze all load cases in order to get the effects of having the actual springs present.
The four steps of this procedure (except the locating of the supports) are done automatically by CAESAR II whenever the user designates a hanger (i.e., a candidate for spring hanger design) in a piping system