Variable & Constant Support Design — MSS SP-58 / SP-69
Ambient temperature when spring is locked/installed
Calcium silicate: 8-14 | Mineral wool: 6-10 | Fiberglass: 3-6
Positive = pipe moves UP (spring compresses). Negative = pipe moves DOWN (spring extends).
Half the distance to adjacent supports on each side of this hanger
Understand spring hanger design principles, variable vs. constant supports, load calculations, and MSS SP-58/SP-69 standards
A variable spring hanger is a pipe support that uses a coil spring to carry the pipe weight while allowing vertical thermal movement. The support force varies as the spring compresses or extends. Variable springs are used when thermal movement is moderate (variability under 25%) and the changing support load does not adversely affect pipe stress or connected equipment nozzle loads.
Variability is the percentage change in support load between hot (operating) and cold (installed) conditions, calculated as (cold load - hot load) / hot load x 100. MSS SP-58 recommends variability below 25% to ensure the support load remains close enough to the design load throughout the thermal cycle. If variability exceeds 25%, a constant support hanger should be used instead.
Hot load is the actual pipe weight that must be supported at operating temperature - it equals the sum of pipe, contents, and insulation weight at the support point. Cold load is the spring force at the installed (ambient) position before thermal movement occurs. Cold load = hot load plus or minus (spring rate times travel). The spring is preset at the factory to the cold load so that when the pipe moves to operating position, the hanger carries the correct hot load.
Constant support hangers should be used when: (1) variability exceeds 25%, (2) thermal movement is large relative to pipe weight, (3) the pipe connects to sensitive equipment like turbines or compressors, or (4) the piping analysis requires constant support force. Constant supports use a mechanism (typically a Trapeze spring with a cam) that maintains nearly constant load throughout the travel range.