Both variable and constant spring hangers exist to solve the same problem: a pipe that moves vertically as it heats up cannot hang from rigid rods without either lifting off its supports or overloading them. The two devices solve it differently — and the difference decides which one belongs on each support tag.
A variable spring hanger is a helical coil spring in a casing. Spring force follows Hooke's law — force equals spring rate times deflection — so as the pipe travels, the supporting force varies. Move the pipe 30 mm on a 100 N/mm spring and the carried load changes by 3 kN, load the piping system must absorb somewhere.
That change is expressed as load variability:
Within that limit, variable springs are the economical choice: shorter, lighter and cheaper than constant supports, available in hanger, base-mounted and trapeze arrangements.
A constant support hanger adds a cam-and-lever (or bell-crank) mechanism between the spring and the load. The geometry is arranged so the moment of the spring force about the pivot stays balanced against the moment of the load through the whole stroke — the supporting force stays effectively constant, held within about ±6% across the full travel range.
That is what critical piping needs: the support keeps carrying its share of the weight throughout expansion, without transferring load into nozzles and neighbouring supports.
| Criterion | Variable Spring | Constant Support |
|---|---|---|
| Supporting force | Varies with travel (F = k·x) | Constant within ±6% |
| Typical travel | Small to moderate (up to ~50 mm) | Large (into the hundreds of mm) |
| Load variability rule | Keep ≤ 25% of hot load | Not applicable — inherently ~constant |
| Where it belongs | General steam, process and utility lines | Main steam / reheat, near turbine and pump nozzles, large risers |
| Relative cost & size | Lower, more compact | Higher, larger — justified by criticality |
Related reading: Spring hanger selection guide with worked example · MSS SP-58 hanger types, explained
One PO covers both — engineered supports plus all the rigid hardware, from one manufacturing base.