Spring hanger

A rigid support carries a pipe only as long as the pipe stays in place. If thermal expansion lifts the pipe, the rod hangs slack and the weight shifts to the neighbouring supports or the nearest nozzle. If the pipe moves down, the rigid support becomes a restraint.

A spring hanger solves this with a coil spring: it carries the weight and allows the vertical movement. The price is that the supporting force changes with spring travel – hence the name variable spring hanger.

A spring hanger is sized for the hot load – the weight it is to carry in the operating condition, taken from the pipe stress analysis. The cold load in the installed condition differs from it by spring rate times travel. If the pipe moves up in operation, the spring relaxes and the cold load is higher than the hot load; for downward movement it is the reverse.

An example: hot load 10 kN, spring rate 100 N/mm, movement 20 mm upwards. The load changes by 2 kN and the cold load is 12 kN. The load variation – change in load relative to the hot load – is 20 per cent. A commonly used upper limit is 25 per cent; close to sensitive connections less is often specified.

Hot load and travel determine the size: the spring has to carry both loads within its working range and keep within the load variation limit. Softer springs with longer travel reduce the variation but need more installation height.

If even the softest spring is not enough because the travel is large or the permitted variation small, a constant hanger is the right component. Spring supports are available as hangers and as base-mounted units on which the pipe rests from below.

Spring hangers are factory-set to the cold load and delivered locked. In the locked condition they act like rigid supports: they carry the empty line during erection and the water-filled line during the pressure test. Only then are the travel stops removed.

At first start-up the load scale shows whether the line behaves as calculated: in the hot condition the indicator should be at the hot load mark.

The spring itself rarely fails. Mistakes almost always arise around it.