Hot Load, Cold Load and Variability

A variable spring support carries the pipe on a coil spring. As the pipe moves vertically, the spring compresses or extends, and the load it carries changes. That change is unavoidable — it is how a spring works — and variability is how we measure it.

This is the single most important concept for working with variable supports, and the reason four different spring series exist.

Why the load changes

A spring obeys Hooke’s law: the force it exerts is proportional to its deflection. Move the pipe up by 30 mm and the spring extends by 30 mm, so the load it carries changes by 30 mm times its spring rate.

The support therefore has two loads, not one:

• Hot load — what it carries in operating position
• Cold load — what it carries in the installed, cold position

The difference between them is the spring rate multiplied by the vertical movement. That is all.

What variability measures

Variability expresses that difference as a percentage of the operating load:

A variability of 10 % means the load the support applies to the pipe deviates by a tenth of its design value over the travel.

Why it matters

The load the support does not carry is transferred to the piping system and its other supports. High variability means the pipe is being loaded differently in cold and hot conditions, which shows up in your stress analysis as changed reactions at nozzles and adjacent supports.

The usual design practice is to keep variability below 25 %, and below 10–15 % for critical lines near sensitive equipment. The software applies a default limit and lets you override it per project or per support.

The three ways to reduce variability

Rearranging the definition gives the whole picture:

So there are exactly three levers:

1. Use a softer spring. The most common fix, and the reason the catalogue offers four series. For the same size, each series trades travel range against spring rate: a series with twice the travel has half the rate, so it produces half the variability for the same movement.

All four cover the same load range in the same number of sizes. What changes is how many millimetres they spread the load change over.

2. Reduce the movement. Usually not yours to change — it comes from the stress analysis — but occasionally a support can be relocated to a point with less vertical displacement.

3. Increase the load. Rarely practical, but it explains why lightly loaded supports with large movements are the hardest cases.

When variability cannot be brought down

If even the softest series exceeds the limit, the movement is too large for a variable spring. That is the point at which a constant spring support becomes the right answer: it carries the same load throughout its travel, so its variability is zero by construction.

The rough guidance is that variable springs suit vertical movements up to around 50–75 mm. Above that, expect to move to a constant.

See How to Setup a Constant Spring Support.

Reading the result

The support datasheet shows hot load, cold load, spring rate and variability together. Two checks are worth making by eye:

• Does the cold load sit on the right side of the hot load? If the pipe rises when hot, the cold load should be lower than the hot load. If it is not, check the sign of the vertical movement.
• Does the variability match the movement? A large movement with a low variability means a soft spring was selected. A small movement with a high variability usually means the load is very low.