Cold spring and preload

Preloading means deliberately introducing a force or deformation in the installed condition so that the operating condition turns out more favourable. In piping the term occurs in four places: cold spring of the line itself, the presetting of spring hangers, the presetting of expansion joints and the bolts of a flanged joint.

All four have in common that the value has to be calculated, set during erection and documented. A preload that nobody remembers later is a source of error.

With cold spring – also called cold pull – the line is fabricated shorter by a fraction of its calculated thermal expansion and pulled together at the final joint. In the cold condition it is therefore under restraint. On heat-up the expansion first relieves this restraint and only then builds it up again in the opposite direction.

With half the expansion as cold spring, the forces are shared roughly equally between the cold and the hot condition. The purpose is to reduce the forces on anchors and connections in operation – when the material is hot and less strong.

The stress range between the cold and the hot condition stays the same; cold spring merely shifts where zero lies. It therefore brings nothing for the fatigue check. ASME B31.3 states explicitly that cold spring is not taken into account in calculating the stress range.

The code is cautious about the reactions as well: because the intended cold spring cannot be achieved reliably on site, ASME B31.3 allows credit for only two thirds of it.

Cold spring is simple in calculation and difficult in practice.

Spring hangers are preset to their installation load and delivered locked. The travel stops stay in place until after the hydrostatic test and are removed before start-up; only then can the hanger follow the movement of the line.

Axial expansion joints are often installed preset – extended or compressed – so that they use their movement range in both directions. The preset depends on the installation temperature and follows the manufacturer’s data. Flange bolts, finally, need a defined preload so that the gasket remains sufficiently compressed in every operating condition.