A sway brace is a special type of variable effort restraint and is built around a standard or non-standard spring. It is used to restrain piping or equipment and is not intended to support. The construction of the sway brace enables a pre-loaded spring which sustains both compression and extension displacement to provide a pre-determined restoring force.
For example, a pipe that is exposed to cross wind will sustain high transverse force during strong winds. If the pipe is subject to thermal expansion and contraction it will have a certain amount of flexibility. If allowed to displace freely during strong winds the pipe may become unstable and possibly sustain permanent deformation.
By installing a sway brace the pipe can be held in position during the application of forces less than the pre-set force within the spring. At higher forces the pipe will be allowed to displace but the further from its neutral position it is pushed the greater the restoring force will become.
When the storm recedes the sway brace will push or pull the pipe back to its neutral position.
Specifying Sway Braces: Like all devices that exert a restoring force to a pipe the magnitude of force that can be applied and the amount of acceptable displacement will be decided by the allowable stresses within the pipe. This information will be defined by piping engineer during his analysis of the system.
The level of pre-load within the sway brace shall be defined by the minimum force required to restore the pipe to its neutral position; it may be a function of the dead weight of the piping and the magnitude of frictional resistance thus created at sliding surfaces or it may be the amount of force required to restore an unstable, out-of-balance mass.
For simplicity, if we consider a pipe crossing a bridge structure, thermal expansion of the pipe is predominantly in the axial direction and so the pipe is carried on three sliding supports each having a coefficient of friction of 0.1. The total supported mass of the pipe is 10,000kg. Therefore the frictional resistance in the transverse plane is 1000kg.
If we select a sway brace that delivers a pre-load of 1000kg and has a spring stiffness of 100kg/mm the minimum transverse resistance to sliding is 2000kg increasing by 100kg/mm of displacement.
Assume now that the wind pressure on the pipe exerts a force of 2500kg; the pipe will displace by 5mm. If the pipe is sufficiently flexible and without the influence of the sway brace it may not be able to generate sufficient elastic energy within itself to return back to its neutral position. Subsequent axial loading may then cause further deformation of the pipe because it is not offering a rigid shape to the applied force.
With the sway brace installed the restoring force is at least that which is necessary to overcome friction and so the pipe is returned to its neutral position.
When in the neutral position the sway brace exerts zero restoring force and so the pipe is free once again to move with the thermal cycle.
Any practical combination of pre-load and spring stiffness may be defined and any spring within our standard range of variable and constant efforts supports can be applied to the product.
In our Variable Effort Supports product catalogue we offer a basic range of sway braces but it will normally be necessary to design the device to suit the specific requirements of the customer.
It is a fact that hydraulic snubbers require maintenance over a period of time in order to replenish the natural leakage of hydraulic fluid. Pipe Supports Limited is working towards a snubber design that will help to minimize the need for such maintenance. However, it cannot be totally eliminated.
In the USA, where there has been very long experience of operating nuclear power plant and where snubbers have been the subject of much discussion and review, there was a move towards mechanical rather than hydraulic snubbers. It was found, however, that a mechanical snubber is also subject to operational problems. The mechanism is prone to wear and fatigue and generally when a mechanical snubber fails in service it seizes and prevents the pipe from being able to move which in turn leads to damage to the piping and connected plant.
Hydraulic snubbers conversely fail in a ‘free’ state – the snubber continues to move with the pipe at the expense of slightly increased drag. However, the resultant stresses on the pipe during normal operation are insignificant. The problem with a ‘failed’ hydraulic snubber is that when the ‘event’ happens it can only provide notional damping by the air that is trapped within it.
Hence in the States there were significant efforts made to minimize the number of snubbers used.
Pipe Supports Group do not intend to manufacture mechanical snubbers; we do, however, have a relationship with a Japanese manufacturer who is willing to supply us mechanical snubbers, although our price will not be as competitive as it would be for hydraulic snubbers.
Struts & Snubbers are designed on the basis of dynamic loading – therefore the loads stated in our catalogue are ‘dynamic’ loads.