By Vincent A. Carucci
The method used to solve a particular vibration problem depends on the type of vibration and the type of piping system. For example, adding restraints to a piping system will not solve a vibration problem caused by high acoustic energy.
It is not always possible to eliminate or isolate the source of vibration in many piping systems. Therefore, other means must be used to control vibration. Some common methods for doing this are briefly described below.
One way to control piping vibration is by adding bracing (i.e., restraints) to the system. This increases the mechanical natural frequency of the piping, thus ensuring that resonance due to low frequency excitation will not occur. It also limits large deflections that could be caused by slug flow, water hammer, etc.
When bracing addition is being considered, it must be confirmed that this will not adversely affect the thermal flexibility of the piping system. Adding restraints increases the system stiffness and can cause higher pipe thermal stresses and end-point reaction loads. New thermal flexibility calculations may be required to confirm that the design is acceptable after adding the restraints.
Vibration amplitudes can be decreased by installing hydraulic or mechanical snubbers. These devices (e.g., dashpots or other frictional devices) increase the system damping by resisting rapid displacements, such as that resulting from vibration. However, they permit movement resulting from slow displacements, such as those from thermal movement. Thus, snubbers may be used in situations where bracing must be added to reduce or prevent vibration movements, but rigid restraints would cause unacceptable thermal displacement stresses or loads.
Surge suppressers may be used to control surge or pulsation-induced vibration. A typical surge suppresser consists of a pilot operated valve which quickly opens after a power failure through the loss of power to a solenoid, or by a sudden large pressure reduction or increase at the surge suppresser. The open valve releases liquid from the line being protected, thus smoothing and reducing the pressure fluctuation. The valve is closed at a slower rate by using a dashpot in order to limit the pressure rise as the liquid flow is shut off.
An accumulator is a pressure vessel that is partially or completely filled with a gas (usually inert). This vessel is then connected by pipe to the main line being protected. In the simplest case, the liquid in the pipe is in direct contact with the gas. In some cases, an elastomer membrane separates the liquid from the gas but transmits pressure between them. In other cases, a rupture disk forms a more rigid barrier between the gas in the accumulator and the liquid in the pipe. For the first two configurations, the device acts instantaneously to a rise in pressure at the gas/liquid interface. In the last configuration, the rupture disk delays the reaction time from 0.2 to 2.0 milliseconds since it must rupture before pressure is transmitted between the two fluids.
Cables or chains can be used to temporarily control large deflections caused by vibration in piping systems. The cable or chain is attached to the pipe, connected back to nearby structure, and tightened to stop the pipe movement. Quite often, even blocks of wood or scrap steel are used as wedges between the pipe or its supports and nearby structure to stop pipe movement. This approach is useful in stopping large amplitude vibration before it can damage the pipe, and to determine the best locations to place permanent bracing. This technique is not a permanent solution to a piping vibration problem.
Since the temporary restraints are installed while the system is in operation (and hot), their presence could restrict pipe thermal movement when the system is shut down. This should be considered when locating the restraints to determine if they would cause excessive thermal stresses or loads when the system is shut down. In extreme cases, it might be necessary to remove the restraints before or as the system is shutting down in order to permit free thermal movement.