By Vincent A. Carucci
This article briefly discusses the most common causes of Piping System Vibration.
Acoustically induced vibration is a potential problem in high capacity, gas flow, pressure reducing systems. The pressure reduction can occur at control valves, at restriction orifices, safety valves, or when sonic flow occurs at a branch connection to a header. Failures can occur in only a few hours since the higher structural and acoustical natural frequencies are excited, and the material endurance limit can be reached in a short time. Failures have occurred in steam desuperheater systems, compressor recycle letdown systems, and safety letdown systems. Severe vibration has also occurred in pipline pressure letdown systems.
The approach to designing such a system consists of:
When the steady-state velocity of a fluid is suddenly altered, a pressure change occurs in the piping. The transient pressure variation is called hydraulic surge or water hammer. The pressure surge moves through the pipe at the speed of sound. Potential consequences are excessive internal pressure, pipe collapse, flange leaks, and large pipe movements.
Common causes of surge include:
Slug flow can cause flow-induced vibration in two-phase fluid systems. In a horizontal line, the vapor above the liquid can travel much faster than the liquid. This creates waves at the liquid surface and entrains some of the liquid into the vapor stream. At high vapor rates, slugs of liquid form across the pipe cross-section and travel at speeds that approach the vapor velocity. When this occurs, a wide range of reaction forces can occur at pipe bends, depending on the size of the slugs that are formed. Reaction forces developed at pipe bends due to slug flow can cause excessive piping vibration and movement unless the piping system is adequately restrained.
Wind can cause piping vibration by vortex shedding from the pipe surface. If wind strikes at a right angle to the axis of a cylinder, aerodynamic forces due to vortex shedding occur at the following frequency that is a function of wind velocity, cylinder diameter, and Strouhal number (0.18 for cylinders in air).
These forces act on the pipe at right angles to the wind direction. Although the forces are small, the amplitude of vibration may be large if the shedding frequency is close to the natural frequency of the piping.
If a problem exists, the stiffness and the natural frequency of the piping should be increased by adding bracing, consistent with still meeting piping flexibility and associated equipment requirements. Mechanical snubbers and shock absorbers may also be used to change the stiffness and add damping to the piping system while still permitting its thermal movement.
Earthquakes can cause piping vibration either directly due to resonance or by the motion of pipe supports or equipment connections. Piping in areas known to experience earthquakes should be checked for forces due to earthquakes.