By Dr. Hyder Husain Ph.D.
January 6, 2011
Cause of Vibration All piping systems typically used in industrial application are made of elastic material. Elastic materials vibrate even under small perturbations due to their elastic properties. Since solid materials have a non-zero stiffness factor for both volumetric and shear deformations, these perturbations can generate waves with different velocities depending upon the deformation mode. Volumetric perturbations produce transverse waves while shear perturbations produce longitudinal waves.
External Perturbation In an ideal situation, pipe vibration would be non-existent if the fluid could flow through the piping system without any disturbances that would cause perturbation. However, in real-life situations, there are many sources that generate perturbation in the piping system and subsequently cause vibration.
Causes of Perturbation Here we can separate the main causes into a few main categories:
(a) Mechanical, (b) Fluid Induced, (c) Transients
(a) Mechanical:
(i) Perturbation originating from the pump or compressor.
(ii) Mechanical perturbation propagating from other moving mechanical components.
(b) Fluid Induced:
(i) Flow turbulence (broad band spectra): Function of Reynolds number
(ii) Multiphase flow: Propagation of slugs (quasi-periodic) and their implosion/explosion may cause serious vibration.
(iii) Bends & elbows: These produce secondary flows causing further interaction and enhancing strong vertical flows of quasi-periodic nature.
(iv) Valves: Valves cause flow separation and/or direction change which leads to high intensity turbulence (Reynolds number dependent).
(c) Transients:
(i) Sudden rupture of pipe
(ii) Sudden closure of valve
(iii) External forces on the pipe or piping components
Causes of Perturbation Thorough plant design should ensure that the Eigen-modes and Eigen-values of the overall system subjected to external perturbations should not match those of the piping system when subjected to those same external perturbations. Low frequency, long waves will cause immediate problems; whereas high frequency, low amplitude vibrations will cause fatigue failures over time. Therefore, one must be careful in designing the piping system and should use various vibration mitigating devices placed at proper locations. In addition, proper process controls should be used to reduce vibration especially in multiphase flows.