Due to the fact that most of the piping design codes (ASME B 31.3, B31.1, B31.4, B 31.8 etc.) does not address the vibration issues in a detailed fashion, the damaging effect is normally ignored during design stage and simple static analysis without attention to vibration is performed on piping systems.
At the same time the vibration tendency of the piping system is increasing to a great extent due to increased flow rates of process industries through pipes and usage of high strength thin walled piping (flexible) material during design. It is seen that Vibration causes many problems in operating plants and the problem should be solved during design phase. Major of the damaging effects of vibration can be mitigated if proper design philosophy is taken while designing the system. This article highlights the major causes of vibration and their effects in short.
Vibration can be defined as a continuous to and fro motion from an equilibrium position. There are many reasons which can cause vibration in a piping system. Few major reasons which could cause the vibration are listed below:
There are a variety of excitation mechanism which can be present in a piping system and can produce vibration and finally failure resulting from fatigue. Few of those causes are listed below:
- Flow induced Vibration: Caused by the turbulence of the flowing fluid.
- Mechanical forces from Equipment: Caused by the excitation forces of reciprocating and rotary equipment’s like pumps, compressors etc.
- Pressure Pulsations from reciprocating equipment.
- High frequency Acoustic excitations generated by relief valves, control valves or orifice plates.
- Water Hammer (Surge) or Momentum changes due to sudden valve closure.
- Cavitation or vapour bubble collapse due to localised pressure drop.
- Due to sudden flashing of fluid.
- Periodic pressure disturbances during a flow past the dead end of branch connection/ instrumental items.
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Effect of Vibration:
Data has shown that out of all failures and downtimes in any individual plant around 10-15% are because of vibration induced fatigue. The major effects of vibration are as follows:
- Vibration causes dynamic stresses (fatigue) in a piping system. If this stress is more than critical value it will initiate a crack which will propagate slowly and end in the failure of the item in concern. The more fatigue sensitive places are the weld point connections where the branch and header is joined together.
- In addition to dynamic stresses, vibration results in wearing of surfaces in contact due to cyclical relative motion between them. This phenomenon is known as Fretting.
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