Piping Vibration can be defined as a continuous to and fro motion from an equilibrium position. Piping vibration problems cause serious integrity risks to operating plants; both onshore and offshore production facilities. The vibration of the Piping System can cause fatigue failure on process piping and small branch connections and reliability problems on equipment. Equipment nozzles, Relief lines, instrumentation ports, drain connections, and valves can also be subjected to Piping Vibration. Data provided by the UK Health & Safety Executive shows that around 21% of hydrocarbon releases are due to vibration-induced fatigue failures.
Due to the fact that most of the piping design codes (ASME B 31.3, B31.1, B31.4, B 31.8, etc.) do not address the vibration issues in a detailed fashion, the damaging effect is normally ignored during the design stage and simple static analysis without attention to vibration is performed on piping systems.
At the same time, the piping vibration tendency 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 Piping Vibration causes many problems in operating plants and the problem should be solved during the 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 piping vibration and their effects in short.
There are many reasons which can cause vibration in a piping system.
Cause of Piping Vibration
There are a variety of excitation mechanism which can be present in a piping system and can produce piping vibration and finally failure resulting from fatigue. Some 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 like pumps, compressors, etc.
- Pressure Pulsations from reciprocating equipment.
- High-frequency Acoustic excitations generated by high-pressure drop at relief valves, control valves, or orifice plates.
- Water Hammer (Surge) or Momentum changes due to sudden valve closure.
- Cavitation or vapor bubble collapse due to localized pressure drop.
- Due to sudden flashing of fluid.
- Periodic pressure disturbances during a flow past the dead-end of branch connection/ instrumental items.
Effects of Piping 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 piping vibration are as follows:
- Piping Vibration causes dynamic stresses (fatigue) in a piping system. If this stress is more than the critical value it will initiate a crack that 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 are joined together.
- In addition to dynamic stresses, vibration results in wearing surfaces in contact due to cyclical relative motion between them. This phenomenon is known as Fretting.
Vibration of plant piping is a significant risk to asset integrity and safety. So, must be addressed. To manage the risk of piping vibration, various analysis and measurement services are performed.
Every piping system has the tendency to vibrate at certain frequencies, called natural frequencies. Every natural frequency is associated with a definite and unique shape, called mode shape. The natural frequencies and modes depend on the distribution of mass and stiffness throughout the piping system, and the distribution is influenced by piping diameter, material properties, wall thickness, location of lumped masses (such as valves), piping supports and fluid density. A mode shape has the locations of zero motion (node) and maximum motion (anti-nodes). The response of the piping to an applied excitation depends on the relationship between the frequency and pattern of the excitation and the piping system’s natural frequencies/modes. When a piping system is excited by a dynamic excitation with a frequency that coincides with one of its natural frequencies, the system undergoes great displacements and stresses. This phenomenon is known as resonance, and it can cause high vibration, even fatigue, and subsequently, failure. Vibration generated in the piping work may lead to high-cycle fatigue of components, such as small-bore connections, or the failure at welds in the main piping itself.
Vibration Problems due to Two-Phase Flow
Two-phase flow refers to the interactive flow of two distinct phases with common interfaces in a piping system, with each phase representing a mass or volume of matter. The two phases can exist as combinations of solid, gas, and/or liquid phases. Although multiphase flow involving three phases can also exist, most multiphase engineering applications are two-phase flow.
Two-phase flow exists in many process piping and power piping components. The flowing fluid is a source of energy that can induce small-amplitude subcritical oscillations and large-amplitude dynamic instabilities. In fact, many practical system components have experienced excessive flow-induced vibrations. To prevent unacceptable flow-induced vibration, Users must understand excitation mechanisms, develop analytical and experimental techniques, and provide reliable design guidelines.
Video tutorial on Solving Vibration Problem due to two-phase flow
In this video collection presented by the Dynaflow research group, the basics of vibration because of two-phase flow, is clarified by the presenter. Hope, most of you will get a good insight to solve the two-phase vibration problems. If you wish to add something more please write in the comments section.
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