Flow Induced Vibration , Noise in Pipes

 

Piping vibrations

Vibration of process plant piping can be a significant risk to asset integrity and safety. This is often due to flow induced vibration (FIV) and acoustic induced vibration (AIV), and is related to the flow of the main process fluid through the piping system.
Other possible sources of piping vibration include:

• Mechanical vibration and pulsations from compressors and pumps;
• Flow induced pressure pulsations related to the pipework configuration and other components and features in the flow;
• Valve configuration and operation;
• Cavitation and flashing across valves in liquid service.

Flow induced vibration

Flow induced vibration is the result of turbulence in the process fluid, which occurs due to major flow discontinuities such as bends, tees, partially closed valves, and small bore connections. The high levels of broadband kinetic energy created downstream of these sources is concentrated at low frequencies, generally less than 100 Hz, and can lead to excitation of vibration modes of the piping and connected equipment. The extent of this problem depends on the piping design, support configuration and stiffness, valve operation, and other related factors which determine the severity of the resulting vibration.

Acoustic induced vibration

A relief or control valve on piping systems in gas service, or other pressure reducing devices, can generate high levels of high frequency acoustic energy, an effect commonly referred to as acoustic induced vibration. In addition to high noise levels arising external to the piping, this excitation can result in high frequency vibration of the pipe wall, with the potential for high dynamic stresses at welded features such as supports and small bore connections. This in turn can lead to the possibility of fatigue cracking within a relatively short period of time (minutes or hours).

Flow induced pulsation

Flow induced pulsation (FIP) can be caused by dead leg branches in pipework, which can be excited as acoustic resonances with discrete frequencies. These resonances can induce large shaking forces in the pipework, leading to integrity and safety risks.

Causes of flow-induced vibration

Flow-induced vibration of pipelines and piping can be caused by a number of mechanisms including:
• Pumps and compressors which could produce pressure pulsations, exciting a response in nearby piping
• Fluctuating flow past obstructions or objects in the flow (for example, thermowells or other intrusions in the flow) and piping dead legs
• Multiphase flow – for cases with multiple phases flowing (for example, gas and liquid), specific multiphase flow regimes and flow frequencies through piping may drive vibration (for example, slug flows where packets of liquid impact the walls of the pipe at bends, elbows and obstructions)
• Rapid changes in flow conditions or fluid properties caused by opening valves, cavitation or other large pressure variations leading to changes in state, for example, flashing of liquids to vapor.