Flow Induced Vibration: What you need to know

What is Flow-induced vibration?

The flow-induced vibration (FIV) is a significant amplitude vibration that occurs in the piping systems carrying high-velocity turbulent fluids. It is prevalent that near-violent sources such as pipe bends, reducers, tee connections, partially closed valves, process equipment bore connections, and more. The fluid flow in the piping system generates high kinetic energy at low frequencies ((generally

What are the causes of flow-induced vibration?

• The flow-induced vibration of piping and pipelines is a complex mechanism caused by various factors including;
• High velocity or increased flow rate of fluids in the piping system results in a high level of turbulent energy
• Usage of thin-walled piping system
• Most of the piping design codes do not address vibration issues in a detailed manner. 
• Rotary types of equipment like pumps and compressors produce pressure pulsations causing excitation forces in nearby piping.
• The pipeline and piping system should be designed to minimize the risk associated with the pipe vibration. 
• Sudden changes in the flashing of fluids caused by opening valves, cavitation, or other rapid pressure variations lead to changes in the state, for example, flashing of liquids to vapor.

Effects of Flow-Induced Vibration

Various studies have shown that out of all failures and downtimes in any individual industrial plant, about 10 – 15% are due to vibration-induced fatigue. Let’s have a look at the significant effects of flow-induced vibration:

• FIV displaces the pipes and piping system in the longitudinal and transverse direction. Moreover, in some cases, it results in damaging the pipe supports.
• Considering the momentum flux of flow - liquid or gases as density * velocity2, gases have a lower density compared to fluids. So, high-density liquids are more prone to flow-induced vibration compared to the gases form.
• The risks associated with the flow-induced vibration can be managed through various screening activities such as pinpointing piping sections of concern, vibration monitoring, and investigating identified challenges, pipework configuration, and more.

FIV Screening

The Design and Engineering Practice (DEP) publications - 31.38.01.26 Gen. by Shell global solutions international provides detailed steps for screening piping systems. It helps in calculating momentum flux (density X velocity2) of flow-induced vibration and categorizes failure into three parts, namely, Negligible, Medium, and High. For instance, in high category vibration fluid, the likelihood of failure (LOF) is calculated following detailed steps mentioned in Energy Institute guidelines for the prevention of vibration induced fatigue failure in process pipework.

It is recommended for the piping engineer to keep LOP below 0.3. However, if LOF is more than 0.3, then it is mandated to take corrective actions to mitigate the vibration possibility in the piping system. 

Mitigation of Flow-Induced Vibration

FIV takes a long time to cause fatigue failure. So, it is generally resolved whenever vibration is observed physically after commissioning the industrial plant. One of the most common techniques to mitigate Flow-induced vibration is by adding supports or restraints. Hence, by adding appropriate guide and line stop supports, i.e., by increasing piping system rigidity, the damaging effect of FIV can be reduced to a great extent. The added supports minimize the shaking tendency of the pipes, which reduces the possibilities of pipe failures. Other mitigation includes;

• Piping System rigidity can be increased by increasing pipe wall thickness
• Ensure the in-between flow pipe and supports is tight
• Reduce fluid velocity by improving pipe size 
• Reduce no turbulent sources like elbows, reducers, and more. 

We hope that this article provided you with adequate information about flow-induced vibration. If you’re looking for pipe stress analysis and flexibility analysis services for your next project, consider hiring a team having a long and wide range of project experience in conducting the full spectrum of stress analysis services for piping systems. Hence, pipe stress analysis is one of the most critical activities in piping design.