What is Flow-Induced Vibration or FIV?
Flow-Induced Vibration or FIV is a large-amplitude, low frequency (generally <100 Hz) vibration that can occur in piping systems carrying high-velocity turbulent fluids. Flow-induced i.e, the fluid flow generates high kinetic energy that forces the piping system to vibrate. It is more prevalent near the turbulent sources such as pipe bends, reducers, branch connections (Tee connections), partially closed valves, process equipment connections, etc.
Which pipes are prone to Flow-Induced Vibration Problem?
In recent days, risk due to flow-induced vibration has increased a lot due to
- Increased flow rate (high velocity) that results in a high level of turbulent energy
- Use of thin-walled piping i.e high D/t ratio
- Long support span which results in flexible piping
Effect of FIV
This type of vibration displaces the piping system in the longitudinal and transverse direction and in some cases leads to damage to the pipe supports.
Considering the momentum flux (density X velocity2), High-density Liquids are more prone to FIV as compared to gases.
DEP 31.38.01.26 (Design and Engineering Practice by Shell Global Solutions International) provides detailed steps for screening piping systems for FIV by calculating momentum flux (density X velocity2) and categorizes failure susceptibility into three groups; Negligible, Medium and High. They also suggest further steps to follow when the susceptibility falls on Medium or High Category. For High category vibration susceptibility fluids, Likelihood Of Failure (LOF) needs to be calculated following detailed steps mentioned in Energy Institute guidelines for the avoidance of vibration induced fatigue failure in process pipework. The aim of the piping engineer will be to keep LOF below 0.3. However, if LOF is more than 0.3 then corrective actions need to be taken to mitigate the vibration possibility.
Mitigation of Flow-Induced Vibration
FIV normally takes a long time to cause fatigue failure. Hence, it is usually resolved when vibration is observed physically after commissioning the plant. The most common mitigation is to add supports or restraints. By adding the appropriate guide and line stop supports, i.e by increasing system rigidity, the damaging effect of FIV can be reduced a lot. These added supports will minimize the shaking tendency of the pipe, thus reducing the tendency of pipe failure. Other mitigations could be
- Reduce fluid velocity by increasing pipe size or by changing process conditions.
- Increase system rigidity by increasing pipe wall thickness.
- Install viscous damper, shock arrestor, snubber, etc in the piping system
- Reduce no turbulent sources like elbows, reducers, etc.
- Tighten the clearances in-between pipe and supports.
- Stress concentration at branches can be reduced by contoured fittings and gussets on Smallbore connections
Some more Resources for You…
References and Further Study
- DEP 31.38.01.26 V43
- Energy Institute guidelines for the avoidance of vibration induced fatigue failure in process pipework