Piping Vibration is the cause of fatigue failures of pipework. This leads to important issues due to the rupture of components, such as welded joints associated with main lines and small-bore connections.
When talking about piping vibrations, two main types can be distinguished, flow induced vibration, FIV, and acoustic induced vibration, AIV. Flow induced vibrations are low frequency vibrations, meanwhile, acoustic induced vibrations are high frequency vibration.
1. Acoustic Induced Vibration
Different sources of excitation can arise in a process piping system. AIV is induced by pressure dropout, which is caused by flow restriction devices such as relief and control valves and restriction orifices. When the upstream to downstream pressure ratio around the restriction reaches its critical value, then choked flow conditions occurs and acoustic vibrations will be generated. Pipelines with high risk of AIV can be identify, both in new design and existing plants, and corrective actions can be applied. The corrective actions can be changes the piping system, such as reduction in mass flow rate or increasing the pipe line, or used of acoustic silencers and low noise control valves, reducing noise levels at the source reduces the risk of acoustic fatigue failure.
2. Flow Induced Vibration
In a process piping system, there are numerous sources of excitation that induce FIV. The first is the internal flow in the pipes, high velocities can cause insatiability in unsupported pipes or even induce buckling in pipes supported at both ends. Oscillatory flow can also cause vibration, which can be generated by reciprocating pumps and compressors. Other components in the line that causes vibrations are bends and small-bore connections, due to be sources of turbulence, and thermowells and dead leg branches, due to be sources of vortex shedding.
A two-phase flow can also induce vibration, for three different reasons:
- Density difference between the two phases.
- Phase changes, such as boiling and condensation.
- Dynamics of various shapes and sizes of bubbles, which induces sloshing, fluctuations and disturbances.
The various sources of FIV mentioned above, and others not explained here, can be detected in a plant, new or existing, and corrective actions can be taken for each case. Actions may consist of changes in the operation and in the piping system or may be specific to the source of excitation.
Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework
The Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework provides a methodology to help minimise the risk of vibration induced fatigue of process piping. The Guidelines can be applied to:
- A new plant/facility.
- An existing plant/facility.
- Changes within an existing plant/facility.
As well as different levels of the plant, such as:
- An operating unit.
- A major area.
- A major piece of equipment.
1. Qualitative Assessment
The first step is to identify the sources of excitation, for this purpose, in the case of a new plant, the Guidelines divides the sources in nine types and provides a qualitative assessment to identify them.
- Kinetic energy of the process fluid, associated to the fluid velocity, induces turbulence and pulsation, in the case of gas flow.
- Chocked flow due to flow restriction devices, induces AIV.
- Rotating and reciprocating machinery, induces mechanical excitation.
- Positive displacement pumps or compressors, induces pulsation.
- Centrifugal compressors, induces pulsation.
- Flashing or cavitation due to flow restriction devices, induces cavitation and flashing.
- Fast acting opening or closing valves as control valves and relief valves, induces surge and momentum changes.
- Intrusive elements, induces vortex shedding.
- Slug flow, two-phases bubbles that induces sloshing, fluctuations and disturbances.
An example of a qualitative assessment is shown in the following table:
2. Quantitative Assessment
After identifying the sources of excitation, the Guidelines provides a quantitative assessment for determining the risk of failure due to each mechanism of vibration. The risk is measure by calculating the likelihood of failure, LOF. The LOF is a form of scoring, not an absolute probability of failure nor an absolute measure of failure. The calculations are based on simplified models to ensure ease of application and conservative assumptions.
For LOF values lower than 0.3, there is not risk of fatigue cracking. For values greater than 0.5, corrective actions shall be examined and a small-bore connection analysis should be assessed. For a LOF between 0.3 and 0.5 only a small-bore connection analysis should be assessed. Despite the LOF value, a visual survey should always be undertaken to check for poor construction, geometry or support, as well as potential vibration transmission from other sources in case of analysing existent facilities.
Example of a quantitative assessment are shown in the following tables:
3. Specialist Predictive Techniques
Once a potential issue has been identified from the quantitative LOF assessment, specialist predictive techniques can be used to explore the theoretical effectiveness of possible corrective actions. Among the various predictive techniques, Structural Finite Elements Analysis, FEA, and Computational Fluid Dynamics, CFD, stand out. FEA can be used to study eight of the mechanism of vibration mentioned above, with the exception of two-phases induced vibration. A CFD can be used for specific mechanism, such as flow induced turbulence or cavitation and flashing.
Example of specialist predictive techniques are shown in the following images:
