Vibration Induced Fatigue Failure in Process Pipework

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 occur and acoustic vibrations will be generated. Pipelines with high risk of AIV can be identified, both in new design and existing plants, and corrective actions can be applied. These may include reducing mass flow rate, increasing pipe diameter, using acoustic silencers or 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 instability in unsupported pipes or even induce buckling in pipes supported at both ends. Oscillatory flow can also cause vibration, generated by reciprocating pumps and compressors. Other components in the line that cause vibrations are bends and small-bore connections (sources of turbulence) and thermowells and dead leg branches (sources of vortex shedding).

A two-phase flow can also induce vibration for three different reasons:

1. Density difference between the two phases.
2. Phase changes, such as boiling and condensation.
3. Dynamics of various shapes and sizes of bubbles, which induces sloshing, fluctuations and disturbances.

These sources can be detected in new or existing plants, and corrective actions applied. These may involve operational changes or modifications in the piping system, tailored to the specific excitation source.

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 provide a methodology to help minimize the risk of vibration-induced fatigue in process piping. They apply to:

• A new plant/facility
• An existing plant/facility
• Modifications within existing facilities

And to different plant levels:

• Operating units
• Major areas
• Individual equipment

1. Qualitative Assessment

The first step is identifying the sources of excitation. For new plants, the Guidelines classify nine excitation types and provide a qualitative assessment framework.

1. Kinetic energy of the fluid: turbulence and pulsation in gas flow.
2. Choked flow from restriction devices: induces AIV.
3. Rotating/reciprocating machinery: induces mechanical excitation.
4. Positive displacement pumps: induces pulsation.
5. Centrifugal compressors: induces pulsation.
6. Flashing or cavitation: induces turbulence and vibration.
7. Fast-acting valves: causes surge/momentum changes.
8. Intrusive elements: induces vortex shedding.
9. Slug flow: induces sloshing and disturbances.

2. Quantitative Assessment

The Guidelines provide quantitative assessment tools to determine the Likelihood of Failure (LOF). This scoring method is based on simplified conservative models. LOF < 0.3 → no fatigue risk. LOF ≥ 0.5 → corrective action and small-bore connection analysis required. LOF between 0.3–0.5 → small-bore connection analysis advised. Visual surveys are always recommended.

3. Specialist Predictive Techniques

Upon identifying potential vibration risks through LOF, specialist techniques like Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) are used to assess corrective measures. FEA handles 8/9 vibration types (except two-phase flows), while CFD is suitable for mechanisms like turbulence or cavitation.