Understanding vibration

Paul Boughton

Neil Parkinson explains the six phases in understanding, correcting and preventing vibration-induced pipework failure.

Vibration induced fatigue of process piping systems is an important and fundamental failure mechanism. Data published by the UK's Health & Safety Executive (HSE) for the offshore industry has shown that in the UK sector of the North Sea, fatigue and vibration failures account for 21% of all hydrocarbon releases. Although overall statistics are not available for onshore facilities, available data for individual plants indicates that in Western Europe, between 10 and 15% of pipework failures are caused by vibration induced fatigue.

Based on the Energy Institute publication 'Guidelines for the Avoidance of Vibration Induced Fatigure Failure in Process Pipework', current best practice is aimed at minimising the risk of incurring loss of containment from vibration-induced failures.

The Energy Institute guidelines break down into two main scenarios - proactive and reactive assessments - and aim to ensure compliance with statutory duty, improve safety and reliability, reduce liability from leakage, and minimise plant downtime. Proactive assessments can be used to routinely evaluate all pipework on a site, whether existing or planned, to ensure that best practice has been adopted and to identify possible areas of concern. Reactive assessments follow, and are used to further investigate known vibration issues or troubleshoot actual failures within both mainline pipework as well as small bore connections (SBCs). There are six key phases to achieving pipework vibration assessments in line with requirements of the guidelines: qualitative assessment; visual assessment; basic vibration monitoring; specialist measurement techniques; specialist predictive techniques; corrective actions.

The qualitative assessment phase is perhaps the most challenging to implement and involves numerous calculations for assessing the likelihood of encountering a vibration-induced fatigue issue - on either an existing or planned plant. This assessment takes into account many relevant factors, from fluid energy, flow velocities and cyclic operation to the construction quality of infrastructure including process machinery and types of valves. It also assesses the chance of flashing or cavitation, and includes a calculation process for scoring likely excitation factors - which are combined with conditional and operational factors to predict the 'likelihood of failure' (LOF) for each pipe branch.

Visual inspection is a quick and effective method for identifying potential areas for concern. Many pipework vibration problems are simply the result of operators not following recommended good practice, and visual inspection by skilled assessors can quickly flag up areas for improvement relating to pipe infrastructure. This may include installing more effective pipe supports or replacing worn or damaged supports, proper bracing of SBCs, avoiding fretting and poor geometry, and allowing for thermal expansion of tubing.

The basic piping vibration measurement phase identifies areas of concern based on measured values of pipework vibration. In this phase specialist engineers will first use a single axis accelerometer connected to a portable data collector to take initial vibration levels, ranging from 1 Hz up to 300 Hz. These measurements are presented as vibration amplitude versus frequency and enable the vibration to be classified as acceptable, concern or problem, based on comparison with assessment criteria in the Energy Institute guidelines.

If vibration is assessed as being at a concern or problem level, or for pipework with a higher frequency vibration of more than 300 Hz, the next phase used by vibration engineers is based on specialist measurement techniques. Here, a variety of more in-depth tests can be deployed depending on need. In addition, engineers can implement specialist predictive techniques, applying sophisticated tools and modelling to provide a more detailed assessment of the dynamics of specific pipelines throughout their lifecycles.

The final stage of any pipework assessment is to recommend corrective actions to reduce vibration levels and the likelihood of future failures. These actions vary from improving the support infrastructure around pipework including bracing and dampening, or modifying the process conditions themselves to reduce fluid loadings.

The design of practical and appropriate corrective actions is important in achieving cost effective yet thorough solutions, and often utilises FEA techniques to predict the effect of remedial repairs, alongside CAD software for mechanical design of supports and bracing systems.

Strain gauging and FEA are powerful tools in this analysis process and although they are often perceived as being distinct and alternative assessment technologies, AVT has long recognised the power of combining practical strain gauge work with theoretical FEA.

AVT's Pipework Vibration Solutions service offers everything from initial on-site surveys and vibration diagnosis, to solution recommendation, design and installation. AVT's service helps customers reduce the risk of major incidents caused by pipework vibration, as well as underpinning HSE responsibilities under the COMAH and Offshore Installations (Safety Case) Regulations 2005.

Neil Parkinson is technical director at asset integrity specialist AV Technology (AVT)

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