Pipework Vibration Analysis

Pipework and Structural Vibration

Pipework and structural vibration pose significant risks to industrial systems, causing fatigue failures, leaks, and safety hazards. 

Key aspects include:

Causes of Vibration

Vibration originates from multiple sources e.g.

• Mechanical forces: Unbalanced machinery (compressors, pumps) transmitting vibration to supports.

• Flow-Induced Vibration (FIV): Turbulence at bends, tees, or valves generating low-frequency (<100 Hz) shaking forces.

• Transient events: Water hammer, slug flow, or rapid valve operations.

Risks and Failure Modes:

• Fatigue cracking: Especially at welds, small-bore connections, or support points.

• Flange leaks and support detachment from excessive movement.

• Resonance: When excitation frequencies match piping natural frequencies, amplifying stresses.

Assessment Methods:

• Proactive surveys: Recommended during design changes (e.g., flow rate increases) to predict vibration hotspots.

• Reactive monitoring: Deployed when visible shaking occurs, using strain gauges and accelerometers to quantify amplitudes.

Screening criteria:

•  FIV: Evaluated via dynamic pressure (ρv²), pipe diameter, and flexibility.

• Fitness-for-service (FFS): Estimates remaining equipment life under vibration loads.

Best Practices:

• Adopt standards: Follow Energy Institute (EI) AVIFF Guidelines for integrity assessments.

• Prioritise small-bore connections: Reinforce with thicker walls or clamps due to high vulnerability.

• Thermowell safety: Ensure compliance with ASME PTC 19.3 to prevent resonance failures.

Proactive vibration management—combining design-stage analysis, operational monitoring, and targeted reinforcement—reduces failure risks by over 50% in high-energy systems.

Acoustic Induced Vibration

Acoustic Induced Vibration (AIV) is a high-frequency structural vibration phenomenon in piping systems carrying vapour or gas, triggered by intense acoustic energy from pressure-reducing devices. It causes rapid fatigue failures at stress-concentration points like small-bore connections or welded supports, often within minutes to hours of operation.

Definition and Mechanism

AIV occurs when high-pressure drops across valves, orifices, or other restrictions in gas/vapour systems generate intense broadband noise (500–2,500 Hz). This acoustic energy excites circumferential "shell-mode" vibrations in the pipe wall, leading to radial displacement and stress amplification at asymmetric features like branches or supports.

Key characteristics:

• Frequency range: 500–2,500 Hz.

·• Excitation sources: Control valves, relief valves, blow-down valves, or any device causing sudden pressure reduction.

• Failure trigger: Resonant vibration at stress concentrations (e.g., welds), where fatigue cracks initiate.

Risks and Failure Modes:

AIV poses severe operational hazards:

• Catastrophic pipe ruptures due to fatigue cracks at branch connections or supports.

• Extremely short failure time: Minutes to hours under high-stress conditions.

Best practices include:

• Screening during design:

• Calculate Sound Power Level (PWL)

• PWL > 155 dB indicates high risk, requiring Likelihood of Failure (LOF) assessment.

Post-construction fixes: For existing systems, reinforcement pads or dampeners reduce dynamic pressure fluctuations.

Industry Standards:

The Energy Institute (EI) Guidelines for the Avoidance of Vibration Induced Fatigue Failure (2008) provides a structured framework for AIV risk assessment and mitigation. 

Key recommendations:

• Prioritise small-bore connections for reinforcement.

•  Use smoother fittings (e.g., B16.9 tees) to minimise stress concentrations.

• Combine AIV and Flow-Induced Vibration (FIV) analyses for comprehensive safety.

Proactive design and adherence to EI standards reduce AIV-related failures by over 50% in high-pressure systems. For operational plants experiencing vibration, real-time monitoring with accelerometers and strain gauges is advised to quantify risk and guide interventions. 

Effective pipework vibration troubleshooting requires identifying excitation sources, resonance conditions, and high-risk components like small-bore connections (SBCs).

Diagnostic Steps:

Proactive troubleshooting, guided by standards like EI AVIFF, prevents catastrophic failures. Priorities SBCs and pulsation-prone areas, combining field data with engineering analysis for robust solutions.

• Measure vibration. characteristics.

• Inspect high-LOF areas.

• Prioritise SBCs, unsupported spans,  bends, and dead legs.

• Check for anomalies like cracks at welds or supports, especially near pulsating equipment (compressors, pumps).

Immediate and Long-Term Solutions

Site fixes:

• Reinforce SBCs: Apply full-encirclement repads and brace connections.

• Install dampers: Use viscous dampers or braces to absorb energy. 

• Add supports: Reduce span lengths to increase natural frequency (>4 Hz).

System redesign:

• Reroute piping: Minimise bends and directional changes to lower turbulence.

• Isolate sources: Use expansion bellows or anti-vibration mounts between equipment and piping.

• Modify operations: Lower flow rates or implement soft-start pumps for surge control.

Advanced Analysis for Persistent Issues

• Fatigue assessment: Apply API 579/ASME FFS-1 if stresses exceed endurance limits.

• Acoustic studies: Design pulsation controls for reciprocating machinery. 

• Continuous monitoring: Deploy IoT sensors for real-time vibration monitoring.

Dynamic Stress Measurement

Dynamic stress measurement is a method for evaluating the behaviour and structural integrity of a component or system subjected to dynamic loads, such as vibration or impact. The primary goal is to understand stress and strain distribution to increase the durability of components, prevent failures, and ensure operational safety. Measurements are often used to validate numerical simulations, such as Finite Element (FE) models, by comparing them with data from real components.

Measurement Techniques

Contact-based strain gauges  i.e. the strain gauges method is, where gauges are physically attached to a component's surface directly measuring strain at the point of application.

Applications of Dynamic Stress Measurement

Dynamic stress measurement is critical in various industrial and engineering fields:

• Turbomachinery: It is a key design factor for turbine and fan blades, where unsteady flow can cause significant vibration. Non-intrusive monitoring systems help isolate potential blade failures before they occur, minimizing machinery damage and risk of injury.

• Piping Systems: In facilities like oil and gas plants, dynamic stress analysis is performed on critical lines, such as those connected to reciprocating equipment like compressors to evaluate dynamic stresses across different operating frequencies. Analysis may lead to recommendations like relocating pipe supports, rerouting pipelines, or using higher-rated piping to ensure structural integrity.

• General Design and Testing: The process is used for structural tests, fatigue strength measurements, and product testing. The results allow engineers to optimize a design to ensure it can withstand its expected loads while minimizing weight and cost>

Pipework Vibration Control (PVC) Clamps Design

Anti-vibration clamps for piping systems are engineered to mitigate dynamic stresses caused by vibration, fluid pulsation, and trasient events. Their design balances stiffness for vibration control with flexibility for thermal movement, preventing fatigue failures. 

Key design aspects include:

Design Principles

Vibration Control:

• Clamps must provide sufficient stiffness to restrain resonant vibration, preventing bolt loosening and fatigue.

Thermal Accommodation:

• This dual functionality prevents static stress buildup (e.g., low-cycle fatigue) without compromising vibration control.

Material and Construction:

• Elastomer inserts minimise surface contact with pipes, mechanically absorbing vibrations and reducing noise.

The finite element method (FEM) is a numerical method for solving problems of engineering and mathematical physics. It is also referred to as finite element analysis (FEA). 

It subdivides a large problem into smaller, simpler parts that are called finite elements. The simple equations that model these finite elements are then assembled into a larger system of equations that models the entire problem. Typical problem areas of interest include structural analysis and fluid flow.