A Guide to Avoiding Vibration-Induced Fatigue in Process Pipework

1. Introduction

Vibration-induced fatigue is a leading cause of hydrocarbon releases and failures in process pipework. A proactive approach to identifying and mitigating vibration is essential for ensuring plant safety, reliability, and integrity.

This guide provides a systematic, screening-based approach to assessing the most common sources of pipework vibration. It is based on industry-standard methodologies (such as those from the Energy Institute) and helps project teams identify which systems require detailed analysis.

2. Definitions and Abbreviations

Key Definitions

Cavitation

The dynamic process of forming and violently collapsing vapor bubbles within a liquid, typically at a point of low pressure (e.g., downstream of a valve).

Deadleg

A piping branch with a closed end, such as a closed valve or a blind flange.

Flashing

The sudden and rapid change of a process fluid from a liquid into a vapor, caused by a pressure drop below its vapor pressure.

Flow Induced Excitation (FIE)

Vibration due to vortex shedding as fluid flows over an object, such as a thermowell or a closed-end branch (deadleg).

Flow Induced Turbulence (FIT)

Also known as Flow Induced Vibration (FIV). This is turbulence associated with high fluid kinetic energy, typically occurring at discontinuities like T-junctions or reducers.

High Frequency Acoustic Excitation (HFAE)

Also known as Acoustically Induced Vibration (AIV). Severe vibration caused by high-frequency acoustic energy generated by pressure-reducing devices (e.g., control valves, relief valves) in gas systems, especially under choked flow conditions.

Mechanical Excitation (ME)

Vibration from dynamic forces produced by rotating or reciprocating equipment, such as pumps and compressors.

Pulsation

A dynamic, periodic fluctuation in the pressure of a process fluid, common in systems with reciprocating or positive displacement machinery.

Small Bore Connection (SBC)

A branched connection on a mainline, typically NPS 2 (DN 50) and smaller, which is highly susceptible to fatigue failure.

Surge

A significant pressure wave caused by the kinetic energy of a moving fluid when it is forced to stop or change direction suddenly (also known as "water hammer").

Common Abbreviations

• AIV: Acoustically Induced Vibration (same as HFAE)
• API: American Petroleum Institute
• ASME: The American Society of Mechanical Engineers
• DN: Nominal Diameter
• EI: Energy Institute
• FIE: Flow Induced Excitation
• FIT: Flow Induced Turbulence
• FIV: Flow Induced Vibration (same as FIT)
• HFAE: High Frequency Acoustic Excitation (same as AIV)
• LOF: Likelihood of Failure
• ME: Mechanical Excitation
• NPS: Nominal Pipe Size
• SBC: Small Bore Connection

3. Vibration Assessment Screening

Vibration assessment can be a time-consuming activity. The scope of the assessment should be managed to avoid unnecessary analysis and focus resources on lines with a significant risk. Pipework should be screened to identify which systems have a medium or high Likelihood of Failure (LOF).

Where the likelihood of excitation is low, a detailed quantitative assessment is generally not required. The following sections describe the screening criteria for the most common vibration mechanisms.

4. Flow Induced Turbulence (FIT/FIV)

This assessment is ideally carried out early in the design phase once flow rates, fluid properties, and line sizes are known. Both continuous and intermittent streams (e.g., recycle, bypass, relief) should be assessed.

Corrective Actions: Solutions include increasing pipe diameter (to reduce velocity), increasing pipe wall thickness, or implementing robust pipe support designs with appropriate stiffness.

5. Mechanical Excitation (ME)

Screening Criterion: A detailed assessment is required for:

• Pipework directly attached to rotating or reciprocating equipment.
• Pipework that is routed close to such equipment (e.g., on the same skid or support structure).
• Pipework that shares supports with another line that is already known to have high vibration levels.

Corrective Actions: Focus on ensuring the pipe's structural natural frequency is not close to the equipment's operating frequency (or its harmonics). This is managed through proper pipe support design, location, and stiffness.

6. Pulsation: Reciprocating Pumps and Compressors

Screening Criterion: An assessment is required for all piping upstream and downstream of screw-type, gear-type, and reciprocating positive displacement machines, extending to the first major vessel.

Corrective Actions: A formal acoustic and mechanical analysis (per API 618 for compressors or API 674 for pumps) is typically required. This is often performed by the equipment vendor. Solutions involve pulsation dampeners (suction and discharge), specialty supports, and careful layout.

7. Pulsation: Rotating Stall

Screening Criterion: Assess systems where an operating case exists:

• At the surge control line (i.e., the anti-surge recycle valve is open).
• Within 10% of the actual surge limit for compressors that do not have anti-surge protection.

Corrective Actions: Ensure the anti-surge control system is correctly designed and tuned. Pipe supports must be designed to handle the potential low-frequency vibration.

8. Pulsation: Flow Induced Excitation (FIE)

Corrective Actions: The goal is to break the resonance. This is typically done by changing the length of the deadleg branch, altering the branch-to-main-pipe connection geometry, or reducing the flow velocity.

9. High Frequency Acoustic Excitation (HFAE)

Corrective Actions: Solutions are critical and may include using multi-stage or low-noise trims in valves, installing silencers, or increasing the pipe wall thickness (e.g., Schedule 80 or heavier) for a calculated distance downstream of the source.

10. Surge / Momentum Changes (Valve Operation)

Screening Criterion: An assessment is required for any system with an automatically operated valve that can close or open quickly. This includes:

• Emergency Shutdown (ESD) valves
• Actuated control valves (especially on-off)
• Blowdown and relief valves
• Check valves (which can slam shut)

Corrective Actions: The most common solution is to increase the valve's opening/closing time. If this is not possible, robust pipe supports designed for the calculated surge forces are required. In severe cases, surge dampeners may be necessary.

11. Cavitation and Flashing

Screening Criterion: Assess all liquid pipework at discrete pressure drop points (valves, orifices) or temperature increase points (heaters) where the local fluid pressure may drop near or below its vapor pressure.

Corrective Actions: Solutions include selecting anti-cavitation or multi-stage control valves, managing the system pressure profile to maintain back-pressure, or using hardened trim materials to resist erosion.

12. Small Bore Connections (SBCs)

Screening Criterion: A specific assessment of the SBC's geometry, weight, and support is required if the main line it is attached to has a medium-to-high Likelihood of Failure (LOF) from any of the other mechanisms listed in this guide.

Corrective Actions: Best practice is to eliminate the SBC where possible. If it must exist, solutions include:

• Providing robust, stiff bracing.
• Using forged "weldolet" fittings instead of fabricated "sockolet" or "threadolet" fittings.
• Avoiding long, unsupported instrument connections.
• Orienting the connection away from the primary flow path.

13. Thermowells and Other Protrusions

Screening Criterion: A formal wake frequency calculation (per industry standards like ASME PTC 19.3 TW) is required for all intrusive elements in the flow stream.

Corrective Actions: If the calculation fails (i.e., resonance is predicted), solutions include shortening the insertion length, increasing the element's root diameter (making it stiffer), or adding a support collar (though this is less preferred).

14. Management of Assessment Results

Appendix: Determination of Fluid Bulk Modulus (K)

The Fluid Bulk Modulus (K) is a measure of a liquid's resistance to compression and is a critical input for surge/momentum calculations (Section 11). It is defined as the pressure change required to produce a unit change in volume.

This can be expressed in terms of volume (V) or density (ρ):

...or, more conveniently for use with simulation software:

• K = Fluid Bulk Modulus (N/m²)
• V = Initial volume (m³)
• ρ = Initial density (kg/m³)
• ΔP = Change in pressure (Pa or N/m²)
• ΔV = Resultant change in volume (m³)
• Δρ = Resultant change in density (kg/m³)

To estimate K for a specific fluid or mixture, use a process simulator:

1. Set the fluid at its normal operating pressure and temperature to get the initial density (ρ).
2. Increase the pressure (at the same constant temperature) to the system's design pressure to get the final density (ρ + Δρ) and the pressure change (ΔP).
3. Calculate K using the formula K = ρ (ΔP / Δρ).