INSIGHTSE2G INDUSTRY

Fall/winter 2015 | Volume 2

Fall/winter 2015 |

Volume 2

E2G Industry Insights

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite O�cesHouston, TXThe Woodlands, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com

ACOUSTICALLY INDUCED VIBRATION

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PROCESS TECHNOLOGY

WHAT IS ACOUSTICALLY

INDUCED VIBRATION?

Acoustically induced vibration (AIV) often occurs when high capacity gas systems flow through pressure- reducing devices such as relief valves, blowdown valves, restricting orifices, pressure-reducing valves, compressor recycle valves, and choke valves. The high acoustic energy generated by these pressure- reducing valves will excite the pipe systems wall and cause high

ACOUSTICALLY INDUCED VIBRATION

JULIE WARDSenior Engineer

CONTACTFor more information on acoustically induced vibration, please contact Julie Ward.

Email: JNWard@EquityEng.com

frequency (>200 Hz) vibration. This vibration can lead to fatigue failure in the pipe system, especially in nearby small-bore connections and welded discontinuities on the pipe (see Figure 1). Fatigue failure can occur in as little as a few hours. The intensity of vibration is governed primarily by the mass flow rate, gas internal energy, flow versus sonic velocity, and pressure drop. The generated sound frequency is typically in the range of 200 Hz to 20 kHz. Typical systems where AIV may occur include compressor recycle systems, steam desuperheater systems, and high capacity safety valve pressure let-down systems. AIV generates through-wall bending vibration in the pipe with no visible pipe movement.

Systems in liquid service are not susceptible to AIV.

Figure 1: Small-Bore Connection Fatigue Failure

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PROCESS TECHNOLOGY

IMPACT OF ACOUSTICALLY

INDUCED VIBRATION

Failure in piping due to acoustically induced fatigue can be considered catastrophic (see Figure 2), as it could happen after only a few hours of operation. It usually results in pipe through-wall fatigue cracks, pipe detachment from saddle supports, and complete detachment of branch connections. Small-bore branches and welded discontinuities on the pipe that are in close proximity to the pressure-reducing device are the most susceptible to acoustically induced vibration fatigue failure.

Fatigue failure of process piping could result in equipment damage, loss of containment for hydrocarbons that can result in potential vapor cloud ignition, environmental impact, and even loss of human life.

The evaluation of piping susceptible to AIV was first proposed by Carucci and Mueller [1]. After several branch connection failures near pressure reduction valves in natural gas production piping, they gathered data on Sound Power Levels (PWL) and the pipe diameters of 9 failures in industry as well as 27 non-failures.

Carucci and Mueller developed a safe design curve by plotting PWL versus the nominal pipe diameter for the range of data gathered. The design curve developed was further refined by F.L. Eisenger [2] by plotting the PWL data as a function of diameter to the wall thickness, D/t, ratio (Figure 3).

Figure 2: Catastrophic Failure of Piping due to AIV

Figure 3: Sound Power Level (PWL) versus Diameter/Thickness (D2/t2)

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METHOD OF ANALYSIS ‒ INITIAL

SCREENING

There are existing design criteria to avoid acoustically induced fatigue based on comparison of generated sound power level to an acceptable sound power level. This criterion is normally used for the design of pressure relief and flare piping where high gas velocity exceeding 50% of the speed of sound (0.5 MACH) is expected.

Current methods used in industry to predict and mitigate AIV failures are based on the Carucci and Mueller studies. The methods involve identifying the systems that require analysis for AIV (e.g., emergency vents, pressure relief valves, compressor letdowns). This is not applicable for liquid systems, as they do not contain a large amount of potential energy to convert to high acoustic energy. For systems that are two-phase, if the system is 50% gas by volume, it would be conservative to assume the total mass flow rate is gas and evaluated as such.

Once a system has been identified as requiring analysis for AIV, initial screening criteria is used to determine when a detailed analysis is needed for gas flow systems.

Pressure letdown systems that meet any of the following criteria should be further evaluated for AIV.

• Line size 16 NPS or greater – mass flow rate is greater than 200,000 pph or a pressure ratio (P1/P2) greater than three

• Line size 8 NPS to 14 NPS – downstream line velocity greater than 50% sonic and P1/P2 greater than three

• Line size is less than 8 NPS but connected to an 8 NPS or larger line – downstream line velocity greater than 50% sonic and P1/P2 greater than three

• Branch connections having a run size of 16 NPS or greater – mass flow rate greater than 200,000 pph and sonic velocity at the branch connection

Based on industry experience, pressure reducing systems that fall under these criteria may present potential problems and should be evaluated in greater detail for potential AIV issues.

To determine if the system represents a safe design, the calculated sound PWL and the piping D/t ratio can be plotted and compared to the design curve established by F.L. Eisenger using industry data. Points that fall under the curve show good operating history with no known failures as a result of AIV. Systems that fall above the curve require further evaluation with consideration given to either reducing the acoustic energy or strengthening the pipe to eliminate the potential fatigue failure.

Figure 4: Diagram from pipe model experience AIV

In this diagram, blue represents inward deflection, red represents outward deflection, and green represents no deflection. Notice the vibration occurs in a consistent pattern that includes circumferential and longitudinal aspects. When this pattern is disturbed (i.e., at a pipe discontinuity), high stresses and fatigue can result.

PROCESS TECHNOLOGY

E2G | The Equity Engineering Group, Inc. 5

Further AIV screening tools were published in “Guidelines for Avoidance of Vibration Fatigue in Process Pipework” (Energy Institute, 2008). This document includes assessments using fatigue life curves for a range of pipe fittings and materials. The method produces a likelihood-of-failure (LOF) value. Any system with LOF greater than or equal to 1 are considered at-risk and should be redesigned, and values less than 1 are considered acceptable.

MITIGATION OF POTENTIAL AIV

Recommendations to mitigate identified AIV issues can include the following:

• Increasing the pipe wall thickness to a D/t<64

• Local branch reinforcement

• Using forged fittings

• Using sweepolets in place of weldolets

• Minimizing the unsupported length of branch connections

• Using low noise trim valves

• Using multiple pressure-reduction devices

• Splitting the flow to reduce mass flow and/or velocity

SUMMARY

AIV can cause pipe fatigue failure in a short time period and can potentially result in catastrophic consequences.

Methods available in industry allow engineers to evaluate their high capacity gas processes to determine if they are at risk for AIV failure, to mitigate the potential risk and increase the reliability and safety of these systems.

REFERENCES

1. Acoustically Induced Piping Vibration in High Capacity Pressure Reducing Systems, ASME Paper No. 82-WA/PVP-8, Carucci, V.A. and Mueller, R.T., 1982.

2. Designing Piping Systems Against Acoustically Induced Structural Failure, Journal of Pressure Vessel Technology, 1997.

3. Guidelines for Avoidance of Vibration Fatigue in Process Pipework, Energy Institute, London, UK, 2008.

Acoustically induced vibration (AIV) often occurs when high capacity gas systems flow through pressure reducing devices such as relief valves, blowdown valves, restricting orifices, pressure reducing valves, compressor recycle valves, and choke valves.

PROCESS TECHNOLOGY

INSIGHTSE2G INDUSTRY

Fall/winter 2015 | Volume 2

Fall/winter 2015 |

Volume 2

E2G Industry Insights

Corporate Headquarters20600 Chagrin Boulevard, Suite 1200Shaker Heights, OH 44122

Satellite O�cesHouston, TXThe Woodlands, TXVictoria, TXAlberta, Canada

216.283.9519www.EquityEng.com