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© 2011 ANSYS, Inc. September 12, 2011

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On the Use of High Fidelity Tools for Root Cause Analysis of Severe Vibration & Noise at Plant Facility

Vishwas Iyengar, Ph.D. Stephen James Marybeth Nored Southwest Research Institute

San Antonio, Texas


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• Modern gas compression facilities have complex and multidimensional piping system responses

• Various components of the compression facility may be subject to severe vibration:

– engine, coolers, main lines,

– anti surge lines, control valve, anti surge valve (ASV) etc.

• Critical as the compressor size and horsepower increase, such as in facilities supporting LNG operations

• The vibrations can occur at normal load conditions (due to bad design) or at part load conditions

• In the case of turbulent induced vibrations or vortex shedding phenomena, it is common for the source of vibration to be in a different location then the location of maximum vibration.

• Vibrations coupled with the structure/piping natural frequencies tend to further amplify the problem.

– Adds complexity while trying to determine the root cause of the vibration

Introduction


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• Audible noise on the facility may also increase to levels that are greater than OSHA limits. Although the instabilities from the compressor components are well known, there is still a need to characterize noise from main lines and anti-surge lines especially at part load conditions.

• Under such circumstances there is a need for a more comprehensive analysis in order to accurately find the root cause of the vibration and noise; such a method is presented in this study.

Introduction (continued)


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The analysis consists of four distinct tasks:

• (I) Mechanical System FEA Analysis of the ASV structure

• (II) Acoustic/1-D Piping Analysis (NOT PRESENTED HERE)

• (III) Computational Fluid Dynamic (CFD) and Noise Analysis; and

• (IV) Integration of Results

Approach


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• Independent mechanical system Finite Element Analysis (FEA) was performed on the overall structure including the 1st stage recycle loop, ASV piping structure, and the expansion loop supports

• The FEA was performed to determine:

– Mechanical natural frequencies and corresponding mode shapes of the piping and structure system

– Forced response amplitudes and severity with assigned forcing frequencies and amplitudes.

• The analysis utilized field vibration data as a means of validating the model predictions.

• The piping and frame structure were initially analyzed separately and later combined into one single model

– The piping comprises the first stage recycle loop. Other piping was not analyzed, because the first stage recycle loop is the main area of interest.

– The frame includes the plant structure from the compressor to the heat exchanger.

Mechanical Analysis - Overview


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Key points extracted from available AutoCAD drawings

Mechanical Analysis – Model Generation


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Line model built from key points



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Automated script to fit beam section to each line



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• A large number of modal frequencies exists for the combined model where many of the modes are pertaining to local bending or torsional models of the structure. Subsequent forced response analysis will indicate modes that could be excited.

Mechanical Analysis – Modal Analysis

MODE #1 MODE #2 MODE #3

MODE #4 MODE #5 MODE #6


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Analysis Summary

• Several mechanical responses in the frequency range of 3.0 – 4.0 Hz

• Vibrations prominent in the location of the anti-surge valve. Excitation force by low frequency vibrations in the valve can excite these modes

• The higher deflections reported in the frequency range of 3.0 – 4.0 Hz, are in close agreement with the field measurements that also show peak vibration amplitudes in the same frequency range

• Entire structure is analyzed based on the assumption that beams in the structure are welded together. Welded joint results in higher stiffness and hence leads to higher natural frequency predictions. In reality, the beams are bolted and hence natural frequencies may be slightly lower than calculations.

• Modal analysis of the system indicated the lowest mechanical natural frequency to be near 3.0 Hz with approximately 17 modes at or below 12.0 Hz, indicating the system is very flexible in general

Mechanical Analysis – Modal Analysis


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• Two different forcing conditions analyzed:

– Dynamic pressure of 2.0 PSI near the valve

– A 0.5 PSI excitation force near the valve

• These two conditions provide a low and high end for the valve turbulent excitation force, as the exact force is difficult to quantify precisely

• Piping tied to the frame using an assumed stiffness –not tied down rigidly, but the stiffness is greater than that of a simple weight support

• Peak velocities and stresses extracted from the location of maximum amplitude at any given frequency

Mechanical Analysis – Forced Response


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• Response plots corresponding (in frequency order) with each of the peaks on the velocity plot is shown below. These plots show the deflection pattern under load, which is similar but not exactly the same, as the modal plot.

Mechanical Analysis – Forced Response

RESPONSE #1 RESPONSE #2 RESPONSE #3

RESPONSE #4 RESPONSE #5 RESPONSE #6


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Conclusions

• The analysis of the large existing structural steel support frame indicated that it is relatively flexible and that vibration amplitude reductions could be achieved by increasing the stiffness of the frame.

• Amplitudes predicted by the Forced Response Analysis vary from a minimum of 0.5 ips to a maximum of 11.0 ips. Values above 0.5 ips are considered too high for this low frequency and large diameter piping.

• The piping in the immediate vicinity of the ASV needs to be more effectively secured to the steel support structure. The stiffer the support, the more effectively it will control vibration.

Mechanical Analysis - Summary


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Analysis

• In this part, the CFD analysis along with a noise analysis is performed to independently assess valves manufactured by three different valve vendors.

• The anti-surge valve (ASV) is an integral part of the compressor recycle loop. Previous results showed very high turbulence levels downstream of the ASV, hence alternative valves are evaluated.

Computational Fluid Dynamics (CFD)


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Operating Conditions • Boundary conditions are prescribed at the inlet and

outlet control surfaces. • Throughout this analysis a mass flow rate condition is

prescribed at the inlet boundary, and a pressure condition is applied at the outlet boundary.

• A typical stack (utilized for high energy, high flow dissipation) is made up of multiple disks, each disk has multistage flow path (90° turn). The number of stages of stack trim is usually determined later once the customer’s needs are described.

• Here, the valve trim internal geometry has not been modeled. The trim geometry is extremely proprietary, and it is custom made upon order.

• Hence, a porous media approach is used to simulate the valve trim. A porous domain is assumed for the appropriate faces where the valve trim would appear.

• The appropriate pressure loss is accounted for by assuming the faces as a porous material.

• This method is commonly used when dealing with complicated flow paths similar to the complex flow path through the ASV trim.

CFD (continued)

Case #1

Flow - 550 MMSCFD MW - 20.14

Pin - 186 psig

Tin - 115 F Pout - 104 psig

Tout - 115 F Case #2

Flow - 865 MMSCFD MW - 17.1

Pin - 535 psig

Tin - 142 F Pout - 254 psig

Tout - 129 F Case #3

Flow – 855333 kg/hr MW – 17.14

Pin – 33.69 bar-A

Tin – 62.78 °C Pout – 17.361 bar-A


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Velocity Vector Comparisons

CFD (continued)

VALVE A VALVE B

VALVE C


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Streamline Comparisons

• High swirl components are noticeable for valves B and C. Swirling flow is greater for the lower flow rates; flow becomes straight and uniform after a short distance downstream of the valve. Valve A shows lesser reversed flow downstream of the valve.

• The CFD Analysis results showed that for the three valves considered and the operating conditions selected for the analysis, the Valve A performs the best in terms of lower exit Mach number and lower flow turbulence. Valve B performance is not at the level of the Valve A but shows some improvement over the Valve C.

CFD (continued)


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• This section summarizes the acoustic predictions performed by the valve manufacturers. The noise predictions furnished by the valve manufacturers are based upon the method given in IEC Standard 60534-8-3, Industrial-process control valves - Part 8-3: Noise considerations - Control valve aerodynamic noise prediction method.

• While each manufacturer used the IEC Standard as a starting point, each manufacturer implements modifications in an attempt to make the prediction more accurate.

• Combined A-weighted sound pressure level 1 meter from the pipe wall, caused by the valve trim and the expander section is given by,

• is the A-weighted sound pressure level 1 meter from the pipe wall generated by the valve trim.

• is the A-weighted contribution to the sound-pressure level 1 meter from the pipe wall caused by pipe expander-induced gas turbulence.

Noise Analysis

1010

10 1010log10 1, PeRmpAe LL

pSL

mpAeL 1,

PeRL


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• C1 (Worst Case) – Valve A 20x42= 76.0 dBA; Valve B 24x24 = 81.8 dBA

• These predictions include the noise generated from the valve body as well as from noise release due to expansion in the piping downstream of the valve.

• The dBA differences between the two valve selections are due to geometric differences between the outlet sections of the two valves. In case of Valve A, virtually all flow expansion is done before the valve exit flange. This reduces downstream noise production. For the Valve B selection, flow expansion occurs aft of the valve flange.

Noise Predictions

Valve A: 20 x 42, 4 turn trim Sound Pressure Level Predictions

Valve B: 24 x 24 Sound Pressure Level Predictions


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Study Summary

• This study demonstrates a multidisciplinary effort to get better identify the root cause for severe vibration.

• Here high fidelity tools (ANSYS and other in-house) are used to develop a framework that involves the structure, the piping, the anti surge valves.

• The combination of tools is presented to facilitate the approach involve Computational Fluid Dynamics (CFD), Finite Element Analysis (FEA), Piping Acoustic Analysis and Noise Analysis.

• The CFD analysis combined with the noise and FEA analysis will help mitigate the turbulence generation near the recycle valve and its interactions with the piping and structures. This will permit the full load operation of the compressor units at the facility.

Take Home Message


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Ben White

Brian Moreland

John Stubbs

Acknowledgments


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QUESTIONS?

Vishwas Iyengar, Ph.D. Stephen James Marybeth Nored Southwest Research Institute

San Antonio, Texas

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