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7/23/2019 FEA-Based Acoustic Fatigue Analysis Methodology - SWRI
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An FEA-Based Acoustic Fatigue Analysis
Methodology
Timothy C. Allison, Ph.D.
Lawrence J. Goland, P.E.
Southwest Research Institute
San Antonio, TX
ANSYS Regional Conference:
Engineering the SystemAugust 31 - September 1, 2011
Houston, TX
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Outline
Introduction and Theory Existing Acoustic Fatigue/AIV Screening
Methods
Carucci-Mueller, Eisinger Energy Institute
SwRI Method
AIV Solutions Conclusions
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Introduction
Acoustically Induced Vibration (AIV) refersto high-frequency vibration (typically 500-1500+ Hz) in piping downstream of apressure-reducing device
E.g. a control valve or pressure relief valve
Can result in high cycle fatigue failures,particularly at branch connections
First identified in 1983 by Carucci andMueller
Often a concern in flare/blowdown pipingwith thin walls and large diameters.
Image Courtesy Tyco Valves & Controls
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Theory Overview
AIV is caused by the following four physicalphenomena:
Excitation from a pressure-reducing valve causes high-frequency pressure fluctuations in downstream piping.
These fluctuations excite higher order acoustic modes inthe pipe with circumferentially varying pressure modeshapes.
The acoustic pulsations couple to shell modes of themain piping.
Branch connections or other welded discontinuities inthe main line serve as stress risers.
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Theory: Acoustic Cross Modes
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Theory: Pipe Shell Modes
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Existing AIV Screening Methods
Carruci-Mueller paper (1983) introduced designlimits based on failure/non-failure experience.
PWL and Pipe Diameter
Eisinger (1997) modified the Carruci-Mueller
limits to include different excitation parameterand wall thickness.
M*P and Pipe D/t Ratio
Eisinger later (1999) used FEA to extend thedesign curve to lower D/t ratios.
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Existing AIV Screening Methods (2)
Carruci-Mueller Design CurveEisinger Design Curve
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Existing AIV Screening Methods (3)
The Energy Institute (2005) introduced a screeningmethodology for AIV: Simple source PWL computation
PWL decay to branch connection and addition of PWLfrom multiple sources at each branch
Estimate of fatigue life from curve-fit data (data from FEmodels calibrated to historical failure/non-failure data)
Fatigue life estimation including reduction due toweldolet fittings and small branch diameter to main line
diameter ratios Likelihood of Failure (LOF) computed from estimatedfatigue life
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SwRI Method Overview
Valve excitation analysis, acoustic analysis andfinite element analysis performed to determinecoincidence of acoustic and pipe shell modes
Forced response analysis of FE model at coincidentmodes performed with shell models to determinestresses at fillet weld and resulting fatigue life. Excitation from valve amplified by acoustic amplification
factor to account for acoustic resonance
Stresses evaluated using mesh-insensitive procedure for
welded joints in accordance with Section 5.5.5 of theASME Boiler and Pressure Vessel Code, Section VIII,Division 2
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SwRI MethodValve Excitation
Valve excitation analysis performedusing control valve noise predictionstandard IEC 60534-8-3 Detailed source PWL prediction
Peak noise frequency from vena
contracta velocity and jet diameter Model PWL decay to branch and
summation of sources at branch insame manner as Energy Institutemethod
Convert PWL to SPL and dynamicpressure
Image Courtesy Floyd Jury, Fisher Controls
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SwRI MethodAcoustic Modes
Closed form solution used tomodel higher-order acousticmodes
Resulting acousticfrequencies and modeshapes validated with ANSYSAcoustic 3D FEA models
Multiply valve excitation byamplification factor to
account for acousticresonance
p1p2
p3
p4
p5
p6
q1
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SwRI MethodPipe Shell Modes
ANSYS APDL scripts constructed to efficientlyconstruct shell element models of piping at
branch connections
Modal analysis performed for each connectionover excitation frequency range
Results postprocessed externally via spatial FFT
to determine dominant nodal diameter patternsin each mode
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SwRI MethodPipe Shell Modes (2)
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SwRI MethodPipe Shell Modes (3)
Circumferential Mode
Shape (n)FFT performed in order to
identify n
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SwRI MethodCoincidence
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SwRI MethodForced Response
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SwRI MethodForced Response (2)
1150 1170 1190 1210 1230 1250 1270 1290
Mesh-SensitivePeakStress
Frequency, Hz
CoincidentMode
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SwRI MethodForced Response (3)
Note: Stresses shown are mesh-sensitive and are not accurate
absolute values. Mesh-insensitive stresses are calculated with
ASME B&PV Code Sec VIII Div 2 procedure
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SwRI MethodForced Response (4)
At Fillet
Weld
Toe
Note: Stresses shown are mesh-sensitive and are not accurate
absolute values. Mesh-insensitive stresses are calculated with
ASME B&PV Code Sec VIII Div 2 procedure
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SwRI MethodFatigue Life
Use relative stressdistribution from mesh-sensitive results to find
location of maximumstress
Use nodal forces andmoments to calculate
bending and membranestresses and assessfatigue life
Images Courtesy ASME Boiler & Pressure Vessel Code, Section VIII, Division 2
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SwRI MethodFatigue Life ASME Div 2 master S-N developed based on a large
amount of welded pipe and plate joint fatigue test data
Fatigue life assessed on -3S-N curve for
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Conclusions
New AIV analysis methodology developed based
on physical principles
Method uses automated implementation of
valve noise prediction standard and exact
acoustic solution for efficient excitation solution
Automated scripting tools applied for efficient
FEA solution of coincident stress at connection
and mesh-independent fatigue life results
FEA-based approach allows for modeling of
various countermeasures
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Method ComparisonCarruci-
Mueller
Eisinger Energy
Institute
SwRI
Method
Calculates PWL X X X X
Includes Pipe Diameter X X X X
Uses historical data X X X See (1)
Includes pipe wall thickness X X X
Includes multiple sources & decay X X X
Includes connection type X X
Includes branch diameter X X
Includes acoustic/structural coincidence X
Includes excitation frequency X
Allows detailed analysis of design
alternatives
X
Fatigue Life Calculation See (2) X
1Future work includes validation of method with test and historical data
2Calculated fatigue life is part of calibrated screening procedure, not end result
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QUESTIONS?