An FEA-Based Acoustic Fatigue Analysis Methodology

Transcription

1 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 System August 31 - September 1, 2011 Houston, TX

2 Outline Introduction and Theory Existing Acoustic Fatigue/AIV Screening Methods Carucci-Mueller, Eisinger Energy Institute SwRI Method AIV Solutions Conclusions

3 Introduction Acoustically Induced Vibration (AIV) refers to high-frequency vibration (typically Hz) in piping downstream of a pressure-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 and Mueller Often a concern in flare/blowdown piping with thin walls and large diameters. Image Courtesy Tyco Valves & Controls

4 Theory Overview AIV is caused by the following four physical phenomena: Excitation from a pressure-reducing valve causes highfrequency pressure fluctuations in downstream piping. These fluctuations excite higher order acoustic modes in the pipe with circumferentially varying pressure mode shapes. The acoustic pulsations couple to shell modes of the main piping. Branch connections or other welded discontinuities in the main line serve as stress risers.

5 Theory: Acoustic Cross Modes

6 Theory: Pipe Shell Modes

7 Existing AIV Screening Methods Carruci-Mueller paper (1983) introduced design limits based on failure/non-failure experience. PWL and Pipe Diameter Eisinger (1997) modified the Carruci-Mueller limits to include different excitation parameter and wall thickness. M* P and Pipe D/t Ratio Eisinger later (1999) used FEA to extend the design curve to lower D/t ratios.

8 Existing AIV Screening Methods (2) Carruci-Mueller Design Curve Eisinger Design Curve

9 Existing AIV Screening Methods (3) The Energy Institute (2005) introduced a screening methodology for AIV: Simple source PWL computation PWL decay to branch connection and addition of PWL from multiple sources at each branch Estimate of fatigue life from curve-fit data (data from FE models calibrated to historical failure/non-failure data) Fatigue life estimation including reduction due to weldolet fittings and small branch diameter to main line diameter ratios Likelihood of Failure (LOF) computed from estimated fatigue life

10 SwRI Method Overview Valve excitation analysis, acoustic analysis and finite element analysis performed to determine coincidence of acoustic and pipe shell modes Forced response analysis of FE model at coincident modes performed with shell models to determine stresses 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 of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2

11 SwRI Method Valve Excitation Valve excitation analysis performed using control valve noise prediction standard IEC 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 in same manner as Energy Institute method Convert PWL to SPL and dynamic pressure Image Courtesy Floyd Jury, Fisher Controls

12 SwRI Method Acoustic Modes Closed form solution used to model higher-order acoustic modes Resulting acoustic frequencies and mode shapes validated with ANSYS Acoustic 3D FEA models Multiply valve excitation by amplification factor to account for acoustic resonance q1 p1 p2 p3 p4 p5 p6

13 SwRI Method Pipe Shell Modes ANSYS APDL scripts constructed to efficiently construct shell element models of piping at branch connections Modal analysis performed for each connection over excitation frequency range Results postprocessed externally via spatial FFT to determine dominant nodal diameter patterns in each mode

14 SwRI Method Pipe Shell Modes (2)

15 SwRI Method Pipe Shell Modes (3) Circumferential Mode Shape (n) FFT performed in order to identify n

16 SwRI Method Coincidence

17 SwRI Method Forced Response

18 Mesh-Sensitive Peak Stress SwRI Method Forced Response (2) Coincident Mode Frequency, Hz

19 SwRI Method Forced 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

20 SwRI Method Forced 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

21 SwRI Method Fatigue Life Use relative stress distribution from meshsensitive results to find location of maximum stress Use nodal forces and moments to calculate bending and membrane stresses and assess fatigue life Images Courtesy ASME Boiler & Pressure Vessel Code, Section VIII, Division 2

22 SwRI Method Fatigue 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 -3σ S-N curve for <1% probability of failure Image Courtesy Dong 2009

23 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

24 Method Comparison Carruci- 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 Includes excitation frequency Allows detailed analysis of design alternatives X X X Fatigue Life Calculation See (2) X 1 Future work includes validation of method with test and historical data 2 Calculated fatigue life is part of calibrated screening procedure, not end result

25 QUESTIONS?