ONLINE DAMAGE DETECTION ON SHAFTS

Transcription

1 ONLINE DAMAGE DETECTION ON SHAFTS USING TORSIONAL AND UNDERSAMPLING MEASUREMENT TECHNIQUES PUBLIC DEFENCE OF MASTERS DEGREE DATE: 13 TH FEBRUARY 2013 Presented by: Vishaal Bhana Supervisor: Prof. Stephan Heyns 1

2 Introduction Turbogenerator shafts needs to function continuously Shafts subjected to complex torsional loadings Often leads to failure due to propagation of cracks Catastrophic and dangerous Failure results in downtime High repair costs 2

3 Introduction Blackouts Cyclic loading, cracknitiation initiation Failures Downtime for repair High repair cost Potential failures High strain on remaining generators High costs for repair Negative effect on industry and public Literature has shown various failure cases Continuous online monitoring is essential 3

4 Introduction Online monitoring system Machine operating constantly Yes Vibration signature normal? No Maintenance done Pre-plan maintenance schedule Investigate irregularity Constantly monitor the system Changes in the signal are further investigated Problem is isolated Machine stopped and repaired/replaced if necessary Pre-plan maintenance programs/ cost effective 4

5 Literature study Lateral and torsional vibration have been successful More focus on torsional methods recently Introduction of fibre-optic sensors and torsional laser vibrometers and non-contact measurement methods Considerable research by Maynard and M. Trethewey with fibre-optic sensors Found damage in shafts and blades DIC methods used for strain visualisation by taking discrete images Impact tests done by Lall Vibration and mode shape measurements by Helfrick Strain gauges ideal for measuring high frequency content Yam investigated strain and displacement modes Various modelling methods implemented (Analytical, Timoshenko beams, commercial software) Different methods used for vibration and specifically torsional vibration but no comparison 5

6 Objectives Introduction Investigate the torsional dynamics of a shaft for Turbogenerator shafts needs to function continuously undamaged and damaged shaft Shafts subjected to complex torsional loadings Comparison Often leads to failure study due to of propagation different of cracks torsional measurement Cracks develop in shafts due to fatigue techniques Failure results in downtime Investigation of undersampling for damage detection Numerical analysis, FEM-model built through use of Patran and the Nastran solver, Normal modes analysis done Verify using lumped spring-mass system Conduct an experimental analysis on a rotor with increasing damage Measurements done using Telemetry strain system, fibre optic sensor, Digital Image Correlation (DIC) system 6

7 System setup Introduction High costs for repair Effects on the public Continuous online monitoring is essential Offline monitoring results in downtime Pre-plan maintenance programs Two disks fixed onto a shaft Coupled to a 3kW DC motor Self-aligning bearings Free-end on the other side 7

8 FEM Introduction Study High costs of for the repair dynamics of the rotor investigating Effects on the public the natural frequencies Continuous online monitoring is essential Sensitivity Offline monitoring analysis results in downtime with damage Pre-plan maintenance programs Key parameters Geometry- Basic structure that would resemble manufactured setup Mesh- Ease of modelling shaft and damage Boundary conditions- representation of motor connection and bearings Material Properties 8

9 FEM- Model Setup Modelled as a single unit Surface Quad 8 elements that is extruded Control over meshing geometry and modelling the damage (non equivalenced nodes Fixed motor end using MPC Node BC applied at bearing location (x and y direction) Mild steel properties used 9

10 FEM- Convergence and verification Introduction Turbogenerator shafts needs to function continuously Shafts subjected to complex torsional loadings Often leads to failure due to propagation of cracks Cracks develop in shafts due to fatigue Failure results in downtime Lumped model Torsional mode Lumped Patran % Difference

11 FEM- Analysis 11

12 FEM- Mode Shape 1 st mode shows high rotational response at free-end Strain comparison showed higher strain response at the motor-end Location of test equipment determined 12

13 Experimental Introduction setup Rotor manufactured consisting of a shaft with two discs Turbogenerator shafts needs to function continuously Damage introduced by cutting a slot in shaft Shafts subjected to complex torsional loadings Rigid coupling to prevent torsional damping Often leads to failure due to propagation of cracks Overall setup with telemetry, Fibre-optic sensor and DIC system Cracks develop in shafts due to fatigue Brake setup for calibration Failure results in downtime Fibre-optic sensor Telemetry 13

14 Techniques Introduction Fibre-optic sensor High costs for repair Black-White coded tape (Square wave output) Effects on the public Continuous online monitoring is essential Offline monitoring results in downtime Changes in widths represent torsional vibration Pre-plan maintenance programs Telemetry DIC Analog strain data Gauges mounted 180 o apart 2 cameras create 3D representation White markers are picked up and their position analysed Strobe lights needed for dynamic testing 14

15 Processing Order content Harmonics of rotational frequency present Causes natural frequency to be hidden Constant time Constant angle sampling FFT to order domain Remove orders Order removed frequency domain 15

16 Processing Undersampling Nyquist theorem f 2 f, if not then aliasing occurs DIC strobe lights operate at 15 Hz s max y sin(2 10 t) sin(2 80 t) 16

17 Processing cont d Fan-paper method Fold to base-band frequency Frequency of interest band-limited Fold causes reverse readings on odd folds Base-band 4.5Hz 17

18 Processing Application of Fan-paper method 18

19 Test procedure 3 test runs conducted Runup test, 2 constant speed tests- 1100Rpm 1640Rpm 2 min constant speed runs 12 averages taken Frequency resolution 0.1Hz Recording of all 3 devices simultaneously Undamaged and damaged case- 5%-66% damage introduced Measure time domain and post-processing in frequency domain Order removal done Investigation of undersampling methods Waterfall plot for runup test View results in frequency domain 19

20 Post Processing-Fibre-optic sensor Waterfall plot shows structural response at B Averaged 2D plot, Response at 120.6Hz A B 20

21 Post Processing- Telemetry Waterfall and averaged 2D plot- Nothing distinct even after order removal Line frequency harmonics 21

22 Post Processing- DIC Capture markers at undersampled 9Hz Continuous marker capture. Create components Images form 3D representation of setup with displacement and location information 22

23 Post Processing- DIC Some images contain no markers- Blind side Create continuous time signal Undersampled FFT- rotational frequency content Expected frequency 3.6Hz 23

24 FEM Update Geometry- Include brake, coupling and damage modelling Material properties- Apply manual updating by adjusting parameters within its range 24

25 Final Results Fibre-optic sensor revealed change in natural frequencies with damage Comparison showed that fibre-optic method most ideal 25

26 Final Results- FEM Comparison Close comparison between FEM and experimental results Change in frequency ratio shows good correlation Error range from 0-3.2% 26

27 Final Results- Undersampling Undersampled fibre-optic sensor Revealed changes Pre-knowledge required for location Damage % Actual frequency Base-band equivalent Undersampled data

28 Conclusions The fibre-optic sensor proved to be the successful torsional measuring equipment for online monitoring and damage detection. The strain gauge provided order content but nothing about dynamics Further investigations showed that high excitation was required The DIC also revealed order content which was successfully obtained through undersampling techniques Undersampling was successfully used with the fibre-optic sensor results to obtain changes in the dynamics for a damaged shaft. 28

29 Recommendations Strain modal analysis may be investigated by applying external excitation or loading on the system to excite the modes of interest By creating a setup with a low enough dynamic frequency and having a high enough strobe light available, one can investigate systems without undersampling techniques. The DIC system in this study only looked at the markers while they were in camera view. Methods by which the markers may be studied during the full rotation and thereby giving a better full scale representation of what happens during each revolution. The implementation of such a setup together with the external loading for higher strain could prove useful. It would be interesting to apply them on a system with an actual crack. Future analysis could be done by growing a crack onto the shaft. 29

30 Acknowledgements Prof. P.S. Heyns- Supervisor Sasol Labs and DSG at U.P George Breytenbach and Herman Booysen- U.P Gerrit Visser and Hennie Klopper- Esteq SANHARP Thank you 30