Monitoring Pump Performance with Vibration. Mitchell Stansloski, PhD, PE

Transcription

1 Monitoring Pump Performance with Vibration Mitchell Stansloski, PhD, PE

2 Centrifugal Pumps

3 Centrifugal pumps Centrifugal Pumps (Radial Flow Pumps): A pump using radial flow impellers that give their energy to fluid primarily by centrifugal force Volute Centrifugal Axial Flow 3

4 Centrifugal pumps Higher pressure, lower flow compared to axial flow Impeller adds energy to fluid by increasing velocity Volute (where fluids goes out expansion cavity) converts velocity into pressure 4

5 Basic Pump Operation Operating range given by intersection of system and pump curves.

6 Pump Curve with Power and Efficiency

7 System Curve Pump friction and head losses.

8 Pump Flow Control Various ways to control pumps.

9 Pump Curve vs. Impeller Pump curve showing different impeller sizes.

10 Pump Curve vs. RPM Pump curve showing different speeds.

11 Pump Curve and Vibration

12 Centrifugal pump severity Machine Rigid mounted centrifugal Resilient mounted centrifugal Displacement (mils, p-p) Velocity (ips, pk) Acceleration (g, pk) NA NA Volute Centrifugal Axial Flow Centrifugal 12

13 Two Types of Pump Vibes Periodic Vane passing frequency Shaft/Coupling/Impeller Unbalance Random Cavitation Turbulence Pump performance directly affects both!!

14 Signal Processing Warning Changing resolution directly affects random vibration amplitude

15 Pump Performance and Vibration Frequencies Periodic Vane passing frequency Load Random Cavitation and Turbulence Efficiency

16 Centrifugal pumps Vibration Diagnostics: Vane Pass Frequency: Vibration caused by movement of each blade in a pump or fan Each time blade passes an arbitrary point on pump, it generates a small vibration Load on pump directly changes amplitude of VPF can detect problems (like blocked suction or discharge) 16

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19 Centrifugal pumps Vibration Diagnostics: Poor Installation: Application of extra force on suction and discharge flanges to align and mate flanges Extra force on pump housing can distort housing Distortion causes hydraulic unbalance Can cause high 1X running-speed and VPF vibrations 19

20 Pipe Design

21 Centrifugal pumps Vibration Diagnostics: Cavitation: Implosion of air bubbles in and around impeller of pump Occurs when net positive suction head is too low: Net Positive Suction Head Required (NPSHR): pressure of fluid at inlet in order satisfy the rated output and overcome frictional or shock losses Net Positive Suction Head Actual (NPSHA): Actual positive suction head determined by system feeding the pump (piping, level of inlet reservoir, etc.) If NPSHA insufficient: Some fluid vaporizes upon entrance to pump housing Vapor bubbles collapse violently near impeller Cause excessive damage and corrosion (throw fan out of balance) 21

22 Centrifugal pumps Vibration Diagnostics: Identifying Cavitation: 1) Random 2) Broad-band (over large frequency range) 3) High frequency (30,000 cpm to 120,000 cpm) --Think of it like implosions act like many tiny hammers that ring natural frequencies of pump housing 22

23 Centrifugal pumps Vibration Diagnostics: Eliminate Cavitation: 1) Reduce NPSHR 2) Increase NPSHA Reduce NPSHR: 1) Restrict discharge 2) Over-size pump 3) Use impeller with larger eye (inlet) --ALL affect efficiency so be careful! Increase NPSHA: 1) Raise height of source fluid 2) Lower pump 3) Reduce pipe friction 4) Pressurize supply tank 5) Reduce pump speed --ALL affect efficiency so be careful! 23

24 Pump Cavitation v op [inch/ s] CHW Sec ondary Pump 1\ Pump\ OB V\ Std Vel Meas\ Spec trum 12/ 13/ :33:55 AM RPM : 1780 (29.67Hz) M(x) : cpm (0.07 Orders) M(y) : inch/ s M / 30/ / 13/ / 29/ f [cpm]

25 Pump Cavitation v op [inch/ s] Tertiary CHW Pump 1\ Pump\ IB V\ Std Vel Meas\ Spectrum 4/ 30/ :20:54 AM RPM : 1780 (29.67Hz) / 30/ / 29/ / 15/ f [cpm]

26 Cavitation Improved v op [inch/ s] FHX CW Pump\ Pump\ IB A\ Std Vel Meas\ Spec trum 1/ 25/ :20:03 AM RPM : 1780 (29.67Hz) / 25/ / 13/ / 13/ f [cpm]

27 Centrifugal pumps Vibration Diagnostics: Turbulent Flow: Irregular motion of fluid particles in directions transverse to the direction of the main flow Occurs when poor piping or recirculation cause rough flow of fluid through pump Poor Piping: Should be at least 10 diameters of constant geometry pipe before and after pump Recirculation: Amount of discharge is too high for outlet piping, fluid will return to volute causing turbulence in housing Random, broad-band vibrations at low frequencies 27

28 Using Vibration to Set Performance Cascade Plot Monitor broadband vibration FFT spectra versus RPM Waterfall Plot Monitor broadband vibration FFT spectra versus valve position

29 Single Channel Signal Processing Cascade Plot: Series of spectra as machine speed varies

30 Cascade Plot

31 Pump Performance

32 Summary Periodic vibration (vane passing frequency) Indicates pump load Random vibration (cavitation & turbulence) Indicates pump efficiency Cascade and Waterfall Plots Vibration vs. RPM or valve position Easily identifies changes in both