Centrifugal pumps
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Volute Centrifugal Axial Flow
Centrifugal Pumps (Radial Flow Pumps): A pump using radial flow impellers that give their energy to fluid primarily by centrifugal force
Centrifugal pumps
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• 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
Centrifugal pump severity
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Volute Centrifugal Axial Flow Centrifugal
Machine Displacement (mils, p-p)
Velocity (ips, pk)
Acceleration (g, pk)
Rigid mounted centrifugal
NA 0.05 - 0.25 0.5 – 1.5
Resilient mounted centrifugal
NA 0.10 – 0.50 0.5 – 1.5
Two Types of Pump Vibes
• Periodic– Vane passing frequency– Shaft/Coupling/Impeller Unbalance
• Random– Cavitation– Turbulence
• Pump performance directly affects both!!
Pump Performance and Vibration Frequencies
• Periodic – Vane passing frequency– Load
• Random – Cavitation and Turbulence– Efficiency
Centrifugal pumps
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Vibration Diagnostics:
•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)
Vane Pass Frequency: Vibration caused by movement of each blade in a pump or fan
Centrifugal pumps
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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
Centrifugal pumps
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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)
Centrifugal pumps
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Vibration Diagnostics:Identifying Cavitation:
1) Random2) 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
Centrifugal pumps
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Vibration Diagnostics:•Eliminate Cavitation: 1) Reduce NPSHR
2) Increase NPSHA
•Reduce NPSHR: 1) Restrict discharge 2) Over-size pump3) Use impeller with larger eye (inlet)
--ALL affect efficiency so be careful!
•Increase NPSHA: 1) Raise height of source fluid2) Lower pump3) Reduce pipe friction4) Pressurize supply tank5) Reduce pump speed
--ALL affect efficiency so be careful!
Pump Cavitation
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0 20000 40000 60000 80000 100000 120000 140000 160000 180000
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
0.028
0.030
0.032
0.034
CHW Secondary Pump 1\ Pump\ OB V\ Std Vel Meas\ Spec trum 12/ 13/ 2011 11:33:55 AM
12/ 13/ 2011
3/ 29/ 2012
4/ 30/ 2012
M
f [cpm]
v op [inch/ s]
RPM : 1780 (29.67Hz)M(x) : 120.00 cpm (0.07 Orders)M(y) : 0.02141 inch/ s
Pump Cavitation
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0 20000 40000 60000 80000 100000 120000 140000 160000 180000
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
0.028
0.030
0.032
0.034
0.036
0.038
0.040
Tertiary CHW Pump 1\ Pump\ IB V\ Std Vel Meas\ Spec trum 4/ 30/ 2012 9:20:54 AM
2/ 15/ 2011
3/ 29/ 2012
4/ 30/ 2012
f [cpm]
v op [inch/ s]
RPM : 1780 (29.67Hz)
Cavitation Improved
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0 20000 40000 60000 80000 100000 120000 140000 160000 180000
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
0.028
0.030
0.032
0.034
FHX CW Pump\ Pump\ IB A\ Std Vel Meas\ Spec trum 1/ 25/ 2012 8:20:03 AM
10/ 13/ 2011
12/ 13/ 2011
1/ 25/ 2012
f [cpm]
v op [inch/ s]
RPM : 1780 (29.67Hz)
Centrifugal pumps
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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
Using Vibration to Set Performance
• Cascade Plot– Monitor broadband vibration FFT spectra versus
RPM
• Waterfall Plot– Monitor broadband vibration FFT spectra versus
valve position
Single Channel Signal Processing
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Cascade Plot: Series of spectra as machine speed varies
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