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Piedmont Chapter Pump Vibration Troubleshooting
by William D. Marscher, P.E.President & Technical Director
Mechanical Solutions, Inc. www.mechsol.com
Whippany NJ
May 13, 2011
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Piedmont Chapter
Open Impeller
Shrouded Impeller
The Centrifugal Pump (End Suction)
Suction
Discharge
Impeller Shaft
Casing
Seal
Flow
VaneShroud
Hub
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Piedmont Chapter
Impeller rotation versus the volute tongue
Volute Tongue
Discharge Nozzle
Impeller
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Potential for Cavitation from Pressure Depression at Impeller Eye
No Cavitation
Cavitation
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Piedmont Chapter
Time (msec) =>
Microphone SPL 85dB
Inboard Pressure +/-150 psi pk
Outboard Pressure +/-150 psi pk
Inboard Accel on Suction Wall +/- 300 G pk
Cavitation Eventv
Scope Traces of Cavitation Event
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Piedmont Chapter
Axially Split Case Dbl. Suction Pump
• 180 degree “drip pocket” allows bearing hsg and stuffing box to flex downward due to pressure pulsations, e.g. at 1x or vane pass
• Double suction impeller typically equalizes thrust side-to-side, but at off-design is prone to axial shuttling
• Suction recirc & cav have been issues
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Boiler Feedwater Ring-Section Pumps
“Donut” Pump with Balance Disk
• 180 Less robust than barrel pump, but fewer Fnproblems.
• Designs where thrust disk totally replaces oil lubricated thrust bearing have encountered rub and binding problems during process upsets
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Hero or Enemy? The Thrust Balancing “Disk”
Can have rubs at start-up or during severe process transients, may permit “axial shutting”.
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Piedmont Chapter
Pacific BFI Barrel Pump
Barrel Pump with Balance Drum and
Stiff Shaft
• 180 degree “drip pocket” allows bearing hsg Fn near vane pass
• Pedestals weaken with age, structural modes drift into 1xRPM resonance
• Diffuser “strong-back” plates can resonate at vane pass
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Piedmont Chapter
Pacific RLIJ
Barrel Pump with Balance Drum, and
Thin Shaft
• Rotor critical speeds sensitive to LomakinEffect and therefore wear ring & drum clearances
• Pedestals weaken with age, structural modes drift into 1xRPM resonance
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Piedmont Chapter
Ingersoll-Rand C
Barrel Pump with Balance Disk
• Balance disk is theoretically prone to “control system” phasing instabilities, causing axial shuttling
• Pedestals weaken with age, structural modes drift into 1xRPM resonance
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Piedmont Chapter
Worthington WNC
Barrel Pump with Composite Balance
Drum/ Disk
• Balance disk portion may have Ax shuttling
• 180 degree “drip pocket” bearing hsgs have Fn’snear vane pass
• Pedestals weaken with age, structural modes drift into 1xRPM Fn
• Develops rotating stall easily at lower flows
• Design is particularly prone to thermal differential growth top v. bottom causing rotor binding/ rubs
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Piedmont Chapter
Axial Split Case Pumps: e.g. Bingham MSD
Axially Split Case Issues:
• Acoustics in x-over
• Top casing half “humps, allowing interstage jet leakage
• Axial shutting at off-design conditions
• High sensitivity of 1st
bend mode to center bushing clearance
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Piedmont ChapterPumps for Wastewater Applications
• Excitation of structural natural frequencies– 1X, 2X, Vane Pass
• Packing vs. mechanical seals• Material selection
– ‘Wear’ parts – rings, etc.– ‘Non-Wear’ parts – casings and impellers
• Vertical with intermediate shafting• Submersible
– Wet pit– Dry pit
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Piedmont Chapter
Chemical & Pharmaceutical Process Pumps
• Nozzle Loads– Distortion– Stress in non-metallic pumps
• Bearings – low precision– Affect on rotor behavior
• Baseplates– Flexibility
• Seals– Nozzle loads effect on mechanical seals– Packing – over-tightening
• Product lubricated bearings in mag drive and canned motor pumps
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Piedmont Chapter
Slurry Pumps
• Similar issues to process pumps, distortion due to nozzle loads is less
• Material selection and specification– Abrasive pumpage– Special coatings and liners
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Vertical Turbine Pump (VTP) Issues:
• Above ground “Reed Freq”
• VFD as a “shaker”
• Lineshaft is “violin string”
• Shaft enclosure tube Fn’s
• Disch nozzle loose joints
• Sump vortices or odd flows
• Column piping acoustics
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Piedmont Chapter
Vertical Pump Shaft Thrust Effects
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The “Concept” of Imbalance
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Vibration Problem No. 1:
1x Running Speed
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What Is Resonance?
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Natural FrequencyVibration & Phase vs. Frequency
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Shaft Response to Imbalance
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Shaft Response to Imbalance
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Balance: Single vs. Two Plane
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Piedmont Chapter Angular Misalignment
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Piedmont Chapter Offset Misalignment
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Vibration Problem No. 2:
1x & 2x .Running Speed
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Observing Alignment Continuously with “Dodd Bars”
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High Energy Density Equipment Alignment Limits
Speed (rpm)
0 1800 3600 10000 20000
Offset, hub-to-hub, mils
1.5
1.0
0.5
0
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Piedmont Chapter
Flow Through Elbows(Courtesy Cheng Fluid Systems, Inc.)
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Piedmont Chapter Stationary Piping Load Sources
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Piping Acoustic Natural Frequencies
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“Lomakin Effect” in Centrifugal Pumps
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Concept of the “Wet Critical Speed
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Lomakin Effect
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Lomakin Result:Critical Speeds Shift Up
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Approximate Calculation of Lomakin Stiffness
R= Radius
L= Length
C= Radial . clearance
P=Pressure . drop
= fric factor
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Some Key Issues in Lomakin Effect Strength:
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Wild Cards: Swirl & Grooving
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Impeller Radial Support Forces:
Sulzer/ EPRI Tests
/x /x
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Some Typical Exciting Frequencies
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Rolling Element Bearing Parameters
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Rolling Element Bearing Frequencies
-
+
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Lateral Excitation Forces (Sulzer)
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Vibration Problem No. 3:High Vane Pass
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Vibration Problem No. 4:High Harmonics of Run Speed
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Marked-up Copy of Pump Curves
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Flow Rate and the “Angle-of-Attack”
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Vane Stalling at Low Flows
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Example of Stalled Blade
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Onset of Internal RecirculationDischarge Recirculation
Suction
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Piedmont Chapter
Vibration & Pulsation vs. Flowrate
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An Unexpected Hydraulic Problem:
Rotating Stall
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Four Stage High Speed Boiler Feed Pump
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Rotating Stall Pulsation Spectrum
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Stall Vibration vs. Speed
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Vibration Problem No. 5:Subsynchronous (below 1x)
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Rotor Dynamics
Critical Speeds & Mode Shapes
Forced Response
Stability
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Rotordynamics Is Best Evaluated by Computer
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Typical Rotor Vibr. Response vs. Speed Exhibits Several Natural Frequencies
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Avoiding Resonance w/ Campbell Diagram
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Rotordynamic Critical Speed Map
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Forward vs. Backward Precession
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Typical Torsional Critical Speeds and Worst Case Excitations
x2/POLES
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Typical Vibration Problems
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Instrumentation Options
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Converting Vibration vs. Time to Vibr vs. f with “FFT”: Fast Fourier Transform
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Meaning of RMS vs. Peak Vibration
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Vibration Measure Numbers
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Vibration Measurement Format
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Housing vs. Shaft Vibration:Which Should Be Measured?
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Piedmont ChapterNatural Frequency
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Approximating Natural Frequency
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Vibration “Mode Shapes”
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Finding Nat. Freq. Resonances with a “Waterfall” Plot of Stacked FFT’s from Test
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Approximate Identification of Natural Frequencies
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A Modal Field Test In-Progress on an Operating Pump
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Short Force Application Excites Wide Frequency Band
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Principle of Time Averaging
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Rotor Impact Testing Case History
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Piedmont Chapter 14 Stage Axially Split Case Pump
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Time Averaging Field Example: Watch the Harmonics Disappear
In the End, Natural Frequencies Are Clear
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Impact Test
Rotordynamic Result
Critical Speeds of 14 Stage Pump
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First Critical Speed, Dbl. Suction SS Pump
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Sources of Damaging Forces in Centrifugal Pumps
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Typical Failures in Centrifugal Pumps
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Risk of Accepting Bad vs. Risk of Rejecting Good
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Stuffing Box & Bearing Skew Case Histories
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Cartridge Pump Seal Rub Example
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Vibration Problem #6: Repeating
Impacts
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Double Suction Pump “Breathing” Tilts Stuffing Boxes
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FEA Model of Twisting Pump Bearing Housing
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Vertical Pump Failure Case Histories
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Typical Structural Vibrations of VTP’s
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Six Vertical Pumps on Platform, with Column Exposed to Air
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Piedmont Chapter______ ___________
|||||
Vertical Pump Column Vibration with & without Water in Column
1xN 1xN
|
With Water Without Water
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Multistage Pump Inboard Bearing Chronic Failure
Case History
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Pump with Inboard Bearing Failures
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Shaft Orbits of Problem Pump
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Vibration Spectra for Problem Pump
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Impact Test Results Showing Shaft Critical Speed at 5280 rpm
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“What If” Analysis for Problem Pump
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Bearing Groove Change from 0.040 in. Deep to 0.010 in. Deep
Vibration decreased a factor of ten
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Vibration Spectrum After Bearing Fix
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Use of Dynamic Absorbersin Constant Speed Equipment to Decrease Vibration Motion
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Dynamic Absorber Design- Vertical Pump/Motor
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Effect of Dynamic Absorber on Vibration
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Gear Box Noise & Wear Problem
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Conclusions
1.There’s More to Pump and System Vibrations than You Might Expect
2.Keys to Success: Knowledge, Experience, the Right Tools
3.Good Condition-Based Methods & Instrumentation Are Getting Better