Subsynchronous Vibration In Vertical Pumps Pump Users ... 2010 Evaluating and Correcting...
Transcript of Subsynchronous Vibration In Vertical Pumps Pump Users ... 2010 Evaluating and Correcting...
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26th Texas A&MInternational Pump Users Symposium
March, 2010
Evaluating and CorrectingSubsynchronous Vibration
In Vertical Pumps
Malcolm E. LeaderKelly J ConnerJamie D. Lucas
Case Study #8
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Case Study Overview
• New Vertical Pumps• Liquid Sulfur Service• Operating Speed – 3,575 RPM• Primary Vibration Component at 1,750 CPM• Shaft Whirling or Whipping Suspected• Structural Resonance near ½ X• Solution
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Pump Layout
164Inches
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Pump System6 Vertical Pumps in 3 Separate Sumps
Single Stage 81 GPM 219 Ft Head 40 HP
Molten Sulfur at 300 °F
Pump and Shaft are Steam Jacketed
4 Radial Bearings/Bushings
Top Bearing is also Thrust Bearing
Line Shaft Bushings are Carbon Graphite
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History• Hard “Sulcrete” Buildup in Sumps
• New Pump Mounting System Devised
• Initial Base Impact Test Showed Structural Natural Frequency in Excess of 5,000 CPM
• Initial Vibration Readings:
First Installation had 0.2 IPS at 1,710 CPM
Second Installation had 0.65 IPS at 1,750 CPM
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Vibration Measurements• High Vibration on All Units
• Virtually no 1X (Running Speed) Vibration
• Most Vibration near ½X Running Speed
Measured Spectrum on Motor
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History• Installed Impact Test Results on Motor
Bump TestLocation
East - WestDirection
North - SouthDirection
East Pump
West Pump
1,734 CPM
1,657 CPM 1,887 CPM
1,887 CPM
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History• Clearly There was a System Natural Frequency near Half of Operating Speed
• What was the “Forcing Function”?
• Sulfur “whirl” Considered
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Initial Attempts• Reduced Weight of Coupling
No Significant Effect
• Possibly Stiffen Structure?
Probably would be Ineffective – Not Done
• Rotordynamics Analysis
Selected Approach
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Rotordynamics Analysis• Gather Data from Disassembled Unit
• Dimensions and Weights of all Rotating Parts
• Evaluate Bushing Dimensions
• Determine Properties of Liquid Sulfur
• Translate into Finite Element Model
• Match Model to Observed Vibrations
• Design New Components to Fix Problem
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Actual System Compared to Model
164Inches
FEAModel
Imbalance
Imbalance
ThrustBearing
PumpBearing
TopBushing
LowerBushing
2.5 InchDiameter
Shaft
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Center Two Rotating Sleeves
OriginalDesign
ModifiedDesign
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Properties of Molten Sulfur• Unusual Material
• Room Temperature Sulfur is a Crystalline Solid
• Fully Liquid at 235°F
• Forms Long-Chain Molecules – Gamma Sulfur
• Highly Non-Linear Viscosity
7.8 cP at 300°F 93,000 cP at 350°F
• Sump Temperature Control Very Important
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Crystalline Solid Sulfur
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Sulfur Viscosity v. Temperature
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Original Bushing Design
Diametral Clearance
16 Mils
= 0
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First Critical SpeedMode Shape
Calculated Frequency = 1,770 CPMWith Original Upper and Lower Bushings
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Original Unbalance Response
Calculated ResponseAt Lower Bushing
1,785 CPM
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Stability with Original Bushings
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Stability with Original Bushings
Non-LinearTime Transient
AnalysisCalculated Spectrum
At Lower Bushing
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Evaluation• Shaft Critical Speed at ½ Operating Speed
• Structural Resonance at ½ Operating Speed
• Inherently Unstable Plain Circular Bushings
• Need To Control The Rotor Vibration
• Center Two Bushings Logical Items to Revise
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Solution Goals• Design New Upper and Lower Bushings
Increase Direct Stiffness
Raise Critical Speed above Operating Speed
Eliminate Instability
Assure Low Operating Speed Vibration
Increase Reliability and Reduce Costs
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Optimized Bushing Design
Diametral Clearance
9 Mils
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Optimized Bushing Pressure Profile
Peak Pressure27 PSI
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Bushing Stiffness and Damping Coefficient Comparison at
3,575 RPMBushing
TypePrincipal Stiffness,
LB/INPrincipal Damping,
LB-SEC/INCross-Coupled
Stiffness, LB/INHorsepower
Loss
Plain Bushing
1,830 407 -69,400 0.65
3-Lobe Bushing
49,100 323 -20,700 0.38
(26.8 Times Stiffer)
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New Design Unbalance Response
Calculated ResponseAt Lower Bushing
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Time TransientAnalysis
Stability with New Design Bushings
Time TransientAnalysis
Calculated SpectrumAt Lower Bushing
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Final Result
Measured Spectrum on Motor
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Conclusion• Vibration Problem Eliminated
Replaced 2 Plain Bushings with Profile Design
Inexpensive
Available Locally
Eliminated Critical Speed
No Structural Modifications Necessary
Reliability Increased and Costs Reduced