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Addressing Pump Reliability Problems
Matthew A. Gaydon
May 9, 2006
Mechanical Solutions, Inc.
11 Apollo Drive
Whippany NJ 07981
973-326-9920
www.mechsol.com
Summary• Pump Basics• Pump Selection• Common Pump Problems
– Imbalance– Misalignment– Suction Conditions– Nozzle Loads– Resonance
• Problem Solving Techniques
Centrifugal Pump Selection
• Rule #1: Match Pump BEP to System Head & Flow
• Rule #2: Require NPSHA greater than NPSHR plus margin
• Rule #3: Use a Long Straight Piping Run to the Suction Nozzle
• Rule #4: Thou Shalt Not Dead-Head
• Rule #5: Avoid Flat or Positive-Slope H-Q Characteristics
• Rule #6: Minimize Nozzle Loads & Use Expansion Joint Tie Bars
• Rule #7: Avoid Structural Natural Frequencies and Rotor Critical Speeds
• Rule #8: Minimize Load Cycling
• Rule #9: Select Materials Based on Corrosion, Galling, Fatigue, Erosion Resistance
• Rule #10: You Get What You Specify & Pay For
Pump Internal Hydraulic Design
Pump Design Fundamentals
• The Fan Laws• Specific Speed (Ns) –
Describes Impeller Design• Suction Specific Speed (Nss) –
Describes Suction Performance• Cavitation Potential
– NPSHA: net positive suction head available at the centerline of the impeller
– NPSHA = (Psuct – Pvap)/fluid density
– NPSHR: Suction head that causes 3% drop in TDH
43
*
HEAD
GPMRPMNs
43
*
RNPSH
GPMRPMNss
Pump Characteristics
• Pumps follow the ‘fan laws’ or ‘affinity laws’
2
1
2
1
N
N
Q
Q
2
2
1
2
1
N
N
H
H
3
2
1
2
1
N
N
HP
HP
Flow
Power
Head
Pump Specific Speed Chart
Basic Pump Components
• Rotor– Shaft
– Impeller(s)
– Coupling(s)
• Casing– Diffusers / Volutes
– Stuffing Box
– Discharge Head (VTP’s)
– Bearing Housings
• Bearings• Seals
Basic Pump Designs
Single Stage End Suction Pump with Open Impeller
Horizontal Split Case Pump
Pacific RLIJ (Barrel Pump)
Pacific BFI (Barrel Pump)
Bingham MSD Pump
Vertical Turbine Pumps
Pump Selection
• A properly selected pump will operate at or near its ‘Best Efficiency Point’ (BEP)
• Pumps operating in parallel will operate at the same ‘head’ point on their curves
• Two identical pumps operating in parallel at different speeds will not operate properly
A pump will operate where its performance characteristic matches the system resistance characteristic
Pump Performance
Typical Pump System Head Curve
Typical Pump System Head Curve
Pump Vibration vs. Flow Rate
Vibration Testing
Instrumentation Options
Data Processing:Converting from time domain to frequency domain with an FFT
Raw Time Signal
Result of FFT
Common Vibration Measurements
Typical Pump Vibration Issues
• Imbalance at 1X RPM (40%)
• Misalignment at 2X and 1X RPM (40%)
• Natural Frequency Resonance (10%)
• Everything Else (10%)– Excessive Vane Pass Forces– Hydraulic Forces, Including Rotating Stall– Motor Electrical Problems
What Does Vibration Do?
• Bearing Failures
• Seal Failures
• Internal Wear (affects performance)
• Increases Power Consumption
Vibration Decreases Pump Reliability
And Increases Cost of Operation
Common Excitation Frequencies:Identifying the Source of the Problem
Balance and Alignment
Vibration Problem #1:1X Running Speed
Vibration Caused by an Oscillating Force - Imbalance
Balance: Single vs. Two Plane
Vibration Problem #2: 1X and 2X Running Speed
Angular Misalignment
Offset Misalignment
Checking Alignment – Reverse Dial Indicator Method
Dodd Bars for Continuous Monitoring of Alignment
(Thermal Effects)
Typical Alignment Limits
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 2 4 6 8 10 12 14 16 18 20
RPM X 1000
Alig
nm
en
t A
cc
ura
cy
(m
ils/in
)
Unacceptable
Acceptable
Good
• General Guideline for Acceptable Misalignment– Offset: less than 2 mils * (3600/RPM)
– Parallel: less than ½ mil * (3600/RPM) per axial inch
• Remember: Alignment when machine is cold and non-pressurized will be different than when machine is hot and pressurized. Machines may have ‘cold offsets’ for best COS alignment, and may need compromise alignment for variable COS
• Beware of ‘soft foot’ (e.g. ‘teetering’ pump or delaminated foundation)
Pump / Driver Alignment Guidelines
Modern Laser Alignment
• Same Principle as Dial Indicator– Eliminates
• sagging indicator brackets• sticking / jumping dial indicators• low resolution / round-off error• reading errors: sign error, parallax error, etc.• looseness in mechanical linkages• offset error due to tilted dial indicator
Piping Design Issues
Suction Piping and Inlet Design
• Hydraulic Considerations– Long Straight Pipe Leading into Pump Suction
• Minimize bends or elbows close to the pump inlet• Minimize restrictions before inlet
– Ample NPSHA vs. 3% Head drop NPSHR
– Operate Near Best Efficiency Point (BEP)
• Mechanical Considerations– Do Not Use Pump Nozzle as Pipe Anchor
– No Unrestrained Expansion Joints
Flow through Elbows(Courtesy Koch Engineering)
Static Piping Load Sources
• Unrestrained Expansion Joint (Like a Rocket Nozzle, F=P*A)
• ‘Bourdon Tube’ Straightening• Thermal Growth / Mismatch
Static Piping Loads are a Common Cause of Casing Deformation and Misalignment
Piping Loads(Misalignment due to Warped Casing)
Vibration Problem #3 High Vane Pass Frequency Vibration
Vane Pass Frequency Vibration
Key Internal Gaps
Vibration Problem #4 High Harmonics of Running Speed
Vibration Problem #5 Excitation of a Natural Frequency (Rotor or Structure)
•All structures have natural frequencies
•Natural frequencies are harmful if they can become excited
•Common excitation frequencies are:
•1X rotational speed
•2X rotational speed
•NX rotational speed (where N = number of impeller blades)
Typical Rotor Vibration Response vs. Speed
Typical Rotor Mode Shape
Avoiding Resonance:Campbell Diagram
Avoiding Resonance:Critical Speed Map
Improving Reliability by Avoiding Natural Frequencies and
Resonance
Natural Frequency
Approximating Natural Frequency
How Natural Frequencies Affect Vibration
Vibration Impact Response(Bump Testing)
• Concept: Provide artificial stimulation to a machinery system to identify rotor or structural natural frequencies
• Practice: Measure vibration response for a known input excitation
Using Waterfall Plots to Identify Natural Frequencies
Natural Frequency
Approximate Identification of Natural Frequencies
Modal Test Results
Test Data
Analytical Prediction
Advanced Problem Solving
• Use test data to identify most likely source of problem
• Model pump to analytically approximate installation
• Adjust analytical model to match site measurements
• Use calibrated model to test possible fixes
• Accurate model allows us to avoid eliminating one problem, but causing a new one
Testing + Analysis = SOLUTION
Conclusions• There’s more to pump and system vibrations
than you might expect• Keys to success: knowledge, experience, and
the right tools• Good rules-of-thumb exist• Good condition-based methods and
instrumentation are getting better