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Addressing Pump Reliability Problems

Addressing Pump Reliability Problems

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Addressing Pump Reliability Problems

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  1. 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

  2. Summary • Pump Basics • Pump Selection • Common Pump Problems • Imbalance • Misalignment • Suction Conditions • Nozzle Loads • Resonance • Problem Solving Techniques

  3. 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

  4. Pump Internal Hydraulic Design

  5. 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

  6. Pumps follow the ‘fan laws’ or ‘affinity laws’ Pump Characteristics Flow Head Power

  7. Pump Specific Speed Chart

  8. Basic Pump Components • Rotor • Shaft • Impeller(s) • Coupling(s) • Casing • Diffusers / Volutes • Stuffing Box • Discharge Head (VTP’s) • Bearing Housings • Bearings • Seals

  9. Basic Pump Designs

  10. Single Stage End Suction Pump with Open Impeller

  11. Horizontal Split Case Pump

  12. Pacific RLIJ (Barrel Pump)

  13. Pacific BFI (Barrel Pump)

  14. Bingham MSD Pump

  15. Vertical Turbine Pumps

  16. 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

  17. Pump Performance

  18. Typical Pump System Head Curve

  19. Typical Pump System Head Curve

  20. Pump Vibration vs. Flow Rate

  21. Vibration Testing

  22. Instrumentation Options

  23. Data Processing:Converting from time domain to frequency domain with an FFT Raw Time Signal Result of FFT

  24. Common Vibration Measurements

  25. 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

  26. What Does Vibration Do? • Bearing Failures • Seal Failures • Internal Wear (affects performance) • Increases Power Consumption Vibration Decreases Pump Reliability And Increases Cost of Operation

  27. Common Excitation Frequencies:Identifying the Source of the Problem

  28. Balance and Alignment

  29. Vibration Problem #1:1X Running Speed

  30. Vibration Caused by an Oscillating Force - Imbalance

  31. Balance: Single vs. Two Plane

  32. Vibration Problem #2: 1X and 2X Running Speed

  33. Angular Misalignment

  34. Offset Misalignment

  35. Checking Alignment – Reverse Dial Indicator Method

  36. Dodd Bars for Continuous Monitoring of Alignment(Thermal Effects)

  37. Typical Alignment Limits Unacceptable Acceptable Good

  38. Pump / Driver Alignment Guidelines • 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)

  39. 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

  40. Piping Design Issues

  41. 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

  42. Flow through Elbows(Courtesy Koch Engineering)

  43. 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

  44. Piping Loads(Misalignment due to Warped Casing)

  45. Vibration Problem #3 High Vane Pass Frequency Vibration

  46. Vane Pass Frequency Vibration

  47. Key Internal Gaps

  48. Vibration Problem #4 High Harmonics of Running Speed

  49. 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)

  50. Typical Rotor Vibration Response vs. Speed