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Troubleshooting cavitation

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How to examine, diagnose, and address cavitation issues inside centrifugal pumping systems. Addresses atmospheric pressure, NPSH, and symptoms of cavitation.

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Troubleshooting cavitation

  1. 1. A LOOK INSIDE YOUR PUMPS: Cavitation, Air Entrainment, & Other Bummers Brian Gongol DJ Gongol & Associates, Inc. March 2, 2017 Iowa WEA Collection Systems Conference Marshalltown, Iowa
  2. 2. What is cavitation?
  3. 3. What is cavitation? The formation of vapor cavities due to a hydraulic imbalance in a kinetic fluid
  4. 4. Where all our trouble begins Pumps create low pressure and discharge to high pressure
  5. 5. Pumps rely on atmospheric pressure
  6. 6. Pumps don't really suck...
  7. 7. ...atmospheric pressure pushes
  8. 8. When there isn't enough "push"...
  9. 9. The liquid behaves like it's being pulled apart
  10. 10. Vapor cavities form
  11. 11. Then the cavities collapse
  12. 12. The collapse releases a blast of energy
  13. 13. Put more formally When atmospheric pressure is insufficient to supply the low-pressure zone created by a pump, suction-side cavitation results
  14. 14. The other half of the story
  15. 15. If the pump is pushing...
  16. 16. ...but the resistance is too great...
  17. 17. ...then internal dynamics will cause cavitation
  18. 18. Discharge-side cavitation, more formally When the pump fails to produce enough discharge pressure to overcome the fluid already in place, discharge-side cavitation results
  19. 19. What does cavitation look like?
  20. 20. Suction-side cavitation illustrated [Let's see the video]
  21. 21. Discharge-side cavitation illustrated [Let's see the video]
  22. 22. Those vapor cavities only look harmless
  23. 23. Cavities form...
  24. 24. ...and then collapse...
  25. 25. ...releasing significant energy... Estimated in the tens of thousands of PSI
  26. 26. ...which creates cavitation wear or pitting
  27. 27. Telltale symptoms of cavitation
  28. 28. A growling sound: Suction cavitation Often like gravel or rocks banging around
  29. 29. A pinging sound: Discharge cavitation Much like the sound of falling hail
  30. 30. Visible damage near the impeller eye: Suction
  31. 31. Visible damage on vane tips: Discharge
  32. 32. Gauges swing left: Suction [Watch the gauges in this video]
  33. 33. Gauges swing right: Discharge [Watch the gauges in this video]
  34. 34. Other symptoms of cavitation (possibly)
  35. 35. Broken shafts
  36. 36. Seal failure
  37. 37. Changes in performance
  38. 38. A deeper dive into the nature of cavitation
  39. 39. NPSH balance Positive Atmospheric pressure
  40. 40. NPSH balance Positive Atmospheric pressure Negative Vapor pressure
  41. 41. NPSH balance Positive Atmospheric pressure Negative Vapor pressure Safety factor
  42. 42. NPSH balance Positive Atmospheric pressure Negative Vapor pressure Safety factor Total dynamic suction head
  43. 43. NPSH balance Positive Atmospheric pressure Negative Vapor pressure Safety factor Total dynamic suction head NPSH required by pump
  44. 44. Vapor pressure: Water versus gasoline
  45. 45. Vapor pressure: Water versus gasoline  Vapor pressure for water at sea level and 100°F:  0.95 psi, or about 2'  Vapor pressure for gasoline at sea level and 100°F:  9 psi, or about 21'
  46. 46. Vapor pressure: Water versus gasoline Higher vapor pressure under the same conditions means gasoline wants to evaporate before water
  47. 47. Vapor pressure: Liquid water vs. boiling water
  48. 48. Vapor pressure: Liquid water vs. boiling water  Vapor pressure at sea level, 70°F:  0.36 psi, or about 1'  Vapor pressure at sea level, 212°F:  14.67 psi, or about 34'
  49. 49. What's the safety factor for?
  50. 50. Atmospheric pressure changes with weather
  51. 51. How much safety factor is enough?  The law  The facts
  52. 52. High vs. low elevations: Different calculations Higher elevations mean less atmosphere above you
  53. 53. High vs. low elevations: Different calculations Less atmosphere above means less available "push"
  54. 54. High vs. low elevations: Different calculations Liquids boil at lower temps because vapor pressures at lower temperatures overcome atmospheric pressure
  55. 55. Consider carbonation in pop in an airplane
  56. 56. Consider carbonation in pop in an airplane Lower atmospheric pressure means more bubbles
  57. 57. Iowa's elevations are very, very flat
  58. 58. Iowa's elevations are very, very flat Low point: 480' High point: 1,670' Difference: 1,190'
  59. 59. 1,190' is a lot if you're on RAGBRAI
  60. 60. But it's 1,353' to the Skydeck
  61. 61. 1190' is about 1.5' of atmospheric pressure
  62. 62. Deduct total dynamic suction head
  63. 63. TDSL even counts against submersibles...
  64. 64. ...and positive-suction applications
  65. 65. Also deduct the NPSHr for the pump
  66. 66. Cavitation issues are really system NPSH issues Atmospheric pressure available minus vapor pressure minus safety factor minus total dynamic suction head minus NPSH required by the pump
  67. 67. The conditions causing cavitation are specific...
  68. 68. ...so the damage is specific as well
  69. 69. Not corrosive damage
  70. 70. Not abrasion damage
  71. 71. But different types of damage can overlap
  72. 72. Some forms of damage can cause/accelerate others
  73. 73. Related but not identical conditions
  74. 74. Some overlap with symptoms of air problems
  75. 75. Vapor cavities are not the same as air
  76. 76. Vapor cavities are not the same as air Vapor is water after a phase change
  77. 77. Air entrainment and its problems
  78. 78. How does air get inside the system?
  79. 79. Cascading water entrains lots of air
  80. 80. Also look for vortex formation
  81. 81. Air also comes out of solution naturally Seasonal temperature changes can enhance the effect
  82. 82. Piping, valves, fittings, and gaskets can leak
  83. 83. How to diagnose air entrainment with gauges [Watch the gauges in this video]
  84. 84. A pump is not a fan Air entrainment is a problem because pumps are meant to move water, not air
  85. 85. Entrained air isn't system hydraulic imbalance
  86. 86. Other similar but non-air problems
  87. 87. Vibration or misalignment
  88. 88. Chemical attack
  89. 89. Abrasive wear
  90. 90. Accelerated corrosion
  91. 91. Where else can you find cavitation?
  92. 92. Valves
  93. 93. Piping
  94. 94. Constrictions
  95. 95. Inlets
  96. 96. Ways to fix cavitation
  97. 97. #1: Fix the system There is no substitute
  98. 98. Reduce losses in piping
  99. 99. Simplify piping Leave the spaghetti bowls at the Olive Garden
  100. 100. Correct valve problems 50% closure is rarely 50% flow
  101. 101. Adjust clearances and tolerances Tighten up!
  102. 102. Replace under-performing parts
  103. 103. Look for seemingly-innocent changes Who moved my VFD?
  104. 104. Consider what's flowing
  105. 105. Temperature is rarely a factor Very small differences in vapor pressure between "cold" and "warm" ...but do keep it in mind if dealing with boilers or HVAC
  106. 106. What's in the water gets in the pipes Wastewater solids Sewer gases Entrained air Mineral deposits
  107. 107. Pipe diameters matter exponentially (item 1) Small constrictions can matter a great deal over long distances
  108. 108. Pipe diameters matter exponentially (item 2)  4" ID pipe  12.56 in2 cross-section  3" ID pipe  7.07 in2 cross-section  25% reduction in diameter  44% reduction in cross-section  Thanks a lot, πr2
  109. 109. #2: Adapt to unresolved problems
  110. 110. Metallurgy: Use hardened parts
  111. 111. Counter-compensate with valves
  112. 112. Avoidance mechanisms Plot today's system head curve and tomorrow's system head curve
  113. 113. Avoidance mechanisms Consider the slope of a changing system head curve versus the slope of your pump performance curve
  114. 114. Steep pump curve / Flat system curve
  115. 115. Flat pump curve / Flat system curve
  116. 116. Flat pump curve / Steep system curve
  117. 117. Steep pump curve / Steep system curve
  118. 118. To recap  Cavitation is a hydraulic matter  It is systemic -- you have to address the system to fix it  Other issues cause similar problems and damage  The issues may overlap  Cavitation won't go away just by hoping
  119. 119. Remember!  Hydraulic problems can happen to any centrifugal pump  Submersible, flooded-suction, and suction lift alike  Hydraulic problems are system problems  Fix the system or the problem will remain
  120. 120. Questions?  Thank you for coming!
  121. 121. Thank you for your attention!  Contact us anytime with questions  Brian Gongol  DJ Gongol & Associates  515-223-4144  info@djgongol.com
  122. 122. References:  Gasoline vapor pressure data:  http://www2.epa.gov/gasoline-standards/gasoline-reid- vapor-pressure  Willis Tower Skydeck elevation:  http://theskydeck.com/for-kids/fun-facts/  Photos of corroded impeller was submitted to our office for troubleshooting assistance; client to remain nameless out of courtesy  All other photos are original work by and copyright reserved to Brian Gongol

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