NPSH and pump cavitation

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Most "pump problems" are really problems with the systems around the pumps. NPSH is a common cause, and cavitation is a common consequence.

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NPSH and pump cavitation

  1. 1. Understanding NPSH and Cavitation Brian Gongol DJ Gongol & Associates March 5, 2015
  2. 2. Do you have any of these symptoms?
  3. 3. Symptoms growling pumps
  4. 4. Symptoms worn-out impellers
  5. 5. Symptoms broken shafts
  6. 6. Symptoms failed seals
  7. 7. Symptoms All leading to... a disappointing change in performance?
  8. 8. You may have low NPSH. Fortunately, there's hope.
  9. 9. NPSH Net Positive Suction Head
  10. 10. A centrifugal perspective  Applies to positive-displacement pumps  Applies to axial-flow and mixed-flow pumps  Also applies to valves
  11. 11. Start with atmospheric pressure
  12. 12. Centrifugal pumps don't suck ...but this parking lot does
  13. 13. Our heavyweight atmosphere: 33.9' at sea level
  14. 14. Centrifugal pumps don't suck They create a low-pressure condition allowing atmospheric pressure to push before discharging to higher pressure
  15. 15. More atmosphere, more pressure
  16. 16. In Iowa
  17. 17. In Iowa  Lowest elevation, highest pressure:  Lee County at 480' above MSL  33.40' atmospheric pressure
  18. 18. In Iowa  Highest elevation, lowest pressure:  Osceola County at 1,670' above MSL  31.96' atmospheric pressure
  19. 19. A difference of 1190', or about 1.5' in atmospheric pressure from northwest to southeast
  20. 20. 1354' to Willis Tower Skydeck
  21. 21. Next: Take deductions Not the taxable kind
  22. 22. Deduction #1 Vapor pressure
  23. 23. Deduction #1 How hard does the atmosphere have to push back to keep the liquid from boiling?
  24. 24. Deduction #1 About 1' for water at room temperature
  25. 25. Deduction #1
  26. 26. Deduction #1 The vapor pressure of water at sea level at 212°F is about 33.9'
  27. 27. Gasoline, by comparison  Summer-blend gasoline vapor pressure  At sea level, 100°F  7.9 psi (18.2')
  28. 28. Deduction #2 Safety factor
  29. 29. Deduction #2 In the civilized world, 2' of safety factor is enough
  30. 30. Deduction #2 2' of safety factor [divided by] 31.96' atmospheric pressure (lowest in Iowa) [equals] 6.25%
  31. 31. Deduction #2 Iowa code requires 6' [Iowa Wastewater Design Standards, Chapter 13]
  32. 32. Deduction #2 Lowest pressure ever recorded at sea level: 29.11' Or...4.79' below standard pressure. In the deepest hurricane ever measured.
  33. 33. Deduction #2 Standard pressure at Denver, Colorado: 28.0' Or...5.9' below standard pressure.
  34. 34. Rocky Mountain High
  35. 35. Deduction #2 If your city's elevation rises by one mile, you have bigger problems. A 6' safety factor is patently unreasonable.
  36. 36. Deduction #2  Highest pressure recorded at Des Moines:  31.06"  Lowest pressure recorded at Des Moines:  28.70"  All-time pressure difference:  2.36", or 2.67 feet of water head
  37. 37. Deduction #3 Total Dynamic Suction Lift
  38. 38. Deduction #3 TDSL = Actual elevation difference + Friction losses in suction pipe
  39. 39. Deduction #3
  40. 40. Deduction #3 Suction friction losses also come from formed suction inlets, strainers, elbows, and stands
  41. 41. NPSH (available)  Atmospheric pressure  minus vapor pressure  minus safety factor  minus total dynamic suction lift
  42. 42. Give the system what it needs NPSH (available) must exceed NPSH (required) for the pump
  43. 43. NPSH (required) All centrifugal pumps have a value for required NPSH
  44. 44. NPSH (required)  self-priming  other suction lift (e.g., with foot valves)  flooded suction (e.g., dry-pit submersibles)  in-line  submersible
  45. 45. NPSHr - axial-flow
  46. 46. NPSHr - end-suction
  47. 47. NPSHr - horizontal split-case
  48. 48. NPSHr - self-priming
  49. 49. NPSHr - submersible
  50. 50. NPSHr - vertical-turbine
  51. 51. NPSH (required) Just because a pump doesn't operate on a suction lift doesn't mean it isn't affected by NPSH
  52. 52. NPSH (required)
  53. 53. Low NPSH Many pump system problems are caused by inadequate accounting for NPSH
  54. 54. Low NPSH Problems are often hidden by other factors Noise is a great example
  55. 55. Hungry like the pump Higher flow rates usually mean higher NPSHr (for common Iowa applications)
  56. 56. Hungry like the pump Relationship of NPSHr to flow depends upon specific speed (centrifugal, axial, propeller, radial...)
  57. 57. Balance your NPSH budget  Credits  Atmospheric pressure  minus vapor pressure  minus safety factor  minus TDSL  Debits  NPSH required by pump
  58. 58. Practical (app) makes perfect Pumps make noise
  59. 59. Practical (app) makes perfect A quiet pump is generally a happy pump
  60. 60. Hear me out Mechanical noises Hydraulic noises
  61. 61. Mechanical noises  Debris  Seals, shafts, and moving parts  Anywhere lubrication is required
  62. 62. Hydraulic noises If a sound isn't mechanical, it's likely a problem of hydraulics
  63. 63. Hydraulic noises Hydraulic problems are usually system problems, not necessarily pump problems
  64. 64. Hydraulic problems  Was the system designed right?  If not, what needs to be fixed?  If so, what changed?
  65. 65. Hydraulic noises  Suction cavitation  Discharge cavitation  Air entrainment (often vortexing)  Other (surges, water hammer, excess velocity)
  66. 66. A bubbling problem
  67. 67. Impeller diagnosis
  68. 68. Suction-side cavitation
  69. 69. Discharge-side cavitation
  70. 70. Abrasion
  71. 71. Suction-side cavitation Let's go to the video
  72. 72. Discharge-side cavitation Let's go to the video
  73. 73. Air entrainment Let's go to the video
  74. 74. Remember Every system is perfectly designed to create the results it produces
  75. 75. Thank you Brian Gongol brian@djgongol.com 515-223-4144
  76. 76. Credits  Gasoline vapor pressure:  http://www.chevron.com/documents/pdf/MotorGasTechReview.pdf  Iowa topographical map:  http://ortho.gis.iastate.edu/client.cgi? zoom=900&x0=469163&y0=4653689&layer=ned_10m&action=pan&pwidth=600&pheight=450  Satellite photo of Earth:  NASA (public domain)  http://earthobservatory.nasa.gov/IOTD/view.php?id=78617&eocn=related_to&eoci=related_image  Hurricane photo:  NOAA (public domain)  Pump performance curves are the work and property of their respective manufacturers (Gorman-Rupp, Patterson, and Wilo)  All other photos are original work by Brian Gongol

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