What causes pump cavitation, and how can it be diagnosed? What makes it different from air-related problems, and how can it be fixed?

1. A LOOK INSIDE YOUR PUMPS:
Cavitation, Air Entrainment, & Other Bummers
Brian Gongol
DJ Gongol & Associates, Inc.
February 1, 2019
Iowa Water Well Association Annual Convention
Altoona, Iowa

2. What is cavitation?

3. What is cavitation?
The formation of vapor cavities
due to a hydraulic imbalance
in a kinetic fluid

4. Where all our trouble begins
Pumps create low pressure
and discharge to high pressure

5. Pumps rely on atmospheric pressure

6. Pumps don't really suck...

7. ...atmospheric pressure pushes

8. When there isn't enough "push"...

9. The liquid behaves like it's being pulled apart

10. Vapor cavities form

11. Then the cavities collapse

12. The collapse releases a blast of energy

13. Put more formally
When atmospheric pressure is insufficient
to supply the low-pressure zone
created by a pump,
suction-side cavitation results

14. The other half of the story

15. If the pump is pushing...

16. ...but the resistance is too great...

17. ...then internal dynamics will cause cavitation

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. What does cavitation look like?

20. Suction-side cavitation illustrated
[Let's see the video]

21. Discharge-side cavitation illustrated
[Let's see the video]

22. Those vapor cavities only look harmless

23. Vapor cavities form...

24. ...and then collapse...

25. ...releasing significant energy...
Estimated in the tens of thousands of PSI

26. ...which creates cavitation wear or pitting

27. Telltale symptoms of cavitation

28. A growling sound: Suction cavitation
Often like gravel or rocks banging around

29. A pinging sound: Discharge cavitation
Much like the sound of falling hail

30. Visible damage near the impeller eye: Suction

31. Visible damage on vane tips: Discharge

32. Gauges swing left: Suction
[Watch the gauges in this video]

33. Gauges swing right: Discharge
[Watch the gauges in this video]

34. Other symptoms of cavitation (possibly)

35. Broken shafts

36. Seal failure

37. Changes in performance

38. A deeper dive into the nature of cavitation

39. NPSH balance
Positive
Atmospheric pressure

40. NPSH balance
Positive
Atmospheric pressure
Negative
Vapor pressure

41. NPSH balance
Positive
Atmospheric pressure
Negative
Vapor pressure
Safety factor

42. NPSH balance
Positive
Atmospheric pressure
Negative
Vapor pressure
Safety factor
Total dynamic suction head

43. NPSH balance
Positive
Atmospheric pressure
Negative
Vapor pressure
Safety factor
Total dynamic suction head
NPSH required by pump

44. Vapor pressure: Water versus gasoline

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. Vapor pressure: Water versus gasoline
Higher vapor pressure under the same conditions
means gasoline wants to evaporate before water

47. Vapor pressure: Liquid water vs. boiling water

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. What's the safety factor for?

50. Atmospheric pressure changes with weather

51. How much safety factor is enough?

The law 
The facts

52. High vs. low elevations: Different calculations
Higher elevations mean less atmosphere above you

53. High vs. low elevations: Different calculations
Less atmosphere above means less available "push"

54. High vs. low elevations: Different calculations
Liquids boil at lower temps because vapor pressures at
lower temperatures overcome atmospheric pressure

55. Consider carbonation in pop in an airplane

56. Consider carbonation in pop in an airplane
Lower atmospheric pressure means more bubbles

57. Iowa's elevations are very, very flat

58. Iowa's elevations are very, very flat
Low point: 480'
High point: 1,670'
Difference: 1,190'

59. 1,190' is a lot if you're on RAGBRAI

60. But it's 1,353' to the Skydeck

61. 1190' is about 1.5' of atmospheric pressure

62. Deduct total dynamic suction head

63. TDSL even counts against submersibles...

64. ...and positive-suction applications

65. Also deduct the NPSHr for the pump

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. The conditions causing cavitation are specific...

68. ...so the damage is specific as well

69. Not corrosive damage

70. Not abrasion damage

71. But different types of damage can overlap

72. Some forms of damage can cause/accelerate others

73. Related but not identical conditions

74. Some overlap with symptoms of air problems

75. Vapor cavities are not the same as air

76. Vapor cavities are not the same as air
Vapor is water after a phase change

77. Air entrainment and its problems

78. How does air get inside the system?

79. Cascading water entrains lots of air

80. Also look for vortex formation

81. Air also comes out of solution naturally
Seasonal temperature changes can enhance the effect

82. Piping, valves, fittings, and gaskets can leak

83. How to diagnose air entrainment with gauges
[Watch the gauges in this video]

84. A pump is not a fan
Air entrainment is a problem because
pumps are meant to move water, not air

85. Entrained air isn't system hydraulic imbalance

86. Other similar but non-air problems

87. Vibration or misalignment

88. Chemical attack

89. Abrasive wear

90. Accelerated corrosion

91. Where else can you find cavitation?

92. Valves

93. Piping

94. Constrictions

95. Inlets

96. Ways to fix cavitation

97. #1: Fix the system
There is no substitute

98. Reduce losses in piping

99. Simplify piping
Leave the spaghetti bowls
at the Olive Garden

100. Correct valve problems
50% closure is rarely 50% flow

101. Adjust clearances and tolerances
Tighten up!

102. Replace under-performing parts

103. Look for seemingly-innocent changes
Who moved my VFD?

104. Consider what's flowing

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. What's in the water gets in the pipes
Wastewater solids
Sewer gases
Entrained air
Mineral deposits

107. Pipe diameters matter exponentially (item 1)
Small constrictions can matter a great deal
over long distances

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. #2: Adapt to unresolved problems

110. Metallurgy: Use hardened parts

111. Counter-compensate with valves

112. Avoidance mechanisms
Plot today's system head curve
and tomorrow's system head curve

113. Avoidance mechanisms
Consider the slope of a changing system head curve
versus the slope of your pump performance curve

114. Steep pump curve / Flat system curve

115. Flat pump curve / Flat system curve

116. Flat pump curve / Steep system curve

117. Steep pump curve / Steep system curve

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. 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. Thank you for your attention!

Contact us anytime with
questions

Brian Gongol

DJ Gongol & Associates

515-223-4144

info@djgongol.com

121. 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