An introduction to cavitation and its causes

1. Cavitation 101
Brian Gongol
DJ Gongol & Associates, Inc.
November 3, 2021
Nebraska Section AWWA Fall Conference
Kearney, Nebraska

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. 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
Remember: Under the right conditions,
any pump can cavitate.
It can happen on the suction side
or on the discharge side.
This includes flooded-suction and submersible pumps!

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. Deduct total dynamic suction head
Static lift plus
any friction losses
through pipes, valves, strainers, etc.

58. TDSL even counts against submersibles...

59. ...and positive-suction applications

60. Also deduct the NPSHr for the pump

61. 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

62. The conditions causing cavitation are specific...

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

64. Not corrosive damage

65. Not abrasion damage

66. But different types of damage can overlap

67. Some forms of damage can cause/accelerate others

68. Related but not identical conditions

69. Some overlap with symptoms of air problems

70. Vapor cavities are not the same as air

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

72. Air entrainment and its problems

73. How does air get inside the system?

74. Cascading water entrains lots of air

75. Also look for vortex formation

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

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

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

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

80. Entrained air isn't system hydraulic imbalance
It's still a serious problem,
but it isn't hydraulic in origin

81. Other similar but non-air problems

82. Vibration or misalignment

83. Chemical attack

84. Abrasive wear

85. Accelerated corrosion

86. Where else can you find cavitation?

87. Valves

88. Piping

89. Constrictions

90. Inlets

91. Ways to fix cavitation

92. #1: Fix the system
There is no substitute

93. Reduce losses in piping

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

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

96. Adjust clearances and tolerances
Tighten up!

97. Replace under-performing parts

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

99. Consider what's flowing

100. 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

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

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

103. 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

104. #2: Adapt to unresolved problems

105. Metallurgy: Use hardened parts

106. Counter-compensate with valves

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

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

109. Steep pump curve / Flat system curve

110. Flat pump curve / Flat system curve

111. Flat pump curve / Steep system curve

112. Steep pump curve / Steep system curve

113. 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

114. 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

115. Questions?

Thank you for coming!

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

117. 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

Photo of diver taken from the public domain:

https://www.loc.gov/resource/thc.5a48300/

All other photos are original work by and copyright reserved to
Brian Gongol