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

1. Understanding NPSH
and Cavitation
Brian Gongol
DJ Gongol & Associates
March 5, 2015

2. Do you have any of these symptoms?

3. Symptoms
growling pumps

4. Symptoms
worn-out impellers

5. Symptoms
broken shafts

6. Symptoms
failed seals

7. Symptoms
All leading to...
a disappointing
change in performance?

8. You may have low NPSH.
Fortunately, there's hope.

9. NPSH
Net Positive Suction Head

10. A centrifugal perspective

Applies to positive-displacement pumps

Applies to axial-flow and mixed-flow pumps

Also applies to valves

11. Start with atmospheric pressure

12. Centrifugal pumps don't suck
...but this parking lot does

13. Our heavyweight atmosphere:
33.9' at sea level

14. Centrifugal pumps don't suck
They create a low-pressure condition
allowing atmospheric pressure to push
before discharging to higher pressure

15. More atmosphere, more pressure

16. In Iowa

17. In Iowa

Lowest elevation, highest pressure:

Lee County at 480' above MSL

33.40' atmospheric pressure

18. In Iowa

Highest elevation, lowest pressure:

Osceola County at 1,670' above MSL

31.96' atmospheric pressure

19. A difference of 1190', or about
1.5' in atmospheric pressure
from northwest to southeast

20. 1354' to Willis Tower Skydeck

21. Next: Take deductions
Not the taxable kind

22. Deduction #1
Vapor pressure

23. Deduction #1
How hard does the atmosphere
have to push back
to keep the liquid
from boiling?

24. Deduction #1
About 1' for water
at room temperature

25. Deduction #1

26. Deduction #1
The vapor pressure
of water at sea level
at 212°F is about 33.9'

27. Gasoline, by comparison

Summer-blend gasoline vapor pressure

At sea level, 100°F

7.9 psi (18.2')

28. Deduction #2
Safety factor

29. Deduction #2
In the civilized world,
2' of safety factor
is enough

30. Deduction #2
2' of safety factor
[divided by]
31.96' atmospheric pressure
(lowest in Iowa)
[equals]
6.25%

31. Deduction #2
Iowa code requires 6'
[Iowa Wastewater Design Standards,
Chapter 13]

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. Deduction #2
Standard pressure at Denver, Colorado:
28.0'
Or...5.9' below standard pressure.

34. Rocky Mountain High

35. Deduction #2
If your city's elevation rises by one mile,
you have bigger problems.
A 6' safety factor is patently unreasonable.

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. Deduction #3
Total Dynamic Suction Lift

38. Deduction #3
TDSL =
Actual elevation difference
+
Friction losses in suction pipe

39. Deduction #3

40. Deduction #3
Suction friction losses also come from
formed suction inlets, strainers,
elbows, and stands

41. NPSH (available)

Atmospheric pressure

minus vapor pressure

minus safety factor

minus total dynamic suction lift

42. Give the system what it needs
NPSH (available)
must exceed
NPSH (required)
for the pump

43. NPSH (required)
All centrifugal pumps
have a value for
required NPSH

44. NPSH (required)

self-priming

other suction lift (e.g., with foot valves)

flooded suction (e.g., dry-pit submersibles)

in-line

submersible

45. NPSHr - axial-flow

46. NPSHr - end-suction

47. NPSHr - horizontal split-case

48. NPSHr - self-priming

49. NPSHr - submersible

50. NPSHr - vertical-turbine

51. NPSH (required)
Just because a pump
doesn't operate on a suction lift
doesn't mean
it isn't affected by NPSH

52. NPSH (required)

53. Low NPSH
Many pump system problems
are caused by
inadequate accounting
for NPSH

54. Low NPSH
Problems are often hidden by other factors
Noise is a great example

55. Hungry like the pump
Higher flow rates
usually mean
higher NPSHr
(for common Iowa applications)

56. Hungry like the pump
Relationship of NPSHr to flow
depends upon specific speed
(centrifugal, axial, propeller, radial...)

57. Balance your NPSH budget

Credits

Atmospheric pressure

minus vapor pressure

minus safety factor

minus TDSL

Debits

NPSH required by
pump

58. Practical (app) makes perfect
Pumps make noise

59. Practical (app) makes perfect
A quiet pump is generally a happy pump

60. Hear me out
Mechanical noises
Hydraulic noises

61. Mechanical noises

Debris

Seals, shafts, and moving parts

Anywhere lubrication is required

62. Hydraulic noises
If a sound isn't mechanical,
it's likely a problem of hydraulics

63. Hydraulic noises
Hydraulic problems are usually
system problems,
not necessarily
pump problems

64. Hydraulic problems

Was the system designed right?

If not, what needs to be fixed?

If so, what changed?

65. Hydraulic noises

Suction cavitation

Discharge cavitation

Air entrainment (often vortexing)

Other (surges, water hammer, excess velocity)

66. A bubbling problem

67. Impeller diagnosis

68. Suction-side cavitation

69. Discharge-side cavitation

70. Abrasion

71. Suction-side cavitation
Let's go to the video

72. Discharge-side cavitation
Let's go to the video

73. Air entrainment
Let's go to the video

74. Remember
Every system is
perfectly designed
to create the results
it produces

75. Thank you
Brian Gongol
brian@djgongol.com
515-223-4144

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