NPSH and pump cavitation
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Understanding NPSHand Cavitation
Brian GongolDJ Gongol & Associates
March 5, 2015
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Do you have any of these symptoms?
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Symptoms
growling pumps
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Symptoms
worn-out impellers
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Symptoms
broken shafts
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Symptoms
failed seals
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Symptoms
All leading to...a disappointing
change in performance?
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You may have low NPSH.
Fortunately, there's hope.
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NPSH
Net Positive Suction Head
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A centrifugal perspective
Applies to positive-displacement pumps Applies to axial-flow and mixed-flow pumps Also applies to valves
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Start with atmospheric pressure
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Centrifugal pumps don't suck
...but this parking lot does
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Our heavyweight atmosphere:33.9' at sea level
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Centrifugal pumps don't suck
They create a low-pressure condition
allowing atmospheric pressure to push
before discharging to higher pressure
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More atmosphere, more pressure
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In Iowa
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In Iowa
Lowest elevation, highest pressure: Lee County at 480' above MSL 33.40' atmospheric pressure
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In Iowa
Highest elevation, lowest pressure: Osceola County at 1,670' above MSL 31.96' atmospheric pressure
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A difference of 1190', or about1.5' in atmospheric pressurefrom northwest to southeast
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1354' to Willis Tower Skydeck
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Next: Take deductions
Not the taxable kind
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Deduction #1
Vapor pressure
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Deduction #1
How hard does the atmospherehave to push backto keep the liquid
from boiling?
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Deduction #1
About 1' for waterat room temperature
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Deduction #1
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Deduction #1
The vapor pressureof water at sea level
at 212°F is about 33.9'
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Gasoline, by comparison
Summer-blend gasoline vapor pressure At sea level, 100°F 7.9 psi (18.2')
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Deduction #2
Safety factor
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Deduction #2
In the civilized world,2' of safety factor
is enough
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Deduction #2
2' of safety factor[divided by]
31.96' atmospheric pressure(lowest in Iowa)
[equals]6.25%
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Deduction #2
Iowa code requires 6'
[Iowa Wastewater Design Standards,Chapter 13]
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Deduction #2
Lowest pressure ever recorded at sea level:29.11'
Or...4.79' below standard pressure.
In the deepest hurricane ever measured.
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Deduction #2
Standard pressure at Denver, Colorado:28.0'
Or...5.9' below standard pressure.
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Rocky Mountain High
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Deduction #2
If your city's elevation rises by one mile,you have bigger problems.
A 6' safety factor is patently unreasonable.
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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
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Deduction #3
Total Dynamic Suction Lift
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Deduction #3
TDSL =
Actual elevation difference+
Friction losses in suction pipe
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Deduction #3
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Deduction #3
Suction friction losses also come fromformed suction inlets, strainers,
elbows, and stands
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NPSH (available)
Atmospheric pressure minus vapor pressure minus safety factor minus total dynamic suction lift
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Give the system what it needs
NPSH (available)must exceed
NPSH (required)for the pump
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NPSH (required)
All centrifugal pumpshave a value forrequired NPSH
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NPSH (required)
self-priming other suction lift (e.g., with foot valves) flooded suction (e.g., dry-pit submersibles) in-line submersible
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NPSHr - axial-flow
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NPSHr - end-suction
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NPSHr - horizontal split-case
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NPSHr - self-priming
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NPSHr - submersible
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NPSHr - vertical-turbine
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NPSH (required)
Just because a pumpdoesn't operate on a suction lift
doesn't meanit isn't affected by NPSH
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NPSH (required)
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Low NPSH
Many pump system problemsare caused by
inadequate accountingfor NPSH
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Low NPSH
Problems are often hidden by other factors
Noise is a great example
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Hungry like the pump
Higher flow ratesusually meanhigher NPSHr
(for common Iowa applications)
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Hungry like the pump
Relationship of NPSHr to flowdepends upon specific speed
(centrifugal, axial, propeller, radial...)
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Balance your NPSH budget
Credits Atmospheric pressure
minus vapor pressure minus safety factor minus TDSL
Debits NPSH required by
pump
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Practical (app) makes perfect
Pumps make noise
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Practical (app) makes perfect
A quiet pump is generally a happy pump
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Hear me out
Mechanical noises
Hydraulic noises
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Mechanical noises
Debris Seals, shafts, and moving parts Anywhere lubrication is required
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Hydraulic noises
If a sound isn't mechanical,it's likely a problem of hydraulics
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Hydraulic noises
Hydraulic problems are usuallysystem problems,
not necessarilypump problems
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Hydraulic problems
Was the system designed right? If not, what needs to be fixed? If so, what changed?
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Hydraulic noises
Suction cavitation Discharge cavitation Air entrainment (often vortexing) Other (surges, water hammer, excess velocity)
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A bubbling problem
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Impeller diagnosis
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Suction-side cavitation
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Discharge-side cavitation
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Abrasion
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Suction-side cavitation
Let's go to the video
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Discharge-side cavitation
Let's go to the video
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Air entrainment
Let's go to the video
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Remember
Every system isperfectly designedto create the results
it produces
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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