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