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A LOOK INSIDE YOUR PUMPS:Cavitation, Air Entrainment, & Other Bummers
Brian GongolDJ Gongol & Associates, Inc.
March 2, 2017
Iowa WEA Collection Systems ConferenceMarshalltown, Iowa
What is cavitation?
What is cavitation?
The formation of vapor cavitiesdue to a hydraulic imbalance
in a kinetic fluid
Where all our trouble begins
Pumps create low pressureand discharge to high pressure
Pumps rely on atmospheric pressure
Pumps don't really suck...
...atmospheric pressure pushes
When there isn't enough "push"...
The liquid behaves like it's being pulled apart
Vapor cavities form
Then the cavities collapse
The collapse releases a blast of energy
Put more formally
When atmospheric pressure is insufficientto supply the low-pressure zone
created by a pump,suction-side cavitation results
The other half of the story
If the pump is pushing...
...but the resistance is too great...
...then internal dynamics will cause cavitation
Discharge-side cavitation, more formally
When the pump fails to produceenough discharge pressure to overcome
the fluid already in place,discharge-side cavitation results
What does cavitation look like?
Suction-side cavitation illustrated
[Let's see the video]
Discharge-side cavitation illustrated
[Let's see the video]
Those vapor cavities only look harmless
Cavities form...
...and then collapse...
...releasing significant energy...
Estimated in the tens of thousands of PSI
...which creates cavitation wear or pitting
Telltale symptoms of cavitation
A growling sound: Suction cavitation
Often like gravel or rocks banging around
A pinging sound: Discharge cavitation
Much like the sound of falling hail
Visible damage near the impeller eye: Suction
Visible damage on vane tips: Discharge
Gauges swing left: Suction
[Watch the gauges in this video]
Gauges swing right: Discharge
[Watch the gauges in this video]
Other symptoms of cavitation (possibly)
Broken shafts
Seal failure
Changes in performance
A deeper dive into the nature of cavitation
NPSH balance
PositiveAtmospheric pressure
NPSH balance
PositiveAtmospheric pressure
NegativeVapor pressure
NPSH balance
PositiveAtmospheric pressure
NegativeVapor pressureSafety factor
NPSH balance
PositiveAtmospheric pressure
NegativeVapor pressureSafety factorTotal dynamic suction head
NPSH balance
PositiveAtmospheric pressure
NegativeVapor pressureSafety factorTotal dynamic suction headNPSH required by pump
Vapor pressure: Water versus gasoline
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'
Vapor pressure: Water versus gasoline
Higher vapor pressure under the same conditionsmeans gasoline wants to evaporate before water
Vapor pressure: Liquid water vs. boiling water
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'
What's the safety factor for?
Atmospheric pressure changes with weather
How much safety factor is enough?
The law The facts
High vs. low elevations: Different calculations
Higher elevations mean less atmosphere above you
High vs. low elevations: Different calculations
Less atmosphere above means less available "push"
High vs. low elevations: Different calculations
Liquids boil at lower temps because vapor pressures at lower temperatures overcome atmospheric pressure
Consider carbonation in pop in an airplane
Consider carbonation in pop in an airplane
Lower atmospheric pressure means more bubbles
Iowa's elevations are very, very flat
Iowa's elevations are very, very flat
Low point: 480'
High point: 1,670'
Difference: 1,190'
1,190' is a lot if you're on RAGBRAI
But it's 1,353' to the Skydeck
1190' is about 1.5' of atmospheric pressure
Deduct total dynamic suction head
TDSL even counts against submersibles...
...and positive-suction applications
Also deduct the NPSHr for the pump
Cavitation issues are really system NPSH issues
Atmospheric pressure availableminus vapor pressure
minus safety factorminus total dynamic suction headminus NPSH required by the pump
The conditions causing cavitation are specific...
...so the damage is specific as well
Not corrosive damage
Not abrasion damage
But different types of damage can overlap
Some forms of damage can cause/accelerate others
Related but not identical conditions
Some overlap with symptoms of air problems
Vapor cavities are not the same as air
Vapor cavities are not the same as air
Vapor is water after a phase change
Air entrainment and its problems
How does air get inside the system?
Cascading water entrains lots of air
Also look for vortex formation
Air also comes out of solution naturally
Seasonal temperature changes can enhance the effect
Piping, valves, fittings, and gaskets can leak
How to diagnose air entrainment with gauges
[Watch the gauges in this video]
A pump is not a fan
Air entrainment is a problem becausepumps are meant to move water, not air
Entrained air isn't system hydraulic imbalance
Other similar but non-air problems
Vibration or misalignment
Chemical attack
Abrasive wear
Accelerated corrosion
Where else can you find cavitation?
Valves
Piping
Constrictions
Inlets
Ways to fix cavitation
#1: Fix the system
There is no substitute
Reduce losses in piping
Simplify piping
Leave the spaghetti bowlsat the Olive Garden
Correct valve problems
50% closure is rarely 50% flow
Adjust clearances and tolerances
Tighten up!
Replace under-performing parts
Look for seemingly-innocent changes
Who moved my VFD?
Consider what's flowing
Temperature is rarely a factor
Very small differences in vapor pressurebetween "cold" and "warm"
...but do keep it in mind if dealingwith boilers or HVAC
What's in the water gets in the pipes
Wastewater solids
Sewer gases
Entrained air
Mineral deposits
Pipe diameters matter exponentially (item 1)
Small constrictions can matter a great dealover long distances
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
#2: Adapt to unresolved problems
Metallurgy: Use hardened parts
Counter-compensate with valves
Avoidance mechanisms
Plot today's system head curveand tomorrow's system head curve
Avoidance mechanisms
Consider the slope of a changing system head curveversus the slope of your pump performance curve
Steep pump curve / Flat system curve
Flat pump curve / Flat system curve
Flat pump curve / Steep system curve
Steep pump curve / Steep system curve
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
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
Questions?
Thank you for coming!
Thank you for your attention!
Contact us anytime with questions
Brian Gongol DJ Gongol & Associates 515-223-4144 [email protected]
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