troubleshooting cavitation

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 info@djgongol.com

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