Cavitation occurs when the static pressure of a moving liquid falls below its vapor pressure, causing vapor bubbles to form and then implode. It can damage pumps by creating pitting on metal surfaces from the shockwaves of collapsing bubbles. To prevent cavitation, the net positive suction head available must exceed the net positive suction head required. Cavitation occurs most commonly at impeller inlets and tips and can reduce pump efficiency and cause component failure over time.

1. Cavitation
Process
Presented by–
Devesh muley

2. Cavitation
A phenomenon which occurs within a moving liquid when
the static pressure at some location falls to or below the vapor
pressure of the liquid.
To prevent cavitation:
NPSHA > NPSHR

3. Cavitation

4. Cavitation – Impeller
Cavitation can occur at the inlet of the impeller if the suction pressure
falls below the liquid vapor pressure.

5. Cavitation – Impeller
Large vapor volumes can
cause reduction in head
generation, or loss of prime.
Large vapor volumes can cause
surging and unstable flows.
• Bubbles collapsing create
intense pressures and
(10,000 psi) shock waves.
• Intense pressures near metal
surfaces exceed material
strength and fatigue metals.
• Creates pitted surfaces.

6. Cavitation
Cavitation is caused by liquid
changing into a
vapor and then back into a liquid.
The
Formation of partial vacuums in a
flowing liquid
as a result of the separation its parts.
When
These collapse, pitting or other
damage is
Caused on metal surface in contact.
There are two forms of cavitation:
• Suction Cavitation, which occurs at the eye of the impeller,
(also referred to as
“true cavitation”.
• Discharge Cavitation, which occurs at the tip of the
impeller, (also referred to as
recirculation cavitation).
Cavitation is NOT AIR in the PUMP!

7. What Damage Does Discharge Cavitation Cause?
• Break Shafts
• Shortens Bearing Life
• Destroys Impellers and Volutes
• Ruins Mechanical Seals
Total Dynamic Head Too “HIGH”

8. What Damage Does Suction Cavitation Cause?
• Shortens Bearing Life
(Increased radial & thrust loads)
• Destroys Impellers
• Destroys Wear Plates
• Ruins Mechanical Seals
(Vibration & thrust loads)
Total Dynamic Suction Lift Too “HIGH”
Discharge Head Too “LOW”

9. Chemical Attack Abrasive Wear
Discharge Cavitation Suction Cavitation

10. Shaft Breakage Problems
Radial Fatigue Break
• Total Dynamic Head Too “High”
• Air (Hydraulic Imbalance)
• Mechanical Imbalance
Torsional Break (Twisting Break)
• Pump Lock-UP
• Reverse Rotation (Open Check
Valve)
• Short Cycling

11. Phase Diagram of Water
STP (Standard
temperature and
pressure)

12. Cavitation – A Simple
Process
 Cavitation is the formation and subsequent
implosion of vapor bubbles in a fluid
 Caused by significant pressure gradients within
process equipment
 Pressure gradients typically generated by high
velocity flow around
 Valves – throttling (partially closed valves)
 Vanes – moving portions of pump impellers
 Protrusions – pitot tubes, vortex shedders, etc.

13. Pressure Gradient Formation
 Arrows indicate flow
direction
 Fluid is compressed above
vane creating a high
pressure region
 Fluid is rarified below vane
creating a low pressure
region
 Behind this low pressure
region is another high
pressure region

14. Impeller Cavitation Regions

15. Typical Impeller Cavitation
Pattern

16. Cavitation Demonstration

17. Microjetting
 Vapor bubbles do not implode symmetrically
 Because of pressure gradients, they tend to
implode towards hard objects

18. Microjetting – High Speed
Photograph
 Microjet has formed
 Vapor bubble is in the
process of imploding
towards the nearby hard
surface
 Shockwaves generated by microjetting can be up to
145,000,000 psi
 This pressure vastly exceeds the compressive
strength of any metal alloy

19. The Alternative to Rigid -
Elastic
 Cavitation energy can be dissipated two ways
 Into a rigid metallic substrate
 Plastic deformation
 Ultimate compressive failure
 Into an elastic substrate
 Hook’s Law
 We modified our Durable & Abrasion
Resistant Elastomer to better withstand
cavitation

20. Belzona 2141 (ACR-Fluid)
 Project sprung from a US Coast Guard
request
 Products had to survive 16+ hours in a high speed
test
 At the time, the best product lasted 12 hours
 Second formulation Belzona submitted lasted 20
hours
 Coast Guard currently in the process of coating all
rudders with Belzona 2141 (ACR-Fluid) to resist
the severe cavitation of their service

21. Equipment
Problem
Solution
Hydro Runner
at a small
Canadian Power
Plant
Mild cavitation
was decreasing
generating
efficiency
Runner was coated with Belzona 2141 (ACR-Fluid Elastomer) to
restore and maintain efficiency. After 2 years it is still in perfect
condition.

22. Equipment
Problem
Solution
ITT 3480
Impeller
Severe cavitation
was necessitating
a full rebuild
every 3 months
Impeller received two coats of Belzona 1321 (Ceramic S-Metal) and
a final coat of Belzona 2141 (ACR-Fluid Elastomer). Impeller now
lasts 28 months between rebuilds!

23. Questions?
Thank You Very Much