Cavitation Indepth

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Cavitation In Depth

1. INTRODUCTION

This document focuses on relatively clear water as found in aquifers, lakes, canals, chiller systems, and

potable water systems. However the principles apply to all incompressible Newtonian (free flowing) fluids.

Pump selection and implementation must account for the properties of the fluid, specifically how that fluid

reacts to the pump operations of intake and impartation of energy into that fluid. Cavitation places definite

upper and lower limits on what pumps can and cannot do with fluids.

There are three mechanisms at work in centrifugal pumps capable of lowering pressures below the vapour

pressure of the pumpage, resulting in cavitation.

LOW INTAKE PRESSURES - Every centrifugal pump can only obtain fluid by one means, by creating a low

pressure area at the eye of the impeller, allowing higher pressures at the fluid source to push fluid to the

pump. If the pump causes intake pressures to drop below the vapour pressure of the fluid, the fluid changes

phases from a liquid into a gas, initiating the cavitation process.

VELOCITY INCREASE PRESSURE DROP - Centrifugal pumps impart energy into the fluid by means of a

velocity increase, causing fluid pressure inside the impeller to first DECREASE (Velocity Increase = Pressure

Decrease - Bernoulli's Principle of Energy Conservation in Fluids). The low pressures caused by the velocity

increase can fall below the vapour pressure of the fluid, causing the fluid to undergo phase change from a

liquid into a gas. The pressure increases only when the fluid is slowed down, causing an energy

transformation from Velocity Energy into Diffusion Energy, a process called Pressure Recovery. Pressure

Recovery occurs in the following places: Impellers, Diffusers, Pump Cases, and in the discharge piping after

leaving the pump. But before pressure recovery occurs, cavitation can occur.

TURBULENCE - High velocity flows occurring in opposing fluid flow lines, and through restricted passages,

can create highly turbulent vortices with resulting localized areas of low pressure that can result in fluid

vaporization, initiating the cavitation process.

Under cavitation conditions water is one of the worst actors regarding equipment damage due to cavitation.

Water is hard on equipment in cavitation conditions for at least two reasons:

• relatively high density (small molecular size and heavy molecular weight), and

• a sharp well defined phase change behaviour.

The combination of small heavy molecules and high cavity wall implosion velocities (resulting from the sharp

and fast rate of phase change), results in the release of extreme inertial energies as the walls of the cavity

strike against each other and against objects in the fluid flow path during the cavity implosion. All pump

components exposed to the fluid can be damaged by cavitation, but the most affected components are

usually impeller vanes and shrouds, pump cases, diffusers, cutwaters, and wear rings.

Liquids with molecules larger and more complex than water, and non-homogenous liquids such as many

petrochemicals, can be much less harmful to equipment in cavitation conditions because they often have a

lower density than water, their larger more complex molecular structure and sometimes non-homogenous

nature causes a "blurred" or less well defined phase change behaviour, both of which reduce the rate of

cavity creation and collapse, and the amount of energy released in the implosion, and therefore the amount

of damage caused by cavitation is reduced.



Cavitation In Depth.doc 2 of 13 4-Jul-08

The data provided below is from research using various liquids including water. Some of the research also

involves experimental methods creating cavitation by means unfamiliar to pump users including the use of

sound waves and lasers.

For ease of writing and clarity, this document makes a sharp differentiation between "bubbles" and

"cavities". Although both terms refer to accumulations of gas phase molecules in a liquid, commonly called

bubbles, there is a huge difference between the two types of "bubbles".

Cavities - Pockets of gas in a pumpage caused by the cavitation process. These pockets of gas originate as

pumpage molecules change phases from a liquid into gas phase, followed by the near instantaneous

implosion of these same cavities as the molecules change phases from gas back to liquid, the implosion

releasing extreme energies in the form of shock wave pressures and heat. The phase change behaviour of

water is near instantaneous, so the cavities are created very fast. At first, the cavities are microscopic. If

conditions are correct, the cavities can coalesce into larger and larger bubbles, eventually becoming

macroscopic. When cavities coalesce into large cavities the term "Super Cavities" or "Super Cavitation" may

be used to describe the cavities.

Bubbles - Bubbles are defined as pockets of gas that, regardless of their source, for the most part do not

involve molecules changing phases, do not involve near instantaneous creation of or the near instantaneous

implosion of those bubbles, and therefore bubbles do not release immense pressures and heat that damage

equipment. Bubbles may cause some damage to equipment by exposing seal faces or by causing the pump

to lose prime. Bubbles can change during the pumping process by compression and by diffusion into or out

of solution in the pumpage. But Diffusion is a slow and orderly process that cannot produce the immense

energies such as occur during the molecular phase changes involved in cavitation.

2. THE CAVITA TION PROCESS

The Latin word "cavus" means hole or cavity, referring to one feature of the cavitation process, the existence

of cavities within the liquid. The interior conditions of these cavities are one of the important differences

between cavitation "bubbles" and gas bubbles.

The multi-stage process of cavitation in a pump is described as follows:

1. Local pressure in a liquid flow falls below the vapour pressure for that liquid.

2. Molecules exposed to low pressure change phases from liquid to gas.

3. When liquid phase molecules change to gas phase molecules, the volume they occupy increases by

a large amount. For water at sea level pressure and close to the freezing point, the volume occupied

by the water molecules increases by a factor of about 1,700.

4. If conditions are conducive to further or increased cavitation, gas phase molecules coalesce or

accumulate into larger and larger pockets of gas. The gas pockets eventually accumulate into large

visible structures appearing as strings, sheets, and flame like shapes. Pockets of gas may become

attached to objects in the flow path.

5. Other substances in solution within the liquid may also experience phase changes or they may

diffuse out of solution. Low pressure may cause a liquid to become supersaturated with a

substance, resulting in that substance coming out of solution, either as a solid or a gas.

6. The last stage of the cavitation process begins when fluid pressure increases as the fluid moves

through the pump or valve, or through or by a restriction. Increasing fluid pressure surrounding the

cavities forces cavity walls inwards, compressing the gas inside the cavity, until the vapour pressure

is reached, at which point the process changes dramatically from compression to phase change.

When pressure in the gas reaches the vapour pressure, near instantaneously, the gas molecules

change phases from gas phase to liquid phase. The same factor of volume expansion now




becomes the factor of contraction, i.e., the volume occupied by the molecules is reduced by a factor

of 1,700 for water at near atmospheric conditions. With gas pressure no longer supporting the

interior walls of the cavity, the walls of the cavity rapidly move inwards

7. The process of liquid re-occupying the cavity is properly called an implosion, because the walls of

the cavity race inwards at extremely high velocities, striking each other with extreme force, releasing

extreme levels of energy, on a microscopic scale, within an extremely short time frame.

8. The implosion is a chaotic process with multiple possible results depending on many variables.

9. Some of the physical phenomena that may result are: formation of a plasma which is the fourth state

of matter where electrons disassociate from atoms, light emission (sonoluminescence), extreme

pressures, extreme temperatures, and shock waves.

3. ENERGY RELEASE

Energy released by cavity collapse is intense, but on a microscopic scale. Even experienced engineers are

surprised by the intensities measured and reported in the literature.

Value ranges found in the literature for cavitation phenomena are:

• PRESSURE - Six sources show a range of 830 Bar to 10,300 Bar(12,000 psi to 150,000 psi)

• TEMPERATURE - Five sources show a range of 6,700 K to 10,000 K.

• VELOCITY - Four sources show a range of 90 m/s to 1200m/s (300 ft./s to 4,000 ft./s)

• VOLUME - 1,700 fold change in volume approximately.

• TIME - One source reports the total time of collapse to be less than 2 microseconds, including two or

more shock waves with durations of 10 nanoseconds and 40 nanoseconds each.

• SIZE - One source measured the diameter of the microjet occurring in the collapse as ranging from

10-100 µm .

The data above was given to show the extreme nature of cavitation. Even if you take only the lowest most

conservative values, the results still exhibit energy levels capable of damaging any known structural material,

and why "cavities" are vastly different than other types of bubbles.

4. TYPES OF PUMP CAVITA TION

There are 4 causes or types of cavitation in centrifugal pumps as follows:

4.1 Suct i on Cavi t a t ion (Due to Low NPSH)

Caused by Insufficient NPSHA. The general concept is that NPSHA should be at least equal to, and if

possible, greater than NPSHR, to avoid Suction Cavitation. The question then arises, How much margin of

NPSHA over NPSHR Is enough ?.

A general consensus within a specific industry, regarding specific pumps and specific fluids, is often the

answer to that question. That solution may seem unsatisfactory to many people, and it should. But the

exact nature of cavitation and pumps is very complex and knowledge is hard to come by if one is seeking

definite answers. There have been studies made that can be helpful.




In the end, this problem highlights a little known fact about pumps, most pumps have some cavitation

occurring within them at all times, incipient cavitation is almost ubiquitous. Incipient cavitation is almost

impossible to totally suppress, but that cavitation is not always very harmful, nor does that type of cavitation

always limit flow through the pump significantly.

So, the answer to the question, How much margin of NPSHA over NPSHR Is enough, is simply, the amount

of NPSHA that produces the least amount of damage to a pump. Now for the complexity, larger margins of

NPSHA over NPSHR often produce more damage in a pump than lower margins, especially when dealingwith cool water (less than about 150 degrees F.).

The best answer may be to hire the services of a true and honest expert with extensive experience in the

specific area of work.

4.2 Reci r cu l at ion Cavi t a t ion

Caused by low flow rate through the pump. There are two types which may occur together or separately:

Suction Side and Discharge Side. Both types of recirculation work by the same phenomena of reverse fluid

flows in close proximity to each other.

When two flow paths within a fluid are moving in opposing directions and in close proximity to each other,

vortices form between the two directions of flow, causing high fluid velocities and turbulence, resulting in

localized pockets of low pressure where cavitation can occur.

How recirculation cavitation occurs within a pump at low flow rates is an inherent function of pump type and

design. In general however, pumps with lower pump specific speed (Ns) and lower suction specific speed

(Nss), are more resistant to recirculation cavitation.

4.2.1 Suct i on Rec i rcu la t ion Cav i ta t ion

Fluid entering the pump suction nozzle is reversed, resulting in high velocity vortexes either in or near the

impeller eye, in the suction nozzle, or in the pipe close to the suction nozzle. High velocities result in low

localized pressures, local pressures may drop below the vapour pressure of the fluid, resulting in cavitation.

Cavitation damage observed on the pressure side of the inlet vanes, near the impeller eye, are a sign of

suction recirculation, and therefore this observation is diagnostic. When looking into the eye of the impeller,

the pressure side of the inlet vanes is on the underside of the vane, and therefore may only be observed

using a mirror.

Noise due to suction recirculation cavitation can be distinctive from other cavitation noise, and is therefore

also diagnostic. Suction recirculation cavitation noise is reported to be a loud popping, crackling,

hammering, or knocking sound, with highest intensity detected at the suction nozzle.

4.2.2 Disch arge Rec i rcu l a t ion Cav i ta t ion

Fluid leaving the impeller discharge side or the pump discharge nozzle, at low flow rates, may be reversed,resulting in high velocity vortexes between the two flow directions, causing localized low pressure areas.

Pressures may drop below the vapour pressure of the fluid resulting in cavitation. Recirculation cavitation

damage also occurs on the discharge side of the impeller periphery, at the cutwater(s), inside the discharge

nozzle, or in the pipe close to the discharge nozzle.

Noise due to discharge recirculation cavitation is generally less noisy than suction side recirculation.

Discharge recirculation cavitation noise is heard mostly at the pump discharge nozzle, and there will not be

the loud popping or crackling noise heard when suction recirculation is occurring.




The two photographs above illustrate the type and location of damage caused by Discharge Recirculation

Cavitation.

4.3 Inc ip ient Cavi ta t io n

First, to understand incipient cavitation one must know the definition of NPSHi (Net Positive Suction Head

Inception), and NPSHR .

Differential Pressure (dP or ∆P) - The pressure differential produced across a pump, as measured at the

intake and discharge nozzles.

NPSHi - That Fluid pressure, as measured at the pump suction nozzle, at which all cavitation inside thepump is suppressed.

NPSHR - The fluid pressure, as measured at the pump suction nozzle, at which a 3% drop in correct ∆P

occurs. The 3% drop in∆P is the result of flow restriction and inefficiency caused by cavitation.

Incipient Cavitation is then defined as: that cavitation occurring inside a pump, from the NPSHR 3% value,

up to the incipient point.

Incipient cavitation occurs in most pumps at all times. The cause is turbulence created by the impeller,

resulting in localized pressure below the vapour pressure of the pumpage. In the general pump market, the




ubiquitous presence of incipient cavitation appears to cause little damage and little loss of performance,

therefore the concept is not commonly discussed. Although this fact may partially be due to under-reporting,

the fact remains that incipient cavitation damage is not a common topic except in specific markets. The topic

is interesting to those markets where high energy suction pumps are used. HVAC cooling towers, chilled

water systems, and boiler feed pumps are well known to have serious incipient cavitation problems.

High margins of NPSHA over NPSHR can result in increasingly severe incipient cavitation damage, the

higher the margin, the more damage that will occur, until the NPSHi value is reached, which is usuallyunachievable. The cooler the water, the more damaging the cavitation. The Hydraulic Institute and others

have established general recommended margins of NPSHA for specific markets.

Factors indicating incipient cavitation may be a problem are:

• Heavy weight liquids such as water, and especially when these liquids are at cooler temperatures, for

water this would be 815OC (1500

OF). or less. Actually water is one of the worst actors in regards to

cavitation damage in general.

• Certain ranges of Pump Specific Speed.

• High Suction Specific Speeds (Nss > 9500), and what the HI calls "High Energy" pumps, which are

determined by a chart published by HI.

• Systems with high ∆P values.

• Systems with high margins of NPSHA over NPSHR .

Incipient cavitation is strongly linked to the Suction Specific Speed of a pump, the higher the suction specific

speed, the more likely that incipient cavitation may become a problem. High Suction Energy pumps require

larger margins of NPSHA over NPSHR, published opinions report this margin as 2 to 20 times NPSHA over

NPSHR . Confused? There are no well defined simple ways to understand and know how to apply these

pumps except by experience. You need extra margin of NPSH, and yet if you supply too much NPSH then

incipient cavitation damage becomes a problem. For some pumps, a small margin works well, for other

pumps higher margins are required.

The reason for this confusion involves the way in which NPSHR values are determined. The standardHydraulic Institute test method for NPSHR sets NPSHR at a point where a 3% drop in correct dP across the

pump occurs as pump inlet pressure is reduced. For low suction energy pumps and low suction specific

speed pumps, that 3% drop in dP represents a small but detectable amount of cavitation. But high suction

energy and high suction specific speed pumps are much more efficient at moving water through the impeller,

so that a 3% dP drop represents a large amount of cavitation that can damage the pump severely and

quickly.

In conclusion, NPSHR 3% does not mean the same thing for all pumps.

4.4 Vane Passi ng Syndro me Cavi t a t ion

Cavitation resulting when the impeller vane tip to cutwater clearance is too small, resulting in excessiveturbulence each time a vane passes the cutwater, resulting in cavitation and also pulsation.

The location of cavitation damage is diagnostic. Typical cavitation type damage may be observed on the

centre of the cutwater, impeller vane tips, discharge edge of the impeller shroud, and possibly to the pump

casing downstream and directly behind the cutwater.

Engineering specifications may attempt to preclude this problem by not allowing pump manufacturers to

supply pumps with the largest impeller diameter available for a given pump family. This is not a

recommended practice for engineers because it presumes that a pump manufacturer will provide a pump

with vane passing syndrome without the manufacturer knowing of the problem, or if they know, they are not




telling the customer. The practice of not allowing a pump manufacturer to provide the largest impeller

diameter, is understandable in view of the behaviour of some pump manufacturers. However, if a pump

manufacturer is honest and trustworthy, they should be allowed to provide the pump with the largest trim

diameter, which also may be a more efficient pump.

5. CAVITATION DAMAGE

Cavitation can destroy pumps and valves, and cavitation causes a loss of efficiency in pumps immediately,

and also a continuously increasing loss of efficiency as the equipment degrades due to erosion of the pump

components by cavitation. Therefore It is important to understand the phenomena sufficiently to predict and

therefore reduce cavitation and damage from cavitation, and also to diagnose and find practical solutions to

cavitation problems.

5.1 Cavi t a t ion Enhanced Chemical Erosi on

Pumps operating under cavitation conditions become more vulnerable to corrosion and chemical attack.

Metals commonly develop an oxide layer or passivated layer which protects the metal from further corrosion.

Cavitation can remove this oxide or passive layer on a continuous basis and expose unprotected metal to

further oxidation. The two processes (cavitation & oxidation) then work together to rapidly remove metal

from the pump casing and impeller. Stainless steels are not invulnerable to this process.

5.2 Mater ia ls Select i on

There is no metal, plastic, or any other material known to man, that can withstand the high levels of energy

released by cavitation in the forms of heat and pressure. In practice however, materials can be selected

that result in longer life and customer value in their ability to withstand cavitation energies, so that attention to

pump construction materials is valuable and productive.

Where cavitation is not a problem or not predicted to be a problem, common materials such as cast iron and

bronze are suitable for pump construction. There are millions of cast iron and bronze pumps that work fine

for 20 years or more without any problem even though many of those pumps experience some cavitation.

Factors indicating use of materials with higher resistance to cavitation are:

• Corrosive Pumpage - Water with chlorine, salt water, or other oxidizers. A metal that normally has no

problem with erosion by a specific chemical can become vulnerable to that chemical If cavitation

occurs. Cavitation can eat away the passivated layer that normally protects the metal from corrosion.

Stainless steel experiences chemical erosion if the passivated surface layer of the stainless is

continuously removed by cavitation, thus continuously exposing fresh unprotected metal to the

oxidizing agent.

• Low Flow Rate - Long term operation at low flow rates can result in both types of Recirculation

Cavitation.

• Low NPSHA - Long term operation with marginal or insufficient NPSHA.

• Heavy Weight (High Density) Liquids - Heavy liquids such as water cause more damage in cavitation

situations. Water molecules are small and dense. Since density is highest at cooler temperatures,

water and similar liquids are more of a problem at temperatures below 815OC (1500

OF).

• High Specific Speed Pumps (Ns>9000)

• High Suction Specific Speeds (Nss>9500)




• Systems with high dP values across the pump.

• Systems with high margins of NPSHA over NPSHR . In these situations reducing NPSH may reduce

or practically eliminate the cavitation damage.

5.3 Mater ia l Resistance

The Materials below are listed in the order of their ability to withstand Cavitation Erosion, Cast Iron havingthe lowest resistance and Stellite the highest resistance to cavitation damage.

• Cast Iron

• Leaded Bronze

• Cast Carbon Steel

• Manganese Bronze

• Monel

• Cast Iron - CA-15, CA6-NM, CF-8M

• Stainless Steel (Cast Precipitation, Cast Duplex)

• Cast Nickel Aluminium Bronze

• Titanium

• Cast Carburised 12% Chromium Stainless, Chrome-manganese austenitic Stainless.

• Stellite




6. DIAGNOSING CAVITA TION

6.1 Diagnose Cavi t a t ion by Sound

Low level cavitation in pumps may be inaudible, but higher levels generate distinctive sounds that we hear

and call cavitation. This sound can be a diagnostic clue to the experienced practitioner. Cavitation makesdifferent sounds depending on the equipment and conditions, and according to the type of cavitation.

Following are some of the descriptions of cavitation sounds:

a) Pumps (water or similar weight liquids)

• Crackling or sizzling

• Small steel shot rapidly striking against metal.

• Hissing, rushing, swishing, or a static like sound similar to radio or television static.

• Suction Recirculation Cavitation can produce loud knocking, hammering, or crackling sounds,

that are distinctive from other cavitation types.

b) Valves (Valve disc riding close to seat)

• High pitch squeal

• High pitch singing

c) Valves (High Flow)

The sound of high flow rate through a full open valve is not proof of cavitation. The high flow rate sound

can be similar to the sound of cavitation in a pump, but there may or may not be cavitation occurring in

the valve. This sound is described as a swishing, rushing, or hissing sound.

The key diagnostic observation to prove or disprove the presence of cavitation in a valve, is to very

slowly increase and decrease flow through the valve. If a well defined point exists, where a very small

change in flow rate causes the noise to appear or disappear, cavitation is the likely cause. If the noise

appears or disappears slowly in response to large changes in flow rate, then cavitation is not a likely

cause.

d) General Sound Levels

If there is no pattern or distinctive sound, or if the person listening cannot distinguish a specific type of

sound, the general sound level can be diagnostic as follows:

• If the sound lessens or disappears as the flow rate is reduced, suction cavitation is probably

occurring in the pump.

• If the sound lessens or disappears as the flow rate is increased, then recirculation cavitation

may be the cause.

• If the sound disappears as suction pressure is increased, suction cavitation may be occurring.

One pump with suction cavitation due to clogged intake screens, produced 86 decibels in the key of C. After

the intake screens were cleared, the sound level dropped to 66 decibels in the key of C.

NOTE: The key of C is commonly used to measure general noise.




Experienced persons may be able to diagnose cavitation by its unique sound qualities. Cavity collapse has

specific sound qualities that distinguish cavitation from sounds made by entrained gas bubbles, and also

from the sound of failed bearings and other machinery noises.

• The trained ear may be able to distinguish just by the sound if cavitation is the source or not.

• Cavitation sounds can start and stop quickly in response to changes in flow rate.

• Cavitation sounds exhibit precise repeatability, the noise is always the same under identical

conditions.

Entrained gas bubbles moving through a pump or valve, make a softer and lower frequency sound than

cavitation because of the immense difference in energy levels. Sound from entrained gasses changes

slowly to variations in flow rate, whereas, cavitation sounds appear and disappear quickly in response to

small changes in flow rate, and sounds caused by cavitation have precise repeatability.

6.2 Diagnose Cavi t a t ion by Visual Examinat io n of Damage

Visual examination of supposed cavitation damage to pump components is often the best way to determine

exact cause. The key observation is usually the location of the damage. The photographs below are a

guide to diagnosing cavitation damage by location.

Discharge Recirculation Cavitation

This close up photograph of an enclosed impeller, shows discharge recirculation cavitation damage to the

vane tips and the outer edges of the front and back vane shrouds.




Discharge Recirculation Cavitation

Explanation of the two photographs above of the same impeller.

Severe discharge recirculation cavitation has damaged the discharge side of the impeller, specifically the

vane tips and the outer edges of the front and rear vane shrouds.

Moreover, discharge recirculation creates cyclic axial thrust loads that can fatigue the shaft, and in this case,

the impeller attaching bolt, causing fatigue failure of the bolt. The impeller bolt can be the weakest axial

component in overhung end suction pumps such as the one in the photographs, so the bolt failed instead of

the shaft.




Suction Cavitation

Two photographs above of the same impeller.




Suction cavitation has damaged the leading edge and suction side of the vane, and also damage is

observed on "corner" surfaces leading into the vane. The suction side of the vane is the side facing the

viewer.

Suction cavitation on this pump was severe enough that cavities formed in the fluid before the fluid reached

the impeller. When the fluid reached the leading edge of the vanes and surrounding areas, the cavities

collapsed onto the vane and surrounding areas eroding the impeller material.

If the pressure side of the vanes were damaged, (back side of the vane that can only be seen with a mirror),

then suction recirculation cavitation would have been the cause.

Suction Cavitation

Close-up photograph of the leading edge of a vane damaged by suction cavitation.

Again, the suction cavitation was severe enough that cavities occurred in the pumpage before the pumpage

reached the pump. When pressure increased in the area just ahead of the vane leading edge the cavities

collapsed onto the vane causing the observed damage.

Cavitation Indepth

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