7/26/2019 Cavitation Indepth http://slidepdf.com/reader/full/cavitation-indepth 1/13 Cavitation In Depth.doc 1 4-Jul-08 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.
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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.
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
• 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.
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.