Cavitation In Depth
Part 3 - Types of Pump
There are 4 causes
or types of cavitation in centrifugal pumps as follows:
1 - Suction Cavitation (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 ?.
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 dealing with 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.
Follow These Internal Links To Learn More About Suction
Glossary - Net Positive Suction Head
Characteristics - NPSHR
Glossary - Cavitation
2 - Recirculation
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
of low pressure where cavitation can occur.
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
Suction Recirculation Cavitation
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 vapor pressure of the fluid, resulting in 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.
Discharge Recirculation Cavitation
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 vapor pressure of the fluid resulting in 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
The two photographs above illustrate the type
and location of damage caused by Discharge Recirculation Cavitation.
The photograph below shows
occurring inside an operating pump.
Click on the photograph to watch the video
of Video - The pump is operating at a low flow rate. When the video
begins the discharge pump discharge nozzle is not seen. As the video
progresses the camera view rotates to bring the discharge nozzle into view.
The nozzle begins to appear on the left hand side of the viewing pane.
Notice that fluid comes off the impeller, enters the discharge nozzle, and then
the fluid is turned around and enters back into the pump, this is discharge
This video is shown with permission of Steve Harrington at
Flometrics. This group of engineers are devoted to their craft.
Please visit their site:
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.
- That Fluid pressure, as measured at the pump suction nozzle, at which all cavitation
inside the pump is suppressed.
- 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
vapor 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,
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 usually
unachievable. 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 1500 F. or less.
Actually water is one of the worst actors in regards to cavitation damage in general.
Certain ranges of Pump Specific
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
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 standard Hydraulic 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 - Vane
Passing Syndrome Cavitation
Cavitation resulting when the impeller vane tip to cutwater clearance is too
small, resulting in excessive turbulence 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
center 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 behavior 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.
End of Cavitation - Part 3