Bulent Aliev wrote:
http://www.buckeyepumps.com/Cavitation.htm
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CAVITATION
for the Technologically Challenged
Haven't you ever wished you could pick up a technical
article that had to do with pumps that would not
concentrate on the theory of fluid in motion but rather
give you hands-on ways to determine if you have a problem
or not? Find the solution. Get on with your life. Well .
. . this is an attempt at giving our customers
nformation which would actually help them make money and
avoid excess theory.
For those of you who crave the technical side, at the end
of the article I will give a number of definitions and
references which will allow you to research NPSH (Net
Positive Suction Head) so that you can, in fact, prove
(with sufficient field testing and data) that you have
cavitation in your system. It really isn't necessary to
do this as that's exactly what buckeye pumps' 18 Field
ales Engineers and 17 Service Technicians are dedicated
to do for you.
All you have to do is identify the problem so you
understand what it is and how to deal with it on the
short term basis until we can help you with a long term
olution.
WHAT IS CAVITATION?
Cavitation is the formation of partial vacuums in a
flowing liquid as a result of the separation of its
parts. There are two types of cavitation. One is suction
ide related and this is by far the most common form
(probably 90% of all known events). The other is
discharge cavitation which occurs rarely (10% of the
time).
SUCTION SIDE CAVITATION in its simplest terms is a
estriction on the suction side of the pump which does not
allow enough fluid to enter the pump and be discharged.
The pump reacts to pressure on the discharge side and
produces a higher flow of liquid than can be on the
suction side. Restrictions either on the suction side of
the pump or atmospheric pressure decreases the flow to
the pump, particularly in suction lift applications. The
pump produces a higher flow of liquid than can be
supplied to it, due to suction side restrictions.
DISCHARGE SIDE CAVITATION is a restriction on the
discharge side of the pump system which constricts the
flow out of the pump. Since liquid can't escape, due to
discharge side restrictions, it is recirculated in the
pump casing, damaging the outer edge of the impeller and
casing or casing ring if there is one.
A common cause of suction side cavitation is related to
the vapor pressure of the liquid. Liquid vapor pressure
is a scale which represents when a liquid is at the
pressure at which it will begin to boil at a given
temperature. For instance, we know that water will boil
at 212°F at sea level. Carbon tetrachloride will boil at
70°F. Benzene boils at 176°F. Dowtherm won't boil or
vaporize until it reaches 494.3°F. Ethylbromide boils at
101°F, etc.
When liquids turn to a gas (boil) they will cause
cavitation in a pump. Solvent transfers from outside
tanks can often become a problem in the summertime when
emperatures heat the liquids to their critical vapor
pressures. By the way, the liquid doesn't have to boil to
be a problem. If it gets close to boiling and the supply
to the pump is restricted - cavitation can be, and often
is, the result.
HOW DO I RECOGNIZE CAVITATION?
Cavitation is relatively easy to recognize. In its
mildest form cavitation will be recognizable due to a
sharp pinging noise that has often been described as
pumping corn kernels or gravel through the pump. If you
suspect cavitation in your pump system but aren't sure
because you don't hear that noise, you might put a
crewdriver onto the casing of the pump and the other end
up to your ear and it will enhance your ability to hear
the noise within the casing of the pump.
Another sure sign of cavitation is that your discharge
pressure gauge on the pump system will fluctuate wildly
over a 5-10 p.s.i. range at a high rate of speed
ndicating uneven discharge flow. One must be careful to
put a new gauge on the system and check the gauge tap
opening so that you can be sure the gauge is operating
correctly. A properly operating system will give a steady
pressure gauge reading with little or no variation during
pump operation.
Cavitation causes a great many undesirable side-effects.
Because the pump is not operating in its proper balance
hydraulically, it is subjected to stresses within the
pump machine that cause shaft deflection and premature
bearing and seal wear. These are two other symptoms
indicating cavitation. If you are constantly replacing
earings and seals in a particular pumping system, chances
are you have severe misalignment or cavitation.
WHAT CAUSES CAVITATION?
The five most common reasons for cavitation are:
A) The pump was oversized by the specifying engineer or
an inexperienced pump field salesperson. Oversizing the
pump occurs because the specifying person does not do a
detailed system analysis and determine the proper head
pressure and flows required to do the job. Even when
calculations are done people have a tendency to "fudge"
the numbers to be "safe." In actuality, when the pump is
first started, the discharge pipes are new and therefore
the losses in the system are less than originally
calculated. The resultant oversizing of the pump is the
most common cause of cavitation.
B) The second most common reason for cavitation is a
change in the system demands. This can be easily
illustrated by a spray system where a given number of
ozzles are used and the back pressure against the pump to
force water through the nozzles at the desired rate of
flow would be, say, 100 lbs. At 100 lbs. our theoretical
pump may discharge 100 gallons per minute on the
performance curve. As the nozzles wear out the openings
through which the water passes are eroded and more water
is allowed to flow through the nozzles while lowering the
head pressure against the pump. The pump attempts to pump
more and more liquid, but supply can't keep up with
demand.
Now the pumping system produces only 50 lbs. of pressure
at the discharge of the pump and the flow through the
pump, depending on the characteristic shape of the
centrifugal performance curve, may be 300 to 500 gallons
per minute. The pump is no longer operating in its best
efficiency range due to a change on the system
performance requirements which may very well creep up on
the user and appear one day seemingly "out of the blue."
I wish I had a dollar for every time I've heard a
aintenance man, project engineer or production foreman
tell me that the pump was working fine yesterday, running
well for years and all of a sudden it began to cavitate .
. . Go figure?
C) The third most common cause of suction side cavitation
is on a suction lift or a pump whose suction side supply
comes from a pit below the centerline of the pump. In
this situation, debris within the sump can block the
suction to the pump and thus starve the pump from the
appropriate amount of fluid it needs to operate in its
most efficient area of performance. Also, leaks can
develop on the suction line and air is introduced into
the pump.
D) As stated earlier, temperature combined with marginal
suction supply can both act to cause cavitation. Changes
in the process or unusual swings in atmospheric
onditions are the most commonly seen reasons observed.
E) Lastly, another explanation for discharge cavitation
is similar to the "B" reason above, except in reverse. As
discharge lines in the system corrode or plug, the pump
is
restricted on its discharge output and discharge
cavitation can occur. Check valves which may not operate
properly on either the discharge or suction side of the
ump can cause cavitation, too.
HOW DO I PROVE MY PUMP IS CAVITATING?
Beyond the obvious and characteristic noise which we
described and the erratic discharge pressure gauge, an
inspection of the impeller in a centrifugal pump will
lso reveal the affects of cavitation.
We have very often talked with customers who tell us that
their impellers are "wearing out." Please understand that
under proper operating circumstances impellers simply do
not wear out. If impellers in your centrifugal pumps look
like "iron worms" have been feasting on them in the
enter of the impeller, you have suction side cavitation.
If you have damage around the outer diameter of the pump
impeller, and in the casing, you probably have discharge
avitation.
This is a good time to let you know that cavitation is
not unique to centrifugal pumps. Cavitation is the
formation of partial vacuums in a flowing liquid as a
esult of the separation of its parts. When these partial
vacuums collapse they pit or damage parts of whatever
they contact, in particular, the metal surfaces or the
lastomeric surfaces of a pump. In other words, cavitation
affects every style of pump whether they are centrifugal,
progressing cavity, gear pumps, sliding vane pumps, air
perated diaphragm pumps or any other style machine that
applies energy to fluid. The laws of physics apply to all
pumps and to all systems.
Back to our ways to prove cavitation. To prove
cavitation, install a combination gauge (reads in vacuum
and p.s.i.) on the suction side of the pump and a
ischarge gauge on the discharge side of the pump and take
the readings. The discharge pressure plus suction
pressure, or vacuum, will be the operating pressure at
which the pump is performing. To save you from doing a
lot of calculations, assume that 1" of mercury on the
vacuum gauge equals 1 foot of head and remember that 1
p.s.i. equals 2.31 feet of head. (Centrifugal pump curves
measure discharge in feet of head NOT in p.s.i. . . .
sorry, it's not my rule!)
To illustrate, if you have a suction lift and vacuum
pressure of 5" of mercury on your suction combination
gauge, convert that to 5 feet of head. If your discharge
pressure gauge reads 100 lbs., multiply that by 2.31 and
your discharge pressure in feet of head is 231'. Add the
5 feet of suction head to 231' and you will find that the
pump is operating at 236 feet of head.
If you have a positive head condition and your suction
gauge and it reads plus 10 p.s.i. then multiply 10 x 2.31
which is 23.10 feet. Deduct that 23.10 feet of head from
the discharge pressure of 231 feet of head and your net
pump operating point is 207.90 ft. of total head. Refer
to the rotating shaft speed of the pump to find the
ump's operating performance curve then determine where
the pump is operating on its performance curve according
to your readings. Make sure that the pump performance
curve is the same speed as your motor. Motors can be
switched from one RPM to another.
You may refine your measurements even further by taking
amp readings on the motor inlet leads and converting them
to brake horsepower. This will allow you to pinpoint the
horsepower performance on the pump curve which will be a
double-check to the pressure readings which you have
taken earlier. If you don't have this formula please call
us - we will get it to you.
If you don't want to conduct any of these tests call
buckeye pumps at any one of the numbers listed at the end
of this article and ask one of our applications Field
Engineers, all of which have at least five years
experience on strictly process pumping systems, to come
to your facility and do the necessary work to determine
here your pump is operating and how serious your
cavitation problem is.
HOW CAN I TEMPORARILY CORRECT MY CAVITATION PROBLEM
UNTIL THE PUMP GUY GETS HERE?
This is pretty easy for suction side cavitation. If you
have a valve on the discharge side of your pump (and you
should), close the valve slowly until the cavitation
noise which you observed to identify the problem
disappears. Conversely, if opening the valve to full open
makes the noise go away, you probably have discharge
avitation. Although other restrictions down stream may
cause your problem to continue even with the open valve.
You may think that this will restrict the flow of liquid
to your system but in reality your system is filled with
liquid separated into gas bubbles which rob you of the
full flow of liquid which you think you are getting. By
returning the pump to its correct operating condition,
you will produce a steady stream of gas free fluid which
will give you the most efficient flow of material from
your pump that can expected under its current operating
conditions.
In the case of discharge cavitation, it may be necessary
to recirculate some of the liquid from the discharge side
of the pump back to the supply of liquid. Do not
recirculate liquid back directly to the suction side of
the pump as this will not alleviate the problem. With
discharge cavitation it is necessary to bypass some of
the fluid out of the discharge line so that the pump
thinks that it is producing more flow than it really is
producing. You'll still get what you were getting but now
the pump will stop self-destructing.
While the above temporary fixes will work quite nicely
for centrifugal pumps they will not work as well for
positive displacement pumps and before you take any
ction to temporarily correct positive displacement pump
cavitation, you should consult with one of our Field
Sales Engineers for their advice.
Never restrict the flow on the discharge side
of a positive displacement pump as it can cause
damage to you and/or the machinery.
WHAT CAN BE DONE TO CORRECT CAVITATION
SO THAT IT DOES NOT REOCCUR?
The only way that cavitation can be corrected properly is
to ask a knowledgeable pump engineer to analyze your
system and determine the system head pressure and flow
requirements. This evaluation produces a system head
curve which can then be used to determine the correct
size and type of pump to do the job. Many times we can
merely trim the impeller or change the speed of the pump
to correct the problem. On other occasions, perhaps 50%
of the time, it is necessary to replace the pump with one
more properly suited for the existing system. The
remaining cavitation problems can be corrected by
altering piping, supply elevation or temperature
regulation.
Lastly, we may recommend that changes be made to the
system such as cleaning out the pipes, removing
obstructions or replacing worn components which will
olve the cavitation problem with little or no expense.
While we are in this corrective mode we should consider
the overall system performance and try to enhance the
system as we correct the cause of the cavitation.
The key here is to reduce costs of operation by reducing
maintenance cost and improving efficiency through the
proper sizing of equipment. Your buckeye Field Sales
Engineer is specifically trained and experienced to do
exactly that. We are here to help you and not to fill you
with endless formulas which seem to place the
esponsibility for solving your problem on you instead of
on us. Your job is to either maintain equipment or
produce a quality product and our job is to help you do
that.
WHAT IF I JUST WANT TO FIX THE PROBLEM?
If you just want to get the problem resolved with the
least amount of time and effort on your part call us now.
We will send an experienced pump engineer to your plant
to analyze the situation and specifically determine the
best course of action to return your system to its
original design for level of operation . . . or improve
it!
GIVE US A LITTLE HELP!
One criteria is that we know the type of pump you have
before we come to your facility so please give us the
nameplate information from the pump and, secondly, please
be able to install gauges on the suction and discharge
side. We know it's cavitating because it makes noise, but
we can't tell why without a combination gauge (registers
vacuum and positive pressure) on the suction side and
discharge gauge on the discharge side. Why do we need
this? Simple! None of us have a big red "S" on our chest
which means that we cannot see through pipes and
determine what's going on inside. We need the gauges to
help us determine what the pump is seeing, that we cannot
see.
WHAT ABOUT ALL THOSE GOOD FORMULAS AND DEFINITIONS?
I will be glad to give you some definitions at this point
because your buckeye pumps Sales Engineer may wish to
talk with you about some of these items when he visits
you.
1. Atmospheric Pressure is 14.7 PSI, or 33.9' of water
nder
standard conditions at sea level.
2. Ambient Temperature. The normal temperature at any
iven
location at any given time.
3. Vapor Pressure. The pressure at which liquid will
egin
to vaporize. This pressure is relative to the tempera-
ture of the liquid.
4. Implosion is to collapse or to burst inward.
5. The Net Positive Suction Head (NPSH) is the total
uction
head in feet of liquid (absolute at the pump centerline
or impeller eye) less the absolute vapor pressure (in
feet) of the liquid being pumped.
If you wish to know more about NPSH or the physical
reasons and conditions which create cavitation, please
write or call me in our Galion office and I will be happy
to send you additional articles which define these
conditions in specific terms.