Paul,
Here is an article on centr pump cavitation. It says
that discharge cavitation can occur but 90% is suction
cavitation.
Bill
PS it was found at
http://www.buckeyepumps.com/Cavitation.htm
So it is "discharge cavitation" intead of stall.
We are finally getting some place here.
Note that this is an open loop system the author is talking about here.
Restricting the output of the pump so the inflow can keep up will reduce
SUCTION side cavitation.
In our case the inflow and the out flow are always the same so we are likely
getting DISCHARGE cavitation at high RPM because of the restriction on the output
side. We will always have some restriction due to the block, rad, etc., etc..
I think I will mock up my cooling system on the dyno test stand and use a
couple of pressure gages. One on either side of the pump to see what is going on.
Thanks Bill.
Paul Lamar
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 information 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 Sales
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 solution.
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 side 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 restriction 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 170°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 temperatures 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 screwdriver 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 indicating 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 bearings 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 nozzles 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 maintenance 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 conditions 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 pump 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 also 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
center 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
cavitation.
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 result 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 elastomeric 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
operated 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 discharge 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 pump'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 where 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 cavitation. 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 action 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 solve 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 responsibility 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 under
standard conditions at sea level.
2. Ambient Temperature. The normal temperature at any given
location at any given time.
3. Vapor Pressure. The pressure at which liquid will begin
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 suction
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.
The Aircraft Rotary Engine Newsletter. Powered by Linux.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org