Some of the basics you need to understand centrifugal
pumps
McNally
Institute
Posted 1-10-05
Fortunately the centrifugal pump business is a logical business
so if you understand seven definitions, three formulas, and
three rules, the whole pump thing will make sense. The following
are some of the basics I teach in my pump seminars.
Let me say here at the beginning that you really do have to
understand the following. You cannot fake it. The good news
is these definitions, formulas, and rules are not complicated
and they will allow you to troubleshoot just about any pump
problem. We will begin with the seven definitions:
Head
If you point the discharge of a centrifugal pump straight
up into the air it will pump the fluid to a certain height
or head called the shut off head. This maximum head is mainly
determined by the outside diameter of the pump's impeller and
the speed of the rotating shaft. The head will change as the
capacity of the pump is altered
The head is measured in either feet or meters. It is important
for you to understand that the pump will pump all fluids to
the same height (air or sulfuric acid, it doesn't make any
difference) if the shaft is turning at the same rpm. The only
difference between the fluids is the amount of power it takes
to get the shaft to the proper rpm. The higher the specific
gravity of the fluid the more power (amps) required.
Capacity
The amount of fluid the pump will move is determined
mainly by the width of the impeller and the shaft speed. Capacity
is normally measured in gallons per minute (gpm.) or cubic
meters per hour (m3/hr). High capacity pumps need a wide impeller
and that is why most manufacturers shift to the double ended
design at high capacity. The bearings on either side of the
shaft do a better job of supporting the wider impeller.
Best efficiency point (B.E.P)
There are two definitions of
a pump's best efficiency point .
- The point where the power going into the pump is the closest
to the power coming out
- The point where the pump shaft experiences the least amount
of vibration.
Brake horsepower
The amount of actual horsepower
going into the pump, not the horsepower used by the motor or
driver. In the metric system
we use the term kilowatts
Specific gravity
A measure of the weight of a liquid compared
to 39°F (4°C)
fresh water. Fresh water is assigned a value of 1.0. If the
product floats on this water the specific gravity (sg.) is
less than one. If the fluid sinks in fresh water the specific
gravity is more than one. Density is a better term and someday
I am sure it will replace specific gravity as the common unit.
Velocity
A measure of how fast the fluid is moving. Velocity
= feet/second, or meters/second in the metric system.
Gravity
G = 32.2 ft/sec2 or 9,8 meters/ sec2 in the metric
system
Next we will learn the three formulas:
First you have to know how to convert head to pressure because
pump curves are shown in feet or meters of head. You have to
know how to make the conversion to be able to reference the
gage readings to the numbers on the pump curve.
Next you have to know how to convert pressure to head because
pressure gages are calibrated in psi or bar and you have to
make the conversion to read the pump curve.
The last formula you need to know is how velocity converts
to head. The only thing a pump can do is impart velocity to
the fluid. Since most pumps run at one speed, the pump can
be described as a constant velocity device. You have to understand
how that velocity converts to head.
Here are the three rules I mentioned at the beginning of this
paper:
Velocity + Pressure = a constant
This means that if the velocity of the fluid increases, the
pressure (90° to the flow) will decrease. If the flow decreases,
the pressure will increase. The two numbers added together
will always come out to the same number. Flow often changes
in a pump meaning that the pressure is changing also.
Velocity x Area = a constant
If the area inside of a pipe decreases, the flow through the
pipe will increase as it passes through the restriction. The
two numbers multiplied together always come out to the same
number. Inside a centrifugal pump there are passages of various
areas and hence various velocities and pressures.
Pressure x Area creates a force.
The unit we use to measure force is pounds, or in the metric
system we use Newtons (kilograms x gravity). Force can deflect
the impeller and rotating shaft so that the pump's wear rings
will come into contact, or the rotating mechanical seal will
hit something that can open the faces or damage a component.
It is important to keep the forces equal around an impeller
to prevent shaft deflection.
If you understand the above definitions, formulas and rules,
you should not have any trouble following the discussions I
have about pumps and seals in these papers
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