THE CONCEPTS YOU NEED TO UNDERSTAND CENTRIFUGAL

PUMPS

The three magic formulas you need to know if you want to understand how centrifugal

pumps function.

Here they are:

1. As the velocity of a liquid increases, the pressure, ninety degrees to the flow,

will decrease, and as the velocity of a liquid decreases the pressure, ninety

degrees to the flow will increase.

2. Pressure acting on an area creates a force.

3. Velocity times area must remain a constant if liquid is to flow.

Let's will look at each of these formulas in detail:

Formula number one explains how airplanes fly. It all started when the Wright brothers

discovered the correct wing shape for an aircraft.

Look at the following diagram. You will note that the air is flowing under the wing at

some velocity. The air going over the top of the wing has a longer path to travel, so its

velocity must increase if it is to join with the air coming underneath the wing.

The air underneath the wing is at atmospheric pressure, but since the velocity is greater

on the top of the wing the pressure falls to some value below atmospheric pressure.

This causes the atmospheric pressure to push on the bottom of the wing lifting it, the

airplane, and all the people inside up into the air. It will continue to do so as long as the

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wing is moving forward and the configuration of the wing does not change. Gravity

offsets this lifting force and the aircraft flies between these two forces.

This same principle explains how an automobile carburetor works, why the shower

curtain comes into the bath tub when you take a shower, and how a sailboat can sail

faster than the wind.

Formula number two explains why the wing lifted into the air:

Pressure x Area = Force

Pressure is measure in pounds per square inch (kilograms per square centimeter)

Area is measured in square inches (square centimeters)

The units for force then become pounds (kilograms) because the square inches (square

centimeters) cancel out.

lbs / in2 x in2 = pounds

It is important for us to know the forces being generated because force over distance, in

a given time period, is a measure of work, energy expended, or heat, depending upon

which units we use.

Formula number three explains the action of a venturi. As the area inside a venturi

decreases the velocity of the fluid increases. This causes the pressure to decrease

(formula #1) allowing atmospheric pressure to push a fluid into the venturi. Look at the

following diagram

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We use the venturi principle to add chemicals to a lawn, remove air from a condenser,

add chemical to a boiler etc. It is the same principle we use to get fuel to the carburetor

of your automobile.

Now we will look at the cross section of a centrifugal pump and these three formulas will

explain why mechanical seals have so much trouble with shaft deflection.

This picture describes a volute pump because the impeller is not in the center of the

casing. You will note that there is less clearance between the impeller and the cut water

than there is between the impeller and the rest of the casing. You will also note that this

area is increasing as you move from the cutwater, around the casing, to the discharge

nozzle. Circular pumps have an equal area around the impeller. They are used to pump

larger quantities of liquid, without having to create a head. The volute design is the most

popular design because it will produce a head.

When we removed the packing from a centrifugal pump we lost a big part of the shaft

support system. It therefore becomes very important that we keep the forces equal

around the impeller to prevent shaft displacement. If the force increases on one side of

the impeller it will deflect the attached shaft and interfere with the performance of the

mechanical seal and pump bearings .

Since the impeller is symmetrical in shape (the area is the same all around the impeller)

It is important that we do not let the pressure vary around the impeller or the resultant

forces will not be equal. (Formula #2).

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To keep the pressure equal around the impeller, you have to keep the velocity of the

liquid constant around the impeller. (Formula #1).

Take another look at the cross section of the volute pump and you will note that the

area (volume) surrounding the impeller is increasing as you move, in the direction of

shaft rotation, from the cut water to the discharge nozzle. Formula #3 states that the

velocity of the liquid times the area must remain a constant, so that means that the

velocity of the liquid is decreasing as the area is increasing.

If the velocity of the liquid decreases, the pressure increases (Formula #1.) Pressure

times area creates a force (Formula #3.)

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.

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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.

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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.

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