Subject: Some of the basics you need to understand centrifugal pumps 13-4.

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'll allow you to troubleshoot just about any pump problem. We'll 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's 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 pumping these 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's 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 .

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'll 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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