SUBJECT: What do we mean by pump efficiency? 6-1

When we talk about automobiles and discuss efficiency, we mean how many miles per gallon, or liters per 100 kilometers. When we discuss centrifugal pumps we are comparing the amount of work or power we get out of the pump to the amount of power we are putting into the pump. As an example:

How do we measure the horsepower or kilowatts coming out of the pump? All we have to do is multiply the pump head by the pump's capacity, and then use a simple conversion number. Let's take an example:

Flow = 300 gallons per minute of fresh water as measured coming from the pump discharge.

Head = 160 feet. We measured it at the discharge side of the pump and corrected it for the fact that the gage was two feet above the pump center line. Look at the following diagram where we have calculated the discharge head from the formula shown on the right hand side of the illustration. If there were any positive head on the suction side of the pump that head would have to be subtracted. A negative suction head would be added to the discharge head.

The centrifugal pump pumps the difference between the suction and the discharge heads. There are three kinds of discharge head:

Suction head is measured the same way.

Here is the formula for measuring the horsepower out of the pump:

Remember that we are using the actual horsepower or kilowatts going into the pump and not the horsepower or kilowatts required by the electric motor. Most motors run some where near 85% efficient.

An 85% efficient motor turning a 76% efficient pump, gives you a real efficiency of 0 .85 x 0.76 = 0.65 or 65% efficient.

A survey of popular pump brands demonstrates that pump efficiencies range from 15% to over 90%. The question then arises, "Is this very wide range due to poor selection, poor design, or some other variable which would interfere with good performance?" The best available evidence suggests that pump efficiency is directly related to " the specific speed number " with efficiencies dropping dramatically below a number of 1000 . Testing also shows that smaller capacity pumps exhibit lower efficiencies than higher capacity designs.

Now that we have learned that pump efficiency is closely related to the shape of the impeller, and the choice of impeller shape is usually dictated by the operating conditions, you should be aware of various conditions that decrease the efficiency of your pump.

These would include:

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