Subject : How the shape of the pump impeller affects the pump curve.12-7

Pump people use the term specific speed to describe the shape of the pump's impeller. In paper Volume 7 Number 3, I have shown you the specific speed formula and how the specific speed number relates to :

Please take a look at the following diagram. It describes how the specific speed number relates to the shape of the impeller :

In this paper we are going to see how the shape of the pump impeller affects both the slope of the pump curve and the amount of horsepower that will be consumed by the pump at various capacities.

Most of the pumps used in the process industry are of the Francis vane type with specific speed numbers between 1500 and 4000. That would be curves number two (#2) and three (#3) in the following diagram. These are the familiar curves you see on most of your pump prints.

In the following diagram you will also learn how this specific speed number (Ns) affects the shape of the pump curve. As you can see, the higher the specific speed number, the steeper the curve.

The trick is to select the correct specific speed number so that the pump has a reasonable chance of accomplishing exactly what you want to do. In other words, the pump curve matches the system curve.

As an example :

The next diagram show how the pump's horsepower consumption relates to each of these curve shapes.

From the above diagram you can draw several conclusions :

If you will refer to other papers I have written about this subject you will learn how the specific speed number of the impeller relates to pump efficiency and need for NPSHR (net positive suction head required)

 

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