Flow determination

Flow is the number of gallons per minute that the pump will discharge.

■ Any pump will generate more flow as the discharge pressure is reduced.

■ FLqually, the pump will generate less flow as the discharge head or pressure requirements are increased. Obviously, both flow and head should be known before selecting a centrifugal pump.


It is not practical to declare the flow without the accompanying head requirements. For this reason, when someone asks for the pump specifications, they need to know the flow in gallons per minute and the head in feet.

■ The available areas in the impeller, and the available area in the volute determine the flow, gprn. There are two critical areas in the impeller, the exit area and the entrance area. For the volute casing, the most important area is the 'cutwater'. All fluid must pass this point.

■ Head or pressure is developed in the pump; when the impeller imparts rotational energy to the liquid (increasing the liquid's velocity), and then the volute converts this energy (by decreasing the velocity) into pressure.

■ The relationship between the 'exit area' of the impeller, and the 'cutwater area' of the volute, generally determine the flow of the pump.

See the illustration below (Figure 5-3):


Know and Understand Centrifugal Pumps Pump efficiency

Numerous factors affcct the pump's efficiency. The impeller is one of the most important efficiency factors.

Affecting the impeller's behavior are:

1. The impeller velocity.

2. The impeller diameter.

3. The number of blades on the impeller.

4. The diameter of the eye of the impeller.

5. The thickness of the impeller.

6. The pitch (angle) of the blades.

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