Matching Centrifugal Pumps to Flow Requirements

1 Pump Characteristic and System Head. The pump characteristic curve needs to be matched with the head loss through a piping system, which is known as the system head. The system head increases approximately in proportion to the square of the flow rate (Ap cc z/2). An example of a system head calculation is given in Section 3.10. 

Different pumps with different characteristic curves will intersect the system head at different flow rates and give different operating points. 

2 Control Valves. A flow system will normally require control and a control valve would normally be inserted into the pipeline. 

3 Net Positive Suction Head - NPSH. The pressure of liquid inside a pump must always exceed the liquid s vapour pressure, otherwise the liquid will tend to vaporise within the pump, causing cavitation. As the fluid passes on through the pump its pressure increases and any vapour bubbles collapse rapidly causing mechanical damage to the 

The point of minimum pressure is not at the pump inlet, but will be somewhere within the impeller of the pump. Therefore, the absolute pressure (or head) of the liquid at the pump inlet, the suction side, must always be greater than the fluid vapour pressure by an amount, called the Net Positive Suction Head or NPSH. Manufacturers will quote the NPSH requirements for their pumps. 

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