Pump Curves, NPSH, and Cavitation

A pump is a liquid transportation device, which must develop enough pressure to overcome the hydrostatic and frictional resistance of the process as it delivers the required fluid. These resistance components are unique characteristics of the process served and can be described by system curves. The system curve of a process relates the pressure (head) required and the amount of fluid flow that is being delivered. 

A generalized equation, describing the system curve of a process is P = H + Ff(Qx), where P is the required to pump discharge pressure, H is the static or elevation head, Ff is the friction factor, Q is the flow rate, and x is an exponent that varies between 1.7 and 2.0 usually, 2.0 is used. 

Post-Oil Energy Technology After the Age of Fossil Fuels 

The system curve crosses the pump curve at the operating point. 

Flow control via pump speed adjustment is less common than the use of throttling with valves, because most AC electric motors are constant-speed devices. If a turbine drive is used, speed control is even more convenient. However the advent of the pulse-width modulated (PWM) adjustable speed drive with sensorless flux-vector control has brought adjustable speed (AS) pumping into the mainstream of everyday applications. 

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