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System Curve and Pump Performance Curve

To select a proper pump for a particular application it is necessary to utilize the system curve and the pump performance curve

The System Curve

A fluid flow system can in general be characterized with the System Curve - a graphical presentation of the Energy Equation.

system curve

The system head visualized in the System Curve is a function of the elevation - the static head in the system, and the major and minor losses and can be expressed as:

h = dh + h_l (1)

where

h = system head

dh = h₂ - h₁ = elevation (static) head - difference between inlet and outlet of the system

h_l = head loss

A generic expression of head loss is:

h_l = k q² (2)

where

q = flow rate

k = constant describing the total system characteristics - including all major and minor losses

Increasing the constant - k - by closing some valves, reducing the pipe size or similar - will increase the head loss and move the system curve upwards. The starting point for the curve - at no flow, will be the same.

Pump Performance Curve

The pump characteristic is normally described graphically by the manufacturer as a pump performance curve. The pump curve describes the relation between flowrate and head for the actual pump. Other important information for proper pump selection is also included - efficiency curves, NPSH_r curve, pump curves for several impeller diameters and different speeds, and power consumption.

pump curve

Increasing the impeller diameter or speed increases the head and flow rate capacity - and the pump curve moves upwards.

The head capacity can be increased by connecting two or more pumps in series, or the flow rate capacity can be increased by connecting two or more pumps in parallel.

Selection of Pump

A pump can be selected by combining the System Curve and the Pump Curve:

pump system curve

The operating point is where the system curve and the actual pump curve intersect.

Best Efficiency Point - BEP

The best operating conditions will in general be close to the best efficiency point - BEP.

Special consideration should be taken for applications where the system conditions change frequently during operation. This is often the situation for heating and air conditioning system or water supply systems with variable consumption and modulating valves.