# The System Curve

A fluid flow system is characterized with theÂ System CurveÂ – a graphical presentation of theÂ Energy Equation.

The system head visualized in the System Curve above is a function of elevation – or the static head and theÂ major and minor lossesÂ in the system and can be expressed as:

h = dh + hlÂ Â Â Â Â Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â Â  (1)

where

dhÂ =Â h2Â – h1Â = elevation (static) head difference between the inlet and outlet in the system (m)

hlÂ =Â major and minor head loss in the systemÂ (m)

A generic expression of major and minor head loss is:

hlÂ = k q2Â Â Â Â Â Â Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  (2)

where

qÂ = flow rate

kÂ = constant describing the total system characteristics – including all major and minor losses

Increasing the constant –Â kÂ – 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 the pump performance curve. The pump performance curve describes the relation between the flowrate and the head for the actual pump. Other important information for a proper pump selection is also included – likeÂ efficiency curves,Â NPSHrÂ curve,Â pump curves for several impeller diameters and different speeds, andÂ power consumption.

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

### Selection of Pump

The proper pump can be selected by combining theÂ System CurveÂ and theÂ Pump Curve:

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

### Best Efficiency Point – BEP

The best operating conditions will in general be close to the BestÂ EfficiencyÂ Point –Â BEP.