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.
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 + hl (1)
h = system head
dh = h2 - h1 = elevation (static) head - difference between inlet and outlet of the system
hl = head loss
A generic expression of head loss is:
hl = k q2 (2)
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, NPSHr curve, pump curves for several impeller diameters and different speeds, and power consumption.
Increasing the impeller diameter or speed increases the head and flow rate capacity - and the pump curve moves upwards.
Selection of Pump
A pump can be selected by combining the System Curve and the Pump 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.
When a pumps operates in the far right of its curve with poor efficiency - the pumps carry out.
Shutoff head is the head produced when the pump operates with fluid but with no flow rate.
A pump is in churn when it operates at shutoff head or no flow.