German standards DIN 24260, Pt.1 define the duty point as the intersection of the pump H-Q and the system H-Q curves.
It indicates the values of Flow and Head which will be obtained at stationary operation with the respective speed-related pump H-Q curve.
The specified duty point is defined to be that point on the system H-Q curve for which a pump is to be selected in line with the calculated hydraulic design criteria. The objective of the selection is (apart from other criteria, such as maximum efficiency) to minimise the deviation between the specified and the actual duty points.
The actual duty point is always located at the intersection of pump H-Q curve and the actual system H-Q curve. At constant speed it moves up the pump H-Q curve with increasing friction losses towards a lower flow rate. The duty point should be chosen as close as possible to the point of optimum efficiency.
Calculating The System H-Q Curve
Pump H-Q Curve