Head, flow rate, power and efficiency are the main performance parameters of centrifugal pump. The relationship between these parameters can be determined by experiments. The production department of centrifugal pump expresses the basic performance parameters of its products with curves, which are called characteristic curves of centrifugal pump. For the reference of pump selection and operation of user departments.
The characteristic curve is measured at a fixed rotating speed, which is only applicable to this rotating speed, so the value of rotating speed n is indicated on the characteristic curve. There are three kinds of curves on the drawing.
1. h-q curve
H-q curve shows the relationship between flow q and head h of pump. The pressure head of centrifugal pump decreases with the increase of flow rate in a large flow range. Different types of centrifugal pumps have different shapes of H-Q curves. If some curves are flat, it is suitable for occasions where the pressure head changes little but the flow rate changes greatly; Some curves are steep, which is suitable for situations where the pressure head varies widely and the flow rate does not change too much.
2. N-Q curve
The n-q curve shows the relationship between pump flow q and shaft power n, and n increases with the increase of q. Obviously, when Q=0, the pump shaft consumes the least power. Therefore, when starting the centrifugal pump, in order to reduce the starting power, the outlet valve should be closed.
3.η-q curve
The η-q curve shows the relationship between the flow q and the efficiency η of the pump. At first, η increases with the increase of Q, and then decreases with the increase of Q after reaching the maximum value. The maximum value of this curve corresponds to the highest point of efficiency. The pump operates at the pressure head and flow rate corresponding to this point, and its efficiency is the highest. Therefore, this point is the design point of centrifugal pump. When choosing a pump, it is always hoped that the pump will work at the highest efficiency, because the operation is the most economical and reasonable under this condition. However, in fact, it is often impossible for the pump to operate under this condition. Therefore, generally, only one working range can be specified, which is called the high efficiency zone of the pump, as shown by the broken line in Figure 2-6. The efficiency of the high efficiency zone should be no less than 92% of the highest efficiency. The nameplate of the pump indicates the flow rate, pressure head and power at the highest efficiency. Centrifugal pump product catalogues and specifications often indicate the range of flow rate, pressure head and power in the highest efficiency area.
