A centrifugal pump , when the working speed n is a fixed value, there is a certain corresponding relationship between the head H, the shaft power Pe, the efficiency η and the necessary NPSHr and the pump flow qv. This relationship curve representing H-qv, η-qv, Pe-qv, and NPSHr-qv is called a pump performance curve or performance curve. If the theoretical analysis method is used to determine the performance curve of the centrifugal pump, various losses in the pump must be calculated. However, these losses have a very complicated relationship with the flow in the pump, and it is difficult to make accurate quantitative calculations at present. One can only qualitatively know the general shape of these curves. The exact performance curve for all types of pumps can only be measured experimentally. Figure 1 shows a centrifugal pump performance curve. It should be noted that, due to experimental conditions and other reasons, the performance curve provided by the pump manufacturer on product samples is often determined experimentally at 20°C (Ï=1000kg/m 3 ). When the density, viscosity and other parameters of the pumping liquid are different from 20 °C clear water, performance conversion is also required.
Figure 1 Centrifugal pump performance curve
The actual performance curve of the centrifugal pump shows that when the pump is operating at a constant speed, corresponding to each flow value qv of the pump, there must be a certain head H, shaft power Pe, efficiency η, and the like correspondingly. From a practical point of view, each performance curve has its own purpose.
1. The H-qv curve is the main basis for selecting and operating the pump. Centrifugal pump H-qv curve is generally divided into flat, steep drop and hump-shaped three, as shown in Figure 2. For general chemical production, the reaction needs to be performed under relatively stable pressure, and when the output of the production can be changed, a centrifugal pump with a flat H-qv curve should be used because of the head (when the suction pressure is changed when the flow rate changes). When there is a certain amount of time, it means that there is little change in the discharge pressure; when the liquid to be transported contains solid particles and other substances, it is easy to block the pipeline and cause the pump discharge pressure to increase. In order to achieve the output of chemical products, the flow rate of the pump is changed very much. For small ones, centrifugal pumps with steep drop shape performance curves should be used, because when the flow rate changes very little, the head (discharge pressure) rises more and can rely on this pressure to open the blocked pipeline. The hump-shaped performance curve is an unstable performance curve, and two different flow values ​​may occur under the same head, making the pump operation unstable. When such a centrifugal pump is selected, its working point should avoid the unstable zone in chemical production. It is best not to use such a centrifugal pump.
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Fig. 2 H-qv curve shape of centrifugal pump
2. The Pe-qv curve is the basis for selecting the prime mover power and operating the pump. Usually, the output power of the prime mover should be determined according to the maximum power in the required flow variation range plus a certain safety margin. When the pump is started, it should be selected under the condition with the minimum power consumption to reduce the starting current and protect the motor. Generally, the relationship between centrifugal pump and qv=0 is minimum, so the valve on the discharge pipe should be closed when starting, and then the valve should be opened after starting.
3. The η-qv curve is the basis for checking the economical efficiency of the pump. The pump should work in an efficient area as much as possible. In engineering, the highest efficiency point of the pump is set as the rated point, which is generally the design operating point of the pump. The parameters corresponding to this point are called rated flow, rated head, and rated power. It is usually specified that the range of operating conditions corresponding to 7% of maximum efficiency is an efficient working area. Some pumps only give high-performance performance curves on the sample.
4. The NPSHr-qv curve is the basis for checking the pump for cavitation. The installation position and use of the pump should leave enough effective NPSH to prevent the pump from cavitation.
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