Centrifugal pumps in water conservancy, chemical and other industries are widely used, the choice of its operating point and the analysis of energy consumption is also increasingly valued. The so-called operating point, refers to the actual water pump device in a moment the amount of water, head, shaft power, vacuum suction and so on, which indicates that the pump's ability to work. In general, the centrifugal pump flow, head pressure may be inconsistent with the pipeline system, or due to production tasks, process requirements change, the need for pump flow regulation, its essence is to change the centrifugal pump operating point. In addition to engineering design stage centrifugal pump selection is correct or not, the actual use of centrifugal pump operating point selection will also directly affect the user's energy consumption and cost. Therefore, how to reasonably change the centrifugal pump operating point is particularly important. The working principle of the centrifugal pump is to convert the mechanical energy of the motor rotating at high speed into kinetic energy and potential energy of the liquid to be lifted, which is a process of energy transfer and conversion. According to this feature, we can see that the operating point of the centrifugal pump is built on the balance between the energy supply and demand of the pump and the pipe system. As long as the situation of one of the two changes, the operating point of the centrifugal pump will be shifted. Changes in operating conditions caused by two aspects: one. Pipeline system characteristics change, such as valve throttling; two. The pump itself changes in the characteristic curve, such as frequency control, cutting impeller, pump in series or in parallel. Here are several ways to analyze and compare: First, the valve throttling The easiest way to change the centrifugal pump flow is to adjust the pump outlet valve opening, while the pump speed remains the same (usually the rated speed), its essence is to change the position of the pipeline characteristic curve to change the pump operating point. As shown in Figure 1, the intersection point A of the pump characteristic curve Q-H and the pipeline characteristic curve Q-Σh is the limit working point of the pump when the valve is fully opened. When the valve is closed, the local resistance of the pipeline increases, and the pump working point moves to the left at point B and the corresponding flow decreases. When the valve is fully closed, it is equivalent to infinite resistance and zero flow. At this time, the pipe characteristic curve coincides with the ordinate. Can be seen from Figure 1, to close the valve to control the flow rate, the pump itself the same water supply capacity, lift the same characteristics, pipe resistance characteristics will change with the valve opening changes. This method is easy to operate, continuous flow, can be adjusted between a certain maximum flow and zero, and without additional investment, for a wide range of applications. However, the throttling regulation consumes the excess energy of the centrifugal pump (shaded part in the figure) to maintain a certain supply, and the efficiency of the centrifugal pump will also decrease, which is not economically reasonable. Second, frequency control Operating conditions deviate from the high efficiency zone is the basic conditions for speed control. When the pump speed changes, the valve opening remains unchanged (usually the maximum opening), the same pipe system characteristics, and water supply capacity and head characteristics change. As shown in Figure 2, A is the pump balance operating point (also known as the operating point), corresponding to the efficiency ηa. To reduce the flow, speed can be reduced, when the operating point is B, corresponding to efficiency ηb, the pump is still in the high-efficiency area. If you use the valve throttling method to adjust, the operating point is C, the corresponding efficiency ηc, the pump efficiency decreases. Thus, in the case of the required flow is less than the rated flow rate, the lift when the frequency control is smaller than the valve throttling, so the frequency of water supply required for regulating speed than the valve throttling small, shaded part of Figure 2 Is the frequency of speed control by the water supply. Obviously, compared with the valve throttling, energy-saving effect of frequency control is very prominent, higher efficiency centrifugal pumps. In addition, the use of frequency control, not only help reduce the possibility of centrifugal pump cavitation, but also by the speed / deceleration time to extend the pre-set start / stop process, the dynamic torque is greatly reduced , Thus largely eliminating the damaging water hammer effect, greatly extending the life of pumps and piping systems. In fact, there are limitations of frequency control, in addition to large investment, high maintenance costs, when the pump speed is too large will cause the efficiency decline, beyond the scope of the proportion of the pump, it is impossible to speed unlimited. Third, cutting impeller When the speed is constant, the pump head, flow and impeller diameter. For the same type of pump, cutting method can be used to change the pump characteristic curve. Set the centrifugal pump impeller diameter D, flow Q, lift H, power P, after cutting the impeller diameter D ', the flow Q', the lift is H ', the power P', then the relationship between : The three types collectively referred to as the pump cutting law. Cutting law is based on a large number of perceptual test data, it believes that if the impeller cutting amount control within a certain limit (the cutting limit and the pump specific speed), then cut the appropriate efficiency before and after the pump can be considered unchanged . Cutting impeller is an easy way to change the performance of the pump, the so-called variable-diameter adjustment, which to some extent, solves the contradiction between the type of the pump, the limited specifications and the diversity of water supply targets, expanding the pump The use of. Of course, the cutting impeller is an irreversible process, the user must be accurately calculated and measure the economic rationality before implementation. Fourth, pumps in series and in parallel Pump in series means the outlet of one pump delivers fluid to the inlet of another pump. Take the simplest two identical centrifugal pump models of the same type and performance as the example: As shown in Figure 3, the series performance curve is equivalent to the single pump performance curve lift at the same flow rate under the same conditions, the series work point A Flow and lift are larger than the single pump operating point B, but less than double the time of a single pump, this is because after the pump in series on the one hand increase in head lift is greater than the increase in pipe resistance, resulting in excess head to promote traffic flow On the other hand, the increase of the flow rate increases the resistance and suppresses the increase of the total head. Pump series operation, we must pay attention to whether the pump can withstand a boost. Before the start of each pump outlet valve to be closed, and then turn the pump and valve to open water. Pumping in parallel means that two or more pumps deliver fluid to the same pressure line, with the purpose of increasing flow at the same pressure head. In the simplest case, two centrifugal pumps with the same type and the same performance are connected in parallel. As shown in Fig. 4, the parallel performance curve is equivalent to the single pump performance curve with the same head lift. The parallel operation point A Of the flow and lift than the single-pump work point B large, but considering the pipe resistance factors, the same can not reach 2 times the single pump. If purely for the purpose of increasing the flow, then whether parallel or series should depend on the flatness of the pipeline characteristic curve, the more flat the pipeline characteristic curve, the closer the parallel flow is to double the single pump operation, When the series flow greater, more conducive to operation. Fifth, the conclusion Although the valve throttling will result in energy loss and waste, in some simple occasions, it is still a fast and easy way to adjust the flow rate. Because of its energy-saving effect and high degree of automation, inverter speed control is more and more favored by users. Cutting impeller is generally used for water pump, due to changes in the structure of the pump, versatility is poor; pump series and parallel only applies to a single pump can not meet the task of conveying the situation, and the number of series or parallel but not economical. In practical applications should be considered from many aspects, in a variety of flow control methods to integrate the best solution to ensure the efficient operation of centrifugal pumps.