| Centrifugal pumps are widely used in water conservancy and chemical industries, and their selection of operating conditions and analysis of energy consumption are also receiving increasing attention. The so-called operating point refers to the actual water output, head, shaft power, efficiency, and vacuum height of the pump unit at a certain moment. It indicates the working capacity of the pump. Generally, the flow rate and pressure head of the centrifugal pump may be inconsistent with the pipeline system, or the flow rate of the pump needs to be adjusted due to changes in production tasks and process requirements. The essence is to change the operating point of the centrifugal pump. In addition to the correctness of the centrifugal pump selection during the engineering design phase, the choice of the operating point of the centrifugal pump will directly affect the user's energy consumption and cost. Therefore, how to reasonably change the operating point of the centrifugal pump is particularly important. The working principle of the centrifugal pump is to convert the mechanical energy of the high-speed rotation of the motor into the kinetic energy and potential energy of the lifted liquid, which is a process of energy transfer and conversion. According to this feature, the operating point of the centrifugal pump is based on the balance between the energy supply and demand of the pump and the pipeline system. As long as one of the conditions changes, the operating point will shift. The change in working conditions is caused by two aspects: The piping system characteristic curve changes, such as valve throttling; The characteristic curve of the pump itself changes, such as variable frequency speed regulation, cutting impeller, water pump series or parallel. Here are some ways to analyze and compare: First, the valve throttle The simple way to change the centrifugal pump flow rate is to adjust the opening of the pump outlet valve, while the pump speed remains the same (generally the rated speed), which essentially changes the position of the pipeline characteristic curve to change the operating point of the pump. As shown in Fig. 1, the intersection point A of the pump characteristic curve Q-H and the pipeline characteristic curve Q-∑h is the limit operating point of the pump when the valve is fully open. When the small valve is closed, the local resistance of the pipeline increases, and the pump operating point moves to the left to point B, and the corresponding flow rate decreases. When the valve is fully closed, it is equivalent to an infinite resistance and the flow rate is zero. At this time, the pipeline characteristic curve coincides with the ordinate. It can be seen from Fig. 1 that when the flow rate is controlled by closing the small valve, the water supply capacity of the water pump itself is unchanged, the lift characteristic is unchanged, and the pipe resistance characteristic will change with the change of the valve opening degree. This method is easy to operate and has continuous flow. It can be adjusted freely between a large flow and zero, and no additional investment is required. However, the throttling adjustment is to maintain a certain amount of supply by consuming the excess energy of the centrifugal pump (shaded portion in the figure), and the efficiency of the centrifugal pump will also decrease, which is not economically reasonable. Second, frequency control Deviation from the high efficiency zone is the basic condition for the pump to adjust speed. When the speed of the pump changes, the valve opening remains the same (usually a large opening), the piping system characteristics remain unchanged, and the water supply capacity and head characteristics change. As shown in Figure 2, A is the pump balance operating point (also called the operating point), corresponding to the efficiency ηa. To reduce the flow rate, the speed can be reduced. At this time, the operating point is B, and the pump is still in the high efficiency zone corresponding to the efficiency ηb. If the valve throttling method is used to adjust, the operating point is C, the corresponding efficiency is ηc, and the efficiency of the pump is lowered. It can be seen that in the case that the required flow rate is less than the rated flow rate, the head of the variable frequency speed regulation is smaller than the throttle of the valve, so the water supply power required for the frequency conversion speed regulation is also smaller than the throttle of the valve, and the shaded part in FIG. 2 indicates The water supply power saved by the frequency conversion speed regulation. Obviously, compared with valve throttling, the energy-saving effect of frequency conversion speed regulation is very prominent, and the centrifugal pump works more efficiently. In addition, the use of variable frequency speed control not only helps to reduce the possibility of cavitation in the centrifugal pump, but also can extend the start/stop process by presetting the up/down time to greatly reduce the dynamic torque. This greatly eliminates the highly damaging water hammer effect and greatly extends the life of the pump and piping system. In fact, the frequency conversion speed regulation also has limitations. In addition to the large investment and high maintenance cost, when the pump speed is too large, the efficiency will drop, which exceeds the pump proportional law range, and it is impossible to adjust the speed without limit. Third, cutting the impeller When the speed is constant, the pressure head and flow rate of the pump are related to the diameter of the impeller. For the same type of pump, the cutting curve can be used to change the characteristic curve of the pump. It is assumed that the original impeller diameter of the centrifugal pump is D, the flow rate is Q, the lift is H, the power is P, the impeller diameter after cutting is D', the flow rate is Q', the lift is H', and the power is P'. : The above three types are collectively referred to as the cutting law of the pump. The cutting law is based on a large number of inductive test data. It is considered that if the cutting amount of the impeller is controlled within a certain limit (this cutting limit is related to the specific number of revolutions of the pump), the corresponding efficiency of the pump before and after cutting can be regarded as constant. . The cutting impeller is a simple and easy way to change the performance of the pump. The so-called variable diameter adjustment solves the contradiction between the limitation of the pump type and specification and the diversity of the requirements of the water supply object to some extent, and expands the water pump. The scope of use. Of course, the cutting impeller is an irreversible process and the user must perform accurate calculations and measure economic rationality before implementation. Fourth, the pump series and parallel Pump in series means that the outlet of one pump delivers fluid to the inlet of another pump. Taking Zui's simple two centrifugal pumps of the same model and the same performance as an example: as shown in Fig. 3, the series performance curve is equivalent to the head of the single pump performance curve superimposed at the same flow rate, and the series working point A Both the flow rate and the lift are larger than the single pump operating point B, but they are less than twice that of the single pump. This is because the increase in the lift is greater than the increase in the line resistance after the pump is connected in series, resulting in a surplus lift that causes the flow to increase. On the other hand, the increase in flow increases the resistance and suppresses the increase in the total lift. When the pumps are operated in series, it must be noted whether the latter pump can withstand the boost. The outlet valve of each pump should be closed before starting, and then the pump and valve should be sequentially opened to supply water. Parallel pump means that two or more pumps deliver fluid to the same pressure line. The purpose is to increase the flow when the head is the same. Still taking the simple two parallel pumps of the same model and the same performance as an example: as shown in Fig. 4, the parallel performance curve is equivalent to the flow of the single pump performance curve superimposed in the case of equal lift, parallel working point A The flow rate and head are both larger than the single pump operating point B, but considering the pipe resistance factor, it is also less than twice the single pump. If the purpose of increasing the flow rate is purely, then whether parallel or series connection should be determined depends on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve, the closer the flow rate after paralleling is to twice the single pump operation, thus The flow in the series is greater and is more conducive to operation. V. Conclusion Although the valve throttling will cause energy loss and waste, it is still a fast and easy flow regulation method in some simple occasions; the frequency conversion speed regulation is more and more favored by users because of its good energy saving effect and high degree of automation; Cutting impellers are generally used for clean water pumps. Due to changes in the structure of the pump, the versatility is poor. The series and parallel connection of pumps are only suitable for the case where a single pump cannot meet the transportation task, and the number of series or parallel is too uneconomical. In practical applications, it should be considered from many aspects, and a comprehensive solution is adopted among various flow regulation methods to ensure efficient operation of the centrifugal pump. (The pictures in this article can refer to the following books) ã€references】 [1] Guan Xingfan. Pump theory and design [M]. Beijing: Mechanical Industry Press, 1987. [2] Zhang Yanbin. SPWM frequency conversion speed application technology [M]. Beijing: Mechanical Industry Press, 2002. |
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