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Analysis of common adjustment methods of centrifugal pump

Centrifugal pumps are widely used in water conservancy, chemical industry and other industries, and their choice of working point and analysis of energy consumption are also increasingly valued. The so-called operating point refers to the actual water output, head, shaft power, efficiency and vacuum height of the pump device at a certain instant, which represents 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 due to changes in production tasks and process requirements, the flow rate of the pump needs to be adjusted, which essentially changes the working point of the centrifugal pump. In addition to the correct selection of the centrifugal pump in the engineering design stage, the selection of the working point in the actual use of the centrifugal pump will also directly affect the user's energy consumption and cost. Therefore, how to reasonably change the working point of the centrifugal pump is particularly important.

The working principle of the centrifugal pump is to convert the mechanical energy of the motor rotating at high speed into the kinetic energy and potential energy of the lifted liquid, which is an energy transmission and conversion process. According to this characteristic, the working 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 two conditions changes, its working point will shift. The change of operating point is caused by two aspects: one. The characteristic curve of the pipeline system changes, such as the throttle of the valve; 2. The characteristic curve of the water pump itself changes, such as frequency conversion speed regulation, cutting impeller, water pump series or parallel.

The following is an analysis and comparison of these methods:

1. Valve Throttling
The easiest way to change the flow of the centrifugal pump is to adjust the opening of the pump outlet valve, and the pump speed remains unchanged (generally the rated speed), which essentially changes the position of the pipeline characteristic curve to change the pump operating point. When the small valve is closed, the local resistance of the pipeline increases, the pump operating point moves to the left, and the corresponding flow decreases. When the valve is fully closed, it is equivalent to infinite resistance and zero flow. At this time, the pipeline characteristic curve coincides with the vertical coordinate. When the small valve is closed to control the flow, the water supply capacity of the pump itself does not change, the head characteristic does not change, and the pipe resistance characteristic will change with the change of the valve opening. This method is easy to operate, continuous flow, can be adjusted between a certain maximum flow and zero, and no additional investment is required, and the application is very wide. However, throttling adjustment is to consume the excess energy of the centrifugal pump to maintain a certain amount of supply, and the efficiency of the centrifugal pump will also decrease accordingly, which is not economically reasonable.

Second, frequency conversion speed regulation
The deviation of the operating point from the high-efficiency zone is the basic condition for the pump to adjust its speed. When the speed of the water pump changes, the valve opening remains unchanged (usually the maximum opening), the characteristics of the pipeline system remain unchanged, and the water supply capacity and head characteristics change accordingly.

In the case where the required flow rate is less than the rated flow rate, the head when the frequency conversion speed regulation is smaller than the valve throttle, so the water supply power required by the frequency conversion speed regulation is also smaller than the valve throttle. Obviously, compared with valve throttling, the energy-saving effect of frequency conversion speed regulation is very prominent, and the working efficiency of centrifugal pumps is higher. In addition, the use of frequency conversion speed regulation not only helps to reduce the possibility of cavitation in the centrifugal pump, but also can extend the start-up / stop process by presetting the speed-up / speed-down time, greatly reducing the dynamic torque. , To a large extent, the extremely damaging water hammer effect is eliminated, and the life of the pump and piping system is greatly extended.

In fact, the frequency conversion speed regulation also has limitations. In addition to large investment and high maintenance cost, when the pump speed is too large, it will cause a decrease in efficiency, which exceeds the scope of the pump proportional law. It is impossible to adjust the speed without limit.

3. Cutting 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 method can be used to change the characteristic curve of the pump.

The cutting law is based on a large amount of perceptual test data. It believes that if the cutting amount of the impeller is controlled within a certain limit (this cutting limit is related to the specific rotation number of the water pump), the corresponding efficiency of the water pump before and after cutting can be regarded as unchanged . Cutting the impeller is a simple and easy way to change the performance of the pump, that is, the so-called variable diameter adjustment, which solves the contradiction between the limited type and specifications of the pump and the diversity of water supply objects to a certain extent, and expands the pump The scope of use. Of course, cutting the impeller is an irreversible process, and the user must carry out accurate calculation and measure economic rationality.

4. Series and parallel connection of pumps
Pump connection in series means that the outlet of one pump delivers fluid to the inlet of another pump. Take the simplest series of two centrifugal pumps of the same model and the same performance as an example: As shown in Figure 3, the series performance curve is equivalent to the head of the single pump performance curve added under the same flow rate, and the series working point A The flow rate and head are larger than the single pump working point B (public number: pump housekeeper), but they are less than twice that of the single pump. This is because the increase in head is greater than the increase in pipeline resistance after the pumps are connected in series. The excess head causes the flow to increase, on the other hand, the increase in flow increases the resistance, which suppresses the increase in the total head. When the pumps run in series, you must pay attention to whether the latter pump can withstand the pressure increase. Before starting, the outlet valve of each pump should be closed, and then the pump and valve should be turned on to supply water outward.

The parallel connection of water pumps means that two or more pumps deliver fluid to the same pressure pipeline, and the purpose is to increase the flow rate when the pressure heads are the same. Still taking the simplest parallel connection of two centrifugal pumps of the same model and the same performance as an example, the parallel performance curve is equivalent to the flow rate of the single pump performance curve is added when the head is equal, and the flow rate and head of the parallel working point A are The working point B of the single pump is large, but considering the pipe resistance factor, it is also less than twice that of the single pump.

If the purpose is purely to increase the flow rate, then whether to use parallel or series connection should depend on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve, the closer the flow after paralleling is to double the single pump operation, which is The flow in series is larger, which is more conducive to operation.

V. Conclusion
Although valve throttling will cause energy loss and waste, it is still a fast and easy flow adjustment method in some simple occasions; frequency conversion speed regulation is more and more popular among users because of its good energy saving effect and high degree of automation; Cutting impellers are generally used for clean water pumps. Due to the change in the structure of the pump, the versatility is poor; the series and parallel pumps are only suitable for the case where a single pump cannot meet the delivery task, and the number of series or parallel units is too large. In practical application, it should be considered from many aspects, and the best plan should be synthesized among various flow adjustment methods to ensure the efficient operation of the centrifugal pump.
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