Centrifugal pumps are widely used in water conservancy, chemical and other industries, and the selection of their working points and the analysis of energy consumption have also attracted increasing attention. The so-called working condition point refers to the actual water output, head, shaft power, efficiency and vacuum suction height of the pump device at a certain moment, which indicates the working capacity of the pump. In general, 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, which is essentially to change the working condition point of the centrifugal pump. In addition to the correctness of centrifugal pump selection in the engineering design stage, the selection of working conditions in the actual use of centrifugal pump will also directly affect the user's energy consumption and cost. Therefore, it is particularly important to reasonably change the working condition of the centrifugal pump.
The working principle of 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 characteristic, the working point of the centrifugal pump is based on the balance of energy supply and demand between the pump and the pipeline system, and as long as the situation of one of the two changes, its working point will be transferred. The change of the working condition point is caused by two aspects: 1. Changes in the characteristic curve of the piping system, such as valve throttling; Two. Changes in the characteristic curve of the pump itself, such as frequency conversion speed regulation, cutting impeller, and series or parallel connection of the pump.
The following is an analysis and comparison of these methods:
1
Valve throttling
The easiest way to change the flow rate of a centrifugal pump is to adjust the opening of the pump outlet valve, while the pump speed remains unchanged (generally the rated speed), which is essentially to change the position of the pipeline characteristic curve to change the working point of the pump. When the small valve is closed, the local resistance of the pipeline increases, the working point of the pump shifts to the left, and the corresponding flow decreases. When the valve is fully closed, it is equivalent to infinite resistance, zero flow, and the characteristic curve of the pipeline coincides with the ordinate. When the small valve is closed to control the flow, the water supply capacity of the pump itself remains unchanged, the head characteristics remain unchanged, and the pipe resistance characteristics will change with the change of valve opening. This method is easy to operate, has a continuous flow rate, and can be adjusted between a certain maximum flow rate and zero at will without additional investment, and is suitable for a wide range of applications. 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, which is not very economically reasonable.
2
Frequency conversion speed regulation
The deviation of the working condition point from the high-efficiency zone is the basic condition for the pump to regulate the speed. When the speed of the pump changes, the valve opening remains the same (usually the maximum opening), the piping system characteristics remain the same, and the water supply capacity and head characteristics change accordingly.
In the case that the required flow rate is less than the rated flow, the head of the frequency conversion speed regulation is smaller than that of the valve throttling, so the water supply power required by the frequency conversion speed regulation is also smaller than that of the valve throttling. Obviously, compared with the valve throttling, the energy-saving effect of frequency conversion speed regulation is outstanding, and the work efficiency of the centrifugal pump is higher. In addition, after the use of frequency conversion speed regulation, it is not only conducive to reducing the possibility of cavitation of the centrifugal pump, but also can prolong the start-up/shutdown process through the presetting of the speed up/down time, so that the dynamic torque is greatly reduced, so as to eliminate the extremely destructive water hammer effect to a large extent, and greatly extend the life of the pump and pipeline system.
In fact, frequency conversion speed regulation also has limitations, in addition to the large investment, high maintenance costs, when the pump speed is too large, it will cause the efficiency to decrease, beyond the range of the pump proportional law, it is impossible to limit the speed regulation.
3
Cutting impellers
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 characteristic curve of the pump can be changed by cutting method.
The cutting law is based on a large number of perceptual test data, which believes 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 water pump), the corresponding efficiency of the water pump before and after cutting can be regarded as unchanged. Cutting 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 limitation of the pump type and specification and the diversity of the requirements of the water supply object to a certain extent, and expands the use range of the pump. Of course, cutting impellers is an irreversible process that must be accurately calculated and economically justified before it can be implemented.
4
Water pumps are connected in series and parallel
A cascade of water pumps is when the outlet of one pump delivers fluid to the inlet of another pump. Take the simplest two centrifugal pumps of the same model and the same performance in series as an example: as shown in Figure 3, the series performance curve is equivalent to the head of the performance curve of a single pump in the case of the same flow, and the flow and head of the series working point A are larger than that of the single pump working point B (public account: pump housekeeper), but they are not up to 2 times of the single pump, this is because the increase of the head of the pump in series is greater than the increase of the resistance of the pipeline, resulting in the excess head to promote the increase of flow, and on the other hand, the increase of flow rate increases the resistance, The increase in the total head is suppressed. When the pumps are operated in series, it is important to pay attention to whether the latter pump can withstand the pressure boost. The outlet valve of each pump is closed before starting, and then the pump and valve are opened sequentially to supply water to the outside.
Parallel pump connection is when two or more pumps deliver fluid to the same pressure line with the aim of increasing the flow rate when the pressure head is the same. Still taking the simplest two centrifugal pumps of the same model and the same performance in parallel as an example, the parallel performance curve is equivalent to the flow rate of the performance curve of a single pump in the case of equal head, and the flow rate and head of the parallel working point A are larger than that of the working point B of the single pump, but considering the pipe resistance factor, it is also less than 2 times that of a 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 rate after parallel connection is 2 times that of a single pump operation, so that the flow rate is larger than that of the series connection, which is more conducive to operation.
5
conclusion
Although valve throttling will cause energy loss and waste, it is still a fast and easy way to adjust the flow in some simple occasions; Frequency conversion speed regulation is more and more favored by users because of its good energy-saving effect and high degree of automation; The cutting impeller is generally used for clean water pumps, and the versatility is poor due to the change of the structure of the pump; The series and parallel connection of water pumps is only suitable for the situation that a single pump cannot meet the conveying task, and it is not economical to have too many units in series or parallel. In practical application, the best solution should be considered from various flow regulation methods to ensure the efficient operation of centrifugal pumps.
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