3. Cutting impeller
When the speed is constant, the pressure head and flow rate of the pump are related to the impeller diameter. For the same type of pump, the cutting method can be used to change the pump characteristic curve.
The cutting rule is based on many rational experimental data. It believes that if the cutting amount of the impeller is controlled within a certain limit (this cutting quantity is related to the specific speed of the water pump), the corresponding power 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 function of the water pump, that is, the so-called variable diameter adjustment. It solves the contradiction between the limited types and specifications of water pumps and the diversity of water supply target requirements to a certain extent, and expands the application scope of the water pump. Of course, cutting the impeller is an irreversible process, and users must accurately calculate and measure the economic rationality before implementation.
4.Water pumps in series and in parallel
Water pumps in series means that the outlet of one pump delivers fluid to the inlet of another pump. Take the simplest example of two centrifugal pumps of the same type and function in series: as shown in Figure 3, the series function curve is equivalent to the head of the single pump function curve superimposed under the condition of the same flow rate. The flow rate and head of the series operation point A are larger than those of the single pump operation point B, but they are less than 2 times that of the single pump. This is because after the pumps are connected in series, on the one hand, the increase in head is greater than the increase in pipeline resistance, resulting in sufficient head to increase flow rate, and on the other hand, the increase in flow rate increases resistance, which restrains the increase in total head. When the water pumps are working in series, it is necessary to pay attention to whether the latter pump can withstand the pressure increase. Before starting, the outlet valve of each pump must be closed, and then the pumps and valves are opened in sequence to supply water to the outside.
Water pumps in parallel means that two or more pumps deliver fluid to the same pressure pipeline, with the intention of increasing flow rate at the same pressure head. Still taking the simplest example of two centrifugal pumps of the same type and function connected in parallel, the parallel function curve is equivalent to the flow of the single pump function curve superimposed under the condition of equal head. The flow and head of the parallel operation point A are larger than those of the single pump operation point B, but considering the pipe resistance factor, it is also less than twice that of the single pump.
If the purpose is simply to increase the flow, then whether to choose 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 parallel connection is to twice that of the single pump, so the flow is larger than that of series connection, which is more conducive to operation.
5.Conclusion
Although valve saving will cause energy loss and waste, it is still a fast and easy flow control method in some simple occasions; variable frequency speed regulation is increasingly popular with users due to its good energy-saving effect and high degree of automation; cutting impellers are generally used in clean water pumps, because the structure of the pump is changed, the versatility is poor; water pumps are only suitable for situations where a single pump cannot meet the transportation task, and too many pumps in series or parallel are not economical. In practical application, many aspects should be considered, and the best solution should be synthesized among various flow control methods to ensure the efficient operation of the centrifugal pump.
