01 Valve throttling
The simple way to change the flow rate of a centrifugal pump is to adjust the opening of the pump outlet valve, while keeping the pump speed constant (usually at the rated speed). Its essence is to change the position of the pipeline characteristic curve to change the operating point of the pump. When the valve is turned down, the local resistance of the pipeline increases, and the operating point of the water pump shifts to the left, resulting in a corresponding decrease in flow rate. When the valve is fully closed, it is equivalent to an infinite resistance and zero flow rate. At this point, the pipeline characteristic curve coincides with the vertical axis. When the valve is turned down to control flow, the water supply capacity of the pump itself remains unchanged, the head characteristic remains unchanged, and the pipe resistance characteristic will change with the change of valve opening.This method is easy to operate, with continuous flow, and can be adjusted freely between a certain high flow rate and zero, without the need for additional investment. It has a wide range of applications. But throttling regulation is to maintain a certain supply capacity by consuming the excess energy of the centrifugal pump, and the efficiency of the centrifugal pump will also decrease accordingly.
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02 Variable frequency speed regulation
The high deviation of operating point and efficiency zone are the basic conditions for water pump speed regulation. When the speed of the water pump changes, the valve opening remains unchanged (usually at a large opening), the characteristics of the pipeline system remain unchanged, and the water supply capacity and head characteristics change accordingly.
When the required flow rate is less than the rated flow rate, the head during variable frequency speed regulation is smaller than the valve throttling, so the required water supply power for variable frequency speed regulation is also smaller than the valve throttling. Obviously, compared to valve throttling, the energy-saving effect of variable frequency speed regulation is very prominent, and the working efficiency of centrifugal pumps is also higher. In addition, adopting variable frequency speed regulation not only helps to reduce the possibility of cavitation in centrifugal pumps, but also extends the start/stop process by presetting the speed increase/decrease time, greatly reducing the dynamic torque and greatly eliminating the destructive water hammer effect, greatly extending the service life of the water pump and pipeline system.
However, variable frequency speed regulation also has limitations. In addition to high investment and maintenance costs, excessive variable speed of the water pump can lead to a decrease in efficiency, exceeding the scope of the pump's proportional law and making it impossible to achieve infinite variable speed regulation.
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03 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 pumps of the same model, the cutting method can be used to change the characteristic curve of the pump.
The cutting law is based on a large amount of inductive experimental data. It believes that if the cutting amount of the impeller is controlled within a certain limit (which is related to the specific speed of the water pump), the corresponding efficiency of the water pump before and after cutting can be considered unchanged. Cutting the impeller is a simple and feasible method to change the performance of water pumps, known as variable diameter adjustment. It solves the contradiction between the limited type and specifications of water pumps and the diversity of water supply object requirements to a certain extent, expanding the scope of use of water pumps.Of course, cutting the impeller is an irreversible process, and users must undergo precise calculations and weigh the economic rationality before implementation.
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04 series and parallel connection of water pumps
Water pump series connection refers to the transfer of fluid from the outlet of one pump to the inlet of another pump. After the pump is connected in series, on the one hand, the increase in head is greater than the increase in pipeline resistance, causing the excess head to increase flow rate. On the other hand, the increase in flow rate also increases resistance, suppressing the increase in total head. When the water pump is running 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 should be closed, and then the pump and valve should be sequentially opened to provide water to the outside.
Parallel connection of water pumps refers to two or more pumps conveying fluid to the same pressure pipeline, with the aim of increasing flow at the same pressure head.
If it is purely for the purpose of increasing traffic, then should we use parallel or series connection? It should depend on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve, the closer the flow rate after parallel operation is to twice that of a single pump, which is greater than the flow rate when connected in series and more conducive to operation.
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