The working principle of a 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 fluid being pushed forward. It is a process of energy transfer and conversion. According to this characteristic, it can be seen that the operating point of the centrifugal pump is based on the balance between the energy supply and demand of the water pump and the pipeline system. As long as one of the two conditions changes, the operating point will be changed. The change of the operating point is caused by two aspects: 1. Changes in the characteristic curve of the pipeline system, such as valve regulation; 2. Changes in the characteristic curve of the pump itself, such as variable frequency speed regulation, cutting impeller, and series or parallel connection of pumps.
The following are several methods for analysis and comparison:
1. Valve Saving
The simplest way to change the flow of a centrifugal pump is to adjust the opening of the pump outlet valve, while the pump speed remains unchanged (generally the rated speed). The essence is to change the position of the pipeline characteristic curve to change the pump's operating point. When the valve is closed, the resistance of the pipeline increases, the pump operating point moves to the left, and the corresponding flow is reduced. 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 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 the valve opening. This method is simple to operate, the flow is continuous, and it can be adjusted freely between a certain maximum flow and zero, and no additional investment is required. It is widely applicable. However, the economical adjustment consumes the excess energy of the centrifugal pump to maintain a certain supply volume, and the power of the centrifugal pump will also decrease accordingly, which is not economically reasonable.
2. Variable frequency speed regulation
The violation of the operating point in the high-efficiency zone is the basic condition for the water pump to be regulated. When the speed of the water pump changes, the valve opening remains unchanged (generally the maximum opening), the characteristics of the pipeline system remain unchanged, and the water supply capacity and lift characteristics change accordingly.
When the required flow rate is less than the rated flow rate, the lift during variable frequency speed regulation is smaller than the valve saving, so the water supply power required for variable frequency speed regulation is also smaller than the valve saving. Obviously, compared with the valve saving, the energy-saving effect of variable frequency speed regulation is very prominent, and the working power of the centrifugal pump is higher. In addition, after adopting variable frequency speed regulation, it is not only conducive to reducing the possibility of cavitation of the centrifugal pump, but also can extend the start-up/shutdown process by presetting the speed increase/deceleration time, so that the dynamic torque is greatly reduced, thereby largely eliminating the extremely destructive water hammer effect, and greatly extending the life of the water pump and pipeline system.
In fact, variable frequency speed regulation also has limitations. In addition to the large investment and high maintenance cost, when the speed of the water pump changes too much, it will cause power loss, exceeding the specified design of the pump ratio, and it is impossible to adjust the speed indefinitely.
