When using a multi-stage centrifugal pump, it is inevitable to encounter some small mistakes in use, but I myself cannot detect them in a timely manner, and sometimes even damage the pump due to such mistakes. So, what are the common misconceptions in the use of multi-stage centrifugal pumps?
1. Large caliber water pump with small water pipe for pumping
Many operators believe that this can increase the actual head, but in fact, the actual head of the water pump is equal to the total head minus the loss head. After the model of the water pump is determined, the total head is fixed; The loss of head mainly comes from pipeline resistance. The smaller the pipe diameter, the greater the resistance, resulting in a greater loss of head. Therefore, reducing the pipe diameter not only fails to increase the actual head, but also reduces it, leading to a decrease in pump efficiency. Similarly, when a small diameter horizontal water pipe is used to pump water, the actual head of the pump will not be reduced. Instead, the loss head will be reduced due to the decrease in pipeline resistance, resulting in an increase in the actual head. Some people also believe that when using a large water pipe to pump water from a small diameter horizontal multi-stage centrifugal pump, it will greatly increase the motor load. They believe that as the pipe diameter increases, the pressure of the water in the outlet pipe will increase, thus greatly increasing the motor load. Little did they know that the magnitude of liquid pressure is only related to the head, and not to the cross-sectional area of the water pipe. As long as the head is constant and the impeller size of the water pump remains the same, regardless of the diameter of the pipe, the pressure acting on the impeller is constant. However, as the diameter increases, the water flow resistance will decrease, resulting in an increase in flow rate and an appropriate increase in power consumption. But as long as it is within the rated head range, regardless of how the pipe diameter is increased, the pump can work normally and reduce pipeline losses, improving pump efficiency.
2. Used for low head pumping
Many operators believe that the lower the pumping head, the lower the motor load. Under this misconception, when choosing a water pump, the head of the pump is often chosen to be very high. In fact, for horizontal multi-stage centrifugal pumps, when the pump model is determined, the power consumption is proportional to the actual flow rate of the pump, and the flow rate of the pump will decrease with the increase of the head. Therefore, the higher the head, the smaller the flow rate, and the lower the power consumption. On the contrary, the lower the head, the greater the flow rate, and the greater the power consumption.
Therefore, in order to prevent motor overload, it is generally required that the actual pumping head of the water pump should not be less than 60% of the calibrated head. Therefore, when used for low head pumping, the motor is prone to overload and heat generation, and in severe cases, it can burn out the motor. If emergency use is required, a gate valve must be installed on the outlet pipe to regulate the water flow rate and prevent motor overload. Pay attention to the temperature rise of the motor. If the motor is found to be overheated, the outlet flow rate should be reduced or the machine should be shut down in a timely manner. This is also prone to misunderstanding. Some operators believe that blocking the water outlet and forcibly reducing the flow will increase the motor load. In fact, on the contrary, the outlet pipes of regular high-power horizontal multi-stage centrifugal pump irrigation units are equipped with gate valves. In order to reduce the motor load during unit startup, the gate valves should be closed first, and gradually opened after the motor starts. This is the principle.
3. The inlet position of the inlet pipe is incorrect
a. The distance between the inlet of the inlet pipe and the bottom and wall of the inlet pool is less than the diameter of the inlet. If there is sediment and other pollutants at the bottom of the pool, and the distance between the inlet and the bottom of the pool is less than 1.5 times the diameter, it will cause poor water intake or suction of sediment and debris during pumping, blocking the inlet.
b. When the water inlet of the inlet pipe is not deep enough, it will cause vortices on the water surface around the inlet pipe, affecting the water intake and reducing the water output. The correct installation method is that the inlet depth of small and medium-sized water pumps should not be less than 300-600mm, and that of large water pumps should not be less than 600-1000mm.
4. If the outlet of the water pipe is above the normal water level of the water outlet tank, although it increases the head of the horizontal multi-stage centrifugal pump, it reduces the flow rate. If due to terrain conditions, the outlet must be higher than the water level of the outlet pool, then elbows and short pipes should be installed at the pipe mouth to make the water pipe siphon and reduce the height of the outlet.
