Many people think that the lower the pumping head, the smaller the motor load. In this kind of misunderstanding, when choosing a pump, the head of the pump is often chosen very high.
In fact, for centrifugal pumps, when the centrifugal pump model is determined, its consumption power is proportional to the actual flow of the pump. The flow of the pump will decrease with the increase of the head, so the higher the head is, the smaller the flow is, and the smaller the power consumption is. On the contrary, the lower the head, the greater the flow, and the greater the power consumption.
Therefore, in order to prevent motor overload, it is generally required that the actual pumping head of the centrifugal pump shall not be less than 60% of the calibrated head. Therefore, when the high head is used for pumping water with low head, the motor is easy to overload and heat, and it can burn down in serious cases. In case of emergency use, a gate valve must be installed on the water outlet pipe to regulate the water output (or block the small water outlet with wood, etc.), so as to reduce the flow, prevent the motor from overload, pay attention to the temperature rise of the motor, and if the motor is overheated, turn down the water outlet flow or shut down in time. This is also easy to misunderstand. Some people think that blocking the water outlet and forcibly reducing the flow will increase the motor load.
In fact, on the contrary, the outlet pipes of normal high-power centrifugal pump irrigation and drainage units are equipped with gate valves. In order to reduce the motor load when the unit starts, the gate valve should be closed first, and then gradually opened after the motor starts.
Many operators think that this can improve the actual head, in fact, the actual head of the centrifugal pump
total head - loss head. When the centrifugal pump model is determined, the total head is certain; the loss head mainly comes from the pipeline resistance. The smaller the pipe diameter is, the greater the resistance is, so the greater the loss head is. Therefore, after reducing the pipe diameter, the actual head of the centrifugal pump can not be increased, but will be reduced, resulting in the decrease of the centrifugal pump efficiency. Similarly, when the small-diameter water pump uses the large water pipe to pump water, the actual head of the pump will not be reduced, but the loss head will be reduced due to the reduction of the resistance of the pipeline, so that the actual head will be increased. It is also believed that the motor load of centrifugal pump will be greatly increased when the small-diameter water pump uses the large water pipe to pump water. They think that when the pipe diameter is increased, the pressure of the water in the outlet pipe on the pump impeller will be large, so the motor load will be greatly increased. As we all know, the liquid pressure is only related to the head, but not to the cross-sectional area of the water pipe. As long as the head is fixed, the impeller size of the water pump remains unchanged. No matter how large the pipe diameter is, the pressure acting on the impeller is certain. Only when the pipe diameter increases, the flow resistance will decrease, and the flow rate will increase, and the power consumption will increase appropriately. However, as long as the pump is within the rated head range, no matter how the pipe diameter increases, it can work normally, and it can also reduce the pipeline loss and improve the efficiency of the pump.
If more elbows are used on the water inlet pipeline, local flow resistance will be increased. And the elbow should turn in the vertical direction, not allowed to turn in the horizontal direction, so as to avoid the accumulation of air.
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