The relationship between the head of the water pump and the inlet/outlet water flow
Many people believe that a lower pumping head means a lighter motor load. Under this misconception, they often select pumps with excessively high heads. In reality, for centrifugal pumps, once the model is determined, the power consumption is directly proportional to the actual flow rate. As the head increases, the flow rate decreases, resulting in lower power consumption at higher heads. Conversely, lower heads lead to higher flow rates and greater power consumption.
Therefore, to prevent motor overload, it is generally required that the actual pumping head of the Water Pump shall not be less than 60% of the rated head. When a high-head pump is used for low-head pumping, the motor is prone to overload and overheating, which may even burn out the motor in severe cases. If emergency operation is necessary, a gate valve (or a wooden block or similar object to plug the small outlet) must be installed on the discharge pipe to regulate the flow and reduce the load on the motor. It is crucial to monitor the motor temperature. If overheating is detected, the outlet flow should be reduced promptly or the motor shut down. This point is often misunderstood, as some operators believe that blocking the outlet and forcibly reducing the flow would increase the motor load. In fact, the opposite is true.
Using a large-diameter Water Pump with a small-diameter pipe for pumping water is a common misconception among many users, as they believe it can increase the actual head. In reality, the actual head of a Water Pump equals the total head minus the head loss. Once the pump model is determined, the total head remains constant. Head loss primarily stems from pipeline resistance, which increases with smaller pipe diameters, resulting in greater head loss. Therefore, reducing the pipe diameter not only fails to increase the pump's actual head but also reduces it, leading to a decline in pump efficiency.
Similarly, when a small-diameter Pump uses a large-diameter pipe to draw water, it does not reduce the actual head of the Pump. Instead, the resistance in the pipeline decreases, which reduces the loss head and improves the actual head. Some users, however, believe that using a large-diameter pipe with a small-diameter Pump will significantly increase the motor load. They argue that the larger pipe diameter increases the pressure of the water in the discharge pipe on the Pump impeller, thereby greatly increasing the motor load.
Little do they know that the magnitude of liquid pressure is solely related to the head height and not to the cross-sectional area of the pipe. As long as the head remains constant and the impeller size of the Pump remains unchanged, the pressure exerted on the impeller will remain consistent regardless of the pipe diameter. However, when the pipe diameter increases, the flow resistance decreases, leading to a rise in flow rate and a corresponding increase in power consumption. Yet, as long as the head remains within the rated range, the Pump can operate normally even with larger pipe diameters, while also reducing pipeline losses and improving Pump efficiency.
