What are the reasons for a decrease in the head pressure of a water pump?

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(1) Blockage in the impeller flow channels: If some of the flow channels within the impeller are blocked, it will affect the impeller’s ability to do work, resulting in a decrease in outlet pressure. Therefore, the pump needs to be disassembled for inspection to remove foreign objects. To prevent this problem from occurring again, a filtering device can be installed in front of the pump inlet if necessary. (2) Internal leakage occurs when the clearance between the rotating and stationary parts within the pump exceeds the designed range, which leads to leakage inside the pump; this is manifested as a decrease in the pump’s discharge pressure, such as in the clearance at the impeller mouth ring or in the inter-stage clearances of multi-stage pumps. At this point, corresponding disassembly and inspection should be carried out, and the components that cause excessive gaps should be repaired or replaced. (3) Cavitation at the inlet: If the pressure at the pump’s suction inlet is too low, below the saturated vapor pressure of the fluid being pumped, cavitation will occur. At this point, it is necessary to check whether there is any blockage in the inlet piping system or if the opening degree of the inlet valve is too low, or else increase the liquid level in the suction tank. (4) The important factors affecting the pump’s head due to a decrease in rotational speed are the outer diameter of the impeller and the pump’s speed. Under constant other conditions, the pump’s head is directly proportional to the square of the speed; it is thus clear that speed has a significant impact on the head. Sometimes, if the pump’s speed decreases for some external reason, the pump’s head will decrease accordingly. At this point, the pump’s speed should be checked; if it is indeed insufficient, the cause should be identified and resolved appropriately. (5) Shift of the operating point toward higher flow rates and lower head: Generally, centrifugal pumps have a continuously decreasing performance curve, with the flow rate increasing as the head decreases. During operation, if the back pressure of the pump decreases for some reason, the operating point of the pump shifts passively along the performance curve toward a point with lower head and higher flow rate. This results in a decrease in head; actually, this is caused by external factors such as changes in the system, and it has little to do with the pump itself. At this point, the problem can be solved simply by increasing the pump back pressure, such as closing the outlet valve slightly. (6) Reverse rotation of the motor: Due to wiring issues, the motor may rotate in the opposite direction to what the pump actually requires. Therefore, it is necessary to check the direction of rotation of the pump at startup; if it is reversed, simply swap any two wires connected to the motor’s terminals.