Thread Content
This post was last edited by B0SS on 2018-6-2 20:23. Centrifugal pumps are widely used in various building or industrial water supply and drainage applications. However, many customers find that after using them for a while, or intermittently, the outlet pressure drops or the head generated is insufficient. So, what are the reasons for a decrease in the outlet pressure or insufficient head of centrifugal pumps? One or more of the following reasons may cause a decrease in the outlet pressure or insufficient head of the centrifugal pump. 1. Motor reversal: Due to wiring issues, the rotation direction of the motor may be opposite to what the pump actually requires. Therefore, it is necessary to check the direction of rotation of the pump when starting it; if it is in the wrong direction, simply swap any two wires connected to the motor’s terminals. 2. The operating point shifts toward higher flow rates and lower head. Under normal conditions, centrifugal pumps have a performance curve that slopes downward, 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 setup, and it has little to do with the pump itself. At this point, the problem can be solved by increasing the pump back pressure, such as closing the outlet valve slightly. 3. Inadequate exhaust before startup, numerous bends in the inlet pipelines, and varying lengths of the pipelines. 4. Reduced speed: An important factor affecting the pump’s head is 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 its speed; it is thus clear that speed has a significant impact on the head. Sometimes, due to some external factors, the pump’s speed decreases, which in turn results in a decrease in the pump’s head. At this point, the pump’s speed should be checked; if it is indeed insufficient, the cause should be identified and resolved appropriately. 5. 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 small, or else increase the liquid level in the suction tank. 6. As the viscosity of the medium being transported increases, the head pressure also decreases. 7. Insufficient water intake by the pump or blocked inlet pipe: If the pump receives an insufficient amount of water while the flow rate at the outlet remains unchanged, then no matter how the pump operates, the outlet pressure (head) will never reach the desired value. It is necessary to check whether there is enough water flowing into the pump’s inlet pipe, to ensure that the fire water inlet is not blocked by debris. 8. Air in the pump: When a large amount of air is drawn into the pump due to insufficient water volume at the suction inlet, the pump operates with high noise and heats up; meanwhile, the pressure (head) at the outlet cannot be achieved. 9. Internal leakage occurs: If the check valve leaks, inspect it. When the gap between the rotating and stationary parts inside the pump exceeds the designed range, internal leakage will occur, which is manifested as a decrease in the pump’s discharge pressure; this can be caused by gaps at the impeller’s mouth ring or between the stages of a multi-stage pump. Repair or replace the components that are causing the excessive gap. 10. Clogging of the impeller flow channels: If some of the flow channels in the impeller become clogged, 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. Methods for identifying problems: 1. Check the pump’s inlet and outlet using pressure gauges; make sure there are no valves or other components between the pressure gauges and the pump ； 2. Measure the motor speed ； 3. Check for blockages inside the pump ； 4. Check whether there is air in the pump during operation; there are two possibilities: cavitation and air inclusion ； 5. If pumping the slurry to check density and viscosity ； 6. Check whether the clearance between the impeller and other components is too large; there are several possible causes, such as a large gap between the side plates and the impeller, or damaged wear rings ； 7. Measure the impeller diameter ； 8. Instrument damage.