Purification Technology Q&A Version [006]

1. Pressure reduction: (1) Motor reversal. Wiring issues can cause the motor to rotate in the opposite direction to what is required by the pump. 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. 　 　　 (2) The operating point shifts toward higher flow rates and lower head. Under normal circumstances, 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 by closing the outlet valve slightly. 　 　　 (3) Reduced rotational speed 　　 An important factor affecting the pump’s head is the outer diameter of the impeller and the pump’s rotational speed. Under constant other conditions, the pump’s head is directly proportional to the square of the speed; thus, the impact of speed on head is very significant. Sometimes, due to some external factors, the pump’s rotational speed decreases, which in turn leads to 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. 　 　　 (4) 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 time, it is necessary to check whether there is any blockage in the inlet piping system or whether the opening degree of the inlet valve is too small, or to increase the liquid level in the suction tank. 　 　　 (5) Internal leakage 　　 When the gap between the rotating and stationary parts inside the pump exceeds the designed range, internal leakage occurs, which is manifested as a decrease in the pump’s discharge pressure; this applies to gaps such as those at the impeller inlet ring and the inter-stage gaps in 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. 　 　　 (6) Clogging of the impeller flow channels 　　 If some of the flow channels in 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 and 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. Rising pressure: (1) The valve in the outlet pipeline is closed partially; (2) The pipeline is blocked; (3) The control valve is closed partially

① The steam pressure of the boiler is unstable. ② The water supply flow rate has been significantly adjusted. ③ The water supply pipe has burst. ④ Frequency and voltage variations. ⑤ The running feed water pump trips or the standby feed water pump starts up accidentally. ⑥ Misoperation of the feedwater pump recirculation valve. ⑦ Boiler leakage or excessive discharge of waste water. ⑧ The coupling of the speed-regulated feed water pump is operating abnormally.

1) Failure in the regulation of the hydraulic coupler； 2) Vaporization of the feed water pump ； 3) Clogging of the feed water pump inlet strainer ； 4) The spoon tube opening is too small ； 5) Feed pump recirculation open circuit ； 6) Front pump failure.

(1) Unstable boiler steam pressure. (2) Significant changes in feedwater flow rate. (3) Breakage in the feedwater pipeline. (4) Changes in frequency and voltage. (5) Tripping of the operating feedwater pump or accidental startup of the standby feedwater pump. (6) Incorrect operation of the feedwater pump recirculation valve. (7) Boiler leakage or excessive blowdown. (8) Abnormal operation of the speed-controlled feedwater pump coupling (inverter).

During operation, the pressure gradually decreases; everything outside is normal, and it is usually due to a decrease in the efficiency of the feed water pump. This is mainly caused by the increased dynamic and static clearances in various parts of the pump as a result of prolonged operation, which leads to a decrease in the pump’s efficiency; replacing the seal rings in those parts can resolve the issue. Another cause is wear of the balance disk or bearings, which leads to the center of the impeller shifting away from the center of the outlet, resulting in reduced efficiency; in this case, it is sufficient to readjust the working clearance.

1) Failure in the regulation of the hydraulic coupler； 2) Vaporization of the feed water pump ； 3) Clogging of the feed water pump inlet strainer ； 4) The spoon tube opening is too small ； 5) Feed pump recirculation open circuit ； 6) Front pump failure.

The reasons for changes in the outlet pressure of the feed water pump are: ① Unstable boiler steam pressure. ② The water supply flow rate has been significantly adjusted. ③ The water supply pipe has burst. ④ Frequency and voltage variations. ⑤ The running feed water pump trips or the standby feed water pump starts up accidentally. ⑥ Misoperation of the feedwater pump recirculation valve. ⑦ Boiler leakage or excessive discharge of waste water. ⑧ The coupling of the speed-regulated feed water pump is operating abnormally.

(1) The valve of the outlet pipeline is closed partially, or the pipeline is blocked. (2) The operating point shifts toward higher flow rate and lower head. (3) Speed decreases. (4) Cavitation occurs at the inlet. (5) An internal leak occurs.

1. A sudden drop may be caused by insufficient input power, such as a reduction in frequency conversion, or faults with the coupler, or disconnection of the coupling； It is also possible for the output traffic to increase suddenly, for example, when high-traffic users start working or due to pipe overloads ； It is also possible that the inlet is suddenly blocked, for example, by the falling of the inlet valve element or by large objects clogging the inlet filter. But all of the above issues will be clearly reflected in parameters such as current, vibration, and noise. 2. Gradual decline: The pressure decreases over time during operation, with everything else remaining normal; this is usually due to a decrease in the efficiency of the feed water pump. This is mainly caused by the increased dynamic and static clearances in various parts of the pump as a result of prolonged operation, which leads to a decrease in the pump’s efficiency; replacing the seal rings in those parts can resolve the issue. Another cause is wear of the balance disk or bearings, which leads to the center of the impeller shifting away from the center of the outlet, resulting in reduced efficiency; in this case, it is sufficient to readjust the working clearance. 3. If pressure drops after maintenance, follow the checks outlined in point 2 as a main guide; also, do not overlook the points mentioned in point 1. This answer was recommended by a netizen

Answer: The reasons for the variation in the outlet pressure of the feed water pump are: ① Unstable boiler steam pressure. ② The water supply flow rate has been significantly adjusted. ③ The water supply pipe has burst. ④ Frequency and voltage variations. ⑤ The running feed water pump trips or the standby feed water pump starts up accidentally. ⑥ Misoperation of the feedwater pump recirculation valve. ⑦ Boiler leakage or excessive discharge of waste water. ⑧ The coupling of the speed-regulated feed water pump is operating abnormally.