2017-6-7 Purification Technology Edition Daily Question

The reasons for changes in the outlet pressure of the feedwater pump during operation are: (1) sudden failure of the automatic feedwater control system, or improper manual adjustment. (2) Fluctuations in plant power supply frequency and voltage. (3) The feed pump recirculation valve opens accidentally, or the feed pump vaporizes. (4) Severe leakage in the feedwater pipes on the machine and furnace sides. (5) Unstable boiler pressure. (6) Boiler leakage, or boiler blowdown. (7) The hydraulic coupling of the feed water pump is not operating properly. (8) The operating feedwater pump trips, or the standby feedwater pump starts up. (9) After the high-pressure heater protection trips, the large bypass of the high-pressure heater fails to open, or the feedwater valve on the boiler side closes accidentally.

The reasons for the 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 feed 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.

⑴Change in the outlet pressure of the feed water pump. ⑵Wear of the balance disc. ⑶The clearance of the throttle bushing in the feed water pump increases (i.e., the radial clearance between the balance disk and the balance ring). ⑷Vaporization of water inside the feed pump.

(1) The boiler steam pressure is unstable. (2) Significant adjustment in feedwater flow rate. (3) Break in the water supply pipe. (4) Changes in the frequency and voltage of the pump motor. (5) Trip of the operating feedwater pump or accidental start-up of the standby feedwater pump. (6) Misoperation of the feedwater pump recirculation valve. (7) Boiler leakage or excessive discharge of waste water. (8) Abnormal operation of the speed-regulating feedwater pump coupling (inverter).

Change in the outlet pressure of the feed water pump. Wear of the balance disc. The clearance of the throttle bushing in the feed water pump increases (i.e., the radial clearance between the balance disk and the balance ring). The pump impeller is defective. The flow in the import and export pipelines is poor. There are problems with the inlet and outlet valves.

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 feed 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 operating point shifts towards higher flow rate and lower head； 2. Motor reversal ； 3. Reduced rotational speed ； 4. Cavitation occurs at the pump inlet ； 5. Internal leakage occurs ； 6. Blockage in the impeller flow channel.

1) The boiler steam pressure is unstable. (2) Significant adjustment in feedwater flow rate. (3) Break in the water supply pipeline. (4) Changes in the frequency and voltage of the pump motor. (5) Tripping of the operating feedwater pump or accidental start-up of the standby feedwater pump. (6) Misoperation of the feed water pump recirculation valve. (7) Boiler leakage or excessive discharge of waste water. (8) Abnormal operation of the speed-regulated feedwater pump coupling (inverter).

1. Shift of the operating point toward higher flow rate and lower head: Under normal circumstances, centrifugal pumps exhibit a continuously decreasing performance trend, with the flow rate increasing as the head decreases. During actual operation, if the back pressure on the pump decreases for some reason, the pump’s operating point 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 time, the problem can be resolved by increasing the pump back pressure, such as by closing the outlet valve. 2. Motor reversal: The motor may reverse rotation due to wiring issues; it is generally necessary to check whether the pump is rotating in the correct direction before starting. If it is reversing, simply swap any two wires to correct this issue. 3. 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 proportional to the square of the speed; it is evident, then, that speed has a significant impact on head. Therefore, a reduction in the pump’s speed will **affect the resulting head. 4. Cavitation at the pump 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 of the pump is too large, internal leakage occurs, which is manifested as a decrease in the pump’s discharge pressure. The pump should be turned on for inspection, and the components that cause an excessive gap should be repaired or replaced. 6. Clogging of the impeller flow channels: If some of the flow channels in the impeller become blocked, it will affect the impeller’s performance and result 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 before the pump inlet.

(1) Sudden failure of the automatic water supply adjustment, or improper manual adjustment. (2) Fluctuations in plant power supply frequency and voltage. (3) The feed pump recirculation valve opens accidentally, or the feed pump vaporizes. (4) Severe leakage in the feedwater pipes on the machine and furnace sides. (5) Unstable boiler pressure. (6) Boiler leakage, or boiler blowdown. (7) The hydraulic coupling of the feed water pump is not operating properly. (8) The operating feedwater pump trips, or the standby feedwater pump starts up. (9) After the high-pressure heater protection trips, the large bypass of the high-pressure heater fails to open, or the feedwater valve on the boiler side closes accidentally.