The same boiler feed pump: the inverter operates normally, but there is overcurrent when operating at line frequency
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Dear friends from Sichuan: Our company has two boiler feed water pumps of the DG150-130×12 type (head: 1500m； Flow rate: 125 m3/h ； Power: 900 KW. There is one main pump and one standby pump. Pump A is a power-frequency pump (used under high load conditions), while Pump B is controlled by an inverter (used under low load conditions). Both pumps have been functioning normally. Upon resuming operation after a recent shutdown, it was found that Pump B was operating normally. However, when switching from Pump B to Pump A, it was observed that Pump A was drawing a current that was about 10A higher than its rated value. Additionally, by checking the records, it was noted that under the same load before and after the shutdown, the feed water pressure supplied by Pump A had decreased by 1.26 MPa ； Pump A was switched to inverter control, while Pump B was switched to mains frequency control. As a result, Pump A operated normally, but Pump B experienced overcurrent. This indicates that there is no problem with the pumps themselves. Inspection of the minimum flow valves for both pumps revealed internal leakage in each case (when the manual valves associated with the minimum flow valves of each pump were closed, the current of both motors decreased by 2–3 A, and the flow rate increased by 5–7 tons). There were no debris in the inlet filters of either pump; the main feed water was routed through the bypass circuit (the high and low pressure heaters were not in use). No signs of water leakage were found in the pipelines, and the supply voltage was within normal limits. I would like to ask my fellow Sichuanese friends: what other factors could cause the above phenomenon?
Based on the information you provided, there were issues during the switching between Pump A and Pump B. Pump A experienced overcurrent problems after being switched from inverter control to mains frequency control, and Pump B also encountered overcurrent problems after being switched from mains frequency control to inverter control. This may indicate that there are other problems in the boiler feedwater system, such as pipe blockages or valve leaks, which increase the system resistance and thus raise the load on the pump, leading to overcurrent. It is recommended to conduct a thorough inspection of the boiler feedwater system, especially the pipes and valves, to eliminate any potential problems. Additionally, it is also possible to check whether the sliding bearings and mechanical seals of the boiler feed water pump are worn or damaged, as this can also lead to overcurrent problems. .
Question 1: After the modification, does Pump A operate under full load? 2. The inlet pressure decreases while the outlet pressure remains unchanged; the load is increasing, and B is operating under overload conditions. 3. The impeller of pump B may be fouled; open it to check.
Thank you for the suggestion! Perhaps I didn’t explain it clearly enough: Pump A functions normally when operated at variable frequency, but experiences overcurrent when operated at fixed frequency. Pump B also functions normally when used at variable frequency, yet it has overcurrent issues when operated at fixed frequency. Both pumps work fine when operated at variable frequency, but they both show overcurrent problems when operated at fixed frequency. I think there is nothing wrong with the pumps themselves.
Thank you for the suggestion! Previously, when Pump A was operated at the power frequency, its power generation capacity was around 4 MW; now, however, an overcurrent phenomenon occurs once the power generation load reaches around 3.3 MW when Pump A is operated at the power frequency. After switching Pump A from power-frequency operation to variable-frequency operation, it functions properly and can achieve its maximum efficiency (with a power generation load of around 3.2 MW). Previously, Pump B also reached around 3.2 MW when operated in variable-frequency mode ； Now that Pump B is in normal operation at power frequency, overcurrent occurs when the load reaches around 3.3 MW. The two pumps behave the same way when operating at variable frequency (running normally), and they also behave similarly when operating at fixed frequency (both experience overcurrent). Furthermore, the inlet pressure of the pumps did not change much; it was the outlet pressures of both pumps that decreased at line frequency.
Based on the information you provided, pumps A and B experience overcurrent when operated at line frequency, but operate normally when operated at variable frequency. At the same time, pressure reduction is also a common feature under power-frequency operation. This may indicate that at line frequency, there is significant resistance in the system, resulting in an increased load on the pump and thus causing overcurrent. At the same time, due to the reduced pressure, there may be other system-related issues, such as pipe blockages or leaks. It is recommended to check the following issues: 1. Check for any blockages or leaks in the boiler feedwater system. 2. Check whether each valve is operating properly, and whether there are any internal or external leaks. 3. Check whether the boiler and its auxiliary equipment are functioning properly, such as pressure relief valves, control valves, etc. 4. Check whether the pump’s mechanical seal is damaged or worn, and whether it needs to be replaced. 5. Check the liquid saturation of the boiler feed water pump to ensure it is within the allowable range. I hope the above suggestions can be helpful to you. .
Check if there is any change in the pump’s flow rate? According to the pump’s H-Q curve, as the outlet pressure decreases, the flow rate increases and the power output rises (the pump moves away from its operating point), but the power generated decreases; it is clear that the water flow is directed elsewhere, so check the return line and similar components.
Thank you for the suggestion! After both pumps were converted to power-frequency pumps, the current increased at the same flow rate (compared with the operation records before any failures occurred); however, when they were used in frequency-conversion mode, the current did not increase at the same flow rate. All pipelines were inspected, and no signs of leakage were found (there is no flow meter on the pump outlet manifold, only one is installed before the economizer, and there was no increase in the flow rate at the desuperheater outlet).