The problem of overcurrent in the boiler feed water pump
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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, one in use and one as backup; 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). They have been operating normally ever since. Upon resuming operation after a recent shutdown, it was found that Pump B was functioning normally. However, when switching from Pump B to Pump A, it was observed that Pump A was operating at a current that was about 10A higher than its rated value. Additionally, by checking the records, it was noted that when the load remained the same 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 fixed-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 flowing through the bypass route (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?
There are several issues in your description that need further clarification: 1. You mentioned that after pump A started up, the current exceeded the rated value by about 10A; so what exactly was the degree of overloading? Is it higher than 10% or 20%? 2. You mentioned that the feed water pressure of pump A has decreased by 1.26 MPa compared to before; is this decrease related to the load? Does it only occur at low load? 3. You mentioned that the minimum flow valves of the two pumps have internal leakage; is this phenomenon directly related to the overcurrent issue? 4. After changing the control method, the B pump experienced overcurrent; did you rule out the influence of the control method? Have you tried moving the frequency converter from Pump A to Pump B for testing? Based on the information you provided, the following issues may exist: 1. It is possible that the pump, pipelines, or other equipment are severely worn or aged, resulting in increased water flow resistance and thus causing the motor to be overloaded. 2. It may be due to an uneven load distribution between pump A and pump B, causing pump A to work under excessive load and resulting in overload. 3. It is likely that the internal components of the minimum flow valve are worn or damaged, resulting in leakage and thus causing the motor to be overloaded. 4. It may be electromagnetic interference or other issues caused by the inverter control method, which lead to overcurrent in Pump B. It is recommended that you continue to investigate the possible issues mentioned above, and further check the condition of equipment such as pumps and pipelines as well as for any other abnormal phenomena. If the problem persists, it is recommended to seek help from a professional for a thorough diagnosis and solution. .
Thank you for the suggestion! To clarify and provide additional information regarding your question: 1. When Pump A is operating at power frequency (before shutdown), the water supply flow rate is 83.2 T/h, the current is 69.7 A, the outlet pressure is 17.41 Mpa, and the power generation capacity is 33 MW ； When pump A is operated at the power frequency (after shutdown and restart), the water supply flow rate is 83.2 T/h, the current is 80 A, the outlet pressure is 16.15 Mpa, and the power generation load is 32.6 MW. 2. The outlet pressure of Pump A changed before and after shutdown; under the same load, it decreased by 1.26 Mpa, while the current increased by about 10 A. 3. Due to suspicions of a leak, various systems were checked and no external leaks were found; therefore, a test was conducted by closing the minimum flow valve, and the current decrease was not significant (about 2–3 A). It appears that internal leakage in the minimum flow valve has little relation to the overcurrent issue. 4. When both pumps were switched to variable-frequency control, they both operated normally (compared with previously recorded data), but when operated at fixed-frequency control, excessive current was observed in both pumps. Explanation: The pump controlled by frequency conversion cannot increase its output beyond around 32 MW under that load, so a conventional frequency-driven pump is used to further increase the load (the frequency conversion pump is no longer used). 5. The two pumps share the inlet and outlet manifolds; the high and low pressure feedwater heaters are not in use (the bypass route for water flow is used).