Parallel circulation water pump is experiencing overcurrent, seeking help
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In a petrochemical circulating water system, the pumps are experiencing overcurrent. There are four circulating water pumps, with three in operation and one as a backup. Pump parameters: Q=1500~2200 m3/h, H=57~48 m. Motor parameters: 6KV, 50HZ, 400KW. Inlet pipeline diameter: DN700; outlet pipeline diameter: DN600; main pipeline diameter: DN900. Operation status: When one valve is fully open (100%), the outlet head is 0.4 MPa. When one valve is opened to 85%, the outlet head is 0.42 MPa. When one valve is opened to 50%, the outlet head is 0.50 MPa. For the two valves that are not fully open, an increase in their opening degree results in overcurrent. Under the current operating conditions, the flow rate in the main pipeline is 6000 m3/h. Experts, please help analyze the cause of this issue.
Is the overcurrent applied to everything as a whole, or just to individual units?
Did this situation exist from the beginning, or did it arise as a result of technical upgrades or changes?
Newly installed equipment, or modified old equipment, or sudden occurrences. Also, does the overcurrent refer to the valve that is opened?
What are the currents of the three pumps respectively?