The issue of low output from the condensate pump under low-load single-cycle operation of gas turbines

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During the commissioning phase of the gas turbine, the unit operated at low load in a single cycle; the pressure in the condensate main was normal at 2.4 MPa, with a flow rate of 420 t/h. At that time, the pressure in the condensate main pipe was 1.2 MPa and the flow rate was 320 t/h. The level of liquid in the condensate sump was 1.2 m high, and the temperature of the condensate was 52 degrees Celsius. The standby condensate pump was started; after it began operating, both the flow rate and pressure increased. After 40 seconds, both the flow rate and pressure dropped significantly, and the current consumption of the standby pump also decreased. When the flow rate in the condensate main pipe dropped to 370 t/h, the operators stopped the standby pump. Eventually, the pressure in the condensate pump main pipe stabilized at 1.4 MPa, while the flow rate stabilized at 340 t/h. Finally, the gas turbine reduced its load; after the condensate pit’s condensate supply and discharge were adjusted so that the temperature dropped to 46 degrees Celsius, the load was increased again, and the pressure and flow rate in the condensate pump main pipe returned to normal. 1. The inlet filter of the condensate pump was cleaned before the unit started up on that day, and the pressure difference across the inlet filter of the condensate pump during its operation did not exceed 4.5 kPa. 2. A rough calculation shows that the net positive suction head of the condensate pump is sufficient. 3. The operators checked for issues such as air leaks on site before starting the backup pump. 4. The pump that is in operation is a conventional frequency pump, while the backup pump is one with permanent magnet speed control. 5. The current consumption of both condensate pumps while they are in operation is lower than the normal operating current. I suspect that the issue was caused by insufficient flow at the inlet of the condensate pumps, but I’m not sure what could be causing the low pressure at the outlet of these pumps. I’d like to hear what others think