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Condenser Pressure and Steam Turbine Output
Condenser pressure shift with no change in steam turbine output.

I am working on troubleshooting a condenser (or plant issue) which is showing some conflicting symptoms.

This is a 2x1 Combined Cycle unit powered by 2 Siemens V94.2 Gas Turbines and a 188 MW output steam turbine.

The main condenser has indicated high condenser pressure overall. This is believed to be the result of air in-leakage and problems with the air removal system. No problem there.

The issue is that at times, for about 24 hour periods, condenser pressure increases 0.6 inHga (20 mbar). This can be seen in multiple pressure indications as well as LP exhaust and condensate temperatures, so the readings are real. The problem is there is 0 shift in the steam turbine output. This happens at both stable loads and during transient conditions. As many of you will know, a lower condenser pressure means more work from the turbine, so why are we not seeing any shift in ST output? At a 0.6 inHga pressure shift, we would expect to see some sort of reaction from the turbine, but there is none.

We think the reason for the pressure is due to a bad vacuum pump, but that does not explain why ST output would not shift with the changing condenser pressure at that magnitude.

Any thoughts or insights would be appreciated. The control system should be putting out whatever the steam turbine can handle. The only thing I can think of would be a cycle isolation issue, but the plant seems confident the very high condenser pressures occurrences are due to a bad vacuum pump, which once again is fine, but the ST output should indicate some sort of shift.

Thank you all in advance for your time!
Jon