Condenser Back-Pressure Excursion During Summer Peak Load
A 250 MW reheat steam turbine is running at 235 MW on a hot afternoon. Condenser vacuum has degraded from 5.5 kPa to 9.8 kPa absolute over four hours. Circulating water inlet temperature is 31 °C; design inlet is 22 °C. The operator notes rising exhaust hood temperature, a slowly falling load demand signal that has not been acted on, and that the turbine last-stage buckets are at their exhaust limit. Circulating water pump B tripped earlier and was not restarted; pump A is at full flow. Condensate subcooling has increased to 4 °C. The control system shows exhaust hood spray valves closed. Grid frequency is nominal and the dispatcher wants full output maintained for another two hours.
1. Diagnose the cause of the vacuum loss and explain the thermodynamic and mechanical consequences of continuing at current load.
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The vacuum loss is almost certainly caused by reduced circulating water flow after pump B tripped, compounded by the high inlet temperature of 31 °C. With only one pump, the condenser cannot reject the same heat load, so saturation temperature and pressure rise. At 9.8 kPa absolute the saturation temperature is roughly 45 °C, versus about 35 °C at 5.5 kPa, so the available enthalpy drop across the last stages shrinks and the exhaust steam becomes wetter and higher in volume flow.
Mechanically, higher back-pressure means higher exhaust hood temperature and greater last-stage bucket loading; the machine is already at its exhaust limit. Continued operation risks blade flutter, exhaust hood overheating, and differential expansion changes as the rotor and casing heat unevenly. Thermodynamically, heat rate worsens because the cycle loses useful work from the low-pressure turbine. The rising subcooling suggests the hotwell is also being affected. This is not a condition to ride out at 235 MW.
2. State your immediate actions and the operating limit you would impose, with justification.
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First, reduce load to bring exhaust hood temperature and last-stage loading back inside limits; a typical protective action is to run back toward roughly 70–80 percent load while monitoring exhaust hood temperature and differential expansion. Second, attempt to restore circulating water flow by investigating the pump B trip and restarting it if safe; verify pump A strainer and discharge pressure. Third, confirm exhaust hood spray is available and verify its supply pressure, since spray valves are shown closed. Fourth, notify the dispatcher that full output cannot be sustained and explain the vacuum limit.
I would impose a hard limit based on the turbine exhaust pressure alarm and trip setpoints, not on the dispatcher's request. If back-pressure continues rising toward the high-exhaust-pressure trip, I would trip the unit deliberately rather than let a protective trip occur at an uncontrolled moment. The justification is that blade and exhaust hood damage is cumulative and the trip envelope is a design boundary, not a target.
3. What monitoring parameters and post-event records would you require before returning to full load?
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Before returning to full load, require stable vacuum at or below the alarm value with both circulating water pumps available, exhaust hood temperatures steady and below alarm, and differential expansion and rotor position within normal bands. Review condenser tube cleanliness and check for air in-leakage using the vacuum decay test, because the subcooling rise hints at either air ingress or tube fouling. Trend heat rate and compare against the corrected curve for the actual circulating water inlet temperature.
Record the event timeline, back-pressure trend, load history, pump trip cause, and all alarm and trip setpoints reached. Confirm last-stage bucket inspection requirements against the manufacturer's guidance for operation above the exhaust-pressure limit. Only after these checks and a documented management-of-change review would I authorize sustained full load.