Condenser Vacuum Low: MFT Is Protection, Not Control

After the protective path is restored, condenser pressure reaching the configured 500 mmHg abs threshold initiates the MFT action and stops continued heat input from fuel. The trip protects a condenser that cannot dissipate the bypass steam heat load with the existing circulating-water flow.

Pressure and vacuum terminology

The term here means condenser absolute pressure, not a vacuum gauge reading. Absolute pressure increases as vacuum deteriorates. A statement such as “condenser vacuum very low” therefore describes high condenser absolute pressure.

Keep 500 mmHg abs in absolute-pressure units throughout the test. Comparing that threshold with a gauge-vacuum indication without converting both values to the same reference can reverse the apparent trip direction.

Heat-load mechanism

During a turbine trip, the turbine stops accepting its normal steam flow. If bypass operation continues dumping steam into the condenser, the condenser must reject that steam’s heat through the circulating-water system. Condensation weakens when the heat-transfer surface, cooling-water temperature, or cooling-water flow cannot carry the imposed duty.

The resulting temperature rise raises condenser absolute pressure and reduces vacuum. Continued steam admission then adds heat to equipment already operating beyond its present heat-rejection capability. The pressure switch and MFT path interrupt the upstream fuel input; they do not correct cooling-water performance or create vacuum.

The specified initiating device is a pressure switch installed at the condenser with a stated threshold of 500 mmHg abs. Confirm the switch’s actual operating direction from its calibration and contact test. A label containing only the threshold does not prove whether the logic expects a contact to open or close on rising absolute pressure.

Check 1 — condenser pressure

1. Read condenser absolute pressure from an independent test instrument. Expect the test reference and the plant indication to agree closely enough that both place the process on the same side of 500 mmHg abs. If they disagree, inspect the pressure tapping, isolation position, trapped liquid, leakage, instrument range, and reference basis before proceeding.
2. Compare the reading with operating state. During the turbine-tripped, bypass-operating condition, expect pressure to move toward poorer vacuum if heat input exceeds the duty removable by the existing circulating-water flow. If pressure remains on the normal side of the threshold, proceed to a controlled switch test rather than forcing the process into poor vacuum.
3. Check the direction of change. Expect rising absolute pressure to represent falling vacuum. If the display reports the opposite trend, correct its engineering-unit conversion or interpretation before evaluating the trip.

A valid process reading sends the decision path to the pressure switch. An invalid reading keeps the test at Check 1; logic changes made before measurement correction conceal the actual defect.

Check 2 — pressure-switch and MFT path

1. Apply a controlled pressure input to the condenser pressure switch. Expect the switch to change state at its calibrated operating point associated with 500 mmHg abs. Record both the applied absolute pressure and contact state.
2. Observe the control-system input. Expect the input indication to follow the physical contact without inversion errors, intermittent transitions, or a stale state. If the switch changes but the input does not, trace field power, wiring, terminations, input-channel status, and configured input polarity.
3. Trace the input through the MFT permissive or trip logic. Expect the asserted low-vacuum condition to satisfy the configured MFT condition. If the input is correct but MFT does not assert, inspect bypasses, inhibits, latches, voting logic, and state-dependent permissives shown in the implemented cause-and-effect logic.
4. Check the final trip output. Expect the MFT command to reach the fuel-trip devices required by the plant design. Use a simulation or approved functional test where a live fuel interruption is not permitted.

MFT here means the configured fuel-trip action associated with the condenser low-vacuum condition. Do not treat it as a condenser pressure controller. Defeating the trip because the condenser pressure switch operates during bypass service removes protection precisely when continued steam dumping can pressurize the condenser.

Check 3 — bypass and circulating-water capacity

Use measured flow rather than pump-running status. A running pump does not prove flow through the condenser. Likewise, bypass command alone does not prove steam admission; compare the command, valve feedback, and process response.

Resolving-branch procedure

1. Correct any pressure-reference, process-connection, calibration, wiring, or input-polarity defect found in Checks 1 and 2.
2. Restore the condenser pressure switch to the approved 500 mmHg abs setting and remove unauthorized logic bypasses or inhibits.
3. Correct the condition limiting circulating-water flow or reduce steam dumping according to the plant operating design. Do not raise the trip threshold to accommodate inadequate heat rejection.
4. Function-test the switch-to-MFT chain with the process protected from an unintended fuel trip. Confirm the physical contact, input indication, MFT logic state, latch behavior, and final output in sequence.
5. Test restoration from the trip condition. Expect the MFT latch and associated outputs to remain in their designed safe state until the approved reset conditions are satisfied.

The wrong practice is to validate only the pressure-switch contact. A complete test follows the signal from applied absolute pressure through the final fuel-trip output and also checks that the bypass and cooling-water conditions causing the pressure rise have been corrected.

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