Condenser pressure rise during a controlled power ascension
The unit is being raised from 70% to 90% power at a controlled 3% per hour. At 78% power the condenser pressure has risen from 4.9 kPa(a) to 7.1 kPa(a) in 50 minutes. Vacuum pump A is running and B is in standby. Condensate dissolved oxygen has climbed from 8 µg/kg to 38 µg/kg; the chemistry manual sets an action level of 20 µg/kg and a limit of 30 µg/kg. Hotwell level is stable at 62% and condensate pump discharge pressure is steady. Condenser air-removal section differential pressure is normal. Cation conductivity is 0.09 µS/cm and sodium is 2 µg/kg, both normal. Circulating water inlet temperature has risen 1.2 °C, matching the ambient rise. Dispatch expects the unit at 90% by 18:00 and the shift supervisor asks you whether the ascent can continue.
1. Determine whether the circulating water temperature rise explains the condenser pressure rise. Show your reasoning and state what the unexplained portion indicates.
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Do the comparison in saturation terms, not in pressure alone.
- 4.9 kPa(a) corresponds to a saturation temperature of roughly 32.5 °C; 7.1 kPa(a) corresponds to roughly 39 °C. Condensing temperature has therefore risen about 6.5 °C.
- Circulating water inlet rose only 1.2 °C. Even allowing the terminal temperature difference to stay constant (a conservative assumption - it normally improves at higher load only slightly), a 1.2 °C rise in cooling water can move condenser pressure by only a few tenths of a kPa, roughly 0.3–0.5 kPa, not 2.2 kPa.
- Roughly 6 kPa of the rise is therefore not explained by the cooling water. In a condenser, that gap behaves like a non-condensable partial pressure: air is accumulating and blanketing tubes.
- The dissolved oxygen jump from 8 to 38 µg/kg independently confirms air ingress into the cycle; a tube leak would normally show rising cation conductivity and sodium, and both are normal.
Conclusion: this is air in-leakage, and the in-leakage is exceeding what one vacuum pump can remove. Continuing the ascent would raise condenser duty and worsen it.
2. Recommend a course of action to the shift supervisor, including what you do with the ascent, the vacuum pumps, and the chemistry limit.
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Recommendation: stop the ascent and hold at 78%, then decide whether to reduce further.
- Continuing to 90% adds heat load to a condenser already carrying an air blanket, which raises back pressure, reduces efficiency, and increases the oxygen ingress that is already above the 30 µg/kg limit. The chemistry limit is not a target to be negotiated against the dispatch schedule.
- Start vacuum pump B and confirm air-removal flow; if pressure recovers with two pumps, that tells you the in-leakage is moderate and one pump was simply insufficient.
- With pressure held and dissolved oxygen still above 30 µg/kg, reduce load further until oxygen returns below the action level, and inform chemistry so they can adjust and trend.
- Begin leak detection: helium or ultrasonic survey of the vacuum boundary, focused on expansion joints, manhole gaskets, vacuum breaker seats, condenser neck flanges and the low-pressure heater/condenser connections. Foam testing of flanges is useful where access permits.
- Notify dispatch with a technical justification, not an apology: the constraint is condenser back pressure and cycle chemistry, and the unit can return to the ascent once oxygen is back within limits and the leak is found or isolated.
3. Two hours later the leak has not been located, but oxygen is 24 µg/kg and condenser pressure is steady at 7.0 kPa(a) at 78% power. What is your recommendation now, and what would you document?
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Steady at 24 µg/kg with a stationary back pressure is an improvement but not a resolution: oxygen is still above the 20 µg/kg action level and the leak source is unknown. I would recommend holding at 78% rather than resuming the ascent, and setting an explicit review trigger.
Reasoning: the parameter is stable only because load is stable and both vacuum pumps may be running. Resuming the ascent re-tests the same boundary with more heat load and more condensate flow; if oxygen crosses 30 µg/kg again you have consumed the margin and will be reducing load anyway, having also exposed the cycle to additional oxygen ingress. Corrosion product transport and crud behaviour are affected long after the oxygen reading normalises.
Documentation: the pressure and dissolved oxygen trend with timestamps and load, the decision and its basis (chemistry action level, dispatch request), vacuum pump lineup, leak-detection efforts and results, notifications made to chemistry, the supervisor and dispatch, and the stated criteria for resuming the ascent (oxygen below 20 µg/kg sustained for a defined period at stable load, leak found or a justified leak rate). Record it as an operating decision with a technical basis, not as an unexplained hold.