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The desired DO operating level in your Condensate/Feedwater will depend on your cycle chemistry philosophy (AVT(O)/(R) or Oxygenated Treatment). Reference EPRI 2011 technical report on Comprehensive cycle Chemistry Guidelines for Fossil Plants for more info. Assuming you have a Deaerator in the system, either on the boiler LP drum or part of the ACC Condensate system, then most likely the DO level should be <7 ppb, as per HEI standard specs for Tray Type Deaerator. We are a 3×1 triple pressure HRSG arrangement, with all ferrous construction in the steam/water cycle, with a tray type vacuum DA on the ACC condensate, and are on a AVT(O) chemistry. Therefore, we target <10 ppb of DO at the Condensate Pump. It is my understanding that with AVT(O) you want some amount of DO (5-20ppb) in the feedwater to prevent FAC. Excursions above these levels are likely a sign of Deaerator performance issues, or excess air-in-leakage on the ACC or associated vacuum pipe network. We recently did an internal inspection of our Deaerator due to very high DO levels (>100 ppb on average) and found that the spray valve and spray header were clogged with tube inserts that liberated from the ACC Dephlegmator (finishing/air removal section) tube bundles. Several spray valves were stuck open or jammed with materials. After clearing all tube inserts from the header and returning all spray valves to good working condition, the deaerator is capable of maintaining below 10 ppb, except when slugged with cold aerated makeup demin water. Ideally cold makeup should be preheated or mixed with the ACC condensate to assist with deaeration performance. What I found is that the Deaerator spray valves functionality is critical to the performance. Good luck.
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