ACC Cold weather operations – Lowest safe Backpressure settings w/o freezing

[nmatesa]

I am interested in other plants that are in cold weather climate with ACC’s. How low can you operate your backpressure setpoint on the ACC during cold (<32F ambient)conditions when operating in a design base-loaded steam load conditions? Please comment on how much air in-leakage you think you are running with as well, and how you are tracking air in-leakage for your ACC.

We run a 50 fan ACC and have been recently running around 3.0″ HgA setpressure for our design conditions and we are seeing some subcooling and excess cold sections of tubes issues in our Dephlegmator bundles when we get into the 20F range. Not as much of an issue as we approach the 30-32F ambient conditions. Our ACC design specs indicate we should be able to operate at 2″ HgA with reduced steam load down to ~33% of design steam load at 0F ambient conditions without needing to isolate any of our fan rows for reduced surface area. We obviously are not able to run this low. We are trying to benchmark ourselves against other plants to help quantify the impacts of our suspected multiple cumulative small air in-leakage points (which are hard to pin-point) vs. long term permanent fouling impacts on performance, and general design limits and logic automatic protection improvement opportunities

Furthermore, we are changing our operations to try and run at a lowest achievable backpressure setpoint to maximize plant heat rate performance, but are closely monitoring the ACC tube temperatures, condensate, and non-condensable subcooling during this change. Freezing tubes is a major concern for our plant. We are monitoring via IR surveying during subfreezing conditions to evaluate the need to burp air pockets or warm up D-cells.
ACC automated freeze protection logic is being tuned as well to maximize system protections, however instrumentation is limited compared to the large areas of tubes for our unit. Understanding that air in-leakage will create further limitations as air pockets affect the thermal performance of the ACC, we utilize helium leak detection and plan to add thermal imaging reports to help target the helium checks on suspect tube leaks. We survey the ACC annually via helium but still have found it inconclusive or not definitive at finding small individual tube leaks. We have found more substantial leaks on rupture disk and vacuum breaker valves, etc and repaired accordingly.

Please reach out to me if you have similar experience or information related to ACC cold weather operations optimizations.

Noah Matesa
Maintenance Manager
noah.matesa@naes.com

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