Survey results – major problem issues with ACCs item 1

[Andy Howell]

A survey was provided for persons familiar with ACCs in March, 2025. The results were presented at the 2025 ACCUG annual meeting in Granbury, Texas (see https://acc-usersgroup.org/wp-content/uploads/2025/09/04-Survey-Primary-Problem-Issues-with-ACCs.-Andrew-Howell-Andy-Howell.pdf for the summary report).

While about 17 issues were identified, the top four were as follows (in no particular order):
– Loss of generating capacity in hot ambient conditions
– Loss of generating capacity and fan / gearbox damage due to wind
– Gearbox issues
– Air in-leakage

This communication is intended to initiate interactive conversation regarding first item listed, “Loss of generating capacity in hot ambient conditions.” Separate threads will be initiated to address the other three “top 4” concerns. Of course anybody is welcome to post separate enquiries or initiate conversation on any of the 17 topics identified, or on any other ACC issue.
To start this topic, please consider the following questions. Expansion of the thread by responding with additional questions is encouraged.

1. What is the MW capacity of your steam turbine?
2. At what ambient temperature will your steam turbine begin to lose generating capacity?
3. Please provide any typical data correlating ambient temperature and lost MW, including the typical maximum temperature at your plant and the corresponding maximum MW loss.
4. Please provide operating backpressures that correspond to the data in item 3.
5. Describe actions taken to manage high backpressure in the ACC as temperatures approach and exceed problem levels.

Hopefully this discussion can develop into an understanding of best practices undertaken at other plants. After about 2 weeks I will follow up with enquiries about options considered and/or undertaken to fully or partially resolve the issue of generation loss with high ambient temperatures.

Andy Howell
Aghowell19@gmail.com
303-870-9785

[bdelcorio]

1. What is the MW capacity of your steam turbine? single shaft 620 NMw
2. At what ambient temperature will your steam turbine begin to lose generating capacity? 80 deg F
3. Please provide any typical data correlating ambient temperature and lost MW, including the typical maximum temperature at your plant and the corresponding maximum MW loss.
80 deg F – 25 NMw derate
85 deg F – 50 Mw derate
9p deg F – 65 NMw derate
4. Please provide operating backpressures that correspond to the data in item 3.
All cases above back pressure at 13 In Hg
5. Describe actions taken to manage high backpressure in the ACC as temperatures approach and exceed problem levels..
Installed Gale Breaker wind screens, impact no noticeable, EPC skipped the upper perimeter wind screen which provided most the benefit during FEA modeling
Installed air removal piping orifice to balance air removal and prevent recirculation of air into other HX. No noticeable improvement.
Performed air in leakage testing, very tight, less than 5 SCFM non condensable ejection. No noticeable improvement.

Other degradation in performance corelated to increase windspeed and to higher solar radiation. 20 mph increase in wind speed usually produces a 2 In Hg increase in BP. Solar radiation increase of 70 w/m2 on low windspeed increases BP by 3.5 In Hg.

Other ACC issue includes excessive fan bridge vibration; gearbox oil breakdown. Performed FEA on ACC structure, including modal analysis, to develop a fan bridge modification to cut seismic vibration by 40%. Greatly reduce gear box seal leaks over time.

[Author]

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