ACC.02: Guidelines for Finned Tube Cleaning in Air-Cooled Condensers

Dr. A.G. Howell, Riad Dandan, Dr. R.B. Dooley, Oscar Hernandez, David Rettke

Original Issue: November 16, 2018. Revision due: November 16, 2021. Publication in whole or in part is allowed provided that attribution is given to the Air Cooled Condenser Users Group.

Steam condensing in air-cooled condensers (ACCs) takes place in specialized heat exchanger tubes designed for optimal heat transfer. Typically, the tubes are composed of carbon steel with exterior (air-side) aluminum coating/cladding, along with external aluminum fins to increase the air-side surface area for heat rejection. Some ACCs, mostly earlier designs, included carbon steel fins on carbon steel heat exchange tubes, with the entire exterior coated with zinc. ACC tubes are normally oval or rectangular in shape and approximately 0.059” (1.50 mm) wall thickness, with closely-spaced fins to facilitate maximum surface area exposure to cooling air flow. Air flow is provided by large fans typically positioned below the array of finned tubes, directing air upward in a so-called “forced draft” situation, although an induced-draft design with fans above the array has been introduced that has some potential advantages. High-purity steam exhausted from the low-pressure steam turbine with a few percent moisture (condensed water) travels through large ducts at relatively high velocity, generally at 120 - 360 fps (35 - 110 mps). Steam velocity is dependent on the vacuum; upon reaching the condenser tubes, flow is redirected and drawn into the tubes with a 90o turn. As the two-phase steam-water mixture moves down the 33 - 36 foot (10 - 11) meter tube length, the remaining steam is condensed to liquid water and releases the latent heat of vaporization.

Flow-Accelerated Corrosion in Steam Generating Plants

Barry Dooley and Derek Lister

Original Issue: September 27, 2018.

Flow-accelerated corrosion (FAC) has been researched for over 50 years at many locations around the world; and scientifically all the major influences are well recognized. However; the application of this science and understanding to fossil; combined-cycle/HRSG and nuclear plants has not been entirely satisfactory. Major failures are still occurring and the locations involved are basically the same as they were in the 1980s and 1990s. This paper reviews the latest theory of the major mechanistic aspects and also provides details on the major locations of FAC in plants; the key identifying surface features of single- and two-phase FAC; the cycle chemistries used in the plants and the key monitoring tools to identify the presence of FAC. The management aspects as well as the inspection; predictive and chemistry approaches to arrest FAC are described; and the different approaches that are needed within fossil; HRSG and nuclear plants are delineated.

REPORTS

ACC.01: Guidelines for Internal Inspection of Air-Cooled Condensers

Dr. A. G. Howell, Gary Bishop, David Rettke, Rene Villafuente, Dr. R.B. Dooley, Hoc Phung

Original Issue: May 12, 2015. Revision due: May 12, 2018. Publication in whole or in part is allowed provided that attribution is given to the Air Cooled Condenser Users Group.

The employment of air-cooled condensers (ACCs) in fossil fueled power-generating facilities has increased dramatically early in the 21st century. These condensers are massive structures at power plant sites, because a large cooling surface is required to compensate for the relatively poor heat capacity of air, in comparison with that of water. As experience has been gained in operating and maintaining facilities with ACCs, it has become clear that corrosion of steam-side surfaces can be a significant problem for unit operation. In particular, iron oxide transport can introduce a large quantity of contaminants to the condensate / boiler feedwater, and through-wall penetrations of cooling tubes can cause significant air inleakage, potentially leading to reduced condenser performance and permitting ingress of air.

Corrosion in Air Cooled Condensers - Understanding and Mitigating the Mechanisms

Setsweke Phala, Denis Aspden, Dr Francois du Preez, Hein Goldschagg, and Keith Northcott

Presented at API PowerChem 2008, Queensland, Australia.

Abstract:

Corrosion in the air cooled condensers has been a cause for concern for several years in Eskom’s direct dry cooling stations. The exact mechanism of the observed corrosion has not been positively identified but is thought to be a form of flow accelerated corrosion.

Eskom has over the years tried several methods to mitigate the corrosion, such as varying the steam/condensate pH as well as the application of certain so-called protective epoxy coatings. There is clearly a link between the observed corrosion and the prevailing pH. Studies have shown a lower rate of iron transport as the pH has been increased, but this has also had an impact on the operation of the condensate polishers. Significant differences in the nature and extent of corrosion of air cooled condensers have been observed between base loaded and two shift operation of these plants.

Research, including international collaboration continues to develop a better understanding of the corrosion mechanisms at play, together with strategies to best mitigate the corrosion and ensure design life is achieved.

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