Inspection guidelines for air-cooled condensers help identify, characterize issues

The ACC Users Group, serving owner/operators of air-cooled condensers since its founding in 2009 by NV Energy, published the organization’s first technical guidance document, Guidelines for Internal Inspection of Air-Cooled Condensers, in May 2015. The document concentrates on inspection and characterization of corrosion mechanisms and locations identified in ACCs worldwide; it also addresses equipment and plant interaction, inspection frequency, and safety.

An all-volunteer organization chaired by Dr Andrew Howell of Xcel Energy, the ACC Users Group helps users:

      • Anticipate issues experienced by others.

      • Reduce elements of surprise.

      • Learn new details on perhaps unfamiliar elements of powerplant science.

      • Tap the minds of a growing and increasingly experienced ACC workforce.

      • Recognize and apply related skills—such as safety and personal risk.

Benefit from the industry’s most recent experience with air-cooled condensers by attending the ACC User Group’s Seventh Annual Conference, Sept 21-24, 2015, at the Wyndham Gettysburg (Pa). Tap into the rapidly growing experience with ACCs in China by participating in the International Conference on Air-Cooled Condensers, Oct 13-16, 2015, at the Empark Grand Hotel in Xi’an.

Highlights of the ACC User Group’s technical guidance document on inspection follow:

ACC inspection 1
 ACC inspection 2
 ACC inspection 3
 ACC inspection 4
 ACC inspection 5
 ACC inspection 6
 ACC inspection 7
 ACC inspection 8
 ACC inspection 9

Offline inspection. Corrosion of steam-side surfaces within an air-cooled condenser can be a significant operating problem. In particular, iron-oxide transport can introduce a large quantity of contaminants to the condensate/boiler feedwater circuit. Plus, corrosion can eat through the thin walls of cooling tubes, allowing ingress of air, which adversely impacts condenser (and plant) performance.      

Carefully planned internal inspections of ACCs during unit outages have become increasingly important and beneficial. For example, the tube entry area in Fig 1 reveals serious corrosion, marked by extensive areas of black deposition adjacent to bare metal. When compared with the relatively good condition shown in Fig 2, this signals a major concern within the upper section of the condenser.

Even more serious is the condition shown in Fig 3. This tube entry evidence indicates widespread holes in the tubing and welding, and would receive the highest severity rating of 5 in the index discussed below.

The Dooley Howell ACC Corrosion Index (DHACI) evaluates and defines these and other ACC characteristics and concerns. The index combines alpha and numeric values applied during inspection of the upper and lower sections, as described below:

      • Upper section, including upper duct/header and cooling-tube entries—rated 1 through 5.

      • Lower section, including turbine exhaust, lower distribution duct, and risers—rated A through C.

A lower-duct rating of B is shown in Fig 4, revealing minor bare-metal areas on generally grey ducts. Some “tiger striping” may also appear with darker grey/black areas indicating flow-accelerated corrosion (FAC), and an overall assessment of minor but noteworthy damage.

Fig 5 would receive a higher rating of C for severe local damage. This photographic evidence clearly shows multiple, widespread areas of bare metal in the turbine exhaust and at abrupt changes in flow direction (where steam flow enters a vertical riser from the lower distribution duct). Bare metal areas (white) clearly indicate metal loss.

The DHACI provides a number (1 to 5) and letter (A to C) to describe and rate the ACC’s condition. A rating of 3C would indicate moderate corrosion at tube entries, but extensive corrosion in the lower ducts. Such information is valuable for tracking changes that occur in a particular ACC resulting from changes in steam-cycle chemistry; or, it can be used to compare the status of several ACCs.

Inspection regions: Tube inlet. It is important to inspect the tube inlet region carefully (Fig 6). Bare metal is commonly found here because of the effects of two-phase flow, a turbulent 90-deg turn into the tubes, and (often) a weld lip that increases local turbulence. Note that this is the only region where through-wall penetrations caused by FAC have occurred; the wall thickness of ACC tubes typically is 0.059 in.

The first few inches into certain tubes are commonly found to have patches of black deposit alternating with either bare metal or flash-rusted metal. In some cases, entries closer to the duct inlet are more severely corroded than those further downstream.

In some ACC designs, the trough between tubesheets retains standing water as indicated in Fig 6, demonstrating that offline rusting contributes iron oxide to condensate. Cross-beams above the tubesheets frequently exhibit bare metal on the side facing steam flow, and often tube entries with the most corrosion are located beneath these cross-bars, where turbulence is greatest.

Turbine exhaust. Steam exiting the LP turbine may impinge directly on baffles or ducts; therefore, the area is susceptible to significant FAC. Metal loss and black iron oxide at this location appear similar to that at the turbine exhaust in a water-cooled condenser (Fig 7). Access the full technical guidance document and get the details on other areas of your ACC to inspect.

Inspection frequency. Thorough inspection during ACC construction and installation is highly recommended. Excess weld flux and construction debris have caused difficulties with initial startup, including failure to achieve steam-cycle purity requirements.

Once in operation, the rate of wall loss caused by steam-cycle corrosion typically is not rapid. Through-wall leaks in thin-walled tubes may take a decade or more to develop. Iron transport, on the other hand, will occur virtually from the time of initial unit operation. To establish a baseline and document corrosion-susceptible areas, a newly commissioned ACC should be inspected within the first few months, if possible.

Subsequently, an annual inspection should be adequate, unless specific concerns exist that would require more frequent examination. If the first three or four annual inspections show little change in corrosion location or extent, and there are no significant changes in operating patterns or chemistry control, biannual inspections may be deemed adequate thereafter.

Photographic documentation of cooling-tube entry points and duct surfaces is essential for characterizing internal corrosion. Although any areas that appear abnormal must be recorded, it is important to document the appearance of “normal” areas as well. Items of particular interest include deposit color, areas where color changes, regions that appear to be bare metal, positions of flow disruptions that may have led to flow-accelerated metal loss, and regions showing heavy deposition or significant depth of metal loss.

Safety awareness. Serious hazards exist for persons inspecting ACCs, and careful planning is critical. Fall hazards are of particular concern (Fig 8). The fan deck of ACCs typically is 80 to 100 ft above ground, generally reached by a lengthy climb on a permanent staircase if elevators are not available. Protective rails are ordinarily well placed to minimize risk.

Reaching the upper distribution duct, however, may be more challenging (Fig 9). While some units are constructed with permanent ladders and platforms for easy access to a manway on the side of the duct, others may require scaffolding or a temporary ladder, followed by a difficult climb on handrails to the top to reach a manway. Fall protection is required.

Once on location, ductwork generally is defined as a “confined space,” and breathing air of acceptable quality should be verified before entering a duct; continue to monitor air quality while personnel are inside the duct. Lighting may be poor, with flashlights perhaps the only light source. Temporary LED lighting may be a better alternative.

Certain areas of ductwork offer a slipping hazard when walking on curved surfaces, particularly in wet areas. Sharp edges may be present on support structures, or may develop because of flow-accelerated metal loss. Covers over drain ports may be displaced, resulting in tripping danger; large open drain piping in the lower duct can constitute a fall hazard.

Personnel may move a considerable distance from entry ports during inspection, and the upper ducts in particular have cross-braces that could obstruct personnel removal in an emergency situation. A specific confined-space rescue plan should be in place.

Finally, the complete technical guidance document available on the ACC Users Group website includes additional photographs, configuration and operation discussions, an inspection worksheet for recording DHACI ratings, and a list of authoritative references.

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