How to improve ACC performance

The large majority of powerplant O&M personnel know that main-condenser vacuum is a primary driver of Rankine (steam) cycle efficiency. The lower the pressure downstream of the steam turbine, the more productive work the unit can do. Most also know condensers may be water- or air-cooled. Today, all but about 1% rely on water as the heat-rejection medium.    

However, the number of air-cooled systems continues to grow because they often are the most practical condensing technology for new sites affected by one or more of the following conditions, among others:

      • Inadequate supply of cooling water.

      • Regulations that militate against once-through cooling.

      • Where the presence of a tower plume (or fog) is a roadway hazard or viewed negatively by farmers and/or homeowners.

The ACC Users Group, established in 2009 by NV Energy and CCJ to foster collaboration among owner/operators of powerplants with air-cooled condensers, meets annually and has an active forum and library to share information on design, operation, and maintenance, as well as best practices and lessons learned.

Next meeting: October 2 – 7 at the Westin Las Vegas. Have information to share? Contact Chairman Andy Howell of Xcel Energy. Questions regarding registration and sponsorships, contact Sheila Vashi.

The upcoming meeting features focus sessions on performance and inspections, given the enthusiasm for these topics at the 2016 conference. Here’s what Chairman Howell had to say at last year’s meeting: “Basically, ACC performance is limited by ambient temperature and tube fouling, parasitic load demand, and potentially by non-optimal ACC design.” The outcome of these limitations, he continued, is increased cost of generating electricity, largely attributed to increased fuel use. “Highly significant,” said Howell, “is high ambient temperature, which can reduce power generation by 10% to 15%.”

ACC design factors therefore include ambient conditions (wind and temperature), anticipated load demand (both internal and external), the steam distribution system, tube design including number of rows, and the fans. Performance can be optimized by numerous design, operation, and upgrade options.

Standard operational improvements available to deck-plates personnel include external tube cleaning, air in-leakage control, air flow management, and spray enhancement (fogging). Implementation of one or more of these solutions typically can deliver the expected results.

Occasionally, the standard performance-enhancement options cannot meet plant objectives and a more complex solution may be necessary. In one example, Howell offered an economic option for large plants in hot weather, especially those with high power replacement costs and adequate water supply: Hybrid cooling. It involves adding a wet cooling system in parallel with the ACC.

Howell explained the hybrid system in detail, including the complete steam cycle and condensate/cooling system. The example given uses a two-cell wet system to supplement the ACC at a combined-cycle plant in Mexico.

A second retrofit example described the addition of a third street of cells to an existing ACC at a nominal 80-MW coal-fired facility in Wyoming. After this street with five larger fans was added, the plant measured 7 MW of additional output at 97F ambient. However, power production decreased in cold weather (less than 32F) due in part to more conservative unit operation.

Howell then addressed fan and motor upgrades, which would be discussed in detail later in the conference. Simply put, if there is no water source to even consider hybrid cooling, another option is to increase air flow through the fan system.

In the example presented, motors were increased from 100 to 200 hp which also meant replacing electrical switchgear, cabling, motors, gearboxes and fans, and conducting detailed structural analyses for load-bearing and resonance issues. For this 100-MW plant, 15 new motors, gearboxes and fans were selected, leading to these results:

      • Improved ACC performance under adverse ambient conditions.

      • No additional water required.

      • Increasing the number of blades to nine from four minimized vibration and resonance issues.

      • Air flow increased from 542 m³/s to 730, static pressure from 71 Pa to 125.

      • Parasitic load increased from 1.12 to 2.24 MW because of the larger fan drive system.

Significant performance results included:

      • Removal of backpressure limitation (sustained improvement at 3.5 in. Hg abs).

      • Increase in power output (could increase condenser load; steam flow through the turbine).

      • Improvement in heat rate (lower condenser pressure/backpressure on the steamer—so-called “free power”).

Howell then highlighted the presentation conclusions:

      1. 1. ACC performance is critical to low-cost unit operation.

      2. 2. Initial ACC design is critical to achieving suitable unit-specific baseline performance.

      3. 3. Careful and consistent operating practices can optimize unit performance.

      4. 4. Retrofit options can improve performance and reduce plant operating costs.

Discussion on tube cleaning began in earnest with a presentation by Huub Hugregtse, of ACC Team Technology. “Fouling,” he explained, “consists of fibers, dust, pollen, and other materials on ACC bundles restricting air flow, reducing heat transfer, and increasing backpressure at the turbine exhaust.” Such fouling normally has a fairly weak bond and often can be removed by spraying with high-pressure water.

Scaling is more difficult. A layer can be deposited on the fins by fumes, spraying water with high dissolved solids on the bundles, or gearbox oil leakage. This layer cannot be removed by water spray alone. Some types of scale have a strong bond from limestone deposits, oil contamination, or chemical fumes mixed with the cooling air. This must be dissolved in water or with chemicals.

After the use of chemicals, solvents and/or blasting with sodium bicarbonate, the residual deposit must be removed by high-pressure cleaning. If residue remains, it can become baked onto the finned tubes, forming a hard layer.

Cleaning is normally required when the internal static pressure rises to a certain level because of restrictions in the spaces between fins, normally detected by high condenser backpressure. Levels are site-specific.

For normal fouling, most cleaning equipment is lightweight and easy to handle. Hubregtse concluded with examples of automatic cleaning systems, noting that most of the setup costs are in the rails. These systems, he said, are relatively simple and moderately priced. Automation, setup time, and cost discussions followed.

Willis Shook of Conco continued with three detailed case studies on water-wash tube cleaning, emphasizing that it is not unusual for an ACC project to encounter the unexpected.

To illustrate: The Yellowstone Power Plant ACC experienced severely leaking tubes. Ambient temperatures covered an extremely wide range, and local industries contributed to fouling. The ACC for the 65-MW facility has eight condensing cells and two reflux cells. Each module contains one fan and six finned-tube bundles. Each bundle has 211 coated carbon steel oval tubes in a three-tube arrangement. Some tubes had been frozen and damaged by debris trapped behind support beams.

Holes also had developed in the top of the tube connections to the steam header, and at the lower condensate header connections (reflux cells)—caused by corrosion from washing with poor quality water.

Yellowstone solutions included the following:

      • Use of underground cable shrink wrap and aluminum duct tape to patch major leaks in tubes.

      • Use of sleeve inserts and outer sleeves to fix tube-to-header connection leaks.

      • Sandblasting and pressure washing of tubes to remove debris from finned areas.

      • Use of epoxy paints to help preserve and close pin-hole leaks in steam header and condensate header connections.

      • Wind fence.

      • Upper wind wall.

Outer tubes were removed for access to inner-row tube leaks. Tube sleeve inserts and outer couplers were installed. Condensate holes were repaired and coated with epoxy paint for added protection.

Air in-leakage testing. Conoco Systems’ Andy Leavitt, followed Shook, covering leak detection equipment and set-up, air in-leakage indicators, and testing challenges specific to ACCs. Standard in-leakage indicators are high backpressure, dissolved oxygen, and continuous hogger use  

His message of caution: The leak is not always in the ACC. It could be in the hogger or hogger exhaust, the gland-seal drain/trap, the crossover bellows, or base weld leaks in retrofit projects. “But even more frightening,” he said, “is finding leaks in new units.” Discussions followed on the best media. “Helium is the best tracer gas,” he concluded.

Fogging enhances dry cooling. Hubregtse returned to the podium and presented a case study on spray-enhanced cooling to improve ACC performance during high ambient temperatures.

Some methods include wetting the heat-transfer surface (more common in Europe) but this can also cause corrosion of the tube bundles, damage to motors and gearboxes, algae on the structure and equipment, and limestone fouling when using hard water.

Fogging is often preferred. A fogging mist is released into the air flowing through the fans and tube bundles. The mist evaporates quickly, cooling the air. The only requirement is a relative humidity of around 50% to 60% (allowing evaporation to take place). The maximum quantity of water used is that needed to achieve 95% to 100% relative humidity.

Hubregtse presented a practical example using a four-cell ACC, ambient temperature of 86F, and relative humidity of 50%, referencing the Mollier diagram. Results included a boost of 2.21 MWh per day.         

Also for this example, evaporation required water droplets of between 25 and 40 microns, calling for either an atomizing or a high-pressure spray nozzle system. The latter, with its large number of nozzles, is considered the most efficient.

He then reviewed, in detail, specifics on nozzles, sensors and controls, and water quality. Soft water must be used.

Hubregtse also stressed the impact on the gas turbine. “If the backpressure at the turbine exhaust reaches the trip point during high ambient temperatures, fogging can lower the vacuum enough to prevent this.”

In the discussions, Hubregtse confirmed that evaporation takes place before entry through the tube bundles, so there is no fouling impact.

Posted in STUG |

Comments are closed.