A power plant being offline for anything other than scheduled maintenance is never a good thing. Corrosion in power plants, however, is one of the one of the greatest risks to an operation because of the generation of steam and surface exposure to hot combustion gases. Using humidity control for protective surface coatings and the regulation of ambient conditions will keep plants online, operational costs lower and workers safer.
Type of Corrosion
Oxidation is the process in which a metal alters its properties to become more stable in an environment. It occurs in the presence of oxygen or moisture, or both. Salts, high temperatures and acids can accelerate oxidation.
Hot, or dry, corrosion is a type of oxidation that occurs in the absence of moisture, when a surface has a thin film of salt in an oxidizing gas coating it. This type of corrosion affects gas turbines, incinerators, boilers and internal combustion engines.
Galvanic corrosion occurs when there are two dissimilar metals or alloys in an electrolyte. In a plant, for example, a humid area with steel pipes and copper fittings could experience this type of corrosion, as one metal would become an anode and the other a cathode, accelerating oxidation.
Erosion is a type of corrosion that occurs when high fluid surface velocities are exposed to aggressive chemical environments. The reaction wears away a metal object’s protective scale or causes a surface coating to fail. As the surface erodes, it begins to thin.
Erosion is a serious problem within turbines, pipelines, valves, heat exchanger tubes and combustion systems. It is one of the main causes of downtime in power plants, accounting for up to 75 percent of the total arrest time. The cost of replacing broken tubes can make up over 50 percent of the total production costs.
Consequences of Corrosion in Power Plants
- Feedwater lines: In 1986, the Surry-2 feedwater line in Virginia failed due to erosion-corrosion, which thinned the walls within a line. The resulting steam explosion in the non-nuclear part of Unit 2 killed seven employees.
- Ductwork: Ducts within a plant direct flue gas from boilers, through flue gas systems and to stacks at high temperatures. The abrasive fly-ash and acids attack the components physically and chemically, leading to oxidation and erosion.
- Stacks: Stacks are vulnerable to the high temperatures and acidic environments if they don’t have protective liners.
- Steel liners: Corroded protective stack liners made of steel allow the escape of environmentally harmful heated, un-scrubbed gases that are likely to cause acid rain, which is a violation of the Clean Air Act.
- Fuel handling areas: Coal is one of the primary fuels in power plants. It is abrasive by nature and leeches sulfurous acid when wet. Therefore, plants must store it in lined areas. If the liners are corroded, the acid in coal might leach out when combined with rainwater.
- Scrubber modules: Scrubbers in flue gas cleaning systems are responsible for removing acids, stripping fly-ash and reducing heat in gas. The harsh environment necessitates protection against corrosion to facilitate an effective scrubbing process.
- Areas that de-mineralized water: Water in the boiler must be free of contaminants. The process to treat the water is aggressive, putting the respective steel and concrete structures at risk of corrosion and failure. Collection sumps are also exposed to caustic and acidic environments created during the de-mineralization process.
- Gas turbines: Cobalt- and nickel-based alloys within fluidized bed combustion (FBC) cogeneration systems are vulnerable to hot corrosion at a wide range of temperatures.
Downtime and Repairs
When a plant is offline, it’s not generating electricity. When it can’t generate electricity, the load becomes greater on other plants in the region. If other plants cannot bear the burden of the extra load, the region might have to purchase power from neighboring areas while the affected power plant undergoes repairs. In the end, preventive measures using humidity control and regular maintenance prove more cost-effective and efficient than repairs.
The Clean Air Act regulates the sulfur dioxide (SO2) and reactive gas emissions that pollute the environment and cause health problems. Some of the primary ways of maintaining compliance with environmental laws are with corrosion prevention and humidity control.
Accidents due to corrosion in a power plant lead to the greatest costs—endangered worker safety and the loss of lives. Smaller accidents most affect employees, while larger failures due to corrosion put the public’s safety at risk.
Corrosion in the power generating industry costs U.S. plants $23 billion annually. From providing temporary humidity control for surface coating applications to creating a safe, comfortable environment for workers, Polygon’s custom environmental control solutions prevent corrosion and lower operating expenses. Talk to a humidity control specialist today to learn more about the benefits of humidity control for your operations.
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