A review of the impact that erosion, corrosion, and foreign objects can have on a power plant and the prevention strategies that can mitigate those power plant damages.
Peak plant protection
A major part of maintaining a plant at peak efficiency—especially an older plant—is dealing with power plant damages from erosion, corrosion, and foreign objects. All of these can be costly, so here's a closer look at how they happen and strategies for prevention.
Foreign objects
While moisture can be considered a foreign object, the most obvious intruder is dust. While outlet air quality is a major issue for managers, inlet air quality is vital in terms of preventing erosion from particulates. Steve Hiner, chief engineer for the Gas Turbine Inlet Filtration business at Clarcor Industrial Air, told Power Engineering International: "A gas turbine takes in a lot of air, and that air is dirty. Depending on where in the world the turbine is operating, there will be different types of contaminant to consider. That contaminant can cause different problems within the machine—for example, causing erosion if particles are big enough."
Even if particles measure two microns or less, there's a significant chance of compressor fouling if the exposure continues for long enough. Although that doesn't cause permanent damage, it will reduce efficiency over time. Eventually, the turbine will need to be washed, and the downtime required to do that could be costly.
Deeper in the machine, in the hot section that includes the combustion chambers and turbine rotating parts, corrosives can pose a serious threat. This is particularly problematic when contaminants like salt mix with the sulfur in the fuel and cause high-temperature corrosion, which attacks the materials used in the hot section.
Erosion and corrosion
Erosion occurs when an aggressive chemical environment—either highly acidic or highly alkali—combined with high fluid surface velocities wears away a surface's protective scale or coating, according to Polygon Group. Erosion is essentially a type of corrosion, although the term also has an environmental context.
In addition to the chemical erosion type of corrosion, there are two others. One is oxide corrosion—the electrochemical process that occurs when metal is exposed to water and changes in composition. The other is galvanic corrosion, where two different metals come into contact, and the resulting electrical reaction leads to and accelerates damage.
This latter type of corrosion is a particular problem at fossil fuel plants, as they have buried storage tanks and underground pipework that is difficult to access and repair. Joints between different metals can often be a challenging issue, especially in older plants that lack cathodic protection or have undocumented connections.
Newer plants will have the benefit of solid-state DC decoupling devices that function as galvanic corrosion prevention—a clear example of how technology has advanced. Safety legislators should ask: "While what you have was good enough when the plant was built, is it still good enough now?"
In thermal power plants, mixtures of metal and water or steam can lead to problems like carryover, where water solids are carried over with the steam. This impacts valves and turbines and can lead to component failure and eventually shutdown. However, there's plenty that a plant manager can do to prevent and control corrosion.
Prevention strategies
Most corrosion specialists with experience in the power-generation industry suggest using a closed-loop system with generators to reduce humidity levels to 35% or lower. Doing so will help reduce corrosion. In addition, preventing corrosion due to foreign objects, such as salt, basically comes down to adhering to filtration methods properly.
Pipes should also be insulated with jackets or protective coatings. When appropriate, improving pipe design to have better flow characteristics can also aid in corrosion prevention. Protective coatings are a good idea on any component exposed to water, condensation, or the environment outdoors.
Often, the water in power plants contains chemicals or organic agents like anaerobic bacteria, so using a water-conditioning agent can be helpful. Other strategies include using drains or vacuums to prevent water from pooling and using dehumidifiers for air that passes through turbines to reduce relative humidity levels. These methods can often be adopted without spending inordinate sums of money.
It's little wonder then that power plant damages from erosion, corrosion, and foreign objects are a constant worry for power plant managers, especially as their plants approach the end of their design life. However, many of those worries can be reduced by undertaking an evaluation of current filtration policy and regularly inspecting at-risk components.
TAKEAWAY
Perhaps the most obvious preventative remedy against power plant damages is a ruthless inspection regime that consistently tests components at risk from corrosion, even ones that have protective surfaces. Such components include turbines, pipes, ducts, areas with demineralized water, welded areas, and scrubber modules.
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