Steam turbines are designed for high energy efficiency, but the turbine blades are subjected to high mechanical stress levels during operation. In addition to planned shutdowns, short-term, medium-term, or long-term shutdowns can occur due to unplanned maintenance or low seasonal power demand. Although the steam used in steam turbines goes through various processing stages, volatile chemicals can accumulate on the blades and other surfaces when the turbine stops. Chemicals such as chloride, sodium, and sulfate can cause corrosion when they come into contact with metal surfaces for extended periods. Chloride ions accelerate corrosion and cause pitting corrosion, shortening the lifespan of turbine blades and other components.
During shutdowns, turbine blades exposed to high humidity levels usually react with oxygen, forming a passive oxide layer on the surface. However, chloride ions penetrate this layer, reaching the underlying metal and causing localized corrosion. This accelerates rust formation and leads to pitting corrosion, where small, deep holes form on the metal surface. The damage caused by high humidity and chemical residues during shutdowns can compromise the structural integrity of the metal, leading to cracks under stress.
Given these concerns, dehumidifiers are used to maintain the relative humidity inside the turbine at 40% RH during shutdowns by enveloping the entire internal volume with dry air. As the relative humidity increases, the non-volatile contaminants in the system accelerate the corrosion process, and the rate of damage increases logarithmically after 60-65% RH. Therefore, integrating dehumidifiers with a proper engineering approach is essential. Ventilation ducts should be designed to create either a "closed" or "open" circuit between the turbine and generator, with automatic dampers and damper motors to control the airflow when the system is shut down. The dehumidifier fans should be selected to provide adequate external static pressure, considering potential sharp turns and obstacles inside the system.
For steam turbine applications, dehumidifiers offer lower initial and operating costs compared to alternative methods (e.g., nitrogen blanket). Dehumidifiers protect components like turbines and condensers from corrosion when the facility is shut down. Using a nitrogen blanket for extended periods is costly and requires additional safety measures due to high nitrogen concentrations. Sending hot air into the turbine can also cause condensation on cold surfaces, accelerating corrosion. Integrating a dehumidifier system with a "closed" circuit design will achieve low dew point values over time, preventing condensation and protecting the internal environment.
Whether for planned maintenance or long-term protection programs, protecting steam turbines with dehumidifiers is a cost-effective and beneficial solution.
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