For power plants to operate reliably, the components within them must have minimal impacts from their environment, including the air, water, fuel oil, and fuel gas. In addition to controlling sulphuric acid to prevent cold-end erosion and comply with environmental laws, plants must consider hygroscopic salt damage and its deliquescent properties. While hygroscopic salts are integral to the inner workings of a plant, the corrosion that it causes will shorten a station’s life if the components don’t have an appropriate surface coating.
Hygroscopic Cycle Explained
When salts remove moisture from an environment, the reaction is hygroscopic. The hygroscopic cycle is a thermodynamic cycle. It uses a steam turbine to convert thermal energy into mechanical power. It’s similar to the Rankine cycle, as it uses water as the motive fluid, but the hygroscopic cycle uses salts (e.g., chlorine salts) and their properties to promote condensation.
Engineers use hygroscopic salts in steam or boiler generators, where clean steam is released and superheated. The heated steam expands and generates power through the steam turbine. During the boiler blow-down process, the concentrated salts preheat the turbine’s condensate and condense the vapors in the steam absorber.
In the steam absorber, hygroscopic compounds cool outlet steam in a manner similar to absorption refrigerators. An air-cooler dissipates the hygroscopic compounds and condensation. The thermal recovery that occurs during the boiler blow-down allows the efficiency of cycles to improve, producing higher electrical outputs and eliminating the need for cooling water. The use of hygroscopic salts also reduces the capital and operating costs of a plant.
The Effect of Hygroscopic Salts
When hygroscopic salts remain dry, the particles are solid and dust-like. As relative humidity levels increase, the salts may humidify themselves with the available moisture and melt in the absorbed water. Types of salt that deliquesce include calcium, zinc, ammonium chloride, ammonium sulfate, and ammonium nitrate.
Deliquescent salts attack metal in a plant as an electrolyte, leaving troughs and pits in the respective component. Depending on the salt’s composition, iron-sulfate or iron-chloride compounds may form along with hydroxides and iron oxides. As the layers of salt build-up, corrosion progresses and the metal’s surface increases in temperature. When this occurs, the salts aren’t able to absorb humidity as well.
The corrosive effects of hygroscopic salts are generally the result of:
- Turbine steam impurities
- The environment surrounding the turbine
- Poor shutdown practices that cause steam to form on metal surfaces because of high relative humidity levels
- The design and properties of components
Surface Coating for Added Protection
Because many of the metal components in a power plant do not form their own protective coating when exposed to moisture, engineers must apply a surface coating to prevent corrosion. Before applying the protective coating, the metal substrate must be properly prepared. Otherwise, corrosion may occur beneath the protective surface, resulting in flaking and delaminating.
Surface Coating Preparation
Coating manufacturers specify how to prepare substrates and the conditions needed for coatings to cure well. While the directions vary by manufacturer, most state to blast the metal with an abrasive material so it’s free of rust, paint, scale, and other debris. Engineers also need to keep the area at a certain temperature (usually around 50 degrees Fahrenheit or above) and relative humidity level (around 50 percent or lower) to prevent corrosion before applying the protective coating. In some cases, plants can only achieve the optimal environmental conditions with temporary humidity control. Temperature and humidity control is also helpful in regards to curing the surface coating successfully and predictably, as it prevents over-time, re-work, budget problems, and schedule delays.
Corrosion at a power plant due to hygroscopic salts is a time-consuming and expensive problem. Polygon’s dry air method and climate control solutions effectively manage the environmental conditions when preparing and applying protective coatings to any area of a plant. The custom cost-effective solutions keep you on schedule and improve business continuity, risk management, and worker safety. Contact Polygon today to learn more about its temporary climate control solutions and for a free quote.
[Photo from IAEA Imagebank via CC License 2.0]