What is boiler out-of-service corrosion? How did it come about? How to prevent or mitigate out-of-service corrosion?

Thread Content

Corrosion damage that occurs when a boiler is in cold standby, hot standby, or during overhaul is called out-of-service corrosion. Boiler out-of-service corrosion is mainly caused by the incomplete drying of the inner metal surface and the continuous leakage of oxygen from the atmosphere during the shutdown period. Although corrosion occurs both internally and externally on the heating surface of the boiler during shutdown, internal corrosion is more serious than external corrosion, so out-of-service corrosion mainly refers to internal corrosion of the heating surface. After the boiler is shut down, as the pressure and temperature decrease, the water vapor in the boiler condenses into water, a vacuum will appear inside the boiler, and external air leaks into the furnace. Oxygen, in the presence of moisture and water vapor, can easily corrode metal. Due to structural reasons of the boiler, such as the use of a vertical superheater, it is impossible to drain the water in the superheater. Therefore, if certain measures are not taken, deactivation corrosion is inevitable. When there is water-soluble salt scale inside the metal heating surface, it will form a highly concentrated salt solution when it absorbs water, which will aggravate the out-of-use corrosion and form internal ulcer corrosion. This situation often exists at the superheater inlet. Because the separated saturated steam always carries a small amount of boiler water when entering the superheater, the saturated steam absorbs heat after entering the superheater, the boiler water evaporates into steam, and the salt contained in the boiler water is deposited on the inner wall of the superheater inlet tube. In order to prevent or mitigate out-of-service corrosion, shutdown protection measures should be adopted. There are many methods for furnace shutdown protection, which are mainly divided into two categories:: Wet protection and dry protection. Wet protection is often used when the outage period is short, such as less than a month. The commonly used method for wet protection is to fill the furnace with oxygen-free water or alkali-containing aqueous solution and maintain a pressure of 0.3 to 0.5MPa to prevent air from leaking in. If the shutdown time is long or the weather is cold, dry protection should be used to prevent freezing damage to the equipment. The method is to drain all the boiler water when the water temperature drops to 70 to 80°C after the boiler is shut down, use the waste heat of the boiler to evaporate all the water in the heating surface, and use compressed air to blow away all the undried water vapor in the furnace, and then place a container containing anhydrous calcium chloride in the steam drum. Add calcium chloride at a ratio of 0.5 to 1.OL per cubic meter of water volume (if quicklime is used, it can be calculated as 2kg/m3), then seal the manhole and check regularly. If the desiccant is found to be powdery, it must be replaced. It can also be filled with ammonia or nitrogen for protection. The preparation work is the same as using desiccant. Because ammonia is lighter than air, when filling with ammonia for protection, ammonia should be introduced from the top and air should be exhausted from the bottom. Nitrogen is slightly lighter than air. Nitrogen can also be introduced from the top and air exhausted from the bottom. In order to prevent air from leaking in, the pressure should be maintained at 0.3~0.5MPa. When the pressure decreases, ammonia or nitrogen should be replenished in time. Boiler emergency shutdown operation steps