Corrosion and Insulation at Power Plants
Corrosion issues at a power plant can be very costly and, in some cases, life threatening. Not too long ago, three men were killed while working under a steam-generating boiler when a boiler supply tube weld ruptured—due in part to corrosion attack—causing hot steam and ash to fall on them. This fatal incident illustrates the importance of frequent inspections of exposed piping systems and taking the necessary steps to prevent corrosion.
The most common types of corrosion found at power plants are oxide and galvanic corrosion. Both can easily be avoided.
- Galvanic corrosion occurs when two or more different or dissimilar types of metals make contact with each other. Galvanic corrosion has the potential to create a hole in the lagging and allow water to penetrate to the plate or casing.
- Oxide corrosion occurs when lagging or pipe jacketing is improperly stored at a power plant. Oxide corrosion attacks the surface finish of the lagging, making it susceptible to a future hole as well as permanently damaging its look. Material with oxide corrosion must be discarded.
Plate corrosion is estimated to cost the power industry millions of dollars in repairs annually. Left unattended, it will always lead to gas or air leaks. The root cause of plate corrosion can be attributed to a failed lagging system, so potential corrosion areas can easily be prevented by periodic inspection of the installed outer lagging. A properly installed outer lagging system will keep water from getting to the plate or casing. While the chances of plate corrosion causing the death of someone working at a power plant, like those three unfortunate men, are minute, a hole in the casing or plate could potentially harm someone walking by the area.
Corrosion on piping systems that carry hot water or steam (above 150°F) can cause serious injury or death if the weld or pipe fails. Preventing corrosion on these systems is normally achieved by using conventional thermal and personnel protection insulation and jacketing systems. Properly applied insulation and outer finish or jacketing keeps water from coming in contact with the pipe or tank shell, preventing corrosion.
For power plants whose operating temperature is above 150°F, the key to preventing most plate and pipe corrosion issues is to properly protect them from water. Water is the medium that causes the electrochemical mechanism to occur that leads to pitting and eventually to holes or cracks on the substrate surface.
The problem is that a power plant has many hot and cold piping systems that are not insulated. Hot piping systems are not always insulated if they are not outside the boiler or exposed to plant personnel. Hot piping systems that do not require insulation are normally inside a vestibule or penthouse enclosure whose outside walls are externally insulated and lagged. These piping systems would, under normal conditions, not be exposed to the weather and would remain free of corrosion.
Unfortunately for the men who lost their lives, the common practice of water washing down the boiler and improper refractory maintenance can lead to pipe and plate corrosion. Improper boiler maintenance and water washing practices can allow water to enter the vestibule or penthouse and begin the corrosion process. The only way to avoid this corrosion is frequent pipe inspections inside the vestibules, proper boiler maintenance, and judicious water washing practices.
A typical power plant has many cold piping systems operating below 150°F (the temperature considered “cold” may be lower in other industries). These low-temperature piping systems are found beyond the boiler island equipment and require special attention to prevent corrosion. Under normal operating conditions, these piping systems would not require insulation. The problems arise with condensation, which is related to dew point, the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into water. The dew point is a saturation temperature; it can change from one region or area to another and is also affected by humidity.
Un-insulated piping and tank systems can be attacked by corrosion due to condensation. To prevent this “dew-point” corrosion, the exposed piping or tank system should be covered by either a moisture-resistant paint or insulation and a weather-preventive finish (e.g., 0.016 in. aluminum jacketing). Although people may not associate insulation with preventing corrosion, insulation with a weatherproof finishing material is a good alternative to painting and is easily maintained.
Corrosion issues can be avoided, and the tragedy of those three men losing their lives should never have occurred. Oxide, galvanic, plate and pipe, and dew-point corrosion can easily be avoided, beginning with frequent plant inspections inside and outside the boiler and proper boiler maintenance and water washing practices.