Understanding Pin Stud Failure in the Power Generating Industry
Studded tubes are found in many types of steam-generating boilers and are used to retain the refractory required inside the furnace wall area and for cooling the refractory surface that comes into contact with corrosive slag. The refractory material protects water wall tubes and pin studs from the environment created by fuel being burned.
When refractory failure occurs, it is a complex problem to solve. When refractory failure occurs in conjunction with a pin stud failure, it is doubly hard to solve. The failure is usually caused by a combination of factors. Finding the root cause of a stud and refractory failure requires an understanding of the slag and boiler environment.
Slag is the formation of molten, partially fused, or re-solidified deposits (ash) on furnace walls. Slag is a function of deposit temperature and deposit composition. Boilers are designed to maintain ash in a fluid state. For slag to adhere to a surface and form deposits, the particles of ash must have a viscosity low enough to wet the refractory-covered studded furnace wall tube. Iron raises all four values of ash fusion temperatures (initial deformation, softening, hemispherical, and fluid). So the greater the iron content in ash, the greater the difference in ash fusibility between the oxidizing and the reducing condition.
The amount of slag deposits formed inside a boiler can also depend on what type of coal is being used. For example, bituminous coal causes low to medium slagging; sub-bituminous coal (such as Powder River Basin or PRB) causes high slagging and has higher moisture content than bituminous coal; and lignite coal causes severe slagging. Along with the type of coal, air and fuel imbalance (stoichiometry) inside the furnace can also cause slagging, especially when the coal being used has a high iron content like bituminous coal does.
Slag deposits on boiler walls not only can cause stud and refractory failure, but can also cause many other boiler problems. Potential problems include the following:
- Reducing furnace heat absorption
- Raising gas temperature at the boiler gas exit, which can affect certain air pollution equipment
- Causing fouling in the convection or heat-recover area of a boiler
- Causing an increase in the spray flow of the attemperator
- Interfering with ash removal
Due to the surface porosity of the refractory, slag can penetrate and cause deterioration of the refractory surface, wearing down exposed pin studs. To prevent this corrosive attack and give longer life to the refractory surface, it is possible to form a “frozen” layer of slag between the refractory surface and the molten slag. This thin, frozen slag layer can only be formed by the combination of the cooling action of studded tube walls, the thickness of the refractory material, and the thermal conductivity of the refractory material. Therefore, the higher the hot face temperatures, the less likely it is that the formation of a frozen slag layer will occur.
When burning coal, these factors inside the furnace can cause the refractory to fail:
- Alkalis in the combustion of coal—such as sodium (Na) or “soda” and potassium (K) or “potash”—can chemically react with the silica found in refractory materials.
- Sulfur found in the combustion of coal can combine with lime and iron oxide, and reduce the strength of a refractory material. In the presence of moisture, sulfur compounds and salts can form sulfurous and sulfuric acids. These acids can react with the “basic” components of refractory materials.
- Hydrocarbons in unburned fuel (ash) can combine with a reducing atmosphere and react with the iron oxides in some refractory materials to form large carbon deposits. These deposits can cause a chemically induced spall on the surface of the refractory.
- Temperature that refractory materials are exposed to can cause refractory failures. When operating temperatures are higher than recommended use limits of refractory materials, it can cause the refractory to melt.
Any number of things can cause a refractory to fail and expose the pin studs to the combustion environment. It is critical to choose a refractory material that can exist inside the furnace environment and not expose the pin studs. However, pin stud failure (when the studs are completely gone in a short time) normally occurs because of a weld failure or chemical attack created by the combination of sulfur and reducing atmosphere found inside the lower furnace. Understanding the welding process helps shed some light on why pin studs fail.
Pin studding is usually done using a semiautomatic stud welder. This apparatus consists of a welding machine, a weld timer, and a welding gun. This allows for the installation of many studs in a short time. A normal studded tube wall surface inside a furnace may have as many as 256 pin studs per square foot that are .375 inch in diameter and .75 inch long after welding. The cost (for material and labor) is almost 40 cents a pin stud, or $102 per square foot. Hand welding of pin studs can cost almost five times this amount. After the pin stud has been welded, it should be field-tested to withstand a 30-foot-pound (ft-lb) load.
Excluding weld failures and normal wearing away of the stud tip due to erosion, stud failure can also occur when the stud is exposed to sulfur and a reducing atmosphere. Sulfur content in coal that is greater than 2 percent, combined with a reducing atmosphere, can attack and destroy calcium-aluminate cement bond refractory materials. This can cause a complete refractory failure and expose the studs to the furnace environment. The sulfur, as found in most PRB-type coals, can change in a reducing atmosphere to a gaseous vapor. This sulfuric acid–type vapor can attack the surface of the studs and penetrate the weld joints between the studs and the tube wall surface. This will, over time, cause the weld joints to fail and the studs to pop off.
Only by understanding the boiler environment created by the fuel being burned can pin stud failures be avoided. In some extreme cases, the pin stud material has to be changed to meet this sulfuric acid condition.
Pin stud failures are a major concern for most power plants and can cost thousands of dollars of lost revenue if a plant is forced to shut boilers down to repair or replace failed and lost pin studs on lower furnace walls. Refractory that is properly selected and installed will last longer and help minimize pin stud wear and loss. However, eliminating pin stud failures begins with understanding ash and slag, as well as the environment created when burning coal inside the furnace area.