Dew point corrosion is likely the most significant challenge faced by tubular air heater operators. In the air heater, dew point corrosion occurs when the surface temperature of the steel drops below the acid dew point. This allows highly acidic liquids contained in the gas streams to condense on the surface of the tube.
Acidic condensation on the tube surface can cause tubes to rapidly corrode and aggressive patterns of corrosion can cause failures to develop in less than six months. As corrosion failures spread throughout the air heater, boiler efficiency is significantly reduced, the performance of pollution control equipment suffers, and parasitic losses substantially increase.
In addition, when the temperature of the tube drops below the acid dew point, the condensation that develops on the tubing acts as a ‘sponge’ which allows inorganic materials contained in the boiler fuel to collect and accumulate on the surface of the steel. This process is referred to as fly ash ‘fouling’ or ‘plugging’ and severe fouling can completely prevent air or gas from flowing through or over the tubing. Fouling not only reduces the capacity for heat transfer but, over time, effectively removes the tube from service.
One commonly explored option to prevent dew point corrosion damage is to install stainless steel tubing in the air heater. However, because of poor heat transfer characteristics, these steels actually remain below the acid dew point longer than many other corrosion resistant options. As a result, air heaters with stainless steel tubes frequently experience extreme rates of fly ash plugging.
Stress corrosion cracking is another common problem when stainless steel tubes are installed in tubular heaters. This damage mechanism leads to decreased service life and reduces equipment performance. Fouling and stress corrosion cracking are two of the main reasons why stainless steel should be ruled out when considering a repair to address a corrosion related problem
CMS has developed a new type of air pre-heater tube that provides the industry’s first long term solution to dew point corrosion and fly ash plugging. The patent pending design, called the CMS Convective Tube Solution™, combines the latest developments in corrosion resistant metallurgy with advanced engineering. The result is an air heater tube that is essentially immune to both dew point corrosion and fly ash fouling.
Convective Tube Solutions have completely resolved extreme cases of dew point corrosion where all other alternatives have proven ineffective. For a Convective repair, CMS offers a 5 year warranty against dew point corrosion and fly ash fouling and the warranty program also includes a comprehensive preventative maintenance package. In less severe cases, CMS recommends installing one of our corrosion resistant steels tailored to meet the demands of your operation.