Inspectioneering Journal

99 Diseases of Pressure Equipment: Flue Gas Dew Point Corrosion

By John Reynolds, Principal Consultant at Intertek. This article appears in the May/June 2005 issue of Inspectioneering Journal.

Most all flue gases produced by the combustion of fuels contain contaminants that can condense into acid droplets. The amount of contaminants will determine the concentration of the acid droplets. Most commonly those contaminants are sulfur or chlorine compounds, and the resulting condensate is sulfuric, sulfurous or hydrochloric acid.

When these aggressive acids condense on carbon and stainless steels in convection sections, flue ducts and stacks, the result can be severe and rapid dew point (DP) corrosion. Hence the industry needs to avoid this acidic condensate, use more resistant construction materials or limit the contaminants in heater/ boiler fuels. The latter is not easy to do, since most fuels contain sulfur compounds and some are contaminated with chlorides; and of course the most economical fuels are often the most contaminated ones.

The amount of contamination and the temperature of the exposed metal surfaces will relate to the amount of corrosion experienced. Sulfuric acid droplets typically begin to form at about 280F (138C) in flue gases, while hydrochloric acid condenses in flue gases at about 130F (54C), both depending upon the concentration of the contaminants in the flue gas. While the sulfuric acids generally produce an aggressive DP corrosion of equipment, the hydrochloric acids can also produce stress corrosion cracking when 300 series SS’s are involved. So most often the prevention for flue gas DP corrosion is to maintain the surface metal temperatures of exposed equipment above the DP or to protect the cooler surfaces with a coating that is resistant to the acidic condensate and will withstand the temperatures to which it’s exposed.

Would you invoke a management of change (MOC) procedure if you were changing heater/boiler fuels that might have different levels of contaminants or if you were going to the change the process in any way that could change the likelihood of fuel gas DP corrosion or areas in your heaters that may be susceptible to dew point corrosion?

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