Effect of Corrosion on Heat Transfer through Boiler Tube and Estimating Overheating

The product of corrosion in the feed water system transported into the boiler and gets deposited on the internal surface of waterwall tubes. Proper treatment of boiler feedwater effectively protects against corrosion of feedwater heaters, economizers, and deaerators. It leads to overheating and on-load corrosion and ultimately tube failure.. Heat is transferred through a thin layer of superheated liquid on a metal surface, the temperature gradient rising to about 10oC as the heat transfer rate increases and the boiling point of the liquid is approached Failure of boiler tubes by corrosion attack has been a familiar phenomenon in power plants resulting in unscheduled plant shut down. The effects of scales and/or corrosion products in reducing thermal conduction leading to higher temperatures in the underlying metal can lead to other modes of failure. Boiler wall tubes are joined to the water drum by expanding and flaring the tubes in the drum plate hole. It seems during fabrication the tube ends were not stress relieved and when these tubes with enough residual stresses came into contact with corrosive condensate, a synergic interaction of tensile stress and a corrodent occurred and resulted into stress corrosion cracking (SCC). There are two principal causes that govern SCC in the boiler environment. First, the metal in the affected region must be stressed to a sufficiently higher level. Second, concentration of a specific corrodent at the stressed metal site must occur. Both the above mentioned factors are observed to play a role ion SCC of the tube. This paper will discuss effect of corrosion on heat transfer rate with varying corrosion condition of boiler tubes and to overcome this, how much extra heat supplied as overheating is also concern of this paper.