Organic compounds pioneered boiler water chemical treatment when lignosulfonates derived from oak bark were used to minimize calcium carbonate scaling. Today's organic boiler treatment methods include the addition of neutralizing amines and oxygen scavengers to feedwater and polymers to boiler water. The literature is rich with documentation of the methods' mechanisms and results as well as the products into which treatment chemicals break down as they are exposed to the temperatures and pressures of the steam cycle.
Both sides debating whether organics do more harm than good to boilers can trot out data to support their position. But history is on the side of the proponents. For years, the vast majority of fossil-fueled and nuclear plants have treated their boiler water with carbon-based chemicals—amines, for example. The positive effect of these chemicals on pH and corrosion prevention are well known. There have been no cases of turbine corrosion that can be directly and unequivocally tied to organic chemical treatment. However, there may have been some instances of corrosion in the turbine and condenser due to the presence of other, larger organic molecules, such as those coming in with makeup water.
Based on experience, utility chemists are primarily concerned about stress-corrosion cracking, corrosion fatigue, and flow-accelerated corrosion in the steam turbine, particularly its low-pressure section (Figures 1 and 2). Corrosion starts where condensation begins, in the phase transition zone, where saturated and superheated steam coexist. The interaction of wet and dry environments in this area enables any steam contaminant to rise in concentration to levels that may be corrosive to materials of turbine blades and rims. Among the chemicals whose corrosion mechanisms have been studied extensively are chlorides and sulfates. But less known is whether carboxylic acids—such as formic and acetic acid—or carbon dioxide (CO2) are potentially as destructive as chlorides and sulfates, and at what concentrations.
1. Weakest link. The final stages of the low-pressure section of a steam turbine are the most vulnerable to corrosion. Courtesy: David Daniels
2. Reasonable doubt. Though organic compounds have never been proven as a cause of stress-corrosion cracking, they're one of the concerns raised by those who want to limit organic compounds in steam. Courtesy: David Daniels
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