This article in Power Engineering by Brad Buecker of Kiewit addresses chemical treatment of HRSG feedwater. During the heyday of coal-fired power plant construction and operation in the last century, many lessons were learned regarding correct water/steam chemistry control in high-pressure, fossil-fuel steam generators. Even seemingly minor issues had the potential to cause serious problems, and some corrosion-induced failures led to injuries and death of plant personnel. Progress in transferring these lessons to the combined cycle power industry has often been slow, and a number of outdated chemistry concepts continue to appear in the specifications for new combined cycle plants. Some problems are magnified by the unique geometrical features of heat recovery steam generators (HRSGs) as compared to their coal unit counterparts. This article will examine three of the most important issues in this regard, as outlined below: • Unless the condensate/feedwater system of the HRSG contains copper alloys (very rare), an oxygen scavenger should not be part of the chemical treatment program. Use of oxygen scavengers, a more accurate term is reducing agent, induces flow-accelerated corrosion (FAC) of plain carbon steel. FAC has caused catastrophic piping and tube failures at a number of facilities over the last three decades, and it continues to occur at many plants. • Tri-sodium phosphate has served as the primary chemical for boiler water treatment in many base-loaded coal units, and the chemistry is often employed in HRSGs. Even at steady load, phosphate treatment is problematic due to the phenomenon known as phosphate hideout. In cycling units, hideout may make phosphate chemistry extremely difficult to control. • While a strong focus should always be placed on operating chemistry, off-line chemistry control is frequently neglected. Air in-leakage into water-filled steam generator networks during down times can significantly damage tubes, piping, turbine blades and rotors, and other equipment. Given the regular cycling nature of most power plants in today's environment, the potential for air ingress and subsequent corrosion may be extensive. Click Here For Complete Article Text
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