Removal of Calcium Carbonate Build-Up in Condenser Tubes Restores Efficiency

Abstract

Water is essential to the power generation process, and for many power generation plants that rely on cooling water systems to condense steam, total hardness in source water is a problem. The high mineral content in many sources of cooling water results in hardened, calcified scale deposits forming on the walls of condenser tubes. These deposits form an insulation barrier inhibiting heat transfer. The scale also reduces the inner diameter of the tubes, restricting the flow of cooling water and further reducing heat transfer. As the heat transfer rate falls, performance of the condenser degrades, which can lead to decreasing megawatt output of the plant. A power station in the United States sources hard lakewater for cooling. Like many plants utilizing hard water, the station relies on a water treatment program to control the formation of hard scale deposits. However, when the plant opened a closed cooling water exchanger (CCW) in its Unit 1, a layer of calcium carbonate was found on the tube walls throughout the bundle. The CCW exchanger was shot with a variety of mechanical cleaners to thoroughly remove the scale and other deposits and debris from the tubes. Plant management suspected that the same problem might be occurring in the main condenser, and they understood that if scale was forming inside the condenser, the resulting loss in heat transfer rate would be a contributing factor in the decreased efficiency of the unit. Since the plant was planning an eddy current test on 100% of the more than 12,000 stainless steel tubes in the main condenser, the tubes were required to be thoroughly cleaned. Upon opening the main condenser, technicians verified the presence of calcium carbonate fouling, confirming their suspicions. While not knowing the condition of the tubes underneath the deposit, plant engineers were concerned about the underlying tube condition. They were also looking for alternative cleaning methods to provide relief from environmental concerns often associated with chemical cleaning. To preserve tube integrity and assure thorough removal of deposits, the plant decided to clean the condenser tubes utilizing the same mechanical cleaning method as the CCW unit. The condenser tubes would be shot with a series of specialized mechanical tube cleaners, one designed to fracture the calcium carbonate and a second designed to remove the fractured scale and other deposits from the tubes. This paper highlights the role of hard water in condenser tube fouling, the need to remove scale from condenser tubes and the mechanical cleaning process that was used to restore condenser performance at a prototypical coal fired power station in the United States.