The Babcock & Wilcox Company
Chapter 42Water and Steam Chemistry, Deposits and Corrosion
Steam generation and use involve thermal and physical processes of heat transfer, fluid flow, evaporation, and condensation. However, steam and water are not chemically inert physical media. Pure water dissociates to form low concentrations of hydrogen and hydroxide ions, H + and OH, and both water and steam dissolve some amount of each material that they contact. They also chemically react with materials to form oxides, hydroxides, hydrates, and hydrogen. As temperatures and velocities of water and steam vary, materials may dissolve in some areas and redeposit in others. Such changes are especially prevalent where water evaporates to form steam or steam condenses back to water, but they also occur where the only change is temperature, pressure, or velocity. In addition, chemical impurities in water and steam can form harmful deposits and facilitate dissolution (corrosion) of boiler structural materials. Therefore, to protect vessels, tubing, and other components used to contain and control these working fluids, water and steam chemistry must be controlled. Water used in boilers must be purified and treated to inhibit scale formation, corrosion, and impurity contamination of steam. Two general approaches are used to optimize boiler water chemistry. First, impurities in the water are minimized by purification of makeup water, condensate polishing, deaeration and blowdown. Second, chemicals are added to control pH, electrochemical potential, and oxygen concentration. Chemicals may also be added to otherwise inhibit scale formation and corrosion. Proper water chemistry control improves boiler efficiency and reduces maintenance and component replacement costs. It also improves performance and life of heat exchangers, pumps, turbines, and piping throughout the steam generation, use, and condensation cycle. The primary goals of boiler water chemistry treatment and control are acceptable steam purity and acceptably low corrosion and deposition rates. In addition to customized boiler-specific guidelines and procedures, qualified operators are essential to achievingSteam 41 / Water and Steam Chemistry, Deposits and Corrosion
these goals, and vigilance is required to detect early signs of chemistry upsets. Operators responsible for plant cycle chemistry must understand boiler water chemistry guidelines and how they are derived and customized. They must also understand how water impurities, treatment chemicals, and boiler components interact. Training must therefore be an integral, ongoing part of operations and should include management, control room operators, chemists, and laboratory staff. General water chemistry control limits and guidelines have been developed and issued by various groups of boiler owners and operators (e.g., ASME,1,2,3 EPRI4 and VGB5 ), water treatment specialists6,7,8 utilities and industries. Also, manufacturers provide recommended chemistry control limits for each boiler and for other major cycle components. However, optimum water and steam chemistry limits for specific boilers, turbines, and other cycle components depend on equipment design and materials of construction for the combination of equipment employed. Hence, for each boiler system, boiler-specific water chemistry limits and treatment practices must be developed and tailored to changing conditions by competent specialists familiar with the specific boiler and its operating environment.
Chemistry-boiler interactionsTo understand how water impurities, treatment chemicals and boiler components interact, one must first understand boiler circuitry, and steam generation and separation processes. Boiler feed pumps provide feedwater pressure and flow for the boiler. From the pumps, feedwater often passes through external heaters and then through an economizer where it is further heated before entering the boiler. In a natural circulation drum-type unit, boiling occurs within steel tubes through which a water-steam mixture rises to a steam drum. Devices in the drum separate steam from water, and steam leaves through connections at the top of the drum. This steam is replaced by feedwater which is supplied42-1
The Babcock & Wilcox Company by the feedwater pumps and injected into the drum just above the downcomers through a feedwater pipe where it mixes with recirculating boiler water which has been separated from steam. By way of downcomers, the water then flows back through the furnace and boiler tubes. Boiler water refers to the concentrated water circulating within the drum and steam generation circuits. Chapters 1 and 5 provide detailed descriptions of steam generation and boiler circulation. Boiler feedwater always contains some dissolved solids, and evaporation of water leaves these dissolved impurities behind to concentrate in the steam generation circuits. If the concentration process is not limited, these solids can cause excessive deposition and corrosion within the boiler and excessive impurity carryover with the steam. To avoid this, some concentrated boiler water is discarded to drain by way of a blowdown line. Because the boiler water is concentrated, a little blowdown eliminates a large amount of the dissolved solids. Since steam carries very little dissolved solids from the boiler, dissolved and suspended solids entering in the feedwater concentrate in the boiler water until the solids removed in the blowdown (boiler water concentration times the blowdown rate lb/h or kg/s) equal the solids carried in with the feedwater (lb/h or kg/s). A small amount of dissolved solids is carried from the drum by moisture (water) droplets with the steam. Because moisture separation from steam depends on the difference between their densities, moisture separation is less efficient at high pressures where there is less difference between the densities. Therefore, to attain the same steam purity at a higher pressure, the dissolved solids concentration in boiler water must generally be lower. In a drum boiler, the amount of steam generated is small compared to the amount of water circulating through the boiler. However, circulation is also largely driven by the difference in densities between the two fluids, so as pressure increases the ratio of water flow to steam flow decreases. At 200 psi (1379 kPa), water flow through the boiler must be on the order of 25,000 pph (3 kg/s) to produce just 1000 pounds per hour of steam. Even at 2700 psi (18.6 MPa), 2500 to 4000 pounds of water circulates to produce 1000 pounds of steam. By contrast, most or all of the water entering a once-through boiler is converted to steam without recirculation. Some boiler operators have asked why boiler water concentrations change so slowly once a source of contamination is eliminated and the continuous blowdown rate is increased. How quickly can excess chemical be purged from a boiler? How much impurity or additive is needed to upset boiler water chemistry? How quickly do chemical additions circulate through the boiler? To answer these questions and explore some other chemistry-boiler interactions, consider for example a typical 450 MW natural circulation boiler, generating 3,000,000 pounds of steam per hour. It has a room temperature water capacity of 240,000 pounds and an operating water capacity of 115,000 pounds. The furnace wall area is 33,000 square feet, about 5800 of which are in the maximum heat flux burner zone. Impurities purge slowly from the boiler because the boiler volume is large compared to the blowdown rate. For example, at maximum steaming capacity with a blowdown rate 0.3% of the steam flow from the drum, 17 hours may be required to decrease the boiler water concentration of a non-volatile impurity by 50%. Almost two hours are required to effect a 50% reduction in the boiler water concentration even at a blowdown rate of 3%. Without blowdown, dissolved sodium with a fractional carryover factor of 0.1% would have a half life of 52 hours. While long periods of time are generally required to purge impurities, mixing within the boiler is rapid. For the boiler being used as an example, the internal recycle rate is about one boiler volume per minute, and steam is generated at a rate of one boiler volume every 5 minutes. The rate of steam generation is such that replacement feedwater must be essentially free of hardness minerals and oxides that deposit in the boiler. For example, feedwater carrying only 1 ppm of hardness minerals and oxides could deposit up to 25,000 lb (11,340 kg) per year of solids in the boiler, so the boiler might require chemical cleaning as often as 3 or 4 times per year. Also, small chemical additions have a large effect on boiler water chemistry. For example, addition of 0.2 lb (0.09 kg) of sodium hydroxide to the boiler water increases the sodium concentration by 1 ppm, which can significantly affect the boiler water chemistry. Similarly, a small amount of chemical hideout can have a large effect on boiler water concentration. Hideout or hideout return of only 0.01 gram per square foot (0.1 g/m2 ) in the burner zone can change the boiler water concentration by 1 ppm.
Control of deposition, corrosion, and steam purityThe potential for deposition and corrosion is inherent to boilers and increases with boiler operating pressure and temperature. Evaporation of water concentrates boiler water impurities and solid treatment chemicals at the heat transfer surfaces. During the normal nucleate boiling process in boiler tubes, small bubbles form on tube walls and are immediately swept away by the upward flow of water. As steam forms, dissolved solids in the boiler water concentrate along the tube wall. Additionally, the boundary layer of water along the wall is slightly superheated, and many dissolved minerals are less soluble at higher temperatures (common phenomenon referred to as inverse temperat