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A pH/phosphate program differs from a standard or residual phosphate program in that the phosphate is primarily added to provide a controlled pH range in the boiler water to provide a buffering counteraction against acid and caustic attack of the internal metallic surfaces, which is described in more detail below. A pH/phosphate control program is very difficult and time consuming to control. The conventional control of such a program is normally based on many assumptions about feedwater phosphate demand, actual boiler cycles and boiler feedwater sodium levels. Feedwater phosphate demand, as that terminology is used herein and generally understood in the boiler field, is phosphate that becomes insoluble within a boiler system, for instance upon the formation of insoluble phosphate species upon combination with calcium or iron ions, or which hides-out (discussed elsewhere) during system load transients. Feedwater phosphate demand is normally a fairly steady value in a well controlled boiler system. Caustic-Gouging Corrosion and pH/Phosphate Programs As noted above, when high purity standards govern feedwater quality, the internal water treatment program of choice for control of scale deposition and corrosion within the boiler system is most often a pH/phosphate program. Stringent feedwater standards are commonly employed for high pressure boilers, but not limited thereto. High pressure boilers are no longer restricted to utilities. The increasing cost of energy is making cogeneration much more popular. The petrochemical, paper, and chemical industries now commonly use 1200-1800 psig boilers to provide cogeneration of both electricity and steam. Condensing and backpressure turbines are used to drive generators, compressors, and the like, while supplying extraction or exhaust steam for plant use. Waste-heat boilers receive heat from process streams for the production of steam. These high pressure cogenerative and waste-heat boilers have feedwater quality standards at least substantially comparable to that of high pressure utility boilers. In many cases, the initial design of high pressure waste-heat boilers may require even stricter feedwater standards than a utility boiler of higher pressure. As boiler pressures and heat transfer rates have increased, water-side caustic corrosion of boiler tubes (caustic gouging) has increasingly become a problem. Caustic gouging, or ductile gouging, starts to occur when caustic is allowed to concentrate against hot boiler metal surfaces, dissolving the normally protective magnetite. Normal boiler water hydrate alkalinity levels are harmless to boiler steel, but localized concentrations of tens of thousands of ppm are very aggressive. At such points, the protective magnetite on the water-side surface of the boiler tube wall is dissolved, as shown in the following Equation 1. Fe3O4 + 4 NaOH à Na2FeO2 + 2 NaFeO2 + 2 H2O Where the protective magnetite film is dissolved, the parent tube metal is exposed and is susceptible to corrosion, as shown in the following Equations 2 and 3. Equation 2: 3 Fe + 4 H2O -> Fe3O4 + 4 H2 Equation 3: Fe + 2 NaOH -> Na2FeO2 + H2 The concentration of boiler water chemicals at boiler surfaces, and the ensuing corrosion, are the results of two prime mechanisms, i.e., deposit formation and film boiling, which are discussed below. Deposit formation on boiler surfaces (dirty boiler tube surfaces) is the most
common cause of localized concentration of chemicals to corrosive levels. These and other contaminants may enter a boiler in soluble form, and then precipitate in the vicinity of the hot boiler surfaces. The higher temperatures at a boiler's heat-exchange surfaces will precipitate contaminants whose water-solubilities decrease at higher temperatures. The precipitation of contaminants leads to deposition on the boiler surfaces. Iron compounds form porous, insulating-types Porous, insulating-types of deposits allow boiler water to diffuse into the deposit where the water becomes trapped and boils. The boiling of deposit-entrapped water produces relatively pure steam which tends to diffuse out of the deposit, leaving behind superheated, non- boiling equilibrium solutions of caustic. Boiler water containing, for instance, 100 ppm of NaOH can form solutions having from about 50,000 to about 400,000 ppm NaOH (5 - 40%) upon diffusion into porous, insulating types of surface deposits. Hydrogen cracking (embrittlement) of boiler steel can occur as an additional consequence of high temperature zone deposit accumulations, (normally found only above 1800 psig). This kind of boiler tube deterioration may accompany caustic gouging. In hydrogen cracking, atomic hydrogen formed as a result of corrosion of the tube surface (from alkali or acid attack) migrates or diffuses into the tube metal where it combines with the carbon contained in the cementite (FeC) to form methane gas, as shown in Equation 4. Fe3C + [H] à Fe + CH4 Discontinuous, intergranular cracks are formed along the grain boundaries due to gas pressure buildup. Film boiling, the second primary cause of caustic gouging, occurs when the
heat input (heat flux) to a given section of boiler tube surface is so Film boiling (which is also called "departure from nucleate boiling" or "DNB", steam blanketing, or steam disengagement) in most instances arises because the affected surface was not intended to receive direct heat input, or the surface orientation (sloped, horizontal, and so forth) is such that inadequate free rinsing occurs even though the heat inputs experienced are normal. Insufficient water flow in a tube due to design or operational considerations may also cause film boiling. Film boiling on the water-side metal surface causes an evaporative concentration of salts. Film boiling and the problems associated therewith are generally seen with increasing frequency when:
The elimination or reduction of surface concentration of caustic and the resulting corrosion requires a boiler water treatment program that minimizes or excludes free hydroxide alkalinity (caustic). Low-alkalinity boiler water treatment programs exemplified by the pH/phosphate approach have become necessary. Todays pH/phosphate control includes :
The "Congruent Control" program operates well within a "captive alkalinity zone", and essentially eliminates the potential for any free caustic. Return condensate should be very high quality, preferably having been polished through powdered resin or deep bed ion exchange units. The congruent / coordinated program take care, not only as not to reduce the risk of the free caustic formation, but to lower the potential of phosphate deposits formation. Due to the fact of newer developments, it is found that the formed phosphate deposits are extremelly corrosive under high pressure conditions, equilibrium phosphate treatment has developped. The main purpose in equilibrium phosphate treartment design is not to have phosphate deposites at all. Select the appropriate link to continue, or press below to return to the home of the current page. |
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The
most prevalent deposits that can cause surface-concentration of boiler chemicals
are derived from iron and copper corrosion products which enter a boiler with
its feedwater. Large industrial condensate return systems or extensive feedwater
heating systems (as found in a utility operation) are major sources of both iron
and copper impurities which can enter the boiler.
of
deposits that are particularly active in promoting
