Boiler water impurities

Power plant boilers are either of the once-through or dmm-type design. Once-through boilers operate under supercritical conditions and have no wastewater streams directly associated with their operation. Drum-type boilers operate under subcritical conditions where steam generated in the drum-type units is in equilibrium with the boiler water. Boiler water impurities are concentrated in the liquid phase. Boiler blowdown serves to maintain concentrations of dissolved and suspended solids at acceptable levels for boiler operation. The sources of impurities in the blowdown are the intake water, internal corrosion of the boiler, and chemicals added to the boiler. Phosphate is added to the boiler to control solids deposition. 

The selection of boiler-water treatment is also dependent on the type of cooling water. When cooling water reaches the boiler, various compounds precipitate before others. For instance, seawater contains considerable magnesium chloride. When the magnesium precipitates as the hydroxide, hydrochloric acid remains. In some lake waters, calcium carbonate is a significant impurity. When it reaches the boiler, carbon dioxide is driven off in the... 

Analytical Methods. A classical and stiU widely employed analytical method is iodimetric titration. This is suitable for determination of sodium sulfite, for example, in boiler water. Standard potassium iodate—potassium iodide solution is commonly used as the titrant with a starch or starch-substitute indicator. Sodium bisulfite occurring as an impurity in sodium sulfite can be determined by addition of hydrogen peroxide to oxidize the bisulfite to bisulfate, followed by titration with standard sodium hydroxide (279). 

All non-volatile impurities entering the boiler must build up in the boiler water. This includes the TDS in the feed, plus most of the conditioning chemicals, of which the... 

Removal of magnetic impurities from industrial wastewater is called magnetic filtration, eg, reconditioning of boiler water and regeneration of condensate in power plants. Sec also Electromagnetic Separation Magnetic Materials and Mass Spectrometry. 

In Universal-Pressure boilers, there are no drams to concentrate the boiler-water salts and impurities, and blowdown is not utilized. Purification takes place by continuously passing all or part of the condensate through demineralizers in a process called condensate polishing. 

Since water impurities cause boiler problems, careful consideration must be given to the quality of water in the boiler. External treatment of water is required when die amount of one or more of the feedwater impurities is too high to be tolerated by the boiler system. 

Steam Purity. The trend toward higher pressures and temperatures in steam power plant practice imposes a severe demand on steam-purification equipment for elimination of troublesome solids in the steam. Carryover may result from ineffective mechanical separation and from the vaporization of boiler-water salts. Total cany-over is the sum of the mechanical and vaporous carry-over of all impurities. 

Mechanical carry-over is the entrainment of small droplets of boiler water in the separated steam. Since entrained boiler-water droplets contain solids in the same concentration and proportions as the boiler water, the amount of impurities in steam contributed by mechanical carry-over is the sum of all impurities ill the boilei water multiplied by the moisture content of the steam. Foaming of the boiler water results in gross mechanical carryover. The common causes of foaming are excessive boiler-water solids, excessive alkalinity or the presence of certain forms of organic mailer, such as oil. 

Attemperation. Maintaining steam temperatures at correct values may require a process called attemperation, where water is sprayed into the inlet of the superheater or reheater to lower the temperature of the entering and, consequendy, exiting steam. Attemperation bypasses the drum of a recirculating boiler. Any impurities in the feedwater are sent direcdy to the turbine. Copper fouling of turbines is commonly associated with high rates of attemperation. Attemperation can also allow salts from a condenser leak to bypass the boiler and deposit on the turbine. 

Plinke process In this process sulfuric acid is distilled at atmospheric pressure with indirect heating by natural gas and oil combustion (combustion chamber temperature 800 to 1100°C). This process is predominantly used in Europe. The to be concentrated waste acid is fed in at the top of the distillation column into boiling 96% sulfuric acid at atmospheric pressure and 320°C in a cast iron boiler. Water is expelled at the column head leaving a 96% acid. By adding nitric acid, which is mainly reduced to nitrogen, organic impurities can be removed oxidatively. 

Boiler Feed Waters.— Impurities in boiler waters not only reduce efficiency and capacity, but also impair quickness of response to demands for steam, increase the rate of deterioration of the boiler, and may produce dangerous conditions. The removal of deposits from a boiler nearly always involves considerable hard labor. Their prevention should be the aim in view. The unintelligent use of proprietary compounds is to be avoided. Some of them contain, besides reagents based on the water analysis, organic compounds which may loosen large masses of scale. 

Roller Water The steam purity limits define boiler-water limits because the steam cannot be purified once it leaves the boiler. For a once-through boiler, the boiler water must have the same specifications as the steam. A recirculating boiler is a still, and there can be considerable purification of the steam as it boils and is separated from the water in the steam dmm. The process of separation is not perfect, however, and some water is entrained in the steam. This water, called mechanical carryover, contains impurities in the same proportions as the boiler water, and its contribution to steam impurity is in those proportions. Typical mechanical carryover is less than 0.25% and often less than 0.1%, but operating conditions in the boiler can affect the mechanical carryover. In addition to mechanical carryover, chemicals can be carried into the steam because of solubility. This is called vaporous carryover. Total carryover is the sum of mechanical and vaporous carryover. The boiler-water specification must be such that the total carryover conforms to the steam purity requirements. For salts, such as sodium phosphate and sodium chloride, vaporous carryover is not a significant problem below approximately 15 MPa (2175 psia). As boiler pressures approach the critical point, vaporous carryover increases rapidly. Above 15 MPa (150 bar), boiler solids concentrations must be carefully controlled to minimize vaporous carryover. Most boilers operating over 18 MPa (180 bar) use all volatile treatment to prevent deposition of salts in turbines. Boiler-water limits for utility boiler are Us ted in Table 2. Recommendations from American Boiler Manufacturers Association (ABMA) for boiler-water limits for drum-type boilers and associated steam purity for watertube boilers are listed in Table 3. 

LEAKAGE - In water treatment, it refers to the passing of impure steam or boiler water through the drum internals. 

Prevention and minimization of corrosion is achieved by a combination of design (steady and vibratory stresses, heat flux, and flow), feedwater and boiler water treatment (control of pH tmd corrosion potential, and control of impurity concentration), and operation (control of impurity ingress and stress and temperature conditions) (Fig. 3). These corrosion control measures are discussed in Refs 4—6 and 9-19. 

Corrosion could be caused by impurities in the boiler water. 

The wastewater from a hydrogen plant typically consists of only the blowdovm from the boiler system. The boiler feedwater that feeds the steam generation system has small amounts of impurities such as sodium, chlorides, silica, and organic carbons. These impurities will accumulate within the boiler system and create sludge, scaling of the boiler tubes, and possible carryover of solids into the process steam. Blowdown of the boiler water is performed to prevent these issues from affecting the operation of the steam system. The blowdown is typically sent to the sewer or the on-site waste treatment plant for treatment and disposal. 

The pre-boiler cycle consisting of feedwater heaters, feed pumps and feed lines are liable to corrode by the condensate return. The main contributors to corrosion are carbon dioxide and oxygen. Corrosion must, therefore, be minimized. This is done by feedwater control, which reduces the ingress of harmful impurities and gases to the boiler water circuit and also to the steam. The best way to prevent corrosion is by controlling the pH, which should be maintained in the range of 8.5-9.2. 

Fiaal purification of propylene oxide is accompHshed by a series of conventional and extractive distillations. Impurities ia the cmde product iaclude water, methyl formate, acetone, methanol, formaldehyde, acetaldehyde, propionaldehyde, and some heavier hydrocarbons. Conventional distillation ia one or two columns separates some of the lower boiling components overhead, while taking some of the higher boilers out the bottom of the column. The reduced level of impurities are then extractively distilled ia one or more columns to provide a purified propylene oxide product. The solvent used for extractive distillation is distilled ia a conventional column to remove the impurities and then recycled (155,156). A variety of extractive solvents have been demonstrated to be effective ia purifyiag propylene oxide, as shown ia Table 4. 

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

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