Boiler scale prevention

Water soluble polymers are of special interest today for biomedical, oil recovery, water purification, boiler scale prevention, coatings and many other applications. Many of the polymers under study contain hydroxyl and/or carboxylic acid functionalities, which contributed needed hydrophilicity, reactivity and other properties to the base polymer. 

Phosphorus is also important in the production of steels, phosphor bronze, and many other products. Trisodium phosphate is important as a cleaning agent, as a water softener, and for preventing boiler scale and corrosion of pipes and boiler tubes. 

Prevention of Soil Crusting. Acid-based fertilizers such as Unocal s N/Furic (a mixture of urea with sulfuric acid), acidic polymers such as FMC s Spersal (a poly(maleic acid) derivative originally developed to treat boiler scale) (58), the anionic polyacrylamides described previously, as weU as lower molecular weight analogues such as Cytec s Aerotil L Soil Conditioner, have all been used successfully in at least some circumstances to prevent the formation of soil cmsts. It is difficult to prove benefits in the laboratory, and field tests may give variable results depending on local weather conditions. 

Use Water-softening, boiler-scale control, component of cleansers, laundry mixes, dishwashing compounds, pitch control in pulp industry, prevention of lime soap deposits in textile operations. 

Jamialahmadi and Muller Steinhagen [1993] conclude that the theory and art of scale prevention in boilers has developed in a rather amorphous way. They cite the use of potatoes in 1821 for scale prevention in boilers. The origin of the technique was apparently that some labourers, wishing to cook their potatoes in the boiler, were distracted so that the potatoes were not retrieved. Some days later when the boiler was due to be cleaned, the operators were astonished to see that the heat transfer surfaces were unusually clean. The cleanliness was explained in terms of the starch particles from the potatoes that provided bulk nucleation sites and the creation of weaker deposits. 

In addition to drinking water and environmental applications, water purity is critical to many industries. Conductivity detectors are used in semiconductor and chip fabrication plants, to monitor cleanliness of pipelines in the food and beverage industry, to monitor incoming water for boilers to prevent scale buildup and corrosion. Any process stream with ions in it can be analyzed by conductometry. Conductivity detectors are part of commercial laboratory deionized water systems, to indicate the purity of the water produced and to alert the chemist when the ion-exchange cartridges are exhausted. The detector usually reads out in resistivity theoretically, completely pure water has a resistivity of 18 MO cm. 

The methacrylamide-MA copolymer has also been studied for purification of industrial waste water/ The disodium salt of MA has been copolymerized with the sodium salt of 2-acrylamido-2-methyl-propanesulfonic acid, using peroxide initiators, to give useful dispersant and deflocculants for water-insoluble compounds of Fe, Ca, Al, and for silt and clay particles/" The copolymers also prevent boiler scale formation, corrosion, etc., without being affected by the hardness of the water. Copolymers prepared in aqueous solutions are claimed to be useful in well wall materials, coatings for microcapsule production, and for paper dry-strength agents. 

Uses Corrosion/scale inhibitor for low pressure steam boiler water treatment provides deposit protection and acts as dispersant to help remove any suspended solids in boiler water prevents hardness ions from precipitating and forming sludge or scale on heating surfs. 

Water Treatment Industrial CleaningPipplications. Boiler and cooling tower waters are treated with lignosulfonates to prevent scale deposition (78). In such systems, lignosulfonates sequester hard water salts and thus prevent their deposition on metal surfaces. They can also prevent the precipitation of certain iasoluble heat-coagulable particles (79). Typical use levels for such appHcatioas range from 1—1000 ppm. 

In dmm boilers sodium hydroxide (caustic), sodium phosphate, or both may be added for pH and scale control. Sodium hydroxide is used more in Europe than in the United States, where sodium phosphate treatment is usually preferred. In boilers operating above 4 MPa (580 psia), caustic concentrations must be carefully controlled to prevent highly corrosive deposits from forming. In the lowest pressure boilers, phosphate treatment may be used to compensate for lower purity feedwater. As the boiler pressure increases, the allowable phosphate concentration decreases, and at 16.5 MPa (2400 psia) or above, equiUbrium phosphate treatment may be used. In this treatment, caustic is added to a low phosphate concentration in the boiler to maintain the proper pH (20). 

Phospha.te Treatment. Calcium phosphate is virtually insoluble in boiler water. Even small levels of phosphate can be maintained to ensure the precipitation of calcium phosphate in the bulk boiler water, away from heating surfaces. Therefore, the introduction of phosphate treatment eliminates the formation of calcium carbonate scale on tube surfaces. When calcium phosphate is formed in boiler water of sufficient alkalinity, a particle with a relatively nonadherent surface charge is produced. This does not prevent the development of deposit accumulations over time, but the deposits can be controlled reasonably well by blowdown. 

In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly ehosen. Thus, the addition of normal phosphates to an unsoftened feed water ean eause a dangerous eondition by elogging the makeup line with preeipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the eombination of ealcium or magnesium with negative ions of soap stearates. 

Thus, to control the buildup of sludge and scale (which may include the prevention of these foulants and deposits in the first place), it periodically becomes necessary to deconcentrate the BW by physically removing some of it from the boiler (blowing down or blowing off). This is achieved via a blowdown valve (BD valve) usually located at the bottom rear of the boiler vessel or WT mud drum. The BD water lost from the boiler is replaced by less concentrated FW. 

The mechanics of deposition in a boiler are often a cycle of cause and effect, wherein an initial low level of scale deposited on boiler surfaces causes a rapid localized rise in wall temperature. The temperature increase leads to localized steam blanketing, which in turn prevents the deposit from resolubilizing. Consequentially, conditions then exist for the further buildup of deposit on the heat transfer surface. 

The need to prevent deposition on boiler surfaces (resulting principally from hardness salt scaling by calcium carbonate and other alkaline earth metals)... 

Phosphate is sometimes present in MU water sources (say, 1-2 ppm or more) usually as a result of field and factory run-off or from the deliberate addition as a city water threshold agent to prevent corrosion and deposition in the mains. The steady growth in the reuse of secondary water sources such as municipal and industrial waste waters means that phosphate is increasingly likely to be present in MU. If the phosphate remains undetected, it likely will scale and foul FW lines by forming amorphous calcium orthophosphate [tricalcium phosphate Caj(P04)2] sludge before it reaches the boiler section. 

Carbon dioxide carryover also occurs following the deliberate addition of soda ash (sodium carbonate) directly to the boiler. Where boiler designs provide for a significant internal drum or shell, the use of soda ash and caustic soda to prevent calcium and magnesium scales by precipitation reactions (internal softening) may be employed. 

W. Smith. The Prevention of Scale Formation by Boiler-Water... 

Silicate scales can form readily in all boilers. Maintenance of a silica content to caustic alkalinity ratio of less than 0.4 1 prevents this. 

---