Water scale forming

A survey by the author while at BNF in the 1950s showed that in scale-forming waters, galvanized hot water tanks and cisterns usually lasted 20 years or more, but otherwise leaks would appear in less than 4 years intermediate performances were less common. 

Ammonia is unsuitable as a neutralizer if copper-base alloys are used, because if used in excess it may destroy the metal faster than acid corrosion. The other neutralizers appear to produce protective films so that corrosion halts after an initial action. If scale-forming water is used, admiralty-metal tubes must be cleaned frequently, because corrosion occurs mainly beneath breaks or porous spots in the scale. The zinc in the brass is dissolved and is replaced by spongy copper. ... 

The iaterrelatioaship of nonalkaline scales (CaSO, CaSO /2H2O, CaSO 2H20) depeads oa temperature and the concentration of CaSO. To assure that no hemihydrate scale forms, MSF operators must mn their plants ia such a manner as to assure that the coaceatratioa of the total dissolved sohds does aot exceed 70,000 ppm at temperatures of 120°C. With average-salinity seawater, plants can operate at a concentration factor of 2, but in the Middle East where water salinity can be as high as 50,000 ppm, the concentration factor should not exceed 1.4. Under no circumstances should the total dissolved soHds exceed 70,000 ppm, ie, twice the concentration of normal seawater at 120°C. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Scale. Scale deposits are formed by precipitation and crystal growth at a surface in contact with water. Precipitation occurs when solubiUties are exceeded either in the bulk water or at the surface. The most common scale-forming salts that deposit on heat transfer surfaces are those that exhibit retrograde solubiUty with temperature. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

Hardness The scale-forming and lather-inhibiting qualities which water, high in calcium and magnesium ions, possesses. 

Chemicals such as disodium or the polyphosphates are used to precipitate scale-forming solids in the water. If alkalinity control is required, caustic soda... 

Figure 4-469 shows the effect on corrosion rates of 1020 steel in different water systems with dissolved hydrogen sulfide. The difference in corrosion rates is due to different corrosion products formed in different solutions. In solution I, kansite forms. Kansite is widely protective as the pyrrhotite coats the surface giving slightly more protection until a very protective pyrite scale is formed. In solution II, only kansite scale forms, resulting in continued increase in the corrosion rate. Finally, in solution 111, pyrite scale is formed as in solution I however, continued corrosion may be due to the presence of carbon dioxide. 

LSI (Langelier Saturation Index) an indication of the corrosive (negative) or scale-forming (positive) tendencies of the water. Hardness the total dissolved calcium and magnesium salts in water. Compounds of these two elements are responsible for most scale deposits. Units are mg/l as CaCOs. 

The effect of pH on the corrosion of zinc has already been mentioned (p. 4.170). In the range of pH values from 5 -5 to 12, zinc is quite stable, and since most natural waters come within this range little difficulty is encountered in respect of pH. The pH does, however, affect the scale-forming properties of hard water (see Section 2.3 for a discussion of the Langelier index). If the pH is below the value at which the water is in equilibrium with calcium carbonate, the calcium carbonate will tend to dissolve rather than form a scale. The same effect is produced in the presence of considerable amounts of carbon dioxide, which also favours the dissolution of calcium carbonate. In addition, it is important to note that small amounts of metallic impurities (particularly copper) in the water can cause quite severe corrosion, and as little as 0-05 p.p.m. of copper in a domestic water system can be a source of considerable trouble with galvanised tanks and pipes. 

Hot water generators and LP steam-raising plants of below 15 psig are designed to operate with minimal blowdown and to suffer negligible circulatory losses. As a result, clean, sediment-free water of almost any characteristic nature (e.g., soft and corrosive or hard and scale-forming) is likely to be suitable as a source of boiler water makeup. 

The design is further claimed to fully treat full flows of water in a single pass when installed direct to mains supplies, and water treated magnetically maintains its state for up to five days. Treatment results in hardness ions being attracted to other ions (rather than to pipework), resulting in a larger neutral body that has no ability to adhere to pipe walls or other contact surfaces, so that no incrustations or hard scales form. ... 

Ryzner, J.W., A new index for determining amount of calcium carbonate scale formed by water, J. Am. Water Works Assoc., 36, 472-486, 1944. 

More than 2 cm3 of scale form during the simulation, making this fluid potentially more damaging than the Miller or Forties formation waters by a factor of five or greater. 

Geothermal aquifer waters are close to saturation with some scale-forming minerals (calcite, pyrite) but undersaturated with others (amorphous silica, amorphous metallic sulphides). Only the slightest degassing suffices to produce calcite oversaturated water. By contrast, extensive cooling may be required to produce amorphous-silica oversaturation. As solubility constants are... 

Variations in the differences, A, between pK FUO) and p 2 (mixed solvent) for each individual solvent composition are relatively small (Tables I, II), indicating that Reaction 4 for different substituted benzaldehydes is influenced almost equally by the change in solvent composition. This fact, together with the existing evidence (19) that for aqueous hydroxide solutions substituted benzaldehydes form a suitable set of J indicators, proves that substituted benzaldehydes can be used also for the establishment of J scales in water-ethanol and water-DMSO mixtures. 

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