Document 4XryD14myr1X5D1JN7R2RjmN

HD! u \ 236 CHAPTER 15 1965 Guide And .Data Book Corrosion >d Deposits 237 ity of phosphate, as shown in Equation 2. Magnesium is most commonly precipitated as the hydroxide, as shown:in Equation 3. .. ' '' Ca(HCOj)j + 2 NaOH ' ' r-* CaCO* +. NaCO* + 2 HtO .. . 0) 3 Ca(HOOt)s + 6 NaOH + 2 Na^h,. *-.* .........................-- Ca,(PO<)*+'6NarfXh + 6H<0-:(2) Mg(HCOi}i +'4 NaOH' - ` Mg(OH), + 2 NaiCO* + HiO. ' (3) There are situations in which it is economically or-otherwise impossible to remove the hardness from;water. ;ln these other measures can be taken to control scale formation.' One common method is to reduce the alkalinity of the water; This very substantially reduces scale formation-because the solubility - of rahhim carbonate-is much less than , that of other calcium salts.. Most of the alkalinity can be removed by means of an anion exchangeresin, in a process analogous to softening.-In this case the alkalinity (bicarbonate).is.retained by the resin..which gives up.to the water an equivalent amount of chloride. When the resin begins.to pass larger amounts of alkalinity .than de-sired, it is backwashed and regenerated by means of. a salt solution, as described above. Usually, this salt solution oon^ tains a email percentage of alkali;- . . . ' Dealkalrang R*a + Hoy -- + a- Reputed Rxtw . Resin (i. , Inasmuch as the hardness of the water remains unchanged' but tiie alkalinity is reduced, this1process is called dealkaha-' mg, rather than softening. 1 ; More commonly, and'particularly in large cooling'towers, sulfuric acid is'used to eliminate most of the alkalinity- The solubility of Mlmnm sulfate is about'1200 ppm, in oontrast ter 35 ppm for calcium carbonate,:thus permitting a much higher- hardness to be present in the circulating water' before'scale can form. The acid feeding procedure involves a considerable, risk'unless careful controls are- employed. The alkalinity`of the'circulating' water is reduced to-such a low figure that a' slight overdose of\add produces'a very low pH and' corre1' spondingly corrosive circulating water. Accordingly, good practice dictates that acid feed be restricted to systems which operate under constant conditions of makeup water compost tion, and evaporation, or that automatic pH control equip ment be employed.' '; For cooling waters in which the above measures cannot be used, two other measures are used in conjunction in order, to minimirft scale formation in cooling systems.-First, the total solids in the circulating water are limited by use of a continu ous bleed or bleed-off to a- maximum value calculated from the T^ngpUw Saturation Index or the Ryznar Stability Indexl Second, scale control adjuncts are added to the water in1order to increase the apparent solubility of the calcium carbonate. Most commonly low concentrations (2-5 ppm) of sodium polyphosphates are used,-and are frequently supple mented by other materials, including organic dispersing agents, such as the various lignin derivatives. -These measures are also used with acid feeding. Biological Groxothe. Algae, bacterial slimes, and fungi are important bacterial growths because of their ability to inter fere with the functioning of cooling systems. Generally speak ing, heating systems do not suffer from the effects of bio logical growths, because:their operating temperatures are sufficient to loll the organisms involved. Algae,' which require light for carTying'on their life proc- yrepg, are likely to cause difficulty in cooling tower head pans; spray ponds, and other areas- where sunlight is abundant: Algae growths can become copious enough in a short time to cause bilking of water distribution piping, nozzles, and troughs. Tn-smaller equipment, blocking off sunlight by the use-of opaque-head pan covers can be a very effective algae control measure. Where this is not possible, biocidal chemicals can be used- ................ " ... Most waters contain slime-producing organisms, but signif icant amountsof slime are produced only when the conditions are such that their life1 processes are favored. These condi tions include sufficient food material from the water or from airborne material, combined \rith optimum temperature conditions, such as often exist on cooling surfaces and in air washers. Equipment nearsources of nutrients are particularly susceptible. to dime formation. Two common examples are air washers in printing plants which have fine paper dust in the`air) and refrigeration-cooling towers located in food storage areas of markets. Slimes can be-formed from bac teria, yeasts, or moulds. Generally, the former form thick, soft Blimp*, and the latter two tend to form tough, rubbery' climw In either case, the use of biocidal materials is indicated.' In- very large systems, particularly once-through cooling systems utilizing river, estuarian/or sea water, macroorgan- jgm* such as barnacles and mussels may form. Although1 antifouling paints can be used on large diameter pipe surfaces to minimize the growth of such macroorganisms, they must be renewed at frequent1 intervals aad are not applicable-^in accessible areas such as the insides of smaller diameter piping. Generally speaking,' chlorine is the only chemical used to destroy such organisms. In once-through Bystems intermit-' tent feeding of chlorine in the'form of chlorine gas or hypo chlorite solutions provides the most effective, economically- feambile method for controlling the growth of microorganisms and slimes. ` '* In other systems,; the effective control of dime and algae- frequently requires a combination of mechanical and chem ical measures. For example,* when a system already contains a considerable accumulation of slime, a preliminary - me-: chftnir*! or detergent cleaning will make the subsequent ap- plication of a biocidal chemical more effective in killing`the- growths and more enduring in5'the- prevention of further-'