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754 CHAPTER 55 0S h 1960 Guide FI*. 3 Adapted from (l) IfU. A Baf. Cbm-, 20 (IKS) 1197--by Balia. (9) JJLC.S. SO (1929) 2068--Freer ft Johnson. Fig. 2 .... Solubility of Calcium Carbonate in Distilled Water Containing Carbon Dioxide (pH Vote** of Approximately 73 F) coolers, are more susceptible to scale and sludge formation than are the cold parts of the same system using the same water. The most common of the unstable soluble salts are the .bicarbonates of caldum, magnesium, and occasionally iron. Under conditions favoring the removal of carbon dioxide, as when the water is strongly aerated or when it is boiled, the bicarbonates are readily converted to the relatively in soluble carbonates. The type of scale most usually en countered is caldum carbonate (often called alkalinity) from the decomposition of caldum bicarbonate according to the following chemical reaction: Ca(HCO,)t + heat -* CaCO, + CO, + HfO Calcium Bicarbonate + heat Caldum , Carbon , Carbonate + Dioxide ^ Wftter Conversely, carbonates are readily converted to the more soluble bicarbonate by the addition of carbon dioxide or other acidic materials. This explains the increase in the ap parent solubility' of caldum carbonate at decreasing pH values (increasing concentration of hydrogen ion) shown in Fig. 2, the carbonate really going into solution largely as bicarbonate. In special cases, primarily with well waters possessing a high natural iron content, deposition of iron made from the decomposition of ferrous bicarbonate takes {dace according to the following reaction: 4 Fe(HCOt)t +0, - 2F*0, + 8 CO, + 4 T*0 Ferrous Bicarbonate + Oxygen Iron Oxide + Carbon Dioxide , water Low water velocity in heat transfer equipment often causes sludges to settle out in the tubes. This has the same' effect on heat transfer as scale formation. When this oc curs, increasing the water velodty often cures the condition. It is sometimes desired to evaluate the tendency in a particular water toward the separation of calcium carbonate, which may be desirable as a means of establishing a corro sion-resistant film on metal surfaces, or in other circum stances may be undesirable because of the impedance of Fi*. 3 Adapted from BulL No. IS, tTuiv. of Mich. Fvrtnatum and Properties of Boiler Scale by P. E. Partndpa. Fig. 3 .... Solubility of Calcium Sulfate and of Caldum Carbonate for Comparison {CaCOj tn EgwCbdasi wrift Nortoaf CO* Content of the Atmosphere] the caldum carbonate film to heat transfer. This tendency is indicated approximately by the Langelier Index* which is obtained by subtracting the actual pH of a particular water from the pH at which it is estimated precipitation of caldum carbonate would just begin. This estimate may be made by the use of Fig. 4. There are various expedients which may be employed for avoiding or mitigating difficulties due to scales: a. The water may be treated before use to remove elements such as calcium, magnesium, and iron, which form relatively insoluble compounds. Ia the various softening processes this removal of these elements is accompanied by the addition of other elements, particularly sodium, the compounds of which are relatively soluble. b. The water may be treated within the equipment to pro mote the separation of dissolved solids as sludges, rather than as scale which is, in most cases, more objectionable. c. The increase in total solids, due to the evaporation of water, may be controlled by the displacement, continuously or intermittently, of some of the used water by fresh supply. d. Substances, such as the polyphosphates, having the property of inhibiting the precipitation of calcium carbonate from solutions supersaturated with it, may be added. e. The pH of the water may be lowered (hydrogen ion con centration raised) to reduce the tendency for precipitation of carbonate. This is permisdble only to such an extent as will not cause a serious increase in rate of corrosion. The choice of the best expedient or combination of ex pedients must be made for each type of equipment, and will be affected by operating and water considerations. Item c above must be used, regardless of which other items are used, for all recirculating water systems from which evaporation takes place. Once-Through Equipment Where abundant supplies of water are available at low cost, the cooling water may pass through the equipment once, undergoing a rise in temperature. Little difficulty from scale should be encountered if the Langelier Index is lower than +0.5, based on the highest water temperature en countered in the system. This often corresponds to a car bonate hardness of less than 200 ppm when water tempera tures are lower than 100 F. However, since many municipal supplies are softened so that incipient calcium carbonate Corrosion and Water-Formed Deposits, Causes and Prevention COLUMN 4 755 COLUMN S to r is - 1537.5 20-i SO 4050O-; 2530-* 4030-- eo-j oo= O too 300 300 400 900 600 700 MO TOTAL OI3SOLVED SOLIDS IN PABTS PCR MILLION 3.0 - ntm1 on article is Oct. 1938 issue of Americas Water Work* Attestation Journal and later eonsetMO far Tobias 2 end 4 of article. Prepared for Charles P. Hoove? of the Calumbos, Ohio, Water Battening end Parifimtion Pleat by M. L. Riehi (Oafa required foe determining pH sotorotio n) (1) Total alkalinity, as parts per million of CaCOi . (1) r'jiwiirw in porta per million. (3) Total dissolved solids, in parts per million. (4) Temperature, Centigrade, at which pH saturation desired. ISO-- 375 200-> 500 ISO-; 20O-j 230-^ 300- O0-- 300soo-: eH iooo= Instructions (or Ueinc Chart:. L Knowing temperature and total direolvod solids, find temperature and total solids constant on CoL 1. X Align this constant with even value of calcium os Col. 3 of Chart, then locate point in Col. 3 of Chart (Pivot Line). 3. Align this point on Pivot Line with given alkalinity oa CoL 5; read pH saturation on CoL 4. Saturation index is pH actual minus pH saturation, eg.--pH actual, pH saturation. Saturation index. 7.8 6.1 --4.5 (corrosive) 6.4 7.6 +4.8 (scale forming). lAppEcabie wftfcen pH Settga 7JO-9J). Fig. 4.... Graph and Nomogram for Determination' of pH Saturation by Langelier's Formula precipitation is induced, the Langelier Index is a more re liable indication of scaling tendency. . Prevention of scale in once-through systems is based pri marily on the adjustment of pH, the application of surfaceactive agents, or a combination of the two. Normally the addition of a few parts per million of a surface-active agent such as a polyphosphate** fed proportionately to a water with a scaling tendency is all .that is required to control scale. Where a given water' does not respond satisfactorily to surface-active agents, reduction of pH with sulfuric acid or carbon dioxide is often employed. The pH is reduced only sufficiently to have it fall within the effective range of the surface-active agent. The use of acid or carbon dioxide is sometimes permitted without polyphosphates when corro sion will not be severe. In this case the pH is reduced so that the langelier Index is less than +0.5. The acid or car bon dioxide must be added with care. Use of automatic pH controllers is required if corrosion is to be prevented. / A special case of scale from ferrous bicarbonate decompo sition was mentioned previously. Ferrous bicarbonate is con siderably less stable than calcium bicarbonate. The preven tion of this decomposition can be accomplished by the addition of polyphosphate in low concentrations.11 The use of polyphosphate in the ratio of two parts by weight to one part by weight of iron is usually mogt effective. The polyphosphate must be added to the water before exposure of the water to air or chlorine if effective stabilization of the iron ia to occur. Closed Recirculating Systems The closed recirculating system is one in which water cir culates through one heat exchanger .where it absorbs heat, has its temperature elevated, and circulates through an other heat exchanger in which its temperature-is lowered. In cold water or chilled water systems scaling is seldom a problem. In hot water systems scaling is seldom a problem unless there is a large amount of make-up water." Open Recirculating Systems _ -Where water from condensers and similar equipment is passed through a spray pond or cooling tower and then returned to the equipment, there is an increase in the con centration of solids because of the evaporation of some of the water into the cooling air, and, moreover, the aeration removes carbon dioxide. Both factors promote the tendency to deposit scale. The corrosive properties of the water can also be increased through the absorption of acid gases such as sulfur dioxide (SO.), from the air." The absorption of gases from the air