Document GmGNX53969oOgBOMEbYpnxBZm

736 CHAPTER 55 1959 Guide -5 Fl* 3 Adapted from (1) Ind. <fc <w- C**w., 10 (1938) 1197--by Bali*. (1) J.A.C.& SO (1919) 1088--Freer * Johnson. Rg. 2 .... Solubility of Calcium Carbonate in Distilled Water Containing Carbon Dioxide (pH VaJuu af 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 calcium, 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 calcium carbonate (often called alkalinity) from the decomposition of calcium bicarbonate according to the following chemical reaction: Ca(HCOi)i + heat -* CaCCh + CO, + H# . Calcium Bicarbonate + heat Calcium Carbonate , Carbon Dioxide + water 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 calcium 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 oxide from the decomposition of ferrous bicarbonate takes place according to the following reaction: iFeiHCO*), + 0. - VPt&t + 8 CO, +4 R& 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 velocity 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 oe *> 4 \a % ' _ T 1 1: OMCTT OCOHCCS Fif. S Adapted (rom Bull. No. IS. Utuv. of ltfinh, Pm iimfi'iin and BmUr ScaUt by P. E. Partridse- Rg. 3 .... Solubility of Caldum Sulfate and of Calcium Carbonate for Comparison (CaCOj ut Equifibrara wtft Normal CO} Content of (ho Alntotphero) the calcium 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 calcium 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. In 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 firoperty of inhibiting the precipitation of calcium carbonate rom 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 permissible 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 +05, 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 COLUMN 5 Corrosion and Water-Formed Deposits, Causes and Prevention 737