Document NGNpqoVean4X9vOz7JzrnDB3D

888 .. CHAPTER 51 1948 Guide carbon clioxide. In-rare instances, hydrogen sulphide, sulphur dioxide, or ammonia are present. In most steam condensing equipment7, 8, the rion-condensable. gases entrained with steam accumulate so that the amount present in the Vapor space is several hundred, times higher than in the incoming steam and, 'the amount dissolved-in the condensate may therefore approach, or even exceed for short periods, the amount entrained by the steam. CAUSES AND PREVENTION OF SCALES AND SLUDGES Scales may be formed on: surfaces of equipment in contact,with water, and sludges in the body of the water, by the separation from the water of dissolved or suspended solids. According/to the nature of a. particular piece of equipment and the method of its operation such separation may be promoted by one or more than one, of several factors: a. The concentration of solids may be increased by the evaporation of water/- b. The dissolved solids may be rendered less soluble in the water by changes in temperature. c. Conditions may favor the decomposition of unstable compounds with the formation of less soluble compounds. - Table 5. Data Typifying the Deleterious Gas Content of Different Atmospheres. Name of " Gas--- Aib Flue Gases Chem ical Rusal Metropolitan Bitum Coal Fuel Oils ^ Natural Gab .' V&Self Partial .Partial %by Pressure % by Pressure Volume Volume . Partial Pressure % by Volume Partial Partial Pressure % fey , .Pressure Volume psia psia psia pain psia Oxygen------------ 0, Carbon Dioxide CO* Sulphur Dioxide so. 21 0.03 None 3.143 0.004 None 21 > 3.143 v 2 . . 0.06 0.009 *15 0.003 0.004 0.07 0.299 2.245 0.010 7 13 0.03 1.048 ; 10 ./ . 1.497. ; 1.946 10 1.497 0.004 0.0001 0.0015 Figs. 2 and 3 show that the solubilities of both calcium carbonate and calcium-sulphate decrease with the rising temperature within a moderate range of temperatures. Surfaces' transferring heat into-water, such as condensers and 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.bicarboriates are readily converted, to the relatively insoluble carbonates'(or;' in the case of iron in the presence of oxygen, ferric hydroxide or oxide may be formed). 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 apparent solubility-of-calcium carbonate at decreasing pH values (in creasing concentration-of- hydrogen -ion) shown in Fig. 2, the carbonate really going into solution largely as bicarbonate. ' It is sometimes desired to evaluate the tendency in a particular water toward the separation of calcium carbonate, which may be desirable asai means of establishing a corrosion-resistant film on metalsiirfaces, or in Corrosion and Water Formed Deposits Causes and Prevention 889 other circumstances.may be undesirable because of the impedance" of-the calcium. carbonate film to-heat transfer. This tendency is indicated 'approximately by' the Langelier Index 9, which is obtained by subtracting the.'actual. pH of a particular water friom 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 fpr avoiding or mitigating difficulties due to scales: a. The water may be treated before use to remove elements such as calcium, mag nesium, 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/ : i b.-The water may be treated within the equipment to promote 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 tshueppdlyis.placement, continuously or intermittently, of some of the used water by fresh 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 concentration 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 must be made for each type of equip ment and will be affected by local considerations.