Document dYJRzo9aeo3M1EDZYMmXByqg0

234 CHAPTER 15 1965 Guide AndDato Book equipment be supplied, &nd that the safety program be en forced at all times, in order to avoid personal injury or dam age to equipment. Also, contamination of drinking water by nonpotable treated or untreated waters must be prevented by eliminating cross-connections between systems or pro viding approved backflow preventers. With very few exceptions, the proper control of water treatment programs which involve the direct addition of chemicals to the water depends upon proportional feeding of the chemicals to maintain a desired concentration level at all times. Particularly in systems that have appreciable makeup rates, intermittent batch or-slug feeding of water treatment chemicals cannot be relied upon to produce satis factory results. Because sound water treatment programs require care and consistent attention, there appear on the market from to time, devices which allegedly prevent sr-al* and corrosion without requiring the operator's attention. Various natural forces, such as electricity, magnetism, or catalysis, generally behaving in some new way are said to be responsible for the effects claimed. Independent investigations of these devices have found them to produce no significant effect in preventing or correcting corrosion and scale formation.**-*1 Corrosion Control. Corrosion^lamage to water systems can be minimised by using corrosion resistant materials of con struction, providing protective coatings to separate the water from the metal surfaces of the equipment, removing oxygen from the water, or altering the water composition by adding corrosion inhibitors and pH control chemicals. Two or more of these methods are often used in the same system. Corrosion control by the selection of corrosion resistant materials of construction is within the province of the equip ment or system designer. Although it is technically possible to build equipment which will show no significant corrosion under almost any operating conditions, economic limitations usually make this impossible. On the other hand, investiga tion of corrosion futures in air conditioning and hunting equipment sometimes reveals design errors which show that. the most elementary principles of corrosion control, dis cussed earlier in this chapter, have been ignored. . Protective coatings are essential for controlling corrosion on external and other surfaces not reached by treated water, but can also be very helpful as adjuncts to water treatment for controlling corrosion at particularly vulnerable locations* For example, metal pans of cooling towers or evaporative condensers operated in areas, such as larger cities, in which the air contains considerable amounts of dust or other .solid matter, should be coated with punt or other suitable protec? tive coating in order to minimize localized pitting of the bottom resulting from the poultice effect produced by a layer of dirt. Such attack-cut take place even when the circulating water in the system is adequately. treated. with corrosion inhibitors. In tiie control of corrosion by the treatment of the water, the removal of oxygen is effective for closed systems in which opportunities for the pickup of additional oxygen are small: 11ms, boiler feedwater may be mechanically deaerated in an open heater or deaerating heater. This process is based upon the reduced solubility of oxygen in water at higher, tem peratures (Table 9) and is made more effective by equip ment design features which reduce the partial pressure of oxygen in the gas above the water. The last traces of oxygen are generally removed chemically by the addition of sodium sulfite or, at higher temperatures, hydrazine. Sulfite: 2 N&jSO, + 0,-2 NfeSO, Hydrazine: NJL + 0* -* N* + 2 HjO Table 9 .... Solubility of Oxygen From Air in Water at Different Temperatures4 Feaperotwe C deg F deg 0 32 5 41 10 50 15 59 20 68 25 77 30 86 40 104 50 122 60 140 80 176 100 212 MfflBilert per liter, lal/t) Alr - Oxygen + Nitrogen 28.64 10.19 + 18.45 ' 25.21 B 8.91 + 16.30 n 22.37 B 7.87 + 14.50 20.11 B 7.04 + 13.07 ' 18.26 B 6.35 + 11.91 ' 16.71 " 5.75 + 10:96 15.39 13.15 11.40 9.78 . 6.00 0.00 * B B B B 5.24 + 10.15 4.48 + 8.67 3.85 + 7.55 ' 3.28 + 6.50 1.97 + 4.03 0.00 + 0.00 ., Undesirable side reactions can occur at higher temperatures leading to acidic gases when sulfite is used, or to ammonia when hydrazine is used. Chemical removal of oxygen is used less often for cold water circuits because of the slow rate of reaction of the sodium sulfite with the dissolved oxygen, although the addition of a very small amount of a cobalt salt acta as a catalyst to greatly speed the reaction. When economically feasible, catalyzed sodium sulfite be used to control corrosion in once-through cooling systems. In open-spray systems, of course, chemical removal of oxygen would be too expensive because the circulating water would be thoroughly oxygenated again with each passage through the spray equipment. Oxygen removal by vacuum deaeration can be used to minimize corrosion in once-through cooling systems. It is particularly applicable .when the water contains an appre ciable carbon dioxide concentration because the deaeration removes this as well as the oxygen. Corrosion control treatment of heating and cooling waters is most often carried out by. pH control or by a combination of pH control and the maintenance of a corrosion inhibitor in the water. Contrary to a widely held belief, adjustment of the pH to 7.0 is not sufficient to stop corrosion. When the oxygen concentration is low, control of pH at certain levels is frequently adequate, as in the case of low pressure heating boilers maintained at a pH above 10.5. In most other cases, the adequate minimization of corrosion requires the use of an inhibitor in addition to pH control. Chromates are by far the most effective and universal cor rosion inhibitors-known for water systems. Depending upon the water temperature and the effectiveness of treatment control, the minimum concentration required may be from 200 ppm (as sodium chromate) to 2000 ppm. The minimum con centration must be carefully maintained .because with chro mates, as with other anodic inhibitors, pitting may develop if the inhibitor concentration is allowed to drop very low. Higher than minimum concentrations are maintained in closed systems because the cost is low, in view of the smalt water losses, and an extra safety factor is thus provided. The chro mates are effective inhibitors over a very wide pH range from about 6.5 up. An upper pH limit is frequently established for purposes of scale control. ,,, When economy of treatment is of primary importance, as in very large cooling towers, it is possible to use low concen tration mixtures of several inhibitors. A number,of com- binations are in use, such as chromate-polyphosphate, zinc- chromate, chromate-polyphosphate-zinc, and chromate- Corrosion and Deposits 235 t wrtMranide. Total inhibitor concentrations of 60 ppm and w are effective with such mixtures, but close pH control is HSntial for good corrosion control. Depending upon the ' Svture used, the desirable pH range may be 6.0 to 6.5, 6.5 least soluble common constituent of waters. The solubility of calcium carbonate depends upon the pH, temperature, and total solids content of a water in addition to the calcium and alkalinity (bicarbonate or carbonate). From these items, by to 7 5 6 0 to 7.0 or other range. Unless tower operating con ditions are constant and continuous, an acid feeder and auto matic pH controller is required to maintain the necessary pH the use of one of several nomographs (such as that in Fig. 2) the pH,, L e., the pH at which any given water is in equilibrium with calcium carbonate, can be calculated. This pH can be used with tiie actual pH of the water in either of two calcula- ^The diromates are sometimes undesirable because of their tions which will indicate whether the water has a tendency to yellow color, disposal problems, and potential hazard if used precipitate calcium Carbonate or to dissolve it. carelessly. Sodium nitrite has been used in place of chromate as an inhibitor. With ferrous metals it is nearly as effective as chromate but must be maintained at a pH.above 7.0 to avoid: The older of is the T-AngpJier Saturation Index. Saturation Index = pH -- pH, breakdown and at a minimum concentration of about 500 ppm. It is also necessary to check for both nitrite and nitrate A positive Saturation Index shows a scale-forming tendency. The larger the index, the greater this tendency. But it is a concentrations at frequent intervals because the nitrite is tendency only, other factors may inhibit scale formation under subject to rapid bacterial oxidation and conversion to ni- - some circumstances. Usually, calcium carbonate precipitates, trate, which has no corrosion inhibiting properties. Sodium ' generally as a scale, when the Saturation Index exceeds +0.5 nitrite has little or no protective effect on nonferrous metals, to +1.0. A negative Saturation Index indicates that calcium and other inhibitors must be added with it, to provide' pro ' carbonate will disolve and that bare metal will remain bare tection. A ' commonly used nitrite-base inhibitor includes borax as a pH buffer, and the sodium salt of mercaptobenzo- and thus accessible for corrosion. Ryznar suggested a modified method for predicting scale thiazole as an inhibitor, for nonferrous metals.Under the proper conditions polyphosphates have been ef-; - fective in reducing tuberculation and pitting. For this purpose polyphosphate concentrations higher than .those used for formation based upon operating performance. He called this the Stability Index. ' Stability Index = 2 pH, -- pH control must be provided, as must close pH control, in-, The Stability Index is always positive. When it falls below 6.0 the 6.0 to 7.0 range. ' formation is possible, and it becomes more probable the Corrosion control treatment of once-through cooling water 1 lower the numerical value of the index. is practical only with very inexpensive chemicals. pH ad Similar, but not as ample, methods have been published justment with caustic soda or lime is sometimes used in an for estimating the scale-forming tendencies of calcium sulfate attempt to form and maintain a thin protective film of cal- anH calcium phosphate, but are not so commonly used. At pinm carbonate on the metal surfaces by adjusting the cooling water temperatures and lower pressure boiler tem T.angplipr Index of the water to about +0.5 to +1.0. Sodium peratures, calcium sulfate is far more soluble than calcium silicate (water glass) is also fairly effective for corrosion con ' carbonate as shown in Fig. 3. ` trol in once-through systems, including potable water sys-; The most fool-proof method for preventing scale formation terns. Sufficient silicate is fed to increase the silica content of' ' is softening the water by passage through an ion exchange the water by about 8 ppm. - . ' (formerly called zeolite), water softener. This external treat- Other chemicals used for corrosion control in heating and' ment process removes all but 2 to 5 ppm of hardness. It is cooling systems include volatile amines (such as morpholine usually "carried out in a closed vertical tank about filled with and cyclohexylamine) and filming amines (such os octade-' cmi>n of synthetic resin, called a cation exchange resin, cylamine) used for steam condensate line protection. Various that has unique chemical properties- At low concentrations, it mixtures are used for glycol or alcohol antifreeze solutions in ,. preferentially absorbs calcium and magnesium ions from the chilled water or snow mplting systems, where the inhibitor - water, yielding, in return, a chemically equivalent amount of must be chemically compatible with the antifreeze agent... .. sodium ion, which is hot scaleforming. The delignification of wood in-cooling towers, while not' . The calcium and magnesium content of the resin ultimately strictly corrosion, is a related deterioration of materials of - rises to the point where they are no longer completely ab- construction. Several distinct types of failure have been - '^Borbed from the water pasting through the resin. The flow of noted, one of which is biological in nature and the other.. water is then reversed, backwashing the resin to remove any chemical. Prevention of this deterioration may be effected .by ' : dirt particles, and the resin is then regenerated by passing treating the wood, either prior,to construction or in place. , - a much higher concentration of salt through it. The sodium Solutions of:copper salts and of chromates are among those-; ions of the strong salt'solutiondisplace the absorbed calcium used for this purpose; Chemical delignification, usually asig- ' ' and magnprinm ions restoring .the resin to its initial sodium nificant problem where the circulating water is high1 in al- ; , ' form. After rinsing- out excess salt, the resin is ready for kalimty and low in hardness, can be minimized by keeping ; another softening cycle. the circulating water pH low (about 7.0) and by limiting _ chlorine to a mftTimum of 1.0 ppm. ^.Settle Control--The methods used for scale control in heat'mg and cooling systems include a variety of both internal and external treatment procedures. The selection of mi ap Softening NaR + Ca++(or Mgr") -R--e-g--e--n-e--r-a-t-i-on- C- aR + 2Na+ Regenerated Exhausted Resin Resin propriate method for any one system requires the evaluation- ! When it is possible to accept and control the presence of of a considerable number of factors. Smaller systems tend to- - suspended matter in .the water, hardness may be eliminated utilize internal treatment methods in which the chemicals by precipitation within the operating equipment. This is are added directly to the water, whereas larger systems may commonly done in lower pressure process steam boilers.' use external treatment if financially justifiable. Alkalis are commonly used to precipitate calcium carbonate Each of these Wale control methods attempts to minimize ,^-in accordance with Equation 1. Phosphates are added to re the opportunity For precipitation of calcium carbonate, the move the last of the hardness, by virtue of the lower sohibfl-