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1052 CHAPTER 42 1957 Guide approximately by the Langelier Index,9 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, mag nesium, and iron, which form relatively insoluble compounds. In the various soften ing 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 promote the separation of dissolved solids as sludges, rather than as scale which is, in most cases, more ob jectionable. 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 concentration raised) to reduce thd 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 expedients 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 evapora tion 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 tempera ture. 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 carbonate hardness of less than 200 ppm when water temperatures are lower than 100 F. However, since many municipal supplies are softened so that incipient calcium carbonate precipitation is induced, the Langelier Index is a more reliable indication of scaling tendency. Prevention of scale in once-through systems is based primarily on the adjustment of pH, the application of surface-active agents, or a combina tion of the two.. Normally the addition of a few parts per million of a surface-active agent such as . a polyphosphate10 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 em ployed. 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 corrosion will not be severe. In this case the pH is reduced so that the Langelier Index is less than +0.5. The acid or carbon dioxide must be added with care. Use of automatic pH controllers is required if corrosion is to be pre vented. A special case of scale from ferrous bicarbonate decomposition was men tioned previously. Ferrous bicarbonate is considerably less stable than calcium bicarbonate. The prevention of this decomposition can be ac- Corrosion and Water Formed Deposits, Causes and Prevention 1U53 8S I* ^0 ililli. * aS3* jgS'ZS. a 2S.o2 a ? , nUa aA; "D 2 .as III 3! H. he ,3 a I I I Mill" flfl1 n Me o o < PPoS . iijimp^ ., >|i|ij.|i|.j o 000 OOQ OO n-- o nwon 0v 0mOe 0000 OgS 3 JOS '2 s* 1 g-f =3 I. a |1 St-- I ijU K 0z < ^ 3a rdo 1 + ooaftooe-h^ennq ill 1 mil niiiniluii Inn I.mlimlmi ti...l unit t nil n il jy.n Pr- ,, **W onOOonOOOO ,, 2 s 8 3 R8 S !! S Rg'PSS 1----1 T l i 1 1 i|iiii|ii'^ ' | 11 'Illlll'lll""l'l'1 '-pT'l n --#) n o bo no^ 00^ 0<0 0*0 00ft 0m 0n on 0n-0n * p 1 1 1 | I 1 1 > ) 1 1 I rp~TT I | I t IT`| I I I I | TT'I I | I l I I | I II I | I I J5*! o n o n fii** b ri o. ` n n" u o n no n ri * P3 Zo a<s mPb<* ^ <2 a |s .a? *2 I *1oz e2 z< 3abWas* 9 3 Q aobs 9- * ^ Is s Oa0<s i* o |o Az (js < 1.2 aB. Iestc as O 3 oft* 3 is -*s d O P S33 .5 og- P- 900t*' aO !Qd 3-a!. lagS a Ha: <3*2Syl TO* 5 -3 | 2i 5o I3 31 2 ,i-= 6 2*a o .5 -^S u55 3d |i | S_: tIo o S_. "S tr- = 2<31". i! s P3-S5 1s