Document g24XGYXwJr8wKOXXoQ7MNrD9e

988 CHAPTER 43 1956 Guide 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 hy 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 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 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 complished 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 most effective. The polyphosphate must be added to the water before exposure of fhe water to air or chlorine if ef fective stabilization of the iron is to occur. Closed Recirculating Systems The closed recirculating system is one in which water circulates through one heat exchanger where it absorbs heat, has its temperature elevated, and circulates through another heat exchanger in which its temperature is lowered. In cold water or chilled water systems scaling is seldom a Corrosion and Water Formed Deposits, Causes and Prevention 3a Ea |S 'S43 989 Z< 3-r s - s gw. =, L fa -s-g i5s3 1g5o -a I1d.2 -gg--Ss'. fa" Iaf.e |.3 rIjsS |s|.| S rt c eo 1 3 PoS 1--r T1 | | |||"i,|i"4|iifii| 11 i|i|i|i|i|.|iiiiiii^,un | |i|'|i|'|>| Jp > O 4)000 000 00 OOOOOOQOO -as-- - n cun 4 no 0 o-- n-- 00o0n 0400 0c0o 3 I H8 g S 03 I `3 "3 a. .0 1 is? 1 ii'ii S i.||l ^ * illll illtt illimti mlnilliinliinlini I...! > v> H nJiintmtiniifmil 0_) 6 3I n pn- o-- n 2^ n *r!- o n oo nnno 9OQn- r- oo no0n0 - n n t- _ -- -- m <\ #>n non "T I T I 1 < ivnn|ii") 1 m i|i|'l`lll'l'1"llllli ' | I | n n nn oo -- no - tv 000000 n 4 no e o-- 00 00 n- o n ) 04 0n ZJI I I I I I 81- 1 11 1 1 r 1 f iTrri 4 " *** *-o 9 BZ *2 3 Cfr S I tq If CQ ** pcH ^'gesz4 H O H p, a PoS 33 oi % PaoS Is ao f * Q 0) Z< Sw PS O 0&4 Stj Tf 3!?? 0 aeo ' 1 ? .M+O --<3T0a03 * Oo 0,0J .20.23^ gggS fl 3i! 5|2? .a eo CD 3 s 1I 1 I" * T3 9 5 1rt o 9^ 5 <3 o0 a3r sd T3 > 6 M cSo 3d os .2 m o 0-5B