Document zzo6wz2yMLEy6wznx57V4B4r0

230 CHAPTER 15 1965 Guide And Dota Book lead alloys in sea water, etc., are more widely used in im pressed current systems. The distribution of protective cur rent on the cathode, power costa, and stray current effects on neighboring structures must be considered when selecting the number, form, and arrangement of the anodes. A recent development in impressed-current cathodic pro tection is automatic potential control.14 In this system, the potential of the structure is continuously measured against a reference electrode by a monitoring system which regulate? the protective current applied to maintain the structure at a preselected protective potential value. Since the protective current requirements vary from place to place and from time to time, the automatic potential control system assures full, protection at all times and prevents excessive cathodic pro tection, which would accelerate coating damage by electro- endosmotic effectand cathodic attack on amphoteric metals such as aluminum and power wastage. . Limitations. Full cathodic protection is dependent upon adequate current flow reaching all surfaces to be protected. On bare sin-faces, the relation between an anode and cathode* configuration determines, in large part, the distribution of protective current. For example, relatively simple anode systems will adequately distribute protective current on the outside of storage tanks, well coated pipelines, tank interiors, and water boxes of heat exchangers. However, the protective current received on the interior of a condenser tube from anodes in the water box diminishes rapidly, within approxh. mately two diameters of the tube entrance. Rapid attenuation of the protective effect occurs on the exterior-of bare pipelines in conductive media. Therefore, protective current sources must be applied at more frequent intervals. Harmful effects of stray currents from a cathodic protec tion system upon neighboring utilities or other isolated metallic systems must be considered. All underground cath odic protection installations should be reported to, and examined in cooperation with, local electrolysis committees. Economics. Since the cathodic protection principle must be adapted to meet the specific needs of each corrosion problem, the costs vary widely. Although the annual costs for pro tecting large coated structures may range from $.002 to $.02 per sq ft per year, the annual cost for protecting similar bare surfaces may vary from $.02 to $.20 per sq ft per year. Environmental Change Another approach in the battle against corrosion is to change the environment so that it is less aggressive to the equipment. One of the more familiar examples of this ap proach in the refrigeration industry is the use of solid desic cants, such as silica gel, for maintaining a low moisture level in refrigerant lines. Another application of desiccants is for minimising atmospheric corrosion by maintaining a -low relative humidity. The familiar moth-balling technique for preserving decommissioned ships and other heavy equipment is an example of this. Vapor phase .corrosion inhibitors can be employed to minimise atmospheric corrosion in certain cases. Dicyclohexylammonium nitrite, for example, is sometimes used for the protec-' tion of steel parts within sealed containers. A related com pound, cyclohexyl&mine dicarbonate, has been used in Great Britain for the protection of idle boQere. Adding alkali to water to raise the pH, and using corrosion inhibitors, such as chromates, are other examples of reducing corrosive attack by changing the environment. Corrosion in hibition in water systems is discussed more fully in the next section. Inhibitors for corrosion control are added to glycol antifreeze solutions,-lubricants,-lithium bromide absorption refrigeration brines, and other liquids. The local environment at a metal surface can sometimes be made less corrosive. This is the principle involved in the addi tion of filming amines to steam in order to protect condensate lines and in the addition of certain additives to fuel oil in order to reduce fireside corrosion in boilers. Finally, some modifications of equipment dreHgn to reduce the likelihood of corrosion are also essentially environmental changes, e.g., the elimination of crevices and the provision of weep holes to prevent water accumulation. WATERSIDE CORROSION AND DEPOSITS The occurrence and correction of corrosion in water systems is so closely interwoven with the occurrence and correction of other water-caused troubles that it is virtually impossible to consider them separately. The most common of them water problems in beating and cooling systems are one or more of the following: (1) Corrosion, (2) Scale formation, (3) Bio logical growths, and (4) Suspended solid matter. The importance of having some knowledge of these prob?. Irens,can be appreciated from the fact that each of them can,to a varying degree, cause serious reductions in the cooling or heating capacity of a system and can cause premature equip ment failure, in some cases wijth widespread- rfamagn and danger to persons in the vicinity. It is rare..that adequate.recognition is given, to the com-, plenty of the control of waterside operating problems, par ticularly in heating and cooling systems. The proper handling of such problems involves water chemistry, engineering, economics, and personnel administration during each stage of the system development, design, construction, installation and operation. The information presented in thin chapter is intended to provide the reader with a background for better understanding of the causes'and handling of water problems in heating and cooling systems. However, it is necessarily far Table 5 .... Analyses of Typical Public Water Supplies Substance location or Arto*-h Unit ID (2) (3) (4) 15) (6) (7) (8) 19) Calcium............ Sodium.............. Potassium......... SKV Fe - Ca Mg. Na K 2 6 12 37 0C 6 5 36 62 92 96 3 155 400 1 2 h m 34 27 2j 46 1,300 2 6 -.7 44 a m 215 7* 11,000 111 i 18 10 3 400 Bicarbonate.... HCO. 14 13 lio 202 339 334 549 210 150 Chloride....... Cl Nitrate... NO. 2 10 13 13 10 280 22 117 19^000 1 0 2 13 0 1 . 3 Dissolved Carbonate Hardness.... C&Ouj Non-Carbonate Hardness.... CaSO. 31 426 434 983 564. 35,000 12 11 9S 165 287 274 172 125 5 7 18 40 58 54 0 295 5,900 All rahue Are parts per miUian af the unit cited to neerat whole -- (em Reference 17). Number* indicate location or ana ac follows: SCatakm eupplr--New Torfc aty . Swamp Water (Colored) Black Creek, Middlcbcig, Florida (3) Niagara River (Filtered) Niagara Fall*, Now York (4) Mbeonri River (Untreated) Avenge (5) WeB Waten--FubBe Supply--Daytoa, Ohio--30-00 ft (6) Well Water--Maywood, Uhnoca--2090 ft (7) Wefl Water--Southfield! Va.--330 ft (8) Wefl Water--Roewell. N. Mexico ' (9) Ocean Water--Average '. ' Corrosion ond Deposits 231 T~hte 6 /.. Variations in Composition of Schuylkill * River Water at Belmont Filter J Plant, Philadelphia, Pa. Port* Per MiBfon Seapl* Cota* January February March April May June July August September -December Maximum 1949. pH Total Hardness ' Total Alkalinity Chloride Sulfate Dissolved Solids 7.0 73 32 6.7 93 38 7.2 105 . 42 7.3 106 43 5 48 123 6 . 59 155 6 52 145 7 60 169 102 39 7 6.9 137 . 49 10 6.9 176 65 15 7.3 166 67 14 66 187 89 218 114 282 87 235 7.2 175 6.9 .205 7.6 181 6:7 184 63 16 70 21 61 - 8 58 109 268 135 . 341 6.7 73 32 7-6 205 . 70 48 123 21 135 were eompontes for the let ten day* of each month. from.sufficient to qualify the reader as an expert and omits many water problems and treatment methods not usually en countered in these systems. Additional information is avail able in References 1, 3, 4, 5, and 16. Water Characteristics Between the time that H falls as rain, sleet, or snow and the time that it is pumped into a user's premises, water dis solves'^ least a little of almost every gas and solid substance with which it comes in contact. It is these dissolved im purities, rather than the water itself, which are the primary causes of the various tcater problems. .. The chemistry of water is not simple. Almost infinite varia tions are found in the compositions of water supplies. Table 5,. showing chemir&l analyses of some typical public water sup^-. plies, gives a slight indication of these variations. Further more, the water received at a given location may vary widely from time to time, either because.supplies from different sources are being used or because the composition of a tingle supply fluctuates, as in the case oi water -supplies obtained from rivers (Table 6). Another, and often `neglected, characteristic which affects water-caused problems is that the composition of the water added to a system does not necessarily remain the same after the system starts to operate.11 Changes in chemical imposi tion take place as'the result of evaporation, aeration, cor rosion, and scale formation. These chemical changes, tem perature changes, formation of biological . growths, and the accumulation of suspended matter all tend to produce operat- Toble 7 .... Conversion Factors for Water Analyses- To Convert > _v_ Into Multiply by ppm ppm ppm ppm : 17 ,14J 1000 Fig. 1 .... Relationship Between Bicarbonate Alkalinity, Free Carbon Dioxide, and pH* ing results which may differ from those anticipated on the of the chemical analysis of the makeup water. Chemical Characteristics. The type and amount of dissolved inorganic materials, including gases, define the chemical characteristics of any water. Typical water analyses such as those shown in Table 5 are not complete, but give only the major constituents and those important for municipal and average industrial use of the water. The many minor constit uents which are present in most water supplies have little or no importance for the overwhelming majority of water uses. In water analyses, all values, except pH, are usually givenas parts per million (ppm), the weight of material dissolved in a million parts by weight of the water or solution. Thus, a 1 percent solution corresponds to 10,000 ppm. It is im portant to keep in mind that, when water analyses are re ported in ppm, the chemical species must also be given. Thus, the same calcium concentration in a single water might be variously expressed as 100 ppm as CaCOt, 56 ppm as CaQ, or 40 ppm as Ca. Other units are also used in reporting water analyses. Table 7 summarizes a few of the more common of these, together with factors for converting their numerical values into ppm. Most water analyses include only dissolved solids and omit the dissolved gases which are also present. Certain of these, such as nitrogen, have virtually no effect upon any use of. water. Others, such as oxygen, carbon dioxide, and hydrogen sulfide, produce important effects in water systems. Carbon dioxide can either be measured directly or can be estimated from the pH and total alkalinity by making use of one of the many available variations of Ttilmann's curves (Fig. 1). Oxygen and hydrogen sulfide, on the other hand, in order to ^ have any moaning must be measured by special techniques at the timo the sample is collected. Because oxygen is so.im portant in the corrosion of metals and because it is so readily dissolved to at least a limited extent as the result of contact with air in many water systems, the water chemist will often simply malm the conservative assumption that any water -supply which has had an opportunity to come-into contact with air will be saturated with oxygen in accordance with its partial pressure in air and the water temperature. . * Of the chemical constituents normally reported in a water analysis, fafa* hardness is probably the item most commonly recognized by laymen. Strictly speaking, this is a measure of the dissolved substances which prevent soap from lathering. - In most cases it corresponds to the calcium and magnesium