Document pRQ1RBOzKJOYqXVoQxEbG7eB

894 CHARTER.SI,A ' 1948 Guide stroy slime-fonriing organisms.- -The others are-presumed to poison marine organisms, most of which recover; when the-chemical, is not used regularly. While chlorine is the most generally used chemical, others may occa sionally proveto;be more practicable. - Choice of the chemical is con ditioned largely by the design and- operation of the system. Open Recirculating Systems :i In spray poinds and cooling towers of the open type, light-loving algae growths are likely to cause blocking of the distribution piping and troughs. These organisms. re most .troublesome,.,in areas accessible to sunlight. Algae slimes are usually "stringy !in character'; , In open recirculating systems, continuous use of small quantities of chlorine is generally most: satsifactOry. .In once-through systems, .where large quantities of water are used, intermittent-treatment a few times each day will usually result in satisfactory, slime removal and- chemical economies. ' - . Neither the phenols nor copper sulphate may be used for the' removal of slime already formed. For this purpose, chlorine gas is used. After being cleaned, the other chemicals may be used to prevent the reestablish-. ment of slime in the system. The removal of green algae from a cooling tower should never be used as an indication that the true slime-forming organisms pn heat exchanger surfaces have been removed. The more resistant slime formers, which so materially reduce heat transfer efficiency, .will often be Unaffected by treatment which completely eliminates algae. Closed Once-Through Systems .. . In equipfitent where jight'is excluded, slime formations are due to fungi. Usually, they predominate on the heat exchange surfaces. Bacteria form thick, softrslime. Yeasts and molds form tough rubbery slimes: Chlorine and hypochlorite solutions, fed intermittently, are used to prevent such slimes. ' UNDER-WATER CORROSION '. When deleterious; substances hre present in? Water, the corrosivity of the solution is increased' in prbportion to the amduntof deleterious substances present, the temperature, arid'usually the rate of flowof the solutioh: over the metal surfaces. - There are other relevant frictorsyfrut' their influence in general is-subordinate to those- mentioned;-Dissolved oxygen; acid gases.andchloridesalts are'; the corrosion-accelerators most-frequently encountered.'- -- "" - : Neutral arid slightly alkaline waters saturated With air, corrode iron at a rate about triple that for the1 same writer free Of air. Hot waterContairiihg oxygen' will-corrodedron at'a rate1 three to four times'that for the srime water when cold.' ` Corrosion of irori decreases as the pH of writer solutions-increases arid- practically ceases rit ri pH of 11. If the metal contains film forming agents, such as chrotniurri;'nickel, rind silicon or if the-water- contains inhibitors such as `silicate's and chromates, corrosion may in: some instances-be minimized. "" ' :''' ' V"'"' ' ' Cold Water Services Where water froin muriicipal -Supplies -is': used industrially in'a closed system with little or.no increase in temperature, it is seldom necessary or Corrosion and Water Formed Deposits ^CdiiseS and Prevention 895 feasible totreat the water to reduce its corrosivity; When it is mandatory, the addition of caustic soda to maintain a pH over 11 ^/wj the addition of sufficient sodiufn sulphite to maintain :a residual of: over 100 ppm (as NasSOs) usually' suffices to prevent-serious troubles.- ; However,-'in some cases the cost may be prohibitive. 1 .- ' When the-use of sodium sulphite or ri comparable Chemicalfor'oxygen removal is prohibited, as in potable writers, the addition of small'rimOunts of lime to maintain'ri Langelier Index (See Fig. 4) of-0.5 Or more may prove helpful. '' : In systerns exposed to the atmosphere, as for exrimple air washers qr storage tanks, both laboratory14 and field tests16 have shown that the addition of alkalies to maintain a pH greater thriri 8.5,'plus the addition of other chemicals that produce 'protective films on the.irietai surfacei will measureably decrease corrosion.; Sodium dichrOmate, sOdiurn 'silidate, arid tri-sodium ` orthophosphate: have . been shown 'to be effective film formers in the order mentioned.' "' - - Caustic soda is usually used to raise the pH value, and sodium dichro mate is mOst often employed as a film former inindustrial waters; In old systems, not previously inhibited, about 500 ppm of sodium dichromate are usually maintained at the start. After two or three months and in new systerns,.a-residual of about 300 ppm of dichromate usually proves effective. When insufficient dichromate is employed, pitting is sometimes accelerated. Aeration does not impair the efficiency of dichromates, but does deplete the caustic soda concentration. In large industrial systems, the use of vacuum deaeration has been shown to be effective16. In small systems, the equipment required can seldom be justified, economically. Soft water, as for example the effluent from zeolite softeners, is likely to be several times more corrosive to iron than hard Waters. In small installations, the rise of copper or brass pipe usually- is a practical ex pedient. Cement lined pipe and tanks suitably resist attack. Where the water contains slime-forming organisms, especially those bacteria that thrive on iron, chlorination of the water is imperative to inhibit tuberculation.and subsequent pitting. ..... r Bitumastic paints,-applied at. regular intervals.'vtpon. well cleaned sur-. faces, will measurably prolong the life, of equipment,handling cold -waters.1 Hot Water Services - ....... , _. ... :.'..v As a usual thing; corrosion does not create important-troubles When temperatures are maintained below 140--F.- .- In closed systems where little fresh water is introduced, guch as in a hot water space heating system, corrosion is usually 'negiiglble'bricriSfe thd oxygen released in heating the water is.purged through, the, vents. ,..... Where large amounts-of fresh water-are1 constantly eriteringrand'rire being, heated, the use.:.of. mechanical deaeration is the most;;un.iversally satisfactory expedientito'employ. Where the use of: such equipment can not; be -justified economically, anti-corrosive- nhemiqrils,; and;-the mse of, corrosion, resistant: metals,, are; .the more practical expedients to be- used; Treating Chemicals. ' Alkalies, such as lirrie and caustic soda; silicatesof soda (water glass), the poly-phosphates of sodri,, sodium sulphite, arid sodium dichromate are usually used. " Organic .coinpounds,-such as the glucosates, dextrines, and tannins are sometimes used, but their, value is