Document NGDqdZXXXmKOpRYLB8vvN447g

756 CHAPTER 55 being processed and the concentration of solids due to evaporation are factors in air washer operation when the air is humidified. The concentration of scale-forming minerals in open re circulated cooling water is limited by' natural drift or wind age loss. This is the loss of water droplets from the system. Windage losses from typical systems may be classified as follows, based on recirculating rates: Ttpe op Ststem % o Rectbcdlating Rate Evaporative Condensers and Air Washers. . 0 to 0.1 1960 Guide 1.0 to 5.0 To show the effect of concentration in a typical mechani cal draft system assume: 1. Recirculating rate * 100 gpm. 2. Drift loss -* 0.2 percent of recirculating rate = 0.2 gpm. 3. Evaporation rate = 1 percent of recirculating rate -- 1.0 4. Calcium bicarbonate in make-up water -- 100 ppm. 5. Calcium bicarbonate in evaporated water -- 0 ppm. Let K concentration of calcium bicarbonate in recirculat ing water " concentration of calcium bicarbonate in drift loss. The process of concentration in the system may be repre sented diagrammatically as follows: Evap. * l gpm Ca(HCO,)t -- 0 ppm T____________ Recirculating Water System K -- Ca(HCOi) icodc. t Make-up 1.2 gpm Ca{HCO)t = 100 ppm Drift 0.2 gpm Ca(HCOt), - K ppm The concentration may be obtained from the equation (Make-Up)[Ca(HCO)i cone.) = (Evap.)|Ca(HCOi)t coqg.] -KDrift)lCa(HCO) conc.J (1.2) (100) - (1)(0) + (0J2)(K) substituting, 120 - 0 + 0.2 K whence, K = GOO ppm Therefore the concentration of calcium bicarbonate in the recirculating water equals 600 ppm. This far exceeds the allowable concentration of 170 ppm at which scaling will occur. To correct this situation, it is imperative to provide a continuous bleed-off or blowdown from the recirculating water circuit. Typical bleed-off requirements are shown in Tig. 5. Reference to the No Treatment curve shows that a bleed-off rate of 12 times the evaporation rate is required when a make-up water contains 100 ppm calcium biear- Ftg. 5 .... Relation of Bleed-Off Requirement to CaCOi in Make-Up Water bonate (or alkalinity) if scale is to be prevented. Calcula tion by a material balance will show that the bleed-off plus drift loss will limit the calcium bicarbonate to 171 ppm. It can be seen from Fig. 5 that very large amounts of bleed-off are required as a make-up water approaches or exceeds 150 ppm of alkalinity. When alkalinity exceeds 175 ppm, bleed-off alone is no longer effective. The use of 5 ppm or less of polyphosphate is extremely effective in re ducing the required bleed-off and making possible the use of make-up waters which will not respond to bleed-off alone. Occasionally, waters are encountered which have excep tionally high alkalinities or which develop high pH values when recirculated. The use of sulfuric acid is common in these cases to maintain pH values between 7 and 8. Poly phosphates are most always used when acid is required. It is very important to control acid feeding carefully to avoid serious corrosion. Automatic pH controllers are often employed for such a purpose. Gosed Hot Wafer Hearing Systems This section excludes residential and also other systems in which leakage or loss of water by use or drainage is insignifi cant and in which consequently, corrosion and formation of scale are not encountered. Corrosion in a closed system is due to the dissolved gases, primarily oxygen, in the make-up water ami to' electrolytic attack arising from contact of dissimilar metals in the same water circuit. The higher the water temperature the more severe will be the corrosion. Corrosion control methods may be either mechanical or chemical. The mechanical method usually deaerates the recirculating water to remove corrosive gases but it removes only dis solved gases and hence does not prevent electrolytic action. Since deaerating equipment can be expensive it usually is not warranted for hot water systems. The chemical methods are of two types: (1) the use of so dium sulfite, tannins, or both and (2) the use of corrosion inhibitors. Oxygen removal by chemical means will not pre vent electrolytic attack which must usually.be prevented by additional chemical treatment. Furthermore, the use of chem ical methods for oxygen removal usually will result in a buildup of total solids in the system and may cause sludges to form and reduce the rate of heat transfer. The most common method for the control of corrosion in closed hot water systems is to use corrosion inhibitors, such Corrosion and Water-Formed Deposits, Causes and Prevention 757 as chromates or nitrites. Corrosion inhibitors, when used at proper concentrations, not only prevent attack by dissolved gases such as oxygen, but also prevent electrolytic attack be tween riifignnilflr metals. Normally, pH adjustment of the re circulated water is not necessary. However, if the pH is below 7.0, an alkaline material should be added to obtain a pH value between 7JO and 8.5. If a pH higher than 8.5 is found, it should be reduced to fall within the 7.0 to 8.5 range by the careful addition of small amounts of mineral acid. The required con centration of corrosion inhibitors depends upon the tempera ture to which the water in the system 13 heated and must be increased as the water temperature becomes higher. New systems normally are contaminated with dirt, grease, oil and other matter which, unless removed from the system, could interfere with heat transfer. They should be cleaned frith an alkaline, detergent-type cleaner before being put in operation. After cleaning, the system should be thoroughly drained and flushed. Low-Pressure Steam Hearing Systems A low-pressure steam heating system is one operating at a pressure not exceeding 15 prig. The amount of condensate re turned to the boiler may be the equivalent of 90 to 100 per cent of the water evaporated. Since air moves in and out of steam systems through the air vent3, the condensate is freely exposed to oxygen, some of which is dissolved and returned to the boiler together with free carbon dioxide and other gases that may also be in contact with the condensate. Make-up water for the system will also introduce the same gases and in some cases may have a high free carbon dioxide content with consequent low pH value. Corrosion in low-pressure boilers is due mainly to dissolved oxygen in the water entering the boiler. Corrosion is therefore usually prevented by chemical or mechanical removal of oxy gen. On some of the larger systems deaerating feed water heaters may be used but since they will not normally remove all of the oxygen enough may remain to cause corrosion. It is customary even when mechanical deaerating equip ment is used, to supplement it with chemical removal of the oxygen. This is usually accomplished by feeding chemical treatment internally to the boiler water. Such p-hf-mical treat ment will consist of sodium sulfite, in conjunction with tan nins and alkalinity-forming chemicals. It is considered good practice to' maintain a pH value in the range of 10.5 to 115 and a sulfite residual of 30 to 40 ppm, at all times. Before proceeding with chemical treatment, enough water should be drained from the bottom of the boiler to remove any sludge or precipitates that have collected. High-Pressure Steam Heating Systems This section deals with proper treatment of water for use in high-pressure systems. The treatment and other considera tions also apply to those low-pressure systems in which large amounts of untreated water are added to the ennHwnsato re turned to the boiler. The problems encountered are due to (1) scale formation, (2) corrosion, (3) pitting, and (4) foaming and priming. Scale is caused primarily by calcium and magnesium salts which precipitate as carbonates and sulfates. In waters hav ing a high silica content, silicate scales may also be encoun tered. In order to prevent the formation of scale in a system, it is necessary to feed a proper phosphate and organic sludge con ditioner, maintain the correct pH, and provide an adequate blowdown. Where extremely hard waters are encountered, or where very large amounts of untreated make-up water are employed, it is sometimes desirable to soften the untreated make-up water in order to reduce chemical treatment costs. When there is little return of condensate to such systems, the method of feed is of utmost importance if adequate control of scale formation is to be obtained. Accepted practice indicates that pH values should be main tained between 105 and 115. In addition, sufficient phosphate should be fed to the boiler water to maintain a phosphate residual of 30 to 50 ppm at all times. There are a number of different types of phosphates which can be used, the se lection of which depends upon operating conditions. In ad dition, a sufficient blowdown must be provided (preferably a bottom blowdown) to maintain total dissolved solids in the boiler water at a satisfactory low level. The permissible con centration of total solids is dependent upon the make-up wa ter characteristics and the method of operating the system. Corrosion and pitting are due primarily to dissolved gases present in the boiler water, but also can be caused by an add condition of the untreated feed water. Pitting and corrosion caused by dissolved oxygen can be prevented by the use of a feed water deaerating heater and chemical deaeration by means of sodium sulfite, hydrazine, or tannin. A sulfite re sidual of 30 to 40 ppm should be maintained at all times. For pH control, an alkaline material should be used when necessary. The use of soda ash should be avoided, because when soda ash is heated by the boiler water, carbon dioxide is liberated. The free carbon dioxide will be carried over with the steam, and cause serious corrosion of steam condensate lines. An alkaline material which does not have a carbonate content should be used whenever possible. For best results in treatment of boiler water, a separate chemical should be used for controlling each of the following: phosphate, pH, and sulfite. In many cases, attempts to con trol all three by means of a ringta compound may not produce the required results. Other problems encountered in heating systems include foaming and priming which will invariably lead to the unde sirable carryover of boiler water into the steam system. Foam ing and priming may be due to the poor quality of untreated boiler feed water which, for example, may contain much or ganic and suspended matter or a large amount of total dis solved solids. Even when feed waters are of relatively good quality, foaming and priming can occur if there is inadequate blowdown or inadequate control of internal chemical treat ment. Anti-foaming chemicals are available for reduction of foaming. Minimizing Steam Condensate Corrosion Corrosion in steam condensate lines is due to the presence of deleterious gases, such as oxygen and carbon dioxide. There are three methods which may be used to minimi*a this type of corrosion: 1. Treatment of the boiler feed water so as to eliminate un desirable gases, such as oxygen, which otherwise would be en trained with the steam. _ 2. Chemical treatment of the condensate so as to neutralise dissolved gases, such as carbon dioxide. 3. Use of film-forming types of chemicals, which will protect or isolate the metallic surfaces from the condensate. Elimination of oxygen in the boiler water, and therefore from the steam itself, can be accomplished either mechani cally or chemically, as described in the section on High-Pres-