INDUSTRIAL WATER SUPPLIES REQUIREMENTS, DEVELOPMENT, AND DESIGN SHEPPARD T. POWELL ?.ND HlLARY E. BACON 330 North Charles Street, Baltimore, Md.
A
MERICAX indust.ries, in nliieli the hnmlliiig of water is of great importance, account for a large portion of total industrial production. A selected group, in which water is of special significance, nianufaetured products valued at almost six billion dollars, according to the census of 1933. Many of tliese indiist,ries, such as the inanufactiire of paper products, textiles, ci~eniicals,and leafrier, iniist also provide for ultiinate diqmal of n.ater used in the plant, which has acquired \&ow waste nmterials from tho inanrifact,uring pmcesxs. In any industrial dcrelopnrent ili which water is a n iinportant processing material or acce plant layout, sliould lie preceded hy careful irivestiga i the quantity arid quality of water required, the supply available and treatment necessary to rnake it suitable for use, and a means of disposal uf the wa.ste liquids incident to tlre entire productive process. Siiali a survey will give consideration to the magnitude of t11e treatrnent plant. and the space required for it, the capacity of pipe liircs, piiinps, and accessorit:s, and t,lre materials of construction best ndnpted to tlre serrirw.
are hy no means rarc, and large c a r h l expenditures could often be avoided if proper investigation had preceded ttir undertaking. To list all of the factors wliicli should influence a decision of this kind would require a detailed study of the needs of a great many industries. JIowever, a number of data are relevant t,o Imct,ically a11 industrial waber supplies: 1. Avsilrthility and seasonal fluctuat,ion of nnder from undcrgiound and surface sowces. 2 . Cheniicd composition mid physiral eharaderistics of all supplies under consideration. 3. Qualities I-evcnled by microscopic and lmcteriological
ariation in q u d i t y rvliicli mny be antieipsted on basis of past records. 5. Predicted influene
tils
dwt.riel r\aste disposal, e 6. Water requiremen cooling, steam generat ion 7. Availability and cost of municipal water supply, and economic study of its Use for sny of the funotions mentioned ahovc. in eomnarison with a nrivate industrial suoolv. 8. ’ Potential future reqdrements for expans&” 9. Interrelation of various water requirements which may permit re-ase and conservation of n.alcr. 10. Modification of temperature, composition,or other qualities of water during plant -e, which may affect its subsequent utility for other purposes, and the influence of these factors on
Systematic Investigation of Factors Involved In practically all industries the provision of an adequatc w n k r supply rcquires ii compretiensirr study and investigation of a nurnhcr of faclors which may 11ni-r a direct bearing on cost of production, Siicli a study should he inaugurated in the early stagcr of a new industrial project. Costly errors have resulted in a miniher of industries diere this necessity has been ignored. Recently a large trnct. was purchased for the cstahlishinent of an industry, before determining the availability of water of adeqnate quality and quantity. It was later discovered that the underground w t i ~ desirable r fur Mle process was not n~nilable,and it was necessary to obtain water froin R distant source a t a hrirdensrrrnp cost,. Sucli cases
the required treatment. 11. Facilities for disposal of rr.aute liquids, and jurisdiotion of
municipal, st.ate, and federal authorit,ies.
S o plant can he adequately rlesigned t.o accomplish the Imt results, from an economic and operating standpoint,
without consideration of the above factors. Securing and developing an adequate water supply for various plant uses ?vi11 depend on intelligent conclusions based on such data. The widely varying requirements of indust,ries make it impossil~leto anticipate sncli concliisions, vliirti must, in the final 615
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1NDUSTRIAL AiXD ENGIXEERING CHEMISTRY
analysis, rest upon specific requireme6%a%~locaI conditions. An investigation on the plan outlined may be most satisfactorily undertaken as a cooperative study by the industrial management and experts in t,he field of water purificat,ion.
Quality of Water Available
VOL. 29, NO. 6
industries also use large amounts of water for cooling and condensing, for steam generation, for potable and sanitary service, and for fire protection. Some of these functions impose no very rigid specifications for quality; others may, in certain industries, require a degree of purity or the presence of Fpccial constituents that can be obtained only by elaborate t,reatrrrent.
The "quality" of water covers a number of characteristics that are generally related to the source of the supply. SurWater Specifications for Process Use face waters contain varying amounts of turbidity; the streams Almost every industry has an individual specification for of the dry interior regions may be many times more turbid than those of the coastal plains. Most surface supplies coiiits process water, and it is possible only to indicate briefly the groups of industries to which specific characteristics of water tain moderate amounts of minerals in solution, and these are dominated by the soluble salts of calcium and magnesium, are of special importance. such as the bicarbonates, sulfates,and chlorides. Sodium and Microorganisms are objectionable not only in foods, heverpotassium usually account for the chloride and nitrate found ages, and fermentation processes, Ixit also in many cases in by analysis, and silica is always present as a minor constituent. which they cause slime deposits in tanks, pipe lines, and In addition to tlie minerals and organic matter leached from other manufacturing equipment. Organic matter responsible the soil and rocks of the drainage area, surface waters also for tastes and odrrrs in t.he m t e r ivill obviously require special acquire pollutidn from sewage and a wide variety of industrial treatnient in the food industries. Turbidity is generally objectionable for all processes and obviously cannot. be tolerated wmtes. Sewage contributes organic constituents and certain where the water is used as a raw material, as in tlie fermentabacterial forms, which draw upon the oxygen dissolved in the water and ultimately produce inorganic nitrates and inert tion and food industries, or as a washing or processing mediimi, as in t.lre textile and paper industries. Wherever an industrial matter by biochemical oxidation. Of the many industrial process aater supply is drawn directly from a surface source, wastes, i t is especially important to mention drainage from coal mines, such as occum in the Ohio River Valley, which it is probable that filtration facilities will be required. Color, which usually derives from vegetable matter with which empties large amounts of sulfuric acid into many streams, and basically alters their chemical characteristics. Organic matwater is in contact, for a considerable period, is generally obiectionahle for its effect on the anuearance of the uroduct. ter from natural sources as well as from uollution. aiid some .. inorganic contaminants frequently confertastes and odors on hut diere this is not significant--as in the Kraft 'and low the water that make i t undesirable for many uses and rewire made naper industry-the treatmrnt of the water would not special corrective treatment. We may saythat tlie most iniLe dicia&d by the urgency of color removal. Iron and manportant constituent8 of surface water arc (1) turbidity, (2) the ganese in very small amounts cause discoloration and stains calcium, magnesium, and silica which confer the undesirable on products and processing equipment and can rarely he neglected if they are present in the water supply in appreciable aspects of hardness, such as scale formation, soap consumption, etc., (3) a great variety of microorganisms, and (4) examounts. Hardness increases the consumption of soap and traneous matter of sewage or industrial origin that may be detergents in washing processes, and the calcinni and magnesium may form insoluble precipitates with other procesying objectionable for a number of reasons. Water from underground sources is generally clear but is chemicals, sucli as the sulfites used in photographic work. apt to be more highly mineralized than surface water. The Where high temperatures are encountered, deposition of scale carbon dioxide in solution enables it to dissolve large amounts or carbonate sludge requires consideration. On the other hand, hardness need not be removed when the water ent.ers of calcium and magnesium salts, iron, and manganese. Although iron and manganese always occur in ininor coneentrainto an acid medium, such as in paper manufacture. Hardness in the folm of calcium sulfate is of benefit to the grorvth tions, they may be extremely troublesoirie since they precipitate as oxidos when the water ia aerated. Occasionally large of yeant in the bread and fermentation industries. concentiations of s o d i n n i are e n c o u n t e r e d . and the g r o u n d waters ire often corrosive because of the low pH value resulting from the carbonic acid present. One of the m o s t i m p o r t a n t characteristics of well waters, froin the i n d u s t r i a l point of view, is thcir low teinperature by c o m p a r i s o n with other s u p p l i e s which are u s u a l l y available. In general, then, underground water supplies are likely to be clear, mineralized to an extent which varies widely with t h e g e o l o g y of the region, and colder than most surface waters. A l t h o u g h for the purp o s e of t h i s d i s o u s s i o n the most i m p o r t a n t u s e Cnirrtery, inlmnafional Filler Compnnp of water is as a raw mateMECHANICAL Eanimem Foa P R O M V P I N ~ FLOC POHM~TIOS TO .hrn IN ~msIneNceor 8nsPENDmD rial or processing mediunr. SOLIDS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I n addition to these characteristics of water which most frequently need consideration, special requirements apply to certain industries. The pH of the water, aside from its effect on the corrosion of equipment, must often be corrected to suit the process in question. As indicated above, calcium sulfate is of benefit in yeast culture and is added to the water used for this purpose. A number of products, such as photographic supplies, refined chemicals, etc., require a degree of purity which can be obtained only in distilled water.
Water Specifications for Other Plant Requirements Cooling and condensing water in enormous volumes is required in many industries, notably in the refining of petroleum and the manufacture of distilled spirits and solvents. It is also necessary in all steam generating plants and in a great variety of other processes. The chief considerations with respect to cooling water are quantity, temperature, corrosiveness, and the tendency to form deposits of mineral matter or organic slimes on the cooling surfaces. Where the required heat absorption is of large magnitude, as in the condensing of steam from turbines used in power generation, economy requires that the cooling medium shall be a surface water that can be obtained with minimum pumping expense. Treatment of such water is usually omitted, with the exception of the application of chlorine or copper sulfate to restrict the growth of microorganisms and occasionally the elimination of gross suspended matter by plain subsidence or auxiliary mechanical apparatus. The undesirable qualities of the water are generally overcome by tHe installation of screens to strain out large solids, the use of corrosion-resisting materials, and special protective measures to retard corrosion. It is feasible to take advantage of the low temperature of well water when the total heat absorption is of smaller magnitude, as in the case of cooling mash before fermentation and certain other processes. The preparation of water for boiler feed purposes is a problem which is widely encountered because of the enormous consumption of steam for process use and power generation. In the absence of a complete treatise on boiler feed water treatment, it may be briefly specified that suspended solids must be absent, that calcium and magnesium must be reduced as much as possible by softening or other means, and often require subsequent treatment in the boilers, that the pH value must be elevated, and dissolved oxygen completely removed to prevent corrosion and that definite concentrations of sodium sulfate must be present to inhibit caustic embrittlement. It is also necessary to assure the absence of large amounts of organic matter, oil, and other extraneous material. The characteristics required in boiler feed water are, in general, dictated by the more rigid specifications for concentrated boiler water, since it is obvious that the feed water must be susceptible of treatment to make it conform to these requirements. More detailed specifications and facilities for boiler feed water purification will be discussed in a later section. The chief requirements for water for fire protection are a large potential supply and high pressure, and it should also be of such chemical composition that it will not deleteriously affect pipes and sprinkler lines. Special supplies and pumping systems are often provided for fire protection, and in some cases a connection with the city mains is maintained. Since the private supply is frequently polluted, care should be taken to avoid cross connections with citymains or drinking water supplies. It is imperative that the pumping facilities installed for fire protection should be of ample capacity and available a t all times. This embodies the use of two or more sources of energy for driving the pumps. If the pumps are electrically driven, there should be two independent sources
617
of power, and auxiliary pumps should be operated either by steam or Diesel engines. The decisions in these matters rest largely upon the recommendations of the fire underwriters but should be based also on independent study by the management. The potable and sanitary water supply for an industrial plant is a matter of regulation by state and municipal health authorities, who have generally adopted the standards of the United States Public Health Service. In brief, such supplies must meet rigid tests which establish limiting concentrations of pathogenic bacteria. When the water supply for process and other plant uses is adequately filtered and chlorinated, it may meet the requirements for potable use. I n many cases a separate system drawing its supply from the municipal mains is installed for drinking purposes. Too frequently industrial management fails to realize the necessity for a safe and adequate drinking water supply for the use of employees. Neglect in this matter creates a potential hazard of the greatest importance. The courts have consistently ruled that the contraction of water-borne diseases by the use of a contaminated water supply furnished to employees is evidence of criminal negligence. Where such neglect has been proved, the courts have awarded liberal damages to the plaintiff. Aside from the potential financial losses which may be incurred, there are likewise many intangible losses resulting from decreased productiveness of employees. There are numerous cases on record in which employers have taken every precaution to make available safe drinking water but have ignored the potential danger from cross connections between safe and unsafe water supplies within their plants and have not eliminated the possibility of employees using unsafe supplies. Dangers of the kind discussed above can be eliminated by providing ample and safe water for drinking purposes and by enforcing strict regulations against any cross connections. The health laws in most states specifically prohibit connections between potable and unsafe supplies. In cases where it is necessary to have accessible connections to unsafe supplies which might be used by employees in violation of orders, such connections should be clearly designated as dangerous and their use should be prohibited. Hazards of this kind are difficult to control. In many large plants authority over the use of water is delegated to a water department. If such departments are properly operated, the danger resulting from the use of impure water is greatly minimized. In addition to this benefit, there is a tendency for the conservation of water and the development of a complete water system layout that facilitates such structural changes as are desired from time to time.
Treatment Facilities for Preparation of Prlocess Water Extensive treatment equipment is often needed to provide the raw water supply and make it satisfactory for process use. To illustrate the magnitude of this equipment and its potential importance for plant layout, it will be profitable to outline briefly the essential elements of a filtration plant for treating a turbid surface water supply. A typical flow diagram is shown in Figure 1. Although the special objective of this treatment is the removal of turbidity, the factors involved are directly applicable to iron and manganese removal, decolorization, and other types of purification. Considerable may be accomplished in the way of clarification by simple presedimentation, where space is available for storage of the water with a long detention period. However, to permit rapid subsidence and efficient filtration, the suspended matter must be flocculated by adding a coagulant, such as an aluminum salt or one of the ferrous or ferric salts. The characteristics of the water and the objectives of the treatment will dictate the choice of coagulants; each is
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 29, NO. 6
adapted to a certain pH range arid t.o t,lie effective precipitation of various types of turbidity, color, etc. In the chemical coagulation ot suspended matter it is necessary to provide equipment for several functions, and in each case the magnitude of the plant should hear a certain relatioii to the quantity of water to Le treated. Chenriea1 feeding devices whieli apply the coagulant and alkali to the water in accurate proportion to flow are reqnired, and their design may Le hased on tlie predicted dose of e i i e n i i e a l s . Ordinarily the quantiiy of coagulant roquired may be 1 or 2 grains pcr gallnu, but with wr5turbid or highly colored water it may hc as much as 5 grains per gallon. I h y chemical feeders (A, Figure l),controllcd Iiy a proportirnring device from a nieter in the main mpply line, are applicable to large systenis, or a variety of solutioii feeders may he used. 'We mixing and reaction of tlie chemicals and the formatiou of $he floc may he effect,ed by siniple bafflcs which give tlie desired velocity and turbulcnce. OT liy inechanical devices ( R , C ) \\.hicti have lieen developed to carry out this part of the t,reatment, with the highest possible efficiency. The benefit8 of mechanical niixiiig and flocculation will often justify the added expense, especially if the raw water is difierilt to treat or if space is a t a promYiim. h m u c l i as 3O-minute rirtrntion inay IE ~~rorirlrdin t,liir part of tlie
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INDUSTRIAL AND ENGINEERING CHEMISTRY
system; thus a million-gallon-per-day plant would devote about 2800 cubic feet to the preparation of coagulated water. The subsidence of the heavier floc is then effected in a basin, D , through which the rate of flow is greatly reduced, and the extent. of removal of solids by this method will depend on the detention time available. A common basis for design is +hour detention which, for the million-gallon-per-day plant, would correspond to about 22,000 cubic feet. Although detention of the water during flocculation and subsidence is often considerably less than suggested above, any deficiencies in this part of the plant impose heavier loads on the filters and contribute to their operating difficulties. After partial clarification by subsidence, the flocculated Jrater is passed through filters of sand, calcite, magnetite, or anthracite coal, E . The water may flow through open filters under gravity head or through closed pressure filters, but in either case the total area of filter bed must be large enough to accommodate the total supply required. Filtration rates of 2 to 3 gallons per minute per square foot or less are consistent with good performance, and control devices should be installed in the effluent lines to prevent the designed rates from being exceeded. Sufficient area should be provided so that rates need not be excessive when part of the system is being backxashed. For the plant with the capacity of a million gallons per day, satisfactory design might require 300 square feet of filter area. Usually the filtered water is discharged to a clear well, F , and it may be desirable to pump it to a head tank, G, to provide flexibility of operation. In designing the plant, provision must be made for water to backwash the filters, which may require special pumps taking suction from the clear well or in some cases from the subsidence basin. Backwash water iiiay be of the magnitude of 15 gallons per minute per square foot of filter area, and sewer lines, H , to accommodate this flow iiiust be provided. If air is used to assist in backwashing, the quantity of water may be reduced. It is cenimon practice t o use pressure filters without pre-
619
coagulation in a subsidence tank. Where this is done, the coagulant is applied to the water as it is discharged to the filter. Such a system is shown in Figure 2. The advantage of this type of system is that it is relatively less expensive than a gravity plant, and the water to be filtered may be passed through the units directly from the pressure main ‘ without repumping. Such installations are not ordinarily as efficient as gravity systems and may not be depended upon to deliver a satisfactory water where strict specifications are imposed for color, turbidity, iron, and manganese removal. They are objectionable also in certain industries, such as rayon and bond paper manufacture, unless the steel tanks are lined to avoid coloration of the water by iron picked up from corrosion. This type of installation may be unsatisfactory because of the short period allowed for coagulation between the point of application of chemicals and the filters. In the choice of pressure filters the so-called vertical units are much to be preferred to horizontal units, since they are subject to less danger of upsetting the filtration bed during backwashing. It is desirable also that pressure filters be operated under a constant rather than a fluctuating head, and operation within a maximum designed capacity should be maintained by means of an orifice in the efRuent line. In designing pressure filtration systems, capacity should be provided for the maximum demand with one unit out of service. Details of the design of the various components of a filtration system cannot be specified, since these will vary with the individual plant and should be worked out by competent water supply engineers. However, the figures mentioned are given to emphasize the space requirements of water purification equipment. Also it is not a matter of indifference whether such space is provided a t different elevations. The floc generated by suspended solids and chemical coagulant is susceptible to disintegration if subjected to violent physical impact or high water velocities. It is therefore desirable to build the coagulating and settling basins and gravity filters, if these are used, on essentially the same elevation, and dimen-
A CHEMCAL
FEECERS
‘iOPORTIONlNG
COAOULATING COMPARTMENT
SUBSIDENCE
-
T O PROCESS
BASIN
.__c
To SUPPLEMENTAW TREATMENT OF BMLER FEEWATER
E FILTER3
H TO S
CLEAR
FIGURE 1.
ARRANGEMENT O F A
WELL
GRAVITYFILTRATION SYSTEM
FOR REMOVINQ SUSPENDED SOLIDS AND COLOR
VOL. 29, NO. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
620
sions of pipes or flumes carrying flocculated water must be large enough to permit the desired velocities. Dropping the water through any considerable height or raising it with pumps is highly prejudicial to the persistence of a satisfactory floc. When pressure filters are used, they may be a t a lower elevation than the discharge of the subsidence basin, since the velocity of flow is throttled a t the effluent valves of the filters. I n view of the relatively rigid specifications which apply to the space occupied by filtration plants, it is obvious that the
bonate, or phosphate, and these are removed by subsidence and filtration. In the other group of softening processes the water is passed through a bed of base-exchange minerals which replace the calcium and magnesium with ions that are not objectionable. Chemical process softening, mentioned above as the first group, is accomplished by adding to the water alkaline reagents such as lime or sodium hydroxide to convert bicarbonates to carbonates, and by supplying sodium carbonate or phosphate to precipitate the sulfates and COAGULANT SOLUTION FEEDER chlorides of calcium or magnesium. Variations in this general scheme include the use of coagulants to accelerate subsidence, and the application of barium salts to reduce the I II I I1 FILTERS concentrations of sulfate and carbonate in the finished water. The latter treatment is somewhat limited by the cost of barium compounds, however. The process may be carried out a t I I A b 4 A h various temperatures and in continuous flow or TO SEWER 4 1 4 1 i intermittent batch tank equipment; the latter TO PROCESS is adapted to cold-process softening in which long reaction and subsidence periods are required. It may be stated in the absence of a n extensive discussion of chemical process softening that equipment must be provided OF A PRESSURE FILTERINSTALLATION FIGURE 2. ARRANGEMENT for the functions of chemical feeding, mixing, reaction., coaeulation. subsidence. and filtralayout of the plant as a whole should give careful consideration. I n these the dimensions of the equipment are related tion to these needs, not only for existing and predicted water to the volume of water to be treated in much the same manner requirements but for any conceivable expansion. An increase as was specified above for a filtration plant. When high temin the capacity of a filtration plant is a major operation when peratures are employed, the detention time can be decreased other structures and equipment have been installed so as to considerably. A schematic diagram of a hot-process continucircumscribe the original coagulating and subsidence basin ous softening plant is shown in Figure 3. and filters. The softening of water by base exchange requires equipment similar in most respects to gravity or pressure filters in The considerations applying to filtration have been given in which the filter medium is replaced by a base-exchange some detail above because they also hold in the case of other mineral. A variety of minerals is available, and recent reforms of purification. The removal of color is accomplished search and invention has greatly increased their field of usein an entirely analogous manner by generating a floc which fulness. The naturally occurring greensand zeolite minerals will absorb the organic coloring matter in the water. The are sodium aluminum silicates which exchange the sodium for removal of iron and manganese is accomplished by oxidizing calcium or magnesium of water which is filtered through the these elements by aeration and raising the p H to precipitate bed. The sodium is restored by regeneration with brine. A the hydroxide. In both cases a ferric floc carries down the number of synthetic zeolites are available, and these have undesirable constituents. The flocculation, sedimentation, properties especially applicable to certain needs, such as high and filtration require equipment similar in every respect to exchange value, etc. There has recently been developed and that specified above for the removal of turbidity, and the placed on the market a synthetic material of carbonaceous same consideration of space should be taken into account. I n composition which will exchange hydrogen ions for sodium, the filtration of water for the removal of iron or manganese, it is d e s i r a b l e to provide compressed air for backVENTED GASES TO ATMOSPHERE CONTINUO'& BOILER BLOW DOWN washing filter beds, in addition to backwash water. This assures much better EXHAUST STEAM removal of the precipitate and cleansing of the sand than can ordinarily be ob'OEAERATING HEATER tained without such provisions. (c
-&
I-
-
,
Equipment for Softening Water Two general methods of softening water are available for consideration in designing the water supply system. In one group of processes insoluble salts are formed, such as calcium carbonate or phosphate and magnesium hydroxide, car-
TO BOILER F E E 0 PUMP
!
FEEDER FOR SOFTENING CHEMICALS
OF APPARATUS REQUIRED FOR BOILER FEEDWATERCONDITIONING BY FIGURE 3. ARRANGEMENT A HOTLIME-SODA PROCESS
magnesium, calcium, and other positive ions, thus replacing carbonates with carbonic acid, etc. Themineral is regenerated withdiluteacid, The" same material may be operated on the sodium cycle and regenerated with brine or other sodium salts, and is of special value where silica must be avoided. Since the carbonic acid which is produced by softening high-carbonate water with the hydrogen zeolite can be driven off by aeration or by raising the temperature, i t is obvious that a reduction in the solids dissolved in the water may be brought about with this method. The choice between chemical-process and baseexchange softening depends on a number of factors, the most important of which is the composition of the raw wster. Extremely high concentrations of bicarbonate hardness are economically removed with l i e ; on the other hand, the simpler o p e r a t i o n a n d m a i n tenance required for zeolite softening may he utilized if the hardness is moderate in amount. Under some conditions it is economical to precede zeolite softening hy cold chemical-process softening. The flow diagram of such B system is shown in Figure 4. When the raw water supply is turbid and requires clarification, this can be carried out incidental to limo-soda softening, which would therefore be indicated as the most economical treatment. When the temperature of the raw water must he raised hecause of its use for cooling purposes, zeolite softening is not feasible, while chemical process softening will he assisted by the high temperature of the water.
Treatment of Boiler Feed Water and Boiler Water I n order to form intelligent conclusions as to the type of boiler feed water treatrnerit required, it is necessary to consider a number of factors related to water treatment for other 'plant needs and to take cognizance of recent trends ill boiler plant design and advanced methods of control. The two more important conditions which have influenced the art of boiler feed water treatment are the increase of boiler operating pressure and tlre employment of back pressure turbines for power generation incidental to the production of steam for Drocess needs. In the oueration of d a n k of this type it is necessary to enforce rigorous specifications on the quality of boiler feed water and the purity of steam. The prevention of scale in boilers is not a difficult problem when
the operating trmperatures are moderate, cmresponding to pressures of not more than 250 pounds per square ineli. As pressures, temperatures, and rates of evaporation per square foot of heating surface are increased, tlre technic of scale prevention requires considerable skill and careful operation. I n maq- plirnts a high percentage of boiler feed water makenn is required, since recorery of steam as -~r condensate is oftcn limited in process industries. T I I ~ quality of foed watt.]. .that mrist be treated is therefore often quite large. These and other i:ircunistances require that preparation of the feed water should accomplish as complete removal of scale-fonning solids as is possible and should be supplemented by internal treatment of the boilers. The extent. and refinement of the treatment will vary with individual plant requirerneuts. Either the chemicalprocess or he-exchange s o f t e n i n g system, both of which have been described, may he installed to meet the requirements of boiler feed water alone when softening of process water is not required, and the considerations mentioned in the foregoing discussion are ~~
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VOL. 29, NO. 6
equally applicable. A recent development in combination the steam which would seriously interfere with certain other processes or with the performance of steam turbines, and, in softening, especially for boiler feed water use, is the combination lime-soda and phosphate treatment. The plant is general, the specifications for steam quality must be established for each plant. Since the concentration of the boiler shown schematically in Figure 5. A typical analysis of the water a t each stage of the process is given in Table I. waters must be maintained below the critical limit by blowing This system was installed to prepare water for boilers operatdown the salines from the boiler drums and replacing them ing a t 750 pounds per square inch pressure and has completely with new make-up water, it is obviously advantageous to inhibited scale formation and corrosion of the boiler and obtain a feed water of the lowest possible mineral conceneconomizer. tration. The economic cost of boiler blowdown is frequently In addition to scale formation, other boiler operating difficulties require supplementary treatmentLON-PRESSURE ECUKJMIZER notably, corrosion, embrittlement of the boiler steel, and contamination of the steam by entrainment of LLME- SODA SOFIEPING T4NG boiler water. When steel is in contact with water a t the temperatures encountered in steam generating equipment, corrosion presents a serious problem, and the most important corrective measures depend on the fact that the rate of corrosion is approximately proportional to the dissolved oxygen content of the water. Modern boiler feed water heaters elevate the tem. perature to the boiling point, where the solubility FIGURE5. FLOWDIAGRAM O F LIME-SODA AND HOT PHOSPHATE of OxvPen is nearlvzero. and bv means of the SweeD BOILSRFEEDWATERSOFTENING SYSTEM of steak and venting of ex,ha;st gases the residual oxygen in the water is reduced to a very small the deciding factor in the choice between alternate methods concentration. However, the occasional persistence of corroof feed water treatment which may be under consideration. sion even under these conditions has led to the use of The types of equipment for boiler feed water treatment chemicals in some plants to remove dissolved oxygen. Chemiwhich should be considered in plant layout will usually be cal deaeration has been most widely accomplished by means designed in conjunction with the installation of the boiler, of sodium sulfite, which is added to the deaerated water to and it is not possible to give general specifications. In addiabsorb traces of remaining oxygen. tion to water softening apparatus, such as has been discussed, it is necessary to provide chemical feeding devices to apply TABLEI. COMPOSITION OF WATERAT VARIOUSSTAGESOF the supplementary treatment which may be needed. These COMBINATION LIME-SODA AND PHOSPHATE SOFTENING may apply the chemicals a t various stages in the feed water Raw River Lime-Soda Phosphate-Softened cycle or direct to the boiler drum, and should be chosen with Water Softener Effluent Water respect to the operating pressure required and the nature of Total hardnee? 169 10 0= 0 46 40 Phenolphthalein alkalinity the material to be handled. 99 77 64 Methyl orange alkalinity .. 2.5 POP DH Q
7.6
9 :6
10.2
Soap test: actual calcium salts, 2 to 4 p. p. m.
A serious operating difficulty which sometimes causes failure of boilers is the cracking of the metal because of a type of deterioration known as caustic embrittlement. This has been the subject of considerable research and discussion and is familiar to those who are responsible for boiler operation. An accumulation of operating data from a large number of plants indicated that embrittlement was inhibited wherever sodium sulfate was present in the boiler water in quantities equal to or larger than the sodium carbonate alkalinity. On the basis of these results, and in the absence of conclusive evidence in favor of any existing theory, the A. S. M. E. Boiler Operating Code was modified to specify that certain ratios must be maintained between the sodium sulfate and the alkalinity in the boiler water, depending on operating pressures. Boiler feed water treatment is therefore customarily designed with a view to establishing ultimately the necessary sulfate concentration, either through building up to the proper ratio in the feed water make-up or adding sulfates direct to the boilers. It should be noted that preliminary water purification which keeps the alkalinity as low as possible may often be a t a premium because of the A.S.M.E. Code requirements and the necessity for the addition of sulfates. The carry-over of boiler water into the steam, while influenced considerably by boiler design and operation, will usually begin to cause difficulty when the concentration of solids in the boiler water reaches a certain limit. Some types of process steam consumption can tolerate impurities in
Consideration of Liquid Waste Disposal in Plant Layout The volume of liquid wastes which is incidental to the productive process may be of very large magnitude, and the chemical and physical characteristics of the material may render it a serious problem. Disposal often takes the form of discharge into nearby surface waters or into municipal sewers. However, it is becoming increasingly difficult to find this means of outlet for liquids which impose heavy pollution loads on streams and rivers or which add to the difficulty and expense of sewage treatment. The jurisdiction of municipal, state, and federal authorities in this field is being continually extended and it is of the greatest importance, in designing a projected industrial plant, to establish in advance a means of waste disposal or of treatment within the plant. The treatment and recovery of industrial wastes has too many and varied aspects to permit a profitable brief discussion of methods. It may be said, however, that some undesirable constituents may be removed by precipitation, coagulation, settling, and filtration, while others may be neutralized. In some cases there is no existing solution except the evaporation of water and disposal of the waste material in concentrated form. It is important in plant layout to take cognizance of the magnitude of the plant which may be required for such treatment and to relate it properly to the water purification, steam generating, and industrial process equipment. RECEIVED May 4, 1937.
