Cerna, Wendell W.,Proc. Seventh Annual Kater Conf., Eng. SOC. of W.Pa. (Jan. 6-8, 1947). Hall, R. E., Proc. Ninth Annual Water Conf., Eng. Soc. of VI‘. Pa. (Oct. 18-20, 1948). Hall, R. E., Trans. A n . Soc. Mech. Engra., 6 6 , 457-88 (1944). Ongman, H. D., Combustion, 24 No. 8 , 40-4 (1953). Purcell, T. E., and Whirl, S.E’., Trans. Am. Soc. M e c h . Engrs., 64, 397-402 (1942). Rivers H. M., and Sonnet:, W. XI.>Combustion, 21, No. 12,41-6
(1950). Ychroeder, W. C., and Berk, A. A,, C . S. Bureau of Xlines. Hull. 443 (1941). Straub, F. G., Mech. EILQ., 61, 199-202 (1939). Thornleg, J. L., Industry and Power, 65, S o . 2, 58--G1, 1953. T ~ a n sAm. . SOC.Mech. Engrs., 64, 393-444 (1942). Ulmer, R. C., Whitney, J. H., arid U’ood, J. W., P I O CAnieilcan . Power Conf.,XIV, 459-67 (1952).
Discussion +
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If 3Fe 4H20 --* FerOI 402 is the main reaction for good film (thin) protection, how do you account for FeaOl and Fe in boiler deposits? What is method of setting FeaOc filnitime, temperature, and alkalinity? R. F. ANDRES: The relative amounts of t,he various iron oxides in boiler deposita are controlled by the oxidizing or reducing characterist)ics and dissolved oxygen content of the water in contact with the metal surfaces. These factors vary throughout the plant cycle and change wit’h temperature, pH, and ferrous ion activity. An excellent discussion of the reactions of iron with mater is presented in a paper by Marcel Pourbaix, ,‘Some Applications of Electrochemical Thermodynamicfi,” C o n m i o n , 6 , No. 12,395404 (1950). The various factors controlling protective magnctic iron oxide film formation have not been thoroughly investigated. Much fundamental research ~ o r kon ii.on-wat,er reactions must be completed before definit,ive rules can be established for t,he hest methods of forming prot,ective iron oxide films on boiler metal surfaces. What concentrations of alkalinity are necessary for destruction of protective ironbxide film on boiler metal? R. F. AMDRES: I cannot define limits for the concentrations of alkalinity that destroy protective iron oxide film in an operating boiler. It has previously been repovted that minimum corrosion of iron results when the caustic concentration in the \rater is in the mnge 40 to 400 p.pni. corresponding to pH values of 11 to 12. Caustic concentrations as low as 5% may be aggTessive to boiler steel according to C. E. Kaufman, I-, hl. AIaroy, and W. H. Trautman, “The Behavior of Highly Concrnt,rsted Boiler Kater,” Proc. S:’zth dnnzcol TT‘aier Cor!f., pp. 23-42 f 1945).
What is the difference in the mechanism of this corrosion and the corrosion at low pH? R. F. ANDRES: The corrosion of iron at low p H values in Kater solutions results from removal of the polarizing hydrogoti film and the absence of any formation of protective oxide films. Caust,ic attack, on the other hand, is charactri.ized by removal of protective iron oxide films as soluble sodium ferrite or ferratp folloFed by reaction of the water with the metal t o replenish thcl oxide film. If the caustic concentration remains high, this film formation is prevented and corrosion continues. At what pH does the corrosion at the so-called excessive caustic concentration occur or accelerate? R. F. ANDRES: Above pH 12 the relative attack of caustic on steel increases very rapidly. I t is impract,ical to attempt to designate the pH above m-hich corrosion may occur because il small increment of pH represents a high increase in caustic, concentration in the pH range 12 to 1.1. I heard a paper recently which reported that nitrogen was added-i.e., a nitrogen atmosphere was maintained-in order to reduce corrosion due to oxygen in water. Have you any experience along these lines or could this be adapted to boilers in any way? The work was laboratory work on the corrosion of metals and alloys in which they proved that removal of hydrogen was as important as the removal of oxygen. R. F. ANDRES: I cannot visualize maintaining a nitrogen atmosphere in contact with n-at8erin an operating boiler. Piitrogen has been used following acid cleaning of boilers to prevent, excessive iron oxide formation which would result if air came in contact mit,h t,he highly reactive pickled metal surfaces. For, protection of idle boilers, a nitrogen blanket could be maint’aine.1 under pressure to exclude oxygen from the boiler. Sulfite breakdown to H,S (not SOa) seems to be associated with dry areas (faulty circulation) in the boiler, whereas SO, formation is a matter of equilibrium. The breakdown is caused apparently by the formation of nascent hydrogen and usually accompanies the formation of excessive magnetic iron oxide in the boiler. Any comments? R. F. ANDRES: The presence of sulfide in a boiler or evidence of hydrogen sulfide in steam is an indication of sulfite breakdown probably resulting from the causes mentioned. Elimination of sulfite h a t m e n t mill not correct the boiler corrosion problem but may prove beneficial in combatting possiblc corrosion from hydrogen sulfide in other portions of the natersteam cycle. The sulfur dioxide evolution with the strain from sulfite trpated boilers is n chemical equilibriii.:u. reaction arid tho quantity is dependent on temperature condit,ions arid the concentrat,ion of sulfitr h i n g maintained in tlic watei..
Foaming in Boilers W. L. DENJIAN Deurhorn Chemical Co., Chicago, Ill.
OIIZR water forming in general is a term applied loosely to cover the formation or generation of steam containing liquid water and is to be differentiated from eo-called spray carryover. In the case of foam, the water is actually present in the steam as free liquid water; whweas, in the case of spray carryover the boiler water ie dispersed in the steam as fine globules. 992
Foaming does not result from the formaticin of a distinct foam layer or blanket on the water surface; rather, it results primarily because the apparent volume of‘ the hoiler water increases dur to the presence of an extremely large number of very small stable steam bubbles within the volume of the boiler water. Small steam bubbles rise much RIOKPI. tha.ri do larger steam bubblrs
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Boiler Water Chemistry-. * and, hence, remain within the volume of the boiler water for a much longer time. Steam bubbles forming in a foaming boiler water have been shown by high speed photography t o be small because of a pronounced tendency to resist coalescence; whereas, steam bubbles forming in a nonfoaming boiling water in a similar manner have been shown to be large because of a very pronounced tendency to coalesce. High speed photographs were first taken of a foaming solution, and then, after a polyamide antifoam agent had been added. The photographs taken of the foaming solution show that numerous small steam bubbles are present and that these steam bubbles remnin in intimate contact without coalescing. The photographs taken after the polyamide antifoam agent had been added show that the small steam bubbles are rapidly coalescing into large steam bubbles. This coalescing occurs a t both the heating surface and in the body of the boiling water. The large steam bubbles are irregular in shape because of distortion during movement in the boiling water and because in many cases the nonspherical steam bubbles formed by coalescence have not had time to round out. FOAM PROMOTERS
The foaming tendency of any given boiler water may be attributed to both the impurities present in the water and to the construction of the boiler. Of the two, impurities present in the water are the more important in contributing toward foaming. Pure water does not foam. Foam formation in a steam boiler is affected by the following factors: 1. 2. 3. 4.
Total dissolved solids in the boiler water Finely divided solidfi in suspension Organic matter-sewage and decayed vegetation Colloidal matter-fine clays, oils, and soaps 5. Design of boiler: Steam space affected by water level Rate of evaporation Locations of steam outlets 6. Variations in load or steam demand 7. Operation of throttle and reverse lever (locomotive boiler only) 8. Operation of pops and whistle (chiefly on locomotive boiler) ’
Foaming of boiler water may be controlled by either deconcentration (blowing down the boiler) or by the addition of antifoam chemicals to the boiler. Boiler blowdown merely dilutes the boiler water with fresh feed water. The use of antifoam chemicals increases the permissible over-all degree of concentration of the boiler water and is justified by the resulting savings in the fuel and water. At times such antifoam chemicals are necessary to operate with limited storage of feed water. EARLY ANTIFOAMS
Among the earliest antifoam chemicals employed were certain so-called black oils. These were crudely refined mineral oils and contained a considerable amount of material of a polar nature. They did not find great favor because of the relatively low degree of antifoam effectiveness. Castor oil early became established as a boiler water antifoam chemical, and for a considerable time it was the standard antifoam chemical used throughout the steam generating industry. It suffered from the serious handicap of very rapid hydrolysis in the presence of the hot alkaline boiler water to form, among other things, the sodium soap of ricinoleic acid. After starting treatment with castor oil the boiler water became contaminated with soap, and unless the treatment was carried on continuously the boiler water foamed more severely than it would without treatment. Because of the disadvantages common t o castor oil, attempts were started early to develop antifoam chemicals which would
May 1954
have a high degree of antifoam effectiveness, which would have a relatively high antifoam boiler life, and which would not contaminate the boiler water with foam stabilizing substances. Among the chemicals arising from such developments were aliphatic amino compounds, polyamides, and the so-called polyoxy chemicals. At the present time, the principal antifoam chemicals used are either polyamides or polyoxyalkyleneglycols and derivatives, the so-called polyoxy chemicals. POLYAMIDE ANTIFOAMS
Polyamides are unusually effective antifoam agents and remain active in the boiler water for a prolonged period of time giving steam of a high purity. The long life of polyamides in boiler water is a result of very marked resistance to hydrolysis and is in marked contrast to the behavior of simple amides which hydrolize very readily. Polyamides used may be derived from diamines, triamines, and tetramines or from polyamines or mixtures thereof, and from a wide range of carboxylic acids. In general, best antifoam activity will be obtained from some particular combination of amine and carboxylic acid. For a given amine, optimum antifoam activity will be obtained over a limited range of carbon atoms in the carboxylic acid, and a carbon content either above or below this range will result in a falling off of antifoam activity. Likewise for a given carboxylic acid, maximum antifoam activity will beobtained from one of the several amines. Boiler water antifoam effectiveness of 1. Diamides of ethylenediamine is peaked a t stearic acid and falls off rapidly with either palmitic acid or behenic acid. 2. Diamides of diethylenetrianiine is peaked between palmitic acid and stearic acid and falls off rapidly - - with either myristic and behenic acid. 3. Triamides of diethylenetriamine is peaked between lauric acid and Dalmitic acid and falls off with fatty acids of greater or lesser carbon content. 4. Distearoyl amides of dibasic acids is peaked with two carbon atoms between the carbonyl groups (adipic acid) and falls off with more or less than two carbon atoms between the carbonyl groups (succinic or sebacic acids). 5. Distearoyl amides of alkylenediamines (18 carbon acyl radicals) is peaked between ethylenediamine and hexamethylenediamine and falls off with either hydrazine or decamethylenediamine. The polyamides useful as antifoam chemicals are insoluble in water, and in practice are added to the feed water in the form of an emulsion. For maximum effectiveness it is essential that this emulsion be both highly dispersed and stable. POLYOXY ANTIF04MS
The polyoxy chemicals consiet of a relatively hydrophylic polyoxy chain condensed to a more or less hydrophobic nucleus. Such condensations may be brought about by adding ethylene oxide to the nucleus until enough has been added t o obtain the required results. One important group of such components is lesignated as high molecular weight diethers of polyoxyalkyleneglycols. Among the diethers covered in this category are the dicetyl, dilauryl, and dioctyl. Also mixed diethers function well as antifoam chemicals. As a class the polyoxy conipounds are quite soluble in cold water and are relatively insoluble in hot water. The molecular weight of the compound used is important and a minimum molecular weight may be required to obtain satisfactory foam inhibition. Such antifoam chemicals may be conveniently fed by standard proportioning devices used for feeding anti-incrustants and the like. In many cases it is common practice t o blend such chemicals with agents such as anti-incrustants, sludge conditioners, and oxygen absorbents. Such chemicals may also be fed in the form of powders, pastes, or bricks.
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There appear to be waters which are treated more effectively with the polyoxy chemicals and other iyaters are treated more effectively with the polyamide chemicals. I n general the polyoxy chemicals work well with low solid Jvaters and t,he polyamide chemicals work well with high solid waters. It has heen observed that ant,ifoani chemicals often function poorly with incompletely softened waters. I n many instances such antifoam chemicals are added as a finishing treatment. Both the polyoxy and polyamide antiloam clierriicals function well t o prevent foaming. HoiT-ever, in a number of instances the polyoxy chemicals have contribut,ed t o continuous spray carry-over. This is evident by t'he fact that even without blowdown a maximum boiler concentration is reached and that further steam generation merely results in the maintenance of this concentrat,ion. I n locomotives, continuous spray carry-over results in the eventual plugging of super heater units, t,lie sticking of front. end t,hrottles, and excessive cylinder viear. I n one instance a locomot,ive mas equipped with sight glasses arid intei,ior illumination of the boiler. Observers noted that a fog or aerosol formed above the surface of boiler water during t,he operation of the locomotive. Thiq fog was part~icularlysevere during the time that water was injected into the boiler.
high quality feed a a t e r is not readily available, and by allowing greater concentration variations over Tvhich steam generation may be carried out without danger of foaming.
Discuss ion When oil is the foaming agent, especially oil with a tallow additive, what effect do you get from antifoams? W. L. DENMAN: The antifoam chemicals described previously will stop boiler water foaming in t,he presence of oil and fatty acid soaps such as you get from tallow. I n fact it is the usual thing to find antifoam chemicals functioning satisfactorily in the presence of oil and fatty acid soaps. How are the antifoam agents affected by different types of organics in general use, such as algins, tannins, and lignins? W. L. DENMAN : Antifoam chemicals in some cases are very markedly affected by such organic additives, and in other cases they are not affected nearly as much. Antifoam chemicals in the prrsence of such organic mat,erials hare a greater effect in decreasing the number of nuclei from u-hich the steam bubbles form arid increasing the tendency of steam bubbles to coalesce than when such organic materials are not present'. The increased effectirenrss impart,ed to boiler water antifoam chemicals by suitable organic materials is due to synergism. Are polyamides derived from adipic and other dibasic acids and diamines highly polymeric, or are the molecular sizes limited by the ratio of the reactants? W. L. DENMAN : The polyamides obtained are regulat'ed by the ratio of the react,ants. Are there any objections to the use of silicone oils as antifoam agents? W. L. DENMAN: One objection of course is the cost. If cone oils were satisfartory boiler water antifoams, and many SI icone oils are not, they would have to be very effective to be usable a t a cost of $3 or 54 a pound. h h o , silicone oils ?onceivably might decompose in the boiler water to give rise to a certain amount of silica. However, if t,hat'did occur, the amount would be very small.
SYXERGISM I Y .4STIFO inIS
In the earliest application of castor oil, the castor oil was soniet.imes blended with organic inatel ials such as tannin or st,arch. Such combinations of arit,ifoani clieniicals with relatively inert organic materials is widely pract'iced today. I n general, the antifoam activity of antifoam chemicals is markedly aided by thc presence of a number of organic materials, among which are tannins, lignins. modified lignins. and humates. Such effects the presence of such are difficult, to predict, and in some ca additives will be much more effective than in other cases. I n summary, modern boiler water ant,ifoarn chemicals are a great improvement over the early ones and have been a great help in reducing the c o d of steam generation by permitting the carrying of considerably higher dissolved solids in the boiler water, by permitting satisfactory steam generation in those cases where
After Boiler Corrosion J. J. RIAGUIRE K ' . H . & L. D. Betz, Philadelphia, Pa.
KTIL a few years ago, in any evaluation of steam and con- -;but is usually exeeded by the labor cost involved. S o t only are line replacements necessary because of corrosive failure, but densate characteristics, the primary concern was the solids content as an indication of possible carry-over of boiler water salt?. I replacements may be required because of plugging of lines with corrosion products originating in other parts of the system. Today, we are awaie that the possible corrosive characteristics of Boiler tuibining or acid washing may be requiied because of iron steam are of equal importance to its solids content. It is not oxide deposits resulting from corrosion in the condensate system. sufficient that steam be free of boiler water solids--modern steam using equipment demands steam that is also noncorrosive. CAUSES OF CORROSION Corrosion of steam and condensate return lines, and of steam using equipment, is a problem of major importance to indudrial The chief cause for corrosive steam and condensate charactwplants. Frequent replacements of lines, valves, and traps can iatics ia the presence of oxygen and carbon diovide in the steam. be cawed bv corrosive gases present in the steam. The replaceWhere corrosion is due to oxygen, it will be shown by tuberculation and pitting of ferrous metals. Carbon dioxide attack is ment cost of the corroded equipment itself may be considerable
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