(4) Cerna, Wendell W., Proc. Seventh Annual Water Conf., Eng. Soc. of W. Pa. (Jan. 6-8, 1947). (5) Hall, R. E., Proc. Ninth Annual Water Conf., Eng. Soc. of W. Pa. (Oct. 18-20, 1948). (6) Hall, R. E., Trans. Am. Soc. Mech. Engrs., 66, 457-88 (1944). (7) Ongman, H. D„ Combustion, 24 No. 8, 40-4 (1953). (8) Purcell, T. E., and Whirl, S. F., Trans. Am. Soc. Mech. Engrs., 64, 397-402 (1942). (9) Rivers H. M„ and Sonnett, W. M., Combustion, 21, No. 12, 41-6 (1950). (10) Schroeder, W. C., and Berk, A. A., U. S. Bureau of Mines, Bull. 443 (1941). (11) Straub, F. G„ Mech. Eng., 61, 199-202 (1939). (12) Thornley, J. L., Industry and Power, 65, No. 2, 58-61,1953. (13) Trans. Am. Soc. Mech. Engrs., 64, 393-444 (1942). (14) Ulmer, R. C., Whitney, J. H., and Wood, J. W., Proc. American Power Conf., XIV, 459-67 (1952).
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Discussion If 3Fe + 4H20 -> Fe304 + 40, is the main reaction for good film (thin) protection, how do you account for Fe203 and Fe in boiler deposits? What is method of setting Fe304 film— time, temperature, and alkalinity? R. F. ANDRES: The relative amounts of the various iron oxides in boiler deposits are controlled by the oxidizing or reducing characteristics and dissolved oxygen content of the water in contact with the metal surfaces. These factors vary throughout the plant cycle and change with temperature, pH, and ferrous ion activity. An excellent discussion of the reactions of iron with water is presented in a paper by Marcel Pourbaix, “Some Applications of Electrochemical Thermodynamics," Corrosion, 6, No. 12, 395-404 (1950).
The various factors controlling protective magnetic iron oxide film formation have not been thoroughly investigated. Much fundamental research work on iron-water reactions must be completed before definitive rules can be established for the best methods of forming protective iron oxide films on boiler
metal surfaces. What concentrations of alkalinity are necessary for destruction of protective iron "oxide film on boiler metal? R. F. ANDRES: I cannot define limits for the concentrations of alkalinity that destroy protective iron oxide film in an operating boiler. It has previously been reported that minimum corrosion of iron results w'hen the caustic concentration in the water is in the range 40 to 400 p.pm. corresponding to pH values of 11 to 12. Caustic concentrations as low as 5% may be aggressive to boiler steel according to C. E. Kaufman, V. M. Marcy, and XV. H. Trautman, “The Behavior of Highly Concentrated Boiler Water,” Proc. Sixth Annual Water Conf., pp. 23-42 (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 pH values in water solutions results from removal of the polarizing hydrogen film and the absence of any formation of protective oxide films. Caustic attack, on the other hand, is characterized by removal of protective iron oxide films as soluble sodium ferrite or ferrate followed by reaction of the water with the metal to replenish the 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. It is impractical to attempt to designate the pH above which corrosion may occur because a small increment of pH represents a high increase in caustic concentration in the pH range 12 to 14. I heard a paper recently which reported that nitrogen was added—Le., 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 water in an operating boiler. Nitrogen has been used following acid cleaning of boilers to prevent, excessive iron oxide formation which would result if air came in contact with the highly reactive pickled metal surfaces. For protection of idle boilers, a nitrogen blanket could be maintained under pressure to exclude oxygen from the boiler. Sulfite breakdown to H2S (not S02) seems to be associated with dry areas (faulty circulation) in the boiler, whereas SOformation 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 treatment will not correct the boiler corrosion problem but may prove beneficial in combatting possible corrosion from hydrogen sulfide in other portions of the watersteam cycle. The sulfur dioxide evolution with the steam from sulfite treated boilers is a chemical equilibrium reaction and the quantity is dependent on temperature conditions and the concentration of sulfite being maintained in the water.
Foaming in Boilers W. L. DENMAN Dearborn Chemical Co., Chicago, III.
water forming in general is a term applied loosely the formation or generation of steam containing liquid water and is to be differentiated from so-called spray carryIn the case of foam, the water is actually' present in the over. steam as free liquid water; whereas, in the case of spray carryover the boiler water is dispersed in the steam as fine globules. to cover BOILER,
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Foaming does not result from the formation of a distinct foam layer or blanket on the water surface; rather, it results primarily' because the apparent volume of the boiler water increases due 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 slower than do larger steam bubbles
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_Boiler 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 to 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 remain 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 at 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 n onspherical steam bubbles formed by coalescence have not had time to round out. FOAM PROMOTERS
The foaming tendency of any given boiler water may be at-
tributed 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
1.
6. 7.
only) 8.
Total dissolved solids in the boiler water Finely divided solids in suspension
Organic matter—sewage and decayed vegetation Colloidal matter—fine clays, oils, and soaps Design of boiler: Steam space affected by water level Rate of evaporation Locations of steam outlets Variations in load or steam demand Operation of throttle and reverse lever (locomotive boiler
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 to castor oil, attempts were started early to develop antifoam chemicals which would May 1954
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high degree of antifoam effectiveness, which would have 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.
have
a
a
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 be,? obtained from one of the several amines. Boiler water antifoam effectiveness of
,
-
1. Diamides of ethylenediamine is peaked at stearic acid and falls off rapidly with either palmitic acid or behenic acid. 2. Diamides of diethylenetriamine is peaked between palmitic acid and stearic acid and falls off rapidly with either myristic and
factors:
2. 3. 4. 5.
Water
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behenic acid.
3. Triamides of diethylenetriamine is peaked between lauric acid and palmitic 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 ANTI FOAMS
The polyoxy chemicals consist 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 to obtain the required results. One important group of such components is designated 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 compounds 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 to 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 waters are treated more effectively with the polyamide chemicals. In general the polyoxy chemicals work well with low solid waters and the polyamide chemicals work well with high solid waters. It has been observed that antifoam chemicals often function poorly with incompletely softened waters. In many instances such antifoam chemicals are added as a finishing treatment. Both the polyoxy and polyamide antifoanr chemicals function well to prevent foaming. However, in a number of instances the polyoxy chemicals have contributed to continuous spray carry-over. This is evident by the fact that even without blowdown a maximum boiler concentration is reached and that further steam generation merely results in the maintenance of this concentration. In locomotives, continuous spray carry-over results in the eventual plugging of super heater units, the sticking of front end In one instance a locomothrottles, and excessive cylinder wear. tive was equipped with sight glasses and interior illumination of the boiler. Observers noted that a fog or aerosol formed above the surface of boiler water during the operation of the locomotive. This fog was particularly severe during the time that water was injected into the boiler. SYNERGISM
high quality feed water is not readily available, and by allowing greater concentration variations over which steam generation may be carried out without danger of foaming.
Discussion 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 the presence of oil and fatty acid soaps such as you get from tallow. In 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 presence of such organic materials have a greater effect in decreasing the number of nuclei from which the steam bubbles form and increasing the tendency of steam bubbles to coalesce than when such organic materials are not present. The increased effectiveness imparted 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 regulated by the ratio of the reactants. Are there any objections to the use of silicone oils as antifoam
IN ANTI FOAMS
In the earliest application of castor oil, the castor oil was sometimes blended with organic materials such as tannin or starch. Such combinations of antifoam chemicals with relatively inert organic materials is widely practiced today. In general, the antifoam activity of antifoam chemicals is markedly aided by the presence of a number of organic materials, among which are tannins, lignins, modified lignins, and humates. Such effects are difficult to predict, and in some cases the presence of such additives will be much more effective than in other cases. In summary, modern boiler water antifoam chemicals are a great improvement over the early ones and have been a great help in reducing the cost 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
agents?
W. L. DENMAN: One objection of course is the cost. If si’icone oils were satisfactory boiler water antifoams, and many silicone oils are not, they would have to be very effective to be usable at a cost of $3 or S4 a pound. Also, silicone oils conceivably might decompose in the boiler water to give rise to a certain amount of silica. However, if that did occur, the amount would be very small.
After Boiler Corrosion J. J. MAGUIRE W'.
if. & L. D, Betz, Philadelphia, Pa,
a few years ago, in any evaluation of steam and condensate characteristics, the primary concern was the solids content as an indication of possible carry-over of boiler water salts. Today, we are aware that the possible corrosive characteristics of steam are of equal importance to its solids content. It is not sufficient that steam be free of boiler water solids—modern steam using equipment demands steam that is also noncorrosive. Corrosion of steam and condensate return lines, and of steam using equipment, is a problem of major importance to industrial plants. Frequent replacements of lines, valves, and traps can be caused by corrosive gases present in the steam. The replacement cost of the corroded equipment itself may be considerable
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but is usually exceeded by the labor cost involved. Not only are line replacements necessary because of corrosive failure, but replacements may be required because of plugging of lines with corrosion products originating in other parts of the system. Boiler turbining or acid washing may be required because of iron oxide deposits resulting from corrosion in the condensate system. CAUSES OF CORROSION
The chief cause for corrosive steam and condensate characteristics is the presence of oxygen and carbon dioxide in the steam. Where corrosion is due to oxygen, it will be shown by tuberculation and pitting of ferrous metala. Carbon dioxide attack is
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