Document ykd614NjZ2j44bENkJkNy5e86

of and 1936American Society Heating Ventilating Engineers Guide, CORROSION2 Corrosion is sometimes encountered in heating work on the outside of buried pipes or the inside of steam heating systems; it is seldom ex perienced in hot water heating systems unless the water is frequently renewed. Piping buried in the ground is quite successfully protected by coatings of the asphaltic type which are usually applied hot and often reinforced with fabric wrappings. Galvanizing by the hot-dip process and painting with specially prepared' mixtures also afford some protection. Internal corrosion in steam heating systems occurs principally in the condensate return pipes and is nearly always caused by oxygen or carbon dioxide, or both, in solution in the condensate. Oxygen may enter the heating system with the steam, owing to its presence in the boiler-feed water, or it may enter as air through small leaks, particularly in systems which operate at sub-atmospheric pressures. When a steam heating system is operated intermittently, air rushes in during each shutdown period and oxygen is absorbed by the condensate which clings to the interior surfaces of the pipes arid radiators. The rate of corrosion depends upon the amounts of oxygen and carbon dioxide present in solution, upon the operating temperature, and upon the length of time that the pipe surfaces are in contact with gas-laden condensate. Another possible cause of corrosion is a flow of electric current some times resulting from faulty electrical circuits which should be corrected. Electrolytic corrosion also may occur-because of the presence of two dis similar metals, such as brass and iron, but the condensate in practically all steam heating systems is such a weak electrolyte that this cause of corrosion is very infrequent. If trouble is experienced from corrosion, oxygen should be eliminated from the feed water by proper deaeration with commercial apparatus. The elimination of the oxygen due to air leakage is more difficult because of the-multitude of small leaks which exist around valve stems and in pipe joints. In vacuum systems, however, an attempt should be made to minimize such leakage. Carbon dioxide in varying amounts is contained in steam produced from the majority of water supplies. It is formed from the breaking down of carbonates and bicarbonates which are present in nearly all natural waters. It can be partly removed by chemical treatment and deaeration, blit there is no simple method whereby it can be entirely eliminated. These gases cause corrosion only when in solution in the condensate; when they are mixed with dry steam their corrosive effect is negligible. The amount of gas in solution depends upon the partial pressure of that gas in the atmosphere above the surface of the solution, in accordance with the well known physical law of Henry and Dalton*. The exact application of this law, however, assumes equilibrium conditions which do not always exist under the flow conditions prevailing in a heating system. *New light on Heating System Corrosion, by J. H. Walker (Heating and Ventilating, May, 1933). Some Fundamental Considerations of Corrosion in Steam and Condensate Lines, by R. . Hall and A. R. Mumford (A.S.H.V.E. Transactions, Vol. 38, 1932). 626 34--Chapter Pipe, Fittings, Welding Distinction should be made between corrosion in heating systems proper nd in the condensate discharge lines from other apparatus using steam, such as water heaters, kitchen equipment, and sterilizers. Experience has shown that in heating systems the partial pressures of the gases do not reach such magnitudes as to cause harmful amounts of gas to become dissolved in the condensate when steam supplies are of reasonable purity. In other kinds of steam-using apparatus which are not ordinarily well vented, the gases tend to accumulate in the steam space and to become dissolved in the condensate in appreciable concentrations. Consequently, corrosion is frequently observed in the condensate discharge lines from such apparatus; but this does not necessarily indicate that equally serious corrosion is taking place in the heating system supplied with steam from the same source. When corrosive conditions are believed to exist, their seriousness should be determined by actual measurement, rather than by inference from isolated instances of pipe failures. The National District Heating Associa tion has perfected a corrosion tester for measuring the inherent corrosive ness of existing conditions. This corrosion tester consists of a frame sup porting three coils of wire which are carefully weighed. After the tester has been inserted in the pipe line for a definite length of time, the loss of weight of the coils, referred to an established scale, indicates the relative corrosiveness of the condensate. Accompanying such corrosion measure ments, a careful chemical analysis should be made of the condensate, and the findings will serve as a basis for an intelligent study of the problem. Corrosion, if found to exist, can. be lessened or overcome by several means. If the steam supply is found to be definitely contaminated, proper chemical treatment of the water, followed by deaeration, is an obvious remedy. The leaks in the piping system, particularly in vacuum systems, should be stopped so far as is practicable. -Sbme success has been reported with the use of inhibitors, chief among which are oil, sodium silicate, and ammonia. Oil may be fed into the main steam-supply pipe by means of a sight-feed lubricator. The type of oil known as 600-W is usually recommended. In the present state of knowledge on this point, the quantity to be fed can best be determined by trial. The use of sodium silicate, fed in a similar manner, is reported to be successful but it has not been widely used. The effect of ammonia is to increase the pH value of the condensate above the point where corrosion is likely to take place. Speller4 reports having injected small quantities of ammonia into a small closed heating system (the entire amount of condensate being returned to the boiler) and finding the pH value maintained at a high point for several months without further additions of ammonia. The concentration of ammonia must be kept low to avoid corrosion of brass parts of the system. The use of ammonia is not to be recommended where steam may come in contact with food or other materials. In view of the fact that corrosion is most frequently found in the return lines from special equipment, which constitute a relatively small part of the total piping in a building, a simple solution of the corrosion Corrosion in Steam Heating Systems, by F. N. Speller (A.S.H.V.E. Transactions, Vol. 34, 1928). 627