Document p2rr0Yz73Z8mgqw5wroKnYXGB

Heating Ventilating Air Conditioning Guide 1939 internal pressure within the radiator closely approaching atmospheri At such times, the vacuum valve will not vent the system any rapidly than the ordinary type. Automatic air valves are provided with 6 . float to close them in case the radiator becomes flooded with water because it does not drain properly. * CORROSION2 Corrosion is sometimes encountered in heating work on the outside of ' buried pipes or the inside of steam heating systems; it is seldom experienced 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 and 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, but 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. 'New Light on Heating System Corrosion, by J. H. Walker {Keating and Ventilating, May, 1933). A.S.H.V.E. Research Report No. 983--Corrosion Studies in Steam Heating Systems, by R. R. Seeber. F. A. Rohrman and G. E. Smedberg, (A.S.H.V.E. Transactions^Vol. 40, 1934. p. 253). A.S.H.V.E. Research Report No. 1037--Corrosion Studies in Steam Heating bystems, by R. R. Seeber, F. A. Kohrman and G. E. Smedberg. (A.S.H.V.E. Transactions. Vol. 42, 1936, p. 203). Corrosion Studies in Steam Heating Systems: by R. R. Seeber and Margaret R. Holley (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, June, 1937, p. 387), 380 Chapter 18. Pipe. Fittings. Welding mount of gas in solution depends upon the partial pressure of that The.a . atmosphere above the surface of the solution, in accordance 8*.,nthe well known physical law of Henry and Dalton*. The exact *1'cation of this law, however, assumes equilibrium conditions which do no always exist under the flow conditions prevailing in a heating ^Distinction should be made between corrosion in heating systems proper , t}je condensate discharge lines from other apparatus using steam, ch as water heaters, kitchen equipment, and sterilizers. Experience has shown that in heating systems the partial pressures of the gases do ot reach such magnitudes as to cause harmful amounts of gas to become d'ssolved in the condensate when steam supplies are of reasonable purity. In other kinds of steam-using apparatus which are not ordinarily well ented, 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 pf 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. Tfie leaks in the piping system, particularly in vacuum systems, should be stopped so far as is practicable. Some success has been reported with the use of inhibitors, chief among which are oil, and sodium silicate. Oil may be fed into the main steamsupply 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. 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 problem may be to use non-corroding materials in those certain portions of the piping system, since the higher cost will usually be an unappreciable portion of the total. Brass and copper are undoubtedly less subject to *Sotrie Fundamental Considerations of Corrosion in Steam and Condensate Lines, by R, E. Hall and A. R. Mumford (A.S.H.V.E. Transactions, Vol. 38. 1932, p. .121). 31