Document bBaNKvZxvN79oG2X8zGbN3V5D

V HEATING VENTILATING AIR CONDITIONING GUIDE 1940 effected by use of- direct-acting radiator valves having a thermostatic element at the valve, or near to it. The direct-acting valve is usually an angle-type valve containing a thermostatic element which permits the flow of steam in accordance with room temperature requirements. These valves usually are capable of adjustment to permit variation in room temperature to suit individual taste. Ordinary steam valves may be used for hot water service by drilling a H6-in. hole through the web forming the seat to insure sufficient circulation to prevent freezing when the valve is closed. Valves made particularly for use in hot water heating systems are of less complex design, one type consisting of a simple butterfly valve, and another of a quick opening type in which a part in the valve mechanism matches up with an opening in the valve body. In one-pipe steam-heating systems, automatic air valves are required at the radiators. Two common types of air valves available are the vacuum type and the straight-pressure type. Vacuum valves_ permit the expulsion of air from the radiators when the steam pressure rises and, in addition, act as checks to prevent the return of air into the radiator when a vacuum is formed by the condensation of steam after the supply pressure has dropped. Ordinary air valves permit the expulsion of air from the radiator when steam is supplied under pressure, but when the pressure dies down and a vacuum tends to be formed the air is drawn back into the radiator. A system operating either continuously or intermittently and supplied with vacuum valves will generally hold heat longer and warm up more quickly than one provided with non-vacuum air valves; thus, it will effect considerable economy of fuel because the idle period during which no heat is delivered is shortened. In those cases, ,where a system is equipped with vacuum air valves and which has been cold for several hours, the system will probably have an internal pressure within the radiator closely approaching atmospheric. At such times, the vacuum valve will not vent the system any more rapidly than the ordinary type. Automatic air valves are provided with a float to close them in case the radiator becomes flooded with water because it does not drain properly. CORROSION* 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 *New Light on Heating System Corrosion, by J. H. Walker (Beating 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 Systems, by R. R. Seeber. F. A. Rohr man and G. E. Smedberg. (A.S.H.V.E. Transactions. Vol. 42. 1936, p. 263). A.S.H.V.E. Research Report No. 1071--Corrosion Studies in Steam Heating Systems, by R; R. Seeber and Margaret R. Holley (A.S.H.V.E. Transactions, Vol. 43, 1937, p. 461). \ 346 CHAPTER 18. PIPE, FITTINGS, WELDING 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. 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. Distinction should be made between corrosion in heating systems proper and 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 a Fundamental Considerations of Corrosion in Steam and Condensate Lines, by R. E. Hall and A. R. Mutnford (A.S.H.V.E. Transactions. Vol. 38. 1932. p. 121). 347