Document 4apzOQJwZJ85YGq2vKm9zvQNe

. 510 CHAPTER 27 1946 Guide heating system requirements. These valves are generally of the angle ' type and are usually made of brass. Graduations on the heads or lever handles are often supplied to. indicate the relative opening of the valve. Automatic control of steam supply to individual radiators can be 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 ije-in. hole through the web forming the seat to insure sufficient circulation to prevent freezing when the valve is closed. Valves made for use in hot water,heating systems are of simpler 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 a vacuum tends to be formed the air is drawn back into the radiator. 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 corrosion6 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 pf 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. Pipe, Fittings, Welding . 511 . 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 correct application of this law, however, requires equilibrium conditions which do not always exist under the flow conditions prevailing in a heating system. There is a distinction between corrosion in heating systems proper and in the condensate discharge lines from other apparatus using steam at relatively high rates, particularly at the times of the cycle when the steam consumption is at its heaviest. In such equipment the gases tend to accumulate in the steam space and to become dissolved in the con- densate in high concentrations, thus greatly increasing the possibilities of corrosion. The condensate will more nearly approach in composition the composition of the steam than will the normal condensate from apparatus such as room radiators, and will, therefore, normally include in solution more contaminants. It is possible that careful venting of such equipment would reduce the amount of contaminants dissolved in the condensate, thus giving less corrosion. There is evidence-that the partial pressures of the gases and the possibility of corrosion are much lower in . heating systems than jn high usage equipment. Hence, corrosion ob served in the condensate discharge lines from high usage equipment does not necessarily indicate that equally serious corrosion is taking place in the heating system. ' The seriousness of corrosive conditions is best determined by actual measurement rather than by inference from isolated instances of pipe failures. The National District Heating Association has perfected a cor rosion tester' for measuring the inherent corrosiveness of existing con ditions. This corrosion tester consists of a frame supporting 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 corrosivensss of the condensate. Accompanying such corrosion measurements, a careful chemical analysis should be made of the condensate, and the findings will serve as a basis for an intelligent study of the problem7. There are some indications that after a. condensate containing carbon dioxide has dissolved some iron and thereby raised its pH value, its cor rosive action is greatly reduced and the solution will remain comparatively inactive until admission of oxygen permits the precipitation of the -dissolved iron ,as ferric oxide. The pH value of the condensate may be ' buffered to a fairly high value by the solution of iron and not correspond to the pH value to be expected in the unbuffered solution containing the same amount of carbon dioxide. -Although inhibitors of.various types have had.considerable trial and experimentation and successes have been reported, they require further.