Document 3QNx63zdoy9mYdZ8aydOe8rQD

420 _______________ ___________CHAPTER 23 ,1946 Guide return mains need not be pitched toward the boiler to maintain steam circulation, they should be pitched for drainage. To improve steam circulation in one-pipe systems quick vent air valves should be provided at the ends and at intermediate points where the steam main is brought to a higher elevation. It is desirable to install the air-vent valves about a foot ahead of the drips, as indicated in Fig. 2, to prevent possible damage to their mechanisms by water. The radiator valves may be the angle-globe, offset-corner pattern or gate type. Straight-globe and straight-comer types should not be used since the raised valve seat would interfere with the flow of condensation through the valve. Graduated valves cannot be used since the steam valves on this system must be fully open or fully closed to prevent the radiators filling with water and creating a dangerous water line condition. With a one-pipe system the heat cannot be modulated at the radiator, the steam being either all on or all off. Systems and devices are available which make it possible to obtain a partial modulating effect from one-pipe heating systems. It is important to keep the lowest points of the steam mains and heating units sufficiently above the water line of the boiler to prevent flooding; The minimum water line difference depends on the initial steam pressure and piping pressure drop plus a safety factor for heating up. Referring to Fig. 6 it will be noted that the boiler, and wet return form a U-shaped container, with the boiler steam-pressure on the top of the water at one end and the steam main pressure on the top of the water at the other end. .The difference between these'two pressures is the pressure drop in the system,'i.e., the friction and resistance to the flow of steam in passing from the boiler to the far end of the main and the pressure re duction in consequence of the condensation occurring in the system.- Steam Heating Systems and Piping 421 The water in the far end will rise sufficiently to overcome this difference in order to balance the pressures, and it will rise far enough to produce a flow through the return pipe and overcome the.resistance of check valves if installed. If a one-pipe steam system is designed, for example, for a total pressure drop of )4 psi, and utilizes a Hartford return connection instead of a check valve on the return, the rise in the water level at the far end of the return due to the difference in steam pressure would be )4 of 28 in. (28 in. Fig. 3. Typical Steam Runout where Fig. 4. Typical Steam Runout where Risers are Not Dripped Risers are Dripped Supply riser- ^Supply main -Drop riser 4s= Air valve- 4= Air vent "Air valve. -Air valve J- mi Hartford return connection" 3 EL - ,______h_____ j__ vWet return h--'' Fig. 5. Typical Down-Feed Gravity One-Pipe Air-Vent System- head being equal to one pound per square inch)', or 3)4 in. Adding 3 in. to overcome the resistance of the return main and 6 in. as a factor of safety for heating up gives 12)4 in. as the distance the bottom of the lowest part of the steam main and all heating units must be above the boiler water line. The same system, however, installed and sized for a total pressure drop of )4 psi, and with a check in the return, would require 34 of 28'in., or 14 in: for the difference in steam pressure, 3 in. for the flow through the return, 4 in. to operate the check, and 6 in. for a factor of safety, making a total of. 27 in. as the required distance. Higher pressure drops would increase the distance accordingly.