Document pp6ZN5QbE1BOLQ7Gn5k44dxqd

442 CHAPTER 23 ____________ 1946 Guide Float and thermostatic traps have both a thermostatic element to release air and a float element to release the water. Impulse traps operate with a moving valve actuated by a control cylinder. When the trap is handling condensate,, the pressure required to lift the valve is greater than the reduced pressure in the control cylinder and consequently the valve opens allowing a free discharge of condensate. As the remaining condensate approaches steam temperature, flashing results, flow through the valve orifice is choked and the pressure builds up in the control chamber closing the valve. Automatic Return Traps In the general heating plant, where thermostatic traps are installed on the heating units, it becomes necessary to provide a means for returning the water of condensation to the boiler, if a condensation or vacuum pump is not used. When the return main can be kept sufficiently high above the boiler water line for all operating conditions, the water of condensation will flow back by gravity, and no mechanical device is required. But actually this does not work out in practice. It follows, therefore, that a direct-return trap is needed for the handling of the condensation even though it may not be called into action except under some operating condition where the pressure differential exceeds the static head provided. The installation of a direct-return trap assures safety for such systems, and guarantees the operation of the plant under varying conditions. Automatic return traps, sometimes called alternating receivers, may be of the counter-balanced, tilting type, or spring actuated. These consist'' of a small receiver with an internal float, .and when,the condensate will not flow into the boiler under pressure, it will feed into the receiver of the trap, and in so doing, raise or tilt the float or mechanism which actuates a steam valve automatically. This admits steam to the receiver, at boiler pressure, and the equalizing of the pressures which follows allows the water to flow into the boiler. Tilting Traps. With this type of trap, water enters a bowl and rises until its weight overbalances that of a counter-weight, and the bowl sinks to the bottom. As the bowl sinks, a valve is opened, thus admitting live steam pressure on the surface of the water, and the trap then dis- charges. After the water is discharged, the'counter-weight sinks and raises the bowl, which in turn closes the valve and the cycle begins again. Tilting traps are necessarily intermittent in operation. They are not ordinarily equipped with glass water gages, as the action of the trap shows when it is filling or emptying. The air relief of tilting traps is taken care of by the valves of the trap. DRIPS A steam main in any type of steam heating system may be dropped to a lower level without dripping if the pitch is downward with the direction of steam flow. Any steam main in any heating system can be elevated if dripped. Fig. 19 shows a connection where the steam main is raised and the drain is to a wet return. If the elevation of the low point is above a dry return, it may be drained through a trap to the dry return in two-pipe vapor, vacuum and sub-atmospheric systems,' Horizontal steam pipes may also be run over obstructions without a change in level if a small pipe is carried below the obstruction to care for the condensation (Fig. 20). Horizontal return pipes may be carried past doorways and other ob- < Steam. Heating Systems and Piping 443 structions by using the scheme, illustrated in Fig. 21. , It will be noted that the large pipe, in this case, runs below Jhe obstruction and the smaller one over it. Branches from steam mains in one-pipe gravity steam systems should use the preferred connection shown in Fig. 22, but where radiator condensa tion does not flow back into the main the acceptable method shown in the same figure may be used. This acceptable method has the advantage of . -Air II At feast 1 inch Fig. 19. Dripping Main Where it Rises to Higher Level Fig. 20. Looping Main Around Beam Fig. 21. Looping Dry Return Main Around Opening Acceptable method Preferred method Fig. 22. Methods of Taking Branch from Main Fig. 23. Constants for Determining Length Offset Pipe Dirt ^ poc**nP3=> Fig. 24. Dirt Pocket Connection ^SMPPfrg Cooling <e( at /HxA V tom. Fig. 25. Dripping End of Main into. Wet Return Fig. 26. Dripping End of Main into Dry Fig. 27. Dripping Heel1 of Riser into Dry giving a perfect swing joint when connected to the vertical riser or radia tor connection, whereas the preferred connection does not give this swing without distorting the angle of the pipe. .Runouts from the steam main are usually made about 5 ft.long to provide flexibility for movement in. the main. .' ................. - Offsets in steam-and return piping should preferably, be made with 90-deg-ells but occasionally fittings of other angles are used, and in such cases the length of the diagonal offset will be found as shown in Fig. 23. .