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680 CHAPTER 29 1956 Guide Typical service connections are shown in Fig. 2 for low-pressure service and Fig. 3 for high-pressure service. The district-heating industry refers to valves according to their functional use. The pressure-reducing valve reduces pressure to a safe point, and. the pressure-regulating valve regu lates pressures to that required by equipment served. Fig. 2 illustrates the use of a pressure-regulating valve with a bypass such as generally provided for larger installations. This bypass permits the operation of the system in case of failure in the pressure-regulating valve. In the smaller sizes, the bypass is usually omitted. When servicing is required, the pressure-regulating valve can be removed, a filler installed, and the gate valves (part 7 in Fig. 2) used to throttle the flow of steam until re pairs are made. valve Fig. 2. Pressure-Regulating Valve with Bypass* 4 From District Heating Handbook, Third Edition. Fig. 3 shows a typical installation where the pressure in the district-, steam main exceeds 50 psig, which pressure is usually higher than can safely be applied to building heating systems, as much heating equipment is not intended to withstand more than 15 psig.1 The first or pressurereducing valve effects the initial pressure reduction. The pressureregulating valve controls the steam at the desired pressure. it is often advisable to install a separate pressure-regulating valve for each steam use in the building such as for water heating, building heating, cooking, so that each system may operate at the optimum pressure. Pressure-reducing valves are more fully described in Chapter 21. For use with, district heating, dead-end type valves of the pilot-operated or spring-loaded. type are generally preferred where applicable. Pressureregulating valves should be sized to supply adequately the maximum steam requirements of the heating system or equipment. This is especially im portant with temperature control systems using intermittent flow of steam District Heating 681 1. Service Pipe 2. Servioe Valve 3. Strainer 4. Flanged Union 5. Chile Valve S/.' rJiUloatla-Oncpeelamnteed Pressure-Reducing Valve - 8. Pressure Gage *. Pilot-Operated Pressure-Reducing Valve 10. Pressure Rebel Valve (optional) 11. Vent 12. Globe Valve 13. Bypass Line Note: AM fittings should be American Standard Class 250 cast Iron. Equipment* (Used Where High-Pressure Steam is Supplied for Low-Pressure Requirements). 4 From District Heating Handbook, Tbird Edition. for 7,^ vu mg' . Pressure-regulating valve capacity recommended or use with various types of temperature control areshown in Table4. Zoning. Where the hours of occupancy or heating loads differ in various sections of the building, it is good practice to install separate supply lines to the different sections. For example, in an office building with stores or restaurants on the first floor which are open'in the evening, a separate main supplying the first floor will permit the steam to be shut off from the remainder of the building in the late afternoon. The division of the build ing into zones, each with a separately-controlled heat supply, is sometimes desirable, as it permits the heat to be adjusted according to variations in sunshine and wind. Fig. 4 shows a typical service installation with a separate line to the Table 4. Suggested Capacity op Pressure-Regulating Valves and Tkaps Type of Contbod Continuous or moHulfttinfl operation, intermit tent with on periods over 3 hr in duration Intermittent operation with on periods of H to c hr in duration intermittent operation with'on periods of 34 hr or tees Pressure-Regulating I Valve Capacity, La/(Ha)j Trap Capacity, Lb/(Hb) (Sq Ft op Pipe and (Sq Ft op Radiation*) Radiation4) HH hH Square Feet EDR. - ~ Except for drip trope on mains which should be 1 lb per sq ft EDR.