Document ZBo7ZYGBakNjaN97jLkLLDMeJ

672 CHAPTER 29 195S 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 by-pass such as generally provided for larger installations. This by-pass permits the operation of the system in case of failure in the pressure-regulating valve. In the smaller sizes, the by-pass 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. 1. Service Pipe-----2. Service Valve 3. Flanged Union 4. Globe Valve 5.-Ground Joint Union - 6."By-pass * .......... ~ ` 7. Gate Valve %'. Pressure-Regulating .Valve 9. Pressure Gage 10. Balance'Pipe to connect into steam main not less, than 10 ft from pressure regulating valve*' ' Fig. 2. Pressure-Regulating Valve with By-Pass* s From District Heating Handbook, Third Edition. Fig. 3 shows a typical installation where the pressure in the districtsteam 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.1 The pressure regulating 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. Pressure regulating 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 673 PLAN VIEW 1. Service Pipe 2. Service Valve 3. Strainer 4. Flanged Union 5. Gate Valve 6. Pilot-0pernted Pressure-Reducing Valve 7. Balance Line 8. Pressure Gage 9. Pilot-Operated Pressure-Reducing Valve 10. Pressure Relief Valve (optional) 11. Vent 12. Globe Valve 13. By-pass Line Note: AU fittings should be American Standard Class 250 cast iron.. Fig. 3. Two-Stage Pressure-Reducing and Pressure-Regulating Equipment* (Used Where High-Pressure Steam is Supplied for Low-Pressure Reguirements). a From District Heating Handbook, Third Edition. to heat the building. Pressure-regulating valve capacity recommended for use with various types of temperature control are shown in Table 4. ZONING AND OPERATION 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 pennit 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 of Pressure-Regulating Valves and Traps Type of Control Pressdbe-Regulating Valve Capacity, Lb/(Hr) (So Ft of Radiation*) Trap Capacity, Lb/(Hb) (Sq Ft of Pipe and Radiation*) Continuous or modulating operation, intermit tent with on periods over 3 hr in duration Intermittent operation with on periods of H to 3 hr in duration Intermittent operation with on periods of Yt hr or less H a \ H .. a Hb * Square Feet EDR. Except for drip tram on mains which, should be V lb per sq ft EDR.