Document oDG1J3Y2XDO505rB3JVkeBm33

Heating Ventilating Air Conditioning Guide 1939 pressure down to that required by the heating system. It has been con. sidered good practice to make the pressure reductions in steps not to exceed 50 lb in each case. For example, in reducing from 100-lb gage to 2-lb gage, two pressure reducing valves would be used, the first redudm, the pressure from 100-lb gage to 50 lb and the second reducing the pressure from 50-lb gage to 2-lb gage. Valves are available that will reduce 100 lb . in one step, and it is questionable whether two valves are now required for. initial pressures of 150 lb or less. The pressure-reducing valve, or pressure-regulator as it is sometimes termed, has ratings which vary 200 to 400 per cent. Some of these ratings are based on arbitrary steam velocities through the valve of 5,000 to 10,000 fpm and it is assumed that the valve when wide open has the same capacity as the pipe on the inlet opening of the valve. At times Table 11. Capacities of Pressure-Reducing Valves (1CO*lb Gags Down to any Pressure--52 lb or Less) Inlet Nominal Pipe Diameter (Inches) x X 1 IX IX 2 2X 3 3X 4 5 6 Pounds Steam per Hour at 100-Lb Gaos 866 1,576 2,459 4,263 5,808 9,564 13.623 21.041 28,213 36,285 56,971 82,336 Equitalbnt Direct SRadiation q Ft Kat Lb 3,464 6,304 9,836 17,052 23,232 38,256 54,492 84,104 112,852 145,140 227,884 329,344 Equitalbnt Direct Radiation Sq Ft at H Lb 2,598 4,728 7,377 12,689 17,424 28,692 40,869 63,123 84,039 108,855 170,913 247,008 Formula: A X V X 3600 X .50 144 X 3.88 pound9 per hour passed by orifice. where A * area of inlet pipe, square inches. V = velocity of steam through orifice (approximately 870 fps). 50 = 70 per cent efficiency of orifice less 20 per cent for factor of safety. 144 = square inches in 1 sq ft. 3600 seconds in one hour. 3.88 " cubic feet per pound at 10$lb gage. ) i it is considered desirable to keep the steam velocity in the high pressure section of the piping and the low pressure section constant. The velocity through the valve port is obviously a function of the pressure drop across the valve. It is well known that steam flowing through an orifice increases its velocity until the pressure on the outlet side is reduced to 58 per cent of the absolute pressure on the inlet side, and that with further reduction of pressure on the outlet side little change in velocity will be obtained. As practically all pressure-reducing valves used for steam- heating work lower the steam pressure to less than 58 per cent of the inlet pressures, only the maximum velocity through such valves need be considered. If it is assumed that the valve, when fully open, has an area equal to that of the inlet pipe size, that the steam is flowing into a pressure less 324 Chapter 16. Piping for Steam Heating Systems -8 er of the initial pressure, that the orifice efficiency is approx- lv 70 per cent, and that 20 per cent more is allowed for a factor of *?tv then the pressure reducing valves will have the working capacities jn Table 11. If the valve, when fully open, does not give an orifice S"Weaual to that of the pipe on the inlet side, then the capacities will be ^oortional to the percentage of opening secured, taking the pipe area ProJS) _er Cent. More frequently, difficulty is encountered from the use nressure reducing valves which are too large in size instead of being * small. Where valves are large in size, the valve tends to work close fbthe seat, causing it to cut out in a relatively short time, as well as being noisy in operation. Most exact regulation of pressure on steam heating systems is secured from diaphragm-operated valves controlled by a pilot line from the low oressure pipe, taken off the low pressure main at least 15 ft from the reducing valve. The reducing valves operating on the proportionalreduction principle will give a variation of steam pressure on the low pressure side if the initial pressure varies between considerable limits. The so-called dead-end valve is used for reduced pressures where the line Less trouble from expansion leaks will occur when the bypass valve is on the same center line as the pressure reducing valve Bypass (same size as high pressure supply line)" High pressure steam Drip Safety valve essure gage Low pressure steam }\ Gate valve T-j-Valve Pressure reducing valve ^ Pilot line Fig. 6. Typical Pressure-Reducing Valve Installation has not sufficient condensing capacity at all times to condense the leakage that might occur with the ordinary valve. Single-disc valves do not give as close regulation as double-disc valves, but the single disc is preferable where dead-end valves are necessary, such as on short runs to thermo statically controlled hot water heaters, central fan heating units and unit heaters. The correct "installation (Fig. 6) of a pressure-reducing valve includes a pressure-reducing valve with a gate valve on each side, a by-pass con trolled by a globe valve, a pressure gage on the low pressure side, and a safety valve on the low pressure main at some point, usually within a reasonable distance of the pressure-reducing valve. Pressure-reducing valves should have expanded outlets for sizes greater than 2 in. Where the steam main is of still larger diameter than the expanded outlet, and in cases where straight valves are used, an increaser is placed close against the outlet of the valve to reduce the velocity immediately after passing through the valve. Strainers are recommended on the inlets of all pressure-reducing valves. A pressure gage may be located on the highpressure line near the valve if desired. Owing to the large variation in steam demand on the average heating system, it is generally advisable to use two pressure-reducing valves con- 325