Document RjpG1mdpQJBpZe2R7vVEM9we7

American Society of Heating and Ventilating Engineers Guide, 1934 Table 10; Capacities of Pressure Reducing Valves (100 lb Gage Down to any Pressure--52 lb or Less) Inlet Nominal Pipe Diameter (Inches) X X l IX IX 2 IX 3 3X 4 5 6 - Pounds Steam per Hour at 100 Lb Gage 866 1,576 2,459 4,263 5,808 9,564 13,623 21,041 28,213 36,285 56,971 82,336 Equivalent Direct SRadiation q Ft at K 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 Equivalent Direct SRadiation q Ft at 1/s 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 3.84 pounds per hour passed by orifice. where A -- area of inlet pipe in 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. 3.8 = cubic feet per pound*at 100 lb gage. that the steam is flowing into a pressure less than 58 per cent of the initial pressure, that the orifice efficiency is approximately 70 per cent, and that 20 per cent more is allowed for a factor of safety, then the pressure reducing valves will have the working capacities shown in Table 10. If the valve, when fully open, does not give an orifice area equal to that of the pipe on the inlet side, then the capacities will be proportional to the percentage of opening secured, taking the pipe area as 100 per cent.. Most exact regulation of pressure on steam heating systems is secured from diaphragm-operated valves controlled by a pilot line from the low pressure 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 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. 2) 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 yalve'vbn 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; arid in 454 Chapter 32--Steam System Piping 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 corinected in parallel. One valve should be large enough for the maximum load and the other should have a diameter approximately half that of the first. The smaller valve can be used most of the time, for it will give much better regulation than the larger one on light or normal loads. Less trouble from expansion leaks will occur when the bypass Fig. 2. Typical Pressure Reducing Valve Installation Control yalves Gate valves are recommended in all cases where service demands that the valve be either entirely open or entirely closed, but they should never be use^ for throttling. Angle globe valves and straight globe valves should be used for throttling, as done on by-passes around pressure reducing valves or on by-passes around traps. EXPANSION IN STEAM AND RETURN LINES Because all steam and return lines expand and contract with changes in temperature, provision should be made for such movement. The expansion in steam supply pipes is normally taken at to in. per 100 ft and in return lines at one-half or two-thirds of this amount. It' may be calculated accurately if the temperature rise arid fall can be determined with reasonable certainty (Table 3, Chapter 34). The tem perature at the time of erection often has a greater expansion effect on piping than the temperature in the building after it has been put irito service. Expansion may be taken care of by any, or all, of three different methods, namely, (1) the spring in the pipe including offsets and expan sion bends, (2) the turning of the pipe on its threads and swing joints, rind (3) the use of expansion joints. By the first scheme, which is the most popular method where space permits, the pipe is offset, or broken, around rooms or corners, and is hung so that the spring in the pipe at right angles to the expansion movement is sufficient to absorb the expansion. If conditions do not lend themselves to this treatment, regular expansion bends of the~Z7 or offset type may be used. In tight places.such as pipe tunnels the expansion joint is pre- ferable. 455 '/