Document yknRLrqa8B661LnzQvD49dnNd

Heating Ventilating Air Conditioning Guide 1938 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 than 58 per cent of the initial pressure, that the orifice efficiency is approx imately 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. More frequently, difficulty is encountered from the use of pressure reducing valves which are too large in size instead of being Table 10. Capacities of Pressure-Reducing Valves (100-lb Gags Down to any Pressure--52 lb or Less) Inlet Nominal Pipe Diameter (Inches) x X 1 IX iX 2 m 3 3X 4 5 6 Pounds Steam peb 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 Equivalent Direct yiRadiation Sq Ft at 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 Radiation Sq Ft at Vi 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: where A XV X 3600 X .50 144 X 3.88 pounds per hour passed by orifice. 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 100-Ib gage. too small. Where valves are large in size, fhe valve tends to work close to the 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 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 sis double-disc valves, but the single disc is preferable where dead-end valves are necessary, such as on short runs to thermo 314 Chapter 16. Piping for Steam Heating Systems 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 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- 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)' lobe valve High pressure steam Drip Low pressure steam Gate valve T.-*~)~Valve Pressure reducing valve Fig. 2. Typical Pressure-Reducing Valve Installation nected 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. Control Valves 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 used 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 1 % to 1H 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 and fall can be determined with reasonable certainty (Chapter 18). The temperature at the time of erection often has a greater expansion effect on piping than the temperature in the building after it has been put into service. 315