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HEATING VENTILATINC AIR CONDITIONING CUIDE 1941 With returns pumped from a vacuum or receiver return pump, the size of the line may be calculated from the water rate on the pump discharge when it is operating, and the line sized for a very small pressure drop, the size being obtained from the Chart for Pressure Drop for Various Rates of Flow of Water, Chapter 45. The relative boiler loads should be considered,1 as in the case of gravity return connections. Boiler header and piping sizes should be based on the total load. HIGH PRESSURE STEAM When high pressure steam is being supplied and lower steam pressures are required for use in heating, domestic hot water, utility services, etc: one or more pressure reducing valves, or pressure regulators, as they are sometimes called, are required. These are used in two classes of service, one where the steam must be shut off tight to prevent the low pressure building up at time of no load, and the other where the low pressure lines will condense enough steam to offset normal leaking through the valve. In the latter case, double seated valves may be used in a manner that reduces the work required of the. diaphragm in closing the valve and' consequently the size of the dia phragm. These valves also control the low pressures more closely under conditions of varying high pressures. Valves that shut off all steam are called dead end type. They are single seated, and some of them have pilot operation that provides close control of the reduced pressure. If a thermostatically controlled valve is installed after, and near, a reducing valve in such a manner as to cut off the passage of steam, the dead end type should be used. It is common practice when the initial steam pressure is 100 lb or higher to install two-stage reduction. This makes a quieter condition of steam flow, as it is apparent that with one reduction, as for example, from 150 to 2 lb, there is a smaller opening with greater velocity across the reducing valve, and consequently, more noise. A two-stage reduction also introduces a source of safety, since if one reducing valve were to build up its discharge pressure, this excess pressure would not be as great as the case might be in a one-stage reduction. If an installation requires single seated valves, and the pilot type cannot be used, it is necessary to use two-stage reduction, as single seated valves require sufficient diaphragm area to overcome the unbalanced pressure underneath the single valve. In. many cases the large diameter of diaphragm required would make it impractical in construction. With a two-stage reduction the diaphragm diameter required would be reduced. If a one-stage reduction is desired, .it is necessary to use a pilot controlled pressure reducing valve, where low pressures are to be maintained closely. In making two-stage reduction, allowance should be made, by in creasing the pipe size, for expansion of steam on the low pressure side of the valve. This also allows steam flow to be at a more nearly uniform velocity. Separating the valves by a distance up to 20 ft is recommended to reduce excessive hunting action of the first valve. When the reduced pressure is approximately 15 lb or lower, the weight and lever diaphragm valve gives the best results with minimum main tenance. Above 15 lb, spring loaded diaphragm valves should be used, 282 CHAPTER 14. PIPING FOR STEAM HEATING SYSTEMS use of the extra weights required on weight and lever type. Equalizing I'e connections should be made not too close to the valve, and into the hottom of the reduced pressure steam main, to allow maximum conden sation to exist in this equalizing line, or the connection is made into the too of the main and a water accumulator used to reduce the variation of the head of water on the diaphragm. Care should be exercised in selecting the size of a reducing valve. The safest method is to consult the manufacturer. It is essential that sizes of piping to and from the reducing valve be such that they will pass the desired amount of steam with the maximum velocity desired. A common error is to make the size of the reducing valve the same size as that of the service, or outlet pipe size. Generally, this will make the reducing valve oversized, and bring about wire-drawing of valve and seat, due to small lift of the valve seat. On installations where the steam requirements are relatively large and variable in mild weather or reduced demand periods, wire-drawing may occur. To overcome this condition, two reducing valves are installed in parallel, with the sizes selected on a 70 and 30 per cent proportion of maximum flow. For example, if 50,000 lb of steam per hour are required, the size of one valve is on the basis of 0.7 + 50,000 lb, or 35,000 lb, and the other on the basis of 0.3 + 50,000 lb, or 15,000 lb. During the mild or reduced demand periods, steam will flow through the smaller valve only. During the remainder of the season, the larger valve is set to control at whatever low pressure is desired, and the smaller one at a somewhat lower pressure. Thus, when steam flow is not at its maximum, the smaller valve is shut, and automatically opens when the maximum steam demand occurs, since this maximum demand of steam creates a slight pressure drop in the service line. The installation of reducing valves in pipe lines requires detailed planning. They should be installed to give ease of access for inspection and repair, and wherever possible with diaphragm downward, except in cases of pilot operated valves. There should be a by-pass around each reducing valve of size equal to one half the size of reducing valve. The glove valve in by-pass line should be of a better type of construction, and must shut off absolutely tight. A steam pressure gage, graduated up to the initial pressure should be installed on the low pressure side. Safety valves located on the low pressure side should be set 5 lb higher than the final pressure but may be 10 lb higher than the reduced pressure if this reduced pressure is that of the first stage reduction of a double reduction. Strainers should always be installed on the inlet to the reducing valve but are not required before a second-stage reduction. If a two-stage reduction is made, it is well to install a pressure gage immediately before the reducing valve of the second-stage reduction also. In sizes 3 in. and above, it is advisable to tap the bodies of the reducing valve on inlet side for purposes of draining condensate accumulation through steam traps. -T 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 283