Document MG5By9XyZOGNvddOa94xnopBx

506 CHAPTER 21 1953 Guide steam demand occurs, because this maximum demand creates a slight pressure drop in the service line. _The installation of reducing valves in pipe lines requires detailed planning. Thfey 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 globe valve in by-pass line should be of a good 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 psi higher than the final pressure, but may be 10 P higher than the reduced pressure if this reduced pressure is that of the first stage reduction of .a double reduction. Strainers are sometimes installed on the inlet to the reducing valve, but are not required before a Fio. 20. The Habtford Return Connection 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 install a drip trap between the two reducing valves. Steam BOILER CONNECTIONS Cast-iron, sectional heating boilers usually have several outlets in the top. Two or more outlets should be used whenever possible to reduce the velocity of the steam in the vertical uptakes from the boiler, and thus to prevent carrying of water into the steam main. Return C^t-inm boilers are generally provided with return tappings on both sides, while steel boilers are generally equipped with only one return Steam Heating Systems 507 tapping. Where two tappings are provided, both should be used to effect proper circulation through the boiler. The return connection should include either a Hartford return connection or a check valve to prevent the accidental loss of boiler water to the returns, with consequent danger of boiler damage. The Hartford return connection is to be preferred over the check valve, because the latter is apt to stick or not close tightly and, furthermore, because the check valve offers additional resistance to the condensate coming back to the boiler, which in gravity systems would raise the water line in the far end of the wet-return several inches. In order to prevent the boiler from losing its water under any circum stances, the use of the Hartford return connection is recommended. This connection for a one- or two-boiler installation is shown in Fig. 20. The essential features of construction of a Hartford return connection are: (1) a direct connection (made without valves) between the steam side of the boiler and the return side of the boiler, and (2) a close nipple, or preferably an inverted Y-fitting connection about 2 in. below the normal boiler water line from the return main to the boiler steam and return pressure balance connection. Equalizing pipe connections between the steam and return are given in Fig. 20, based on grate areas, but in no case shall this pipe size be less than the main return piping from the system. - Sizing Boiler Connections Little information is available on the sizing of boiler runouts and steam headers. Although some engineers prefer an enlarged steam header to serve as additional steam storage space, there ordinarily is no sudden demand for steam in a steam heating system, except during the heating-up period, at which time a large steam header is a disadvantage rather than an advantage. The boiler header may be sized by first computing the maximum load that must be carried by any portion of the header under any conceivable method of operation, and then applying the same schedule of pipe sizing to the header as is used on the steam mains for the building. The horizontal runouts from the boiler, or boilers, may be sized by cal culating the heaviest load that will be placed on the boiler at any time, and sizing the runouts on the same basis as the building mains. The difference in size between the vertical uptakes from the boiler, which should be of same size as the boiler outlet tapping, and the horizontal main or runout, is compensated for by the use of reducing ells. Return connections to boilers in gravity systems are made the same size as the return main itself. Where the return is split and connected to two tappings on the same boiler, both connections are made the full size of the return line. Where two or more boilers are in use, the return to each may be sized to carry the full amount of return for the maximum load which that boiler will be required to carry. Where two boilers are used, one of them being a spare, the full size of the return main would be earned to each boiler, but if three boilers are installed, with one spare, the return line to each boiler would require only half of the capacity of the entire system, or, if the boiler capacity were more than one-half the entire system load, the return would be sized on the basis of the maximum boiler capacity. As the return piping around the boiler is usually small and short, it should not be sized to the minimum. 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 operated, and the line sized for a very small pressure drop.. The relative boiler loads should be considered, as in the case of gravity