Document O17GnQDdXnD4mow05Kar9B9we

Heating Ventilating Air Conditioning Guide 1939 pipe, is dropped down into the boiler return, or into the top of the Hart- ford Loop, which is described in a following paragraph. As a result, any water carried over from the boiler follows the direction of steam flow v toward the rear and is discharged into the rear drip, or balance pipe ' without being carried over into the system. ' Return Cast-iron boilers are generally provided with return tappings on both sides, but steel boilers often arh equipped with only one return tapping. A boiler with side return tappings will usually have a more effective cir culation if both tappings are used. Check valves generally should not be used on the return connection to steam heating boilers from one and two pipe gravity systems because they are not always dependable inasmuch as a small piece of scale or dirt lodged on the seat will hold the tongue open and make the check useless. These valves also offer a certain amount of resistance to the returns coming back to the boiler, and in gravity systems will raise the water line in the far end of the wet return several inches4. However, if check valves are omitted and the steam pressure is raised with the boiler steam valve closed, the water in the boiler will be blown out into the return system with the accompanying danger of boiler damage. These objections are largely overcome with the Hartford return connection. Hartford Return Connection In order to prevent the boiler from losing its water under any circum stances, the use of the Hartford Connection, or the Underwriters Loop, is recommended. Fig. 5 shows this connection for a two-boiler installation. For a. single boiler installation the connection is made as is indicated for one boiler. The essential features of construction of a Hartford Loop 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 con nection about 2 in. below the normal boiler water line from the return main to the boiler steam and return balance connection. Sizing Boiler Connections Little authentic information is available on the sizing of boiler runouts and steam headers. Although many 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 com puting 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 calculating the heaviest load that will be placed on the boiler at any time, and sizing the runout on the same basis as the building 4Scc method of calculating height above water line for gravity one-pipe systems in Chapter 15. 322 Chapter 16. Piping for Steam Heating Systems The difference in size between the vertical uptakes from the .jn*'onH the horizontal main or runout is compensated for by the use of -SS ells (Figs- 3 and 4). ' ^`^'Retum connections to boilers in gravity systems are made the same *- the return main itself. Where the return is split and connected to >"?'e >appings on the same boiler, both connections are made the full size ' V^the return line. Where two or more boilers are in use, the return to .- v may be sized to carry the full amount of return for the maximum load ; , ?a, ich that boiler will be required to carry. Where two boilers are used, ' -:?he of them being a spare, the full size of the return main would be ~'Tarried to each boiler, but if three boilers are installed, with one spare, the Fig. 6. The Hartford Return Connection 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 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, Fig. 3, Chapter 43. The relative boiler loads should be considered, as in the case of gravity return connections. Boiler header and piping sizes should be based on the total load. HIGH PRESSURE STEAM When steam heating systems are supplied with steam from a high pressure plant, one or more pressure-reducing valves are used to bring the 323