Document rmZGVrY7myk1kpkd6jMq72Gq

952 CHAPTER 50 . 1959 Guide FRICTION LOSS IN HEAD IN LBS. PER SQ. IN. PER IOO FT. LENGTH Water Services 953 Fig. 5. Flow Chart for Copper Tubing At this point it is necessary to make some estimate of the equivalent pipe length of.the fittings on the direct line from the street main to the highest fixture. The exact equivalent length of the various fittings cannot now be determined since the pipe sizes of the building main, riser, and branch leading to the highest fixture are not known as yet, but a first approximation is necessary in order to make a tentative selection of pipe sizes. If the computed pipe sizes differ from those used in deter mining the equivalent length of pipe fittings, a recalculation will be necessary, using the computed pipe sizes for the fittings. For the.purposes of this example assume that the total equivalent length of the pipe fittings is 50 ft. -i Then the permissible pressure loss per 100 ft of equivalent pipe is 12 X 100/(100 + 50) = 8 psi. Assuming that the corrosive and caking properties of the water are such that Fig. 3 for fairly rough pipe is applicable, a 2 in. building main is found adequate.--;: The sizing of the Stanches of the building main, the risers, &n&'fixture branches follow the-principles outlined..; For example, assume'that one of the branches of the building main carries the cold water supply for 3 wafer closets, 2 bath tubs, and 3 lavatories. Using the.permissible pressure loss of 8 psi per 100 ft, the size of branch determined from Table 1 arid Figs. 1 and 3 is found to be 1)4 in. Items entering the computation of pipe size are given in Table 4. The principles involved in sizing either up-feed or down-feed systems are the same. The principal difference in procedure is that in the downfeed.system the difference-in elevation between the house tank and the fixtures provides the'pressure required to overcome pipe friction. The water demand for hose bibbs or other large demand fixtures taken off the building main is frequently the cause of inadequate water supply to the upper floor of a building. This robbing of upper floor fixtures of water may be prevented by sizing the distribution system so that the pressure drops from the street main to ail fixtures are the same. It is good practice to maintain the building main of ample size (not less than 1 in. where possible) until all branches to hose bibbs have been'connected. Where the street main pressure is excessive and a pressure reducing valve is used to prevent water hammer or excessive pressure at the fixtures, it is fre quently desirable to connect hose bibbs ahead of the reducing valve. Table 4. Computation of Branch Size in Example 1 Fixtures No. and Kind 3 flush valves.________ 2 bath tubs.... 3 lavatories Total-- ............ .Fixture Units ,. (From Table 1 and Notsc) 3x6 =18 . Vs (2x2) = ,3 ... M (3x 1) = 2.25 23.25 Demand (From Fig. 2)' Gfm Pipe Size (From Fig.- 3) . In*. 38 m "