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94 Am. Soc. of Heat.-Vent. Engineers Guide, 1922 Table 3 gives a method for computing the sizes of mains and risers and the pipe sizes given are based upon a pressure drop in the main riser of 10 lb. per 100 ft. of run; other values such as 5 lb. or 7 lb. can be used from Table 5 if it is found advisable on account of very long runs. It will be seen that 60 per cent of the total amount of water required for all the fixtures on any floor is given as the factor of use and that a further reduction is made by deducting 10 per cent on each floor below the top floor until 40 per cent is reached. This of course is a matter of judgment and experience, but the volume of flow will be adequate for the average case; providing the pipe sizes selected are ample for the drop in pressure admissable. WHEN TANK IS ON ROOF If tank is elevated.about 35 ft. above highest fixture, which would be about 25 ft. above the roof, all the computations given herein will apply for branch connections and main risers except that the main riser will have its greatest diameter at the top. It will be seen that 35 ft. elevation will give the necessary 15 lb. pressure at the highest fixture. FRICTION IN ELBOWS Friction caused by elbows should be added to straight pipe friction. Each elbow in a line will add friction equal to a length of straight pipe at forty times the diameter of the pipe approximately as follows: Pipe size ........................... % 1 Equivalent length of . straight pipe in feet.. 2.5 3.3 114 114 4.1 5 2 214 S 8.3 3 10 314 4 11.7 13.3 The pressure loss due to friction inthe main leading tobase of riser can readily be added tothe pressure requiredat the base of a riser, thus: Required--the size of riser and main for a 10-story building 100 ft. high where the water use per floor as given by Table 1 for fixtures aggregates 100 gal. per floor: The computations as given in Table 3 show that the water flow re quired on the various floors is as follows: TABLE 7. GIVING EXAMPLE OF RISER SIZES FOR DIFFERENT PRESSURE DROPS PER 100 FT. RUN Floor Gal. per min. 51b. 71b. 101b. 201b. 10 60 2 2 9 108 24 . 24 8 144 3 3 7 168 34 3 6 180 34 3 5 180 . 34 3 . 4 184 34 3 " 3 192 34 3 2 216 34 3 1 '240 34 34 2 14 24 14 24 2 3 24 3 24 3 24 3 24 3 24 3 24 3 24 If the system of mains and risers is based upon 10 lb. pressure drop per 100 ft. run, the pressure required at base of-riser when water is flow ing will be 69 lb., see Table 6 at TOO ft. height; or at 20 lb. drop it would be 79 lb. while at 5 lb. drop it would only be 64 lb. Pressure required at the main 100 ft. away with,10 lb. drop per 100 ft, would, be. 69_dLl0 = 79 lb. or-50-ft.-away-69'+ 5 = 74 lb, , ~----- --7 Am. Soc. of Heat.-Vent. Engineers Guide. 1922 WATER SUPPLY FORMULA 95 CF -- Cu. ft. per min. discharged G -- Gal. per min. discharged H = Friction head of water in feet = pressure X 2.31; if water is raised vertically, deduct number of feet raised, from head due to pressure. L = Length of pipe in feet--including horizontal and vertical runs. (3d)5 X 3---H-CF - 0.16 \ (1) G -- 1.2 (3d)5 X 3 H L (0.16 X CFy X L H~ (3d)5 (2) (3) (1.2 C)2 XL H = --------------------- (3d)5 (4) The above formula neglect the head due to entry, which need not be computed except when L is very short.' '0.83 G Hx = ,d2 X 13 Hl = head due to entry in feet. 0.16 CF V ,d2 X 13 / Example-- Required the discharge of a 2 in. main with pressure 30 lb. 100 ft. horizontal run and 30 ft. vertical run. H = 30 X 2.31 -- 30 = 39.3 Formula (2) G = 1.2 1(3 X 2,)5 X 3 X 39.3 \-------------------------- = 100.8 100 -f- 30 In the above case the head due to entry would be / 0.83 X 100.8 \ 2 H, = (----------------------) = 2.56 ft. \ 2 X 2 X 13 / Usually this can be neglected except for very close calculations.