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HEATING VENTILATING AIR CONDITIONING GUIDE 1943 governed by the amount of pressure which can be spared in overcoming resistance to flow of a given volume of water per unit of time. After the approximate amount of- water required has been computed, a minimum delivery pressure at the highest fixture may be determined, which should be approximately 15 lb per square inch. It should be remembered that for every foot in height there will be a hydrostatic loss of head of 0.433 lb. The pressure loss through a water meter may be significant as may be seen from Table 2. The pressure losses through filters or other water conditioning apparatus also must be considered. After evaluating the previously mentioned factors, the total allowable friction loss for the Fig. 1. Chart Showing Relation Between Maximum Flow and Probable Usage system may be determined by subtracting from the street main pressure, the sum of the following four items: 1. Minimum allowable pressure at top fixture. 2. Meter loss. 3. 0.433 X height in feet from main to top fixture. 4. Loss for filters, softener, etc. PIPE MATERIAL x The material used in the water piping affects its carrying capacity. For example, copper or brass pipe is not as likely to retain interior incrustation as is ferrous pipe, and galvanized pipe will not rust as quickly as uncoated pipe. Some waters tend to deposit salts, rust, and the like on the interior surfaces of pipes, greatly reducing their capacity. In some cities it has been found necessary to allow for as much as 50 per cent reduction in carrying capacity after 15 years of service. The data given in this chapter are based on use of galvanized steel piping and on water which is not notoriously inclined to leave deposit. If the building is in a zone having untreated water, known to carry precipitable solids,, the pipe sizes should be increased at least one size and no water pipe should be smaller than % in. - . ... 822 CHAPTER 46. WATER SUPPLY PIPING AND WATER HEATING Table 2. Pressure Loss Through Water Disc Meters4 A. W. W. A. Standards Rate of Flow Gpm 5 10 15 20 25 30 35 40 45 50 100 % 1.5 6.0 14.0 25.0 175200 250 300 350 400 500 GOO 800 1000 Approx. Pressure Loss Through Meters, Lb per Sq In. f \ Pipe Size (In.) H1 0.5. 2.0 5.0 :. 9.0 ' 13.5 19.5 012 1.0 2.0 3.5 ' 5.5 8.0 11.0 14.0 18.0 22.0 1H . 0.2 0.6 1.0 1.5 2.0 3.0 4.0 5.0 6.0 14.0 25.0 2 ; 0.2 . 0.4 0.6 0.9 1.0 1.5 2.0 2.5 5.5 10.0 15.0 - 22.0 3 0.7 1.5 2.8 4.0 6.0 8.0 10.4 16.0 23.0 4 1.0 1.5 2.2 3.0 4.0 6.0 9.0 12.0 16.0 25.0 6 1.0 1.5 2.2 3.0 4.0 6*.5 9.0 16.0 25.0 Minimum Size of Service Recommended Safe Maximum Delivery of Meters Flow Gpm Approx. Minimum Pipe Size of Service,Maximum Length (Ft) 30 75 100 150 200 1-20 H 20-30 H 30-50 1. 50-100 ' "iM 100-150 lM H 1 m l}4 2 1. 1 2 2 11 1 lH IH ' 1M 2 2. 2X 2)4 Meter Size In. X .X l 1M 2 3 4. 6' Capacity. Gpm Based on 25 Lb Loss Through Meter 20 34 53 100 160 315 , 500 1000 ^Pressure loss through compound and current meters is less than shown in table. For exact information consult manufacturers. ALLOWANCE for fittings Before applying charts for pipe friction, the resistance due to fittings and valves should'be evaluated. Table 3 gives this resistance expressed in equivalent' feet of straight pipe. To use Table 3, the size of the valve or fitting must be known- Table 3 is therefore of little use in the original design of a system, since the valve and fitting allowances must be made before the pipe size is known. Experience indicates, however, that an average increase of 50 pec cent in the length of the longest measured pipe will account,for the fittings, and thus a tentative length can be assumed for computing the pressure drop per 100 ft of run. The values of Table 3 823 '