Document NGq6BMkKMOJMzv7XKbV56ybdg

American Society of Heating and Ventilating Engineers Gbide, 1935 pipe friction plus that introduced by the pipe fittings, all of which produce an excessive drop when compared to the conditions existing with a down feed riser. To size an up-feed riser the minimum pressure of the street main or other source of supply, should be ascertained and from this should be subtracted the pressure to be maintained at the highest fixture, namely 15 lb per square inch, plus the height in feet above the source of water pressure, multiplied by 0.43 to change from feet of head to pounds of pressure. The total length of run from the source of pressure to the farthest and highest fixture should be ascertained, and this should-be ^changed-to-equivalent length of run~to"allow for theTloss occasioned by the pipe fittings. Table 3 gives the additional lengths necessary to allow for the various fittings and valves. The drop allowable in pressure per 100 ft of run may then be obtained by multiplying the surplus pressure (over that required for the gravity head and to supply 15 lb a,t the fixture) by 100 and by dividing this by the equivalent length of run to the farthest or highest fixture. Example 2. Assume a street pressure of 60 lb, the height of the highest fixture 50 f, and the length of the longest run 200 ft. Without knowing the additional length of pipe to be added for the fittings it will be assumed that this is about 100 ft. The surplus pressure which will be available for pressure drop will then be 60 lb - (15 lb + 50 ft X 0.43 lb) = 60 lb - (15 lb + 21.5 ib) = 23.5 lb To change this into drop per 100 ft: 23.5 lb X 100 200 ft + 100 ft 7.8 lb per 100 ft. The pipe may then be sized from the maximum probable flow by selecting a size that does not give a drop in excess of 7.8 lb per 100 ft. It will be seen from Example 2 that it is impossible to size up-feed risers without determining the drop allowable in both the horizontalffeed mains and the toilet room branches. Having once ascertained this allow able drop, it is simply a matter of applying it throughout the system. Table 3. Approximate Allowances for Fittings and Valves in Feet of Straight Pipe Sob of Pipb (Inches) % 1 mm . 2 2H 3 4 5 6 90-Deg Elbow 4 s 5 6 7 7 10 12 18 25 30 45-Deg Elbow 3 3 3 4 5 5 7 8 13 18 21 Type of Fitting ob Valve Return Bend Gate Valve 82 10 3 10 3 12 3 14 4 14 4 20 5 24 6 36 9 50 13 60 15 Globe Valve 48 60 60 72 84 84 120 144 216 300 360 8 10 10 12 14 14 20 24 36 so 60 604 \ y <> Chapter 35--Water Supply Piping HORIZONTAL SUPPLY MAINS The horizontal mains supplying the risers at the top of a down-feed tem must be liberally sized unless the house tank is set at a much higher elevation than usual. To provide a gravity head on the highest fixtures of 15 lb per square inch it is necessary for the water line in the house tank to be nearly 40 ft higher, and with the line loss considered this becomes about 45 ft. Such heights are not often practical and as a result the pressure on the highest fixtures either is reduced to 7 lb (which is sufficient to operate a flush valve), or flush_tank .water-closets, are sub stituted, "or aTseparate cold "and hot water supply is installed with a small pneumatic tank to give the increase in pressure necessary. The chief objection to the use of a pneumatic tank is that a separate hot water heater is required and this heater must be located either sufficiently below the highest fixtures to obtain a gravity circulation, or it must be provided with a circulating pump in order to force the hot water to the top floor level. The most common solution is to place the house tank as high as the structural and architectural conditions will permit and then to use liberally-sized lines between the house tank and the upper fixtures, say for the two top stories, below which the riser sizes may be reduced to those indicated in Fig. 2 and Table 2. Where the house tank is only one story above the top fixtures, flush tank water-closets must be used and the drop in the entire run from the house tank down to the farthest fixture should not exceed 1 lb; the less, the better. This means that if the total equivalent run to the farthest top fixtures supplied is 300 ft, the drop per 100 ft should not exceed 1 10^ or 0.33 lb per 100 ft. The friction curves shown in Fig. 3 may be used for quickly determining the proper size of pipe to give any desired drop in pounds per 100 ft of equivalent run. OVERHEAD DISTRIBUTION MAIN Example 3. Suppose an installation has a house tank in which the water line is 20 ft above the level of the top fixtures to be supplied and that the length of run to the farthest fixtures on this level is 400 ft with the pipe fittings adding another 200 ft, making an equivalent length of 600 ft. What would be the size of main coming out of the tank where a maximum flow rate of 400 gpm may be expected, of the horizontal main where a maximum flow rate of 200 gpm may be expected, and of the riser down to the fixture level where the maximum flow rate is approximately 100 gpm? Here the level of the water in the house tank is 20 ft above the faucet of the highest fixture and the gravity pressure will be 0.43 lb X 20 ft = 8.6 lb and, if a total pressure drop of 1 lb is assumed, the pressure on the farthest fixture under times of peak load will be 8.6 lb - 1 lb = 7.6 lb while the drop per 100 ft of equivalent run will have to be 1 lb X 100 600 0.16671b. Referring to Fig. 3 it will be noted that where the flow through the main is 400 gpm, an 8 in. pipe would be required; that where the flow is reduced to 200 gpm, a 6-in. pipe 605