Document yZYnoY8wOqp2DDEKL2Z0wrqX

748 :HAPTER 56 1959 Guide termination of the design rate of flow in any particular sec tion of piping. The next general step is to determine the size of piping. As water flows through a pipe, the pressure continually de creases along the pipe, due to loss of energy from friction. The problem is then one of ascertaining the minimum pres sure in the street main, and the minimum pressure required for the operation of the topmost fixture. (A pressure of 15 psi is'ample for flush valves, but reference should be made to the manufacturers' requirements. A minimum of 8 psi should be allowed for other fixtures.) The pressure differ ential thus obtained will be available for overcoming pressure losses in the distributing system, and in overcoming the dif ference in elevation between the water main and the highest fixture. The pressure loss, in pounds per square inch, caused by the difference in elevation between the street main and the high est fixture, may be obtained by multiplying the difference in elevation in feet by the conversion factor 0.43. When water flows through a pipe, friction occurs as the re sult of the sliding of water particles past one another. If the pipe wall is rough, the roughness projections cause addi tional friction, owing to the development of increased turbu lence in the flowing water. As the water flows along a uni form pipe, the pressure decreases as a result of a dissipation of energy arising from the internal friction set up by viscosity of the water. This loss in energy is shown by the loss of pressure. The pressure loss is proportional to the length of straight uniform pipes, and varies greatly with flow velocity, pipe diameter, roughness of pipe, and water temperature. Table 2....Demand Weights of Fixtures in fixture Units* fixfar* or Grocpb Occupoitey Type of Supply Confrol Weigh! m Fixtvro (Mb' 10 2 Kitchen sink................... Hotel or restaurant... Faucet.......... 4 6 Pri In. for closet.. for closet.. 8 6 Laundry trays (1-3) -- Private....... Faucet.......... Combination fixture.... Private....... Faucet.......... 2 3 3 * For supply outlets likely to impose continuous demands, estimate continu ous (apply separately sad add to total demand (or fixture*. b For fixtures not listed, weights may be assumed by comparing the fixture to a listed one using water in similar quantities and at similar rates. The given weights are (or tote! demand. For fixtures with both hot and eoid water supplies, the weights (or maximum separate demands may be taken es X the listed demand for the supply No. J for system predomtnonfly f*r Hvth valves. No. 2 for tysfoa prodomioanfty for fkish (asks. fig. \.... Estimate Curves for Demand Load On the basis of inside surface conditions, pipes may be classified as smooth, fairly rough, and rough, as follows: Smooth. The pipe surface shows no perceptible roughness. Pipes made of copper, brass, or lead may usually be classified as smooth. Fairly Rough. All ordinary pipes, such as wrought iron, gal vanized iron, steel, and cast iron, after a few years of usage, may be called fairly rough. Rough. Pipes that have deteriorated fairly rapidly for some 10 or 15 years after being laid, are classified as rough. Figs. 3, 4, and 5 give the pipe frictioa losses corresponding to these three types of pipes for various nominal diameters for a water temperature of 50 F.1 Example 1 will illustrate the use of the charts. Example 1: A 2Vi-in. fairly rough pipe supplies 100 gpm of water. Find the friction loss in head if the pipe length is 200 ft. Solution: Enter Fig. 4 at 100 gpm, and move along this line until it intersects the 2W-in. diameter line. From this intersec tion point, move vertically down and read 4.5 psi friction loss per 100 ft of pipe length. Then the total friction loss will be 2 X 45 -- 9 psi. The pressure losses in the distributing system will consist of the pressure losses in the piping itself, plus the pressure losses in the pipe fittings, valves, and the water meter, if any. Approximate design pressure losses for disc-type meters for various rates of flow are given in Fig. 6. Flow limits for disctype meters, which may be regarded as the limits of recom mended ranges in capacities, are given in Table 3. Manufac turers should be consulted for data on exact pressure losses and capacities since these vary for meters of different internal design. Wafer Services FRICTION LOSS IN HEAD IN LBS. PER SQ. IN. PER IOO FT. LENGTH 749 Fig. 3 .... Flow Chart for Copper Tube1 Fig. 7 shows the variation of pressure loss with rate of flow for various types of faucets and cocks, based on experimental data obtained at the State University of Iowa. The loss of pressure through any fitting or valve can be expressed in pounds per square inch for any given rate of flow. Experience has shown, however, that the simplest method of expressing losses in fittings and valves is to use the concept of an equivalent length of straight pipe. Thus it has