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Q00 0

Online Encyclopedia
Originally appearing in Volume V14, Page 69 of the 1911 Encyclopedia Britannica.
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Q00 0.984 loo° 1.26o 110° 1.556 120° 1.861 130° 2.158 140° 2.431 =0.131 -}-1.847 (d/2p) r, (I) (4a) value of d by assuming if the average demand is 25 gallons per head per day, the mails should be calculated for 50 gallons per head per day. § 86. Determination of the Diameters of Different Parts of a Water Main.—When the plan of the arrangement of mains is determined upon, and the supply to each locality and the pressure required is ascertained, it remains to determine the diameters of the pipes. Let fig. 97 show an elevation of a main ABCD..., R being the reservoir from which the supply is derived. Let NN be the datum line of the levelling operations, and Ha, Hb...the heights of the main above the datum line, Hr being the height of the water surface in the (b) A second method is to obtain a rough '=a. This value is d' V (32Q2/gr2i)V a =o.6319 V, (Q2/i)V a. Then a very approximate value of 1• is =a(I+I/I2d'); and a revised value of d, not sensibly differing from the exact value, is d" =V (32Q2; gr2i)V i'' °0.6319 V (Q2/i)V r'. (c) Equation 7 may be put in the form d =V (32aQ2/gr2i) V (1+111 ad). (9) Expanding the term in brackets, _ tE E 11 (I+I/12d) =1+I/6od—I/1800d2... Neglecting the terms after the second, d=J(32a!gr2)V(Q2/i).(I+I/6od) X z =V(32a/gr2)V(Q2/2)+o.o1667;(9a) il'° b and V (32a/gr2) =0.219 for new pipes s =o•252forincrusted pipes. § 85. Arrangement of Water Mains 4'e for Towns' Supply.—Town mains are •-.-- --..P-' n-~.Txs-.°-' : r ea- w .:_..~ ~,. usually suppliedby gravitation from FIG. 98. a service reservoir, which in turn is supplied by gravitation from a storage reservoir or by pumping from a lower level. The service reservoir should contain three days' supply or in important cases much more. Its elevation should he such that water is delivered at a pressure of at least about too ft. to the highest parts of the district. The greatest pressure in the mains is usually about 200 ft., the pressure for which ordinary p)aes and fittings are designed. Hence if the district supplied has great variations of level it must be divided into zones of higher and lower pressure. Fig. 96 shows a district of two zones each with its service reservoir and a range of pressure in the lower district from loo to 200 ft. The total supply required is in England about 25 gallons per head per day. But in many towns, and especially in America, the supply is considerably greater, but also in many cases a good deal of the supply is lost by leakage of the mains. The supply through the branch mains of a distributing system is calculated from the population supplied. But in determining the capacity of the mains the fluctuation of the demand must be allowed for. It is usual to take the maximum demand at twice the average demand. Hence reservoir from the same datum. Set up next heights AA, BB1,... representing the minimum pressure height necessary for the adequate supply of each locality. Then A1B1C1D1... is a line which should form a lower limit to the line of virtual slope. Then if heights I)0, 1)b, I)c... are taken representing the actual losses of head in each length lo, lb, la... of the main, AoBoCo will be the line of virtual slope, and it will be obvious at what points such as Do and Eo, the pressure is deficient, and a different choice of diameter of main is required. For any point z in the length of the main, we have Pressure height =Hr—Hz—Oa.+b+- • .~s)• Where no other circumstance limits the loss of head to be assigned to a given length of main, a consideration of the safety of the main from fracture by hydraulic shock leads to a limitation of the velocity of flow. Generally the velocity in water mains lies between I i and 42 ft. per second. Occasionally the velocity in pipes reaches so ft. per second, and in hydraulic machinery working under enormous pressures even 20 ft. per second. Usually the velocity diminishes along the main as the discharge diminishes, so as to reduce somewhat the total loss of head which is liable to render the pressure insufficient at the end of the main. J. T. Fanning gives the following velocities as suitable in pipes for towns' supply: Diameter in inches 4 8 12 18 24 30 36 Velocity in feet per sec. . . 2.5 3'0 3'5 4'5 5'3 6.2 7.0 § 87. Branched Pipe connecting Reservoirs at Different Levels.—Let A, B, C (fig. 98) be three reservoirs connected by the arrangement of pipes shown,—l1, d1, Qi, v1; 12, d2, Q2, v2; l3, d3, Q3, v3 being the length, diameter, discharge and velocity in the three portions of the main pipe. Suppose the dimensions and positions of the pipes known and the discharges required. If a pressure column is introduced at X, the water will rise to a height XR, measuring the pressure at X, and aR, Rb, Rc will be the lines of virtual slope. If the free surface level at R is above b, the reservoir A supplies B and C, and if R is below b, A and B supply C. Consequently there are three cases: I. R above b; Q1=Q2+Q3. II. R level with b; Q1=Q3; Q2=o To determine which case has to be dealt with in the given conditions, suppose the pipe from X to B closed by a sluice. Then there is a simple main, and the height of free surface h' at X can be determined. For this condition ha —h' = I'(v12/2g)(411/d1) = 323"Q'2l1/gr2d15 ; Ii'—ha =I-(v32/2g)(4l3/d3) =32f'Q'2l3/gr2d35; where Q' is the common discharge of the two portions of the pipe. Hence (ha—h')/(h'—ha) =11d35/13d15, from which h' is easily obtained. If then h' is greater than hb, opening the sluice between X and B will allow flow towards B, and the case in hand is case I. If h' is less than h5, opening the sluice will allow flow from B, and the case is case III. If h'=hb, the case is case II., and is already completely solved. 4•---~-_Nigh eve/• Zone 1 i Low Level Zone=•-~_. 1 _ soi zo6 I -`I > orssi Fm. 97. The true value of h must lie between h' and hb. Choose a new value of h, and recalculate QI, Q2, Q3. Then if Q,>Q2+Q3 in case I., or Q,+Q2>Q3 in case III., the value chosen for h is too small, and a new value must be chosen. If Qi End of Article: Q00 0
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