Open main menu
This page needs to be proofread.
66
[sTeADv FLOW IN PIPES
HYDRAULICS


(b) A second method is to obtain a rough value ofdby assuming § '=a.. This value is

d' =i/ (32Q'/g1f'i)%/ a =0-6319 if (Q”/i>'/ a-Then a ver) approximate value of 3' is

§ '=<=<1+1/12d'>;

and a revised value of d, not sensibly differing from the exact value, IS

fi” =i/ (32Q2.i§ "'2i)§ / I' =0°6319 V (Q'/DV I'(c) Equation 7 may be put in the

if the average demand is 25 gallons per head per day, the mains should be calculated for 50 gallons per head per day. § 86. Determination of the Diameters of Dijferent 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 Hg. 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 Hn, Hz, ...the heights of the main above the datum line, Hi- being the height of the water surface in the form Q

d=§ /(32aQ2/g1r2i)%' (I +1/12d). (9) , § ; -, ? -'~'~'H~2::; ° ';::fr' '°" “ ' """ """ ""' “"' °""""""°" Expanding the term in brackets, 1”'*'- """"~3§ -=, ?;f: @.... ' & 6 W (1 +1/1 zd) = 1 -i-I/60d-I/ISOO112... -B 11 d, i ""' Slowking4Farmbrough's revenge. ' T" A Neglecting the terms after the second, ' I 'Q ' Qz@~';' 'B: d = 2/ (321, /gw2)v (Q2/i).{ 1 + 1 /sed; g XYZ* nz” '-~s = V (32¢1/gr”)§ ~/(Q”/1)+0-01667;(9r1): = ' 51, '~-and L, - of l 5 '~~ 0

M(32U.:/g .2) =o-219 for new pipes ' 5 Z i3”" f " 311% N T =O'252fOI'iI'1CI'l1St€dpip€S. Jd3Q.?T{;-E ':'T'EH"* § 8'. Arrangement of Water Mains 5 ' ' -64: for To:'iv11.s' Suppl .-Town mains are - — .v.....!Z.... .H ""P...~.I!!?!'.l'l'-»~.»—¥»-»~~—~»- —"—'~~°'”"“"! °"" "°' ' ""'°“ °°"' '°~-5-“*°° usually suppliediiy gravitation from

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 be such that water is delivered at a pressure of at least about loo ft. to the highest parts of the district. The greatest ressure in the mains is usually about 200 ft., the pressure for which ordinary pines and fittings are designed. Hence if the district supplied has l*” “"1:7H' L J Z .. .. . .-4-

-1- 49" eve one I A(

s if "I 0

I

'=91 |<¢!. Q' 'l9;T'l .' , ' "“i

1' 'V I

A | .?: -5 4

“L mf 2?° "

s ' 1

aw “', "

f I £56

I m y ILO, !

~ 4 1

56:4

L . . . . . i . . L . . . ... i . .. FIG. 96.

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 100 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 FIG. 98. B V K V

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 ba, bb, bc... are taken representing the actual losses of head in each length la, lb, l¢... of the main, AUBOCO will be the line of virtual slope, and it will be obvious at what points such as D0 and EU, 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 2 Pressure height=H, -H, -(fy, -|~f)1, +...§ ,). 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 1% and 4% ft. per second. Occasionally the velocity in pipes reaches 10 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.

]. 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-o § 87. Branched Pipe connecting Reservoirs at Different Levels.-Let A, B, C (fig. 98) be three reservoirs connected by the arrangement of pipes shown, -ll, dl, QI, 111; lg, 112, Q2, 112; 13, da, Q3, '03 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

. § ; T g, rg —— ..- ~- . <-, —~-~- -T-~-~-»———~.q——»=~»- —=¢———= — r»— R is below b, A and B supply C.

% Q Consequently there are three cases 1-7§ ";T "l ~§ &"f § § § } I. R above b; Q1=Q2-I-Q3. R i ¥ Q 5 II. R level with b; Q1=Q3; Q2=o f A f Zm Q 5- 5 111. Rbe1Qwb;Q, +Q, =Q, L 20 i' ' 'Ui 1 : 5; < Ego To determine which case has to be t i 'Q dealt with in the given conditions, 5 ' " A “' 5 5 E Q suppose the pipe from X to B closed Hr: § ; ~-, ;C, : , Q; by a sluice. Then there is a simple

2 Q . » is I ~, F main, and the height of free surface

1 ' 3 I E aB; ' -' 5;0. '%E~-;% h' at X can be determined. For this ffm; = ;, Q is condition

2; Ha; T i In-h'=r<vf/2g><4li/do,

4: 5 » 5 II 5 D; E " =32§ “Q'l1/g1r'di5; Q 2 fe 2; -F 1-'-fi=f<»-2/2g><41-/dg., ,,

- J. - .131 . § ' . . L .. = . 5 ' Q i =32s' Sgr- 3i

—Y —— —- -~~»- 71 —ff- ——~ 35 — ~J-:!— —~—~.~—5—»~i»-~...-~...-5 .... - where Q' is the common discharge N N of the two portions of the pipe.

FIG. 97. Hence

a good deal of the supply is lost by leakage of the mains. The su ply through the branch mains of a distributing system is calculated liiom the population supplied. But in determining the capacity of the mains the Euctuation of the demand must be allowed for. It is usual to take the mximum demand at twice the average demand. Hence (he -W)/(h'-he) =lid3“/lid1',

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 hb, opening the sluice will allow flow from B, and the case is case III. If h'=h, ,, the case

is case II., and is already completely solved.