Since flow data is usually given in gallons per minute and pipe
diameters in inches, it is more convenient to express the Darcy
Weisbach equation as:
Where:
Hf = friction head loss, in ft
f = dimensionless friction factor based on the Rn and the inside
roughness of pipe
L = length of pipe, in ft
Q = flow, in GPM
d = inside diameter of the pipe, in inches
0.031 = Constant
Hf = 0.031xfLxQ,,
d5
Example: Calculate frictional head loss, given the following information
and using the following equation.
Pipe size = 8.415 inches.
Fuel = DF-2 at 50F.
V = 4 ft/sec
Length of pipe = 4000 ft.
Hf = fxLxV,,
2gxD
Step 1:
Calculate Reynolds number
K = 4.9 = 0.0000527
Y = 92900
R = VD = 4 x .701 = 53206.8317(52307) or 5.3 x 10
.0000527
R = 3.160 xQ
dxK
To obtain Kinematic viscosity in centistokes, use Figure 1-1; move up the
scale to DF-2, move to the left and read 4.9.
Step 2: find the friction factor using the Reynolds number. Figure 1-2.
Locate 5.36 x 10, move up the chart to 8 inch pipe, move to the left and
read .0216 or .022.
Step 3:
Hf = fxLxV,,
2gxD
Step 4:
2g = 2x32.2 = 64.4
Step 5:
Diameter of pipe in ft. = .701
Step 6:
Hf = .0216x4000x16 = 1382.4
64.4 x .701
45.14
12-14
QM 5099
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