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3 6
PLUMBING CONNECTION
WINTER 2016
The pressure loss per metre length of pipe is directly
proportional to the roughness of the pipe. We use a
dimensionless friction factor (f), to calculate pipe pressure
losses along the pipe due to friction. The f value is variable
and depends on the roughness of the pipe material, the
pipe diameter, the density and viscosity of the fluid, and the
velocity of flow.
Typical f values for most practical applications range
between about 0.012 and 0.030. Major Losses are not
affected by gravity so it doesn’t matter how much slope a
pipe has, or even if it is vertical.
The main thing affecting Major Losses is the velocity of
the flow in the pipe. Major losses are applied to the velocity
head of the pipe flow. The velocity head is directly proportion
to the velocity squared, so small changes in velocity
can result in large increases in pressure loss. The major
(friction) loss (H
f
) in a pipe is calculated using (the Darcy-
Wiesbach) Equation 1.
EQUATION 1
where:
H
f
= Major (friction) loss (m)
f = Pipe friction factor (dimensionless)
L = Pipe length (m)
D = Pipe diameter (m)
V = average pipe flow velocity (m/s)
g = gravitational force on earth (9.81m/s
2
)
Example 1
Calculate the major headloss (m) in the pipe shown in
Figure 3 if it is a Ø150mm, PVC (f = 0.015) pipe, the flowrate
[Q] is 65L/s, and the distance between points (1) and (2) is
4.6m.
Solution:
1. First we need to calculate the velocity in the pipe. Velocity
equals flowrate divided by pipe area (V = Q/A).
2. To calculate velocity, we need the pipe’s area:
3. Area =
ʌ
D
2
/4 =
ʌ
x 0.15
2
/ 4 = 0.0177m
2
4. V = Q/A: Velocity = Q (0.065 m
3
/s) / Area (0.0177 m
2
) =
3.68 m/s
5. Now insert values into Eq. 1:
So the major (friction) head loss in the pipe between
points (1) and (2) is 0.317m (ans).
H
f
f
=
L
D
V
2
2g
H
f
f
=
L
D
V
2
2g
0.015
0.317 m (317mm)
=
=
4.6
0.15
3.68
2
2*9.81
Order.
Chaos.
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