Hydraulic Machines: Pumps - Wastewater Operators - page 115

important manual also shows velocities in different pipe diameters at
varying flows, as well as the resistance coefficient ( k ) for valves and fit-
tings (Wahren, 1997). friction head (in feet) is the amount of energy
used to overcome resistance to the flow of liq-
uids through the system. It is affected by the
length and diameter of the pipe, the rough-
ness of the pipe, and the velocity head. It is
also affected by the physical construction of
the piping system. The number and types of
elbows, valves, T's, etc. will greatly influence
the friction head for the system. These must be converted to their equiv-
alent length of pipe and included in the calculation:

Key Point: For centrifugal pumps, good
engineering practice is to try to keep
velocities in the suction pipe to 3 fps or
less. Discharge velocities higher than 11
fps may cause turbulent flow or erosion in
the pump casing.

ngth
Diameter

Le Velocity

2

2

Friction Head (ft)Roughness Factor () = f × × (3.10)

g

The roughness factor ( f ) varies with length and diameter as well as
the condition of the pipe and the material from which it is constructed;
it is normally in the range of .01 to .04.

 Example 3.6

Problem: What is the friction head in a system that uses 150 ft of 6-in.-
diameter pipe when the velocity is 3 fps. The valving of the system is
equivalent to an additional 75 ft of pipe. Reference material indicates a
roughness factor ( f ) of 0.025 for this particular pipe and flow rate.

Solution:

2

Length
D

Velocity
2

Friction Head = Roughness Factor (f) × ×

iameter g

(150 ft + 75ft) (3 fps)

232ft/s

2

= 0.025 × × ×

0. 5ft

22

225 ft
0.5ft

× 9ft s
64 ft/s

= 0.025 × = 0.025450 × × 0.140ft1.58f = t

2

It is also possible to compute friction head using tables. Friction
head can also be determined on both the suction side of the pump
and the discharge side of the pump. In each case, it is necessary to
determine:

1. The length of pipe

2. The diameter of the pipe

3. Velocity

4. Pipe equivalent of valves, elbows, T's, etc.