Environmental Engineering Reference
In-Depth Information
units of foot pounds (ft-lb). It is convenient to express hydraulic energy in terms of
energy head
in feet of water. This is equivalent to foot-pounds per pound of water
(ft-lb/lb = ft).
Energy Available
Energy available is directly proportional to flow rate and to the hydraulic head. As
mentioned, the head is equivalent to stored potential energy. This is shown as
Head =
m
×
g
×
h
where
m
= Mass of water.
g
= Acceleration due to gravity (can be taken as 10 m/s
2
in most applications).
h
= Head difference.
The diameter of a pipe must be large enough to handle the volume of water flow-
ing. Friction in the pipes will reduce the effective head of water so larger diameters
are used, although cost is a factor. Ideally, the pipes should narrow as they proceed
downhill; however, friction losses are highest where the velocity is highest, so usu-
ally the pipe diameter changes very little. Friction losses in piping are classified as
either major head loss or minor head loss (Tovey, 2005).
Major Head Loss
Major head loss consists of pressure decreases along the length of pipe caused by
friction created as water encounters the surfaces of the pipe. It typically accounts for
most of the pressure drop in a pressurized or dynamic water system. The components
that contribute to major head loss are roughness, length, diameter, and velocity:
•
Roughness—
Even when new, the interior surfaces of pipes are rough. The
roughness varies, of course, depending on pipe material, corrosion (tuber-
culation and pitting), and age. Because normal flow in a water pipe is tur-
bulent, the turbulence increases with pipe roughness, which, in turn, causes
pressure to drop over the length of the pipe.
•
Pipe length—
With every foot of pipe length, friction losses occur. The lon-
ger the pipe, the greater the head loss. Friction loss because of pipe length
must be factored into head loss calculations.
•
Pipe diameter—
Generally, small-diameter pipes have more head loss than
large-diameter pipes. In large-diameter pipes, less of the water actually
touches the interior surfaces of the pipe (thus encountering less friction)
than in a small-diameter pipe.
•
Water velocity—
Turbulence in a water pipe is directly proportional to the
speed (or velocity) of the flow; thus, the velocity head also contributes to
head loss.
DID YOU KNOW?
For the same diameter pipe, when flow increases, head loss increases.
