Environmental Engineering Reference
In-Depth Information
t
Total
d
ynaMiC
h
ead
(t
Total
s
ysteM
h
ead
)
Total head = Static head + Friction head + Velocity head
(12.8)
p
ressure
and
h
ead
The pressure exerted by water or wastewater is directly proportional to its depth or
head in the pipe, tank, or channel. If the pressure is known, the equivalent head can
be calculated:
Head (ft) = Pressure (psi) × 2.31 ft/psi
(12.9)
■
Example 12.6
Problem
: The pressure gauge on the discharge line from an influent pump reads 75.3
psi. What is the equivalent head in feet?
Solution:
Head = 75.3 × 2.31 ft/psi = 173.9 ft
h
ead
and
p
ressure
If the head is known, the equivalent pressure can be calculated by
Pressure (psi) = Head (ft) ÷ 2.31 ft/psi
(12.10)
■
Example 12.7
Problem
: A tank is 15 ft deep. What is the pressure in psi at the bottom of the tank
when it is filled with wastewater?
Solution:
Pressure (psi) = 15 ft ÷ 2.31 ft/psi = 6.49 psi
Before we look at a few example problems dealing with force, pressure, and head,
it is important to summarize the key points related to force, pressure, and head:
1. By definition, water weighs 62.4 pounds per cubic foot.
2. The surface of any one side of a 1-ft cube contains 144 square inches (12
in. × 12 in. = 144 in.
2
). Therefore, the cube contains 144 columns of water
1 foot tall and 1 inch square.
3. The weight of each of these pieces can be determined by dividing the weight
of the water in the cube by the number of square inches.
Weight = 62.4 lb ÷ 144 in.
2
= 0.433 lb/in.
2
(psi)
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