Environmental Engineering Reference
In-Depth Information
substance has a specific gravity outside this range, so the 8.34 factor
must be adjusted. Multiply 8.34 lb/gal by the specific gravity to obtain
the adjusted factor:
8.34 lb/gal × 0.94 = 7.84 lb/gal (rounded)
Then convert 1455 gal to pounds using the adjusted factor:
1455 gal × 7.84 lb/gal = 11,407 lb (rounded)
2.4 forCe and Pressure
Water exerts force and pressure against the walls of its container,
whether it is stored in a tank or flowing in a pipeline. There is a dif-
ference between force and pressure, although they are closely related.
force
is the push or pull influence that causes motion. In the English
system, force and weight are often used in the same way. The weight of
1 ft
3
of water is 62.4 lb. The force exerted on the bottom of a 1-ft cube is
62.4 lb (see Figure 2.1). If we stack two cubes on top of one another, the
force on the bottom will be 124.8 lb.
Pressure
is a force per unit of area.
In equation form, this can be expressed as:
f
a
P
=
(2.4)
where:
P
= pressure.
f
= force.
a
= area over which the force is distributed.
Earlier we pointed out that pounds per square inch (lb/in.
2
or psi) or
pounds per square foot (lb/ft
2
) are common expressions of pressure. The
pressure on the bottom of the cube is 62.4 lb/ft
2
(see Figure 2.1). It is nor-
mal to express pressure in pounds per square inch. This is easily accom-
plished by determining the weight of 1 in.
2
of a cube 1 ft high. If we have
a cube that is 12 in. on each side, the number of square inches on the
bottom surface of the cube is 12 × 12 = 144 in.
2
; dividing the weight by the
number of square inches determines the weight on each square inch:
62.4lb/ft
144in.
psi
=
=
0.433 psi/ft
2
This is the weight of a column of water 1-in. square and 1 ft tall. If the
column of water were 2 ft tall, the pressure would be 2 ft × 0.433 psi/ft
= 0.866.
Note:
1 foot of water = 0.433 psi.
With the above information, feet of head can be converted to psi by
multiplying the feet of head times 0.433 psi/ft.
