Environmental Engineering Reference
In-Depth Information
The specific gravity of water is 1, which is the standard, the reference against
which all other liquid or solid substances are compared. Specifically, any object that
has a specific gravity greater than 1 will sink in water (e.g., rocks, steel, iron, grit,
floc, sludge). Substances with a specific gravity of less than 1 will float (e.g., wood,
scum, gasoline). Considering the total weight and volume of a ship, its specific grav-
ity is less than 1; therefore, it can float. The most common use of specific gravity in
water operations is in gallon-to-pound conversions. In many cases, the liquids being
handled have a specific gravity of 1 or very nearly 1 (between 0.98 and 1.02), so a
value of 1 may be used in the calculations without introducing significant error. For
calculations involving a liquid with a specific gravity of less than 0.98 or greater
than 1.02, however, the conversions from gallons to pounds must consider specific
gravity. The technique is illustrated in the following example.
■
Example 4.4
Problem:
A basin contains 1455 gal of a liquid. If the specific gravity of the liquid is
0.94, how many pounds of liquid are in the basin?
Solution:
Normally, for a conversion from gallons to pounds, we would use the factor
8.34 lb/gal (the density of water) if the specific gravity of the substance is between
0.98 and 1.02; in this instance, however, the 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 correction factor:
1455 gal × 7.84 lb/gal = 11,407 lb (rounded)
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. Force and pressure are different, although they
are closely related. Force and pressure are defined below. 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 4.2
)
. If we stack two 1-ft cubes on top of
one another, the force on the bottom will be 124.8 lb. Pressure is the force per unit of
area. In equation form, this can be expressed as:
F
A
P
=
(4.4)
where:
P
= Pressure.
F
= Force.
A
= Area over which the force is distributed.
