Environmental Engineering Reference
In-Depth Information
Pounds per square inch (lb/in.
2
or psi) or pounds per square foot (lb/ft
2
or psf) are
commonly used to express pressure. The pressure on the bottom of our 1-ft cube is
This is easily accomplished by determining the weight of 1 in.
2
of a 1-ft cube. 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:
62.4 lb
144 in.
Pounds persquare inch (psi)
=
=
0.433 psi
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 ft of water = 0.433 psi.
With this information, feet of head can be converted to psi by multiplying the feet of
head times 0.433 psi/ft.
■
Example 4.5
Problem:
A tank is mounted at a height of 90 ft. Find the pressure at the bottom of
the tank.
Solution:
90 ft × 0.433 psi/ft = 39 psi (rounded)
Note:
To convert psi to feet, divide the psi by 0.433 psi/ft.
■
Example 4.6
Problem:
Find the height of water in a tank if the pressure at the bottom of the tank
is 22 psi.
Solution:
Height = 22 psi ÷ 0.433 psi/ft = 51 ft (rounded)
DID YOU KNOW?
One of the problems encountered in a hydraulic system is storing the liquid.
Unlike air, which is readily compressible and is capable of being stored in
large quantities in relatively small containers, a liquid such as water cannot be
compressed. It is not possible to store a large amount of water in a small tank;
62.4 lb of water occupies a volume of 1 cubic foot, regardless of the pressure
applied to it.
