Force, Pressure, Head, and Velocity Calculations - Basic Mathematics for Water and Wastewater Operators - page 181

Note: The flow through a pipe (cfs) can be determined if the velocity and pipe
diameter are known. The area of the pipe must be calculated in square feet and
then multiplied by the velocity (fps).

FORCE, PRESSURE, HEAD, AND VELOCITY EXAMPLES
■ Example 12.9
Problem : Convert 40 psi to feet of head.

Solution:

1ft
0.433 psi

40 psi × = 92 . ft

■ Example 12.10
Problem : Convert 40 feet of head to psi.

Solution:

0.433 psi
1ft

40 ft × = 17.32 psi

As the above examples demonstrate, when we convert psi to feet we divide by
0.433, and when we convert feet to psi we multiply by 0.433. This information can be
most helpful in clearing up any confusion as to whether to multiply or divide. There
is another way to think about it, though—one that may be more beneficial and easier
for many operators to use. Notice that the relationship between psi and feet is almost
two to one. It takes slightly more than 2 ft to make 1 psi. Therefore, when looking at
a problem where the information provided is in pressure and the result should be in
feet, the answer will be at least twice as large as the starting number. For example, if
the pressure is 25 psi, we intuitively know that the head is over 50 ft. Therefore, we
must divide by 0.433 to obtain the correct answer.

■ Example 12.11
Problem : Convert a pressure of 45 psi to feet of head.

Solution:

1ft
0.433 psi

45 psi × = 104 ft

■ Example 12.12
Problem : Convert a pressure of 15 psi to feet of head.

Solution:

1ft
0.433 psi

15 psi × = 34 . ft