Estimation of Flow
An easy way to make a quick estimate of flow from a pipe can be made by a few simple measurements. (*The pipe must be flowing full in order to make the estimate.)
First, measure the distance, "D", horizontally from the end of the discharge pipe to a point which is exactly one foot above the falling stream of water, as indicated in the drawing. The distance "D" in inches, multiplied by the area of the pipe in square inches, will give the quantity in gallons per minute, which should be within 5 percent of the correct amount.
*The formula for calculating the estimate of flow is:
Q (quantity in gpm) = A (area in sq. in.) x "D" (in inches)
*For example, in making an estimate of flow from a pump, suppose "D" is 24 inches and the discharge pipe is 6 inches in diameter. The area of a 6-inch pipe is 28.3 sq. in.:
A = pi(r)2 = 3.14 x (3)2 = 28.3
28.3 x 24 = 680 gpm
Therefore, a safe estimate would be 700 gpm.