Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 68

■ Example 2.126
Convert flow in cubic feet per second to million gallons per day.

Flow (MGD) = Flow (cfs) ÷ 1.55 cfs/MG

Problem: The flow in a channel is determined to be 3.90 cfs. What is the flow rate in
million gallons per day?

Solution:

Flow = 3.90 cfs ÷ 1.55 cfs/MG = 2.5 MGD

■ Example 2.127
Problem: The water in a tank weighs 670 lb. How many gallons does it hold?

Solution: Water weighs 8.34 lb/gal; therefore,

670 lb ÷ 8.34 lb/gal = 80.3 gal

■ Example 2.128
Problem: A liquid chemical weighs 64 lb/ft 3 . How much does a 5-gal can of it weigh?

Solution: Solve for specific gravity, determine lb/gal, and multiply by 5.

3

Weight of chemical
Weight of

64 lb/ft
62.4 lb/ft

Specificgravity = = =103 .

3

ffwater

Weight of chemical
8.34 lb/gal

1.03 =

103 . × 8.34 lb/gal = Weight of chemical = 8.59 lb/gal

8.59 lb/gal × 5gal = 43 lb

■ Example 2.129
Problem: A wooden piling with a diameter of 16 in. and a length of 20 ft weighs 50
lb/ft 3 . If it is inserted vertically into a body of water, what vertical force is required
to hold it below the water surface?

Solution : If this piling had the same weight as water, it would rest just barely sub-
merged. Find the difference between its weight and that of the same volume of
water—that is the weight needed to keep it down:

62.4 lb/ft 3 (water) - 50.0 lb/ft 3 (piling) = 12.4 lb/ft 3

Volume of piling = 0.785 × (1.33) 2 × 20 ft = 27.8 ft 3

12.4 lb/ft 3 × 27.8 ft 3 = 344.7 lb