Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 32

Flow through a Pipeline

Q (cfs) = 0.785 × ( D ) 2 (ft) × Velocity (ft/sec) (2.5)

■ Example 2.64
Problem: What is the flow in cfs for a 2-ft-diameter pipe flowing full at a velocity
of 2.8 ft/sec?

Solution :

Q = 0.785 × ( D ) 2 (ft) × Velocity (ft/sec) = 0.785 × (2 ft) 2 × 2.8 ft/sec = 8.79 cfs

When converting flow rates, the most common flow conversions are 1 cfs = 449
gpm and 1 gpm = 1440 gpd. Also, MGD × 700 = gpm. To convert gallons per day to
MGD, divide the gpd by 1,000,000. For example, convert 150,000 gallons to MGD:

150,000 gpd ÷ 1,000,000 = 0.150 MGD

In some instances, flow is given in MGD but is needed in gpm. To make the con-
version from MGD to gpm, two steps are required.

1. Convert the gpd by multiplying by 1,000,000.

2. Convert to gpm by dividing by the number of minutes in a day (1440 min/
day).

■ Example 2.65
Problem: Convert 0.135 MGD to gpm.

Solution:

1. Convert the flow in MGD to gpd:

0.135 MGD × 1,000,000 = 135,000 gpd

2. Convert to gpm by dividing by the number of minutes in a day (24 hr/day ×
60 min/hr = 1440 min/day):

135,000 gpd ÷ 1440 min/day = 94 gpm

To determine flow through a pipeline, channel, or stream, we use the following
equation:

Q = V × A (2.6)

where
Q = Cubic feet per second (cfs).
V = Velocity in feet per second (ft/sec).
A = Area in square feet (ft 2 ).