The minimum sizes which may be used

1 percent. At the intersection of the

for

TO

construction

are

4-inch

1-percent line and the 8-inch diameter

diameter pipe for a house connection

line, a line is drawn parallel to the

and 6-inch diameter pipe for any other

velocity lines.

The new line lies

sewer.

between the line of 3 feet per second

and 3.5 feet per second. Its value is

3.2 feet per second.

The acceptable limits for the sewage

c. Discharge ratio.

The discharge

velocity are 2 feet per second to 10

ratio is the ratio of the actual

feet per second.

Velocities lower

discharge (flow) (QA) to the full

than this will tend to deposit solids

discharge (Qc).

Thus the discharge

in the sewer and velocities higher

ratio is found by dividing the actual

will scour out the invert of the

flow by the full flow.

For the

sewer.

Occasionally a choice is

example started above the actual flow

forced upon the designer of using a

is 300 gallons per minute and the full

lower velocity than 2 feet per second

flow

is

500

gallons

per

minute.

or of putting in an automatic lift

Therefore the discharge ratio is

station. If it can be shown that the

costs incurred in keeping the sewer

d. Velocity ratio.

The velocity

clean, and perhaps replacing it, are

ratio is the ratio of the actual

cheaper over the design life of the

velocity (Va) to the velocity at full

system

than

the

procurement

and

flow (Vc). Since Va is not known yet

maintenance cost of the lift station

this cannot be found by division. The

or other special facility, then the

velocity ratio is found by use of

actual velocity may be decreased to

figure 15.

The chart is used by

1.5 feet per second at peak flow.

entering along the top or bottom at

There are five steps to finding actual

the value of the discharge ratio.

velocity.

Move vertically along the discharge

ratio value until the discharge curve

a. Full flow. The full capacity of

is intersected.

From this point move

the sewer is found by entering the

horizontally to the right until the

chart at the given slope and moving

velocity curve is intersected.

At

all the way up to the chosen pipe

this point move vertically up or down

size.

Moving horizontally to the

and read the velocity ratio at the top

right

from

this

point

the

full

or

the

bottom

of

the

chart.

capacity can be read.

Continuing the

Continuing the example, the chart is

example started above, the chart is

entered along the bottom at the value

entered at the 1-percent slope line.

of discharge ratio 0.6.

Moving up to

Moving up to the 8-inch line and

the discharge curve, across to the

reading to the right, a full flow of

velocity

curve,

and

down

to

the

500 gallons per minute is obtained.

bottom, a value of 1.045 is read.

Thus the velocity ratio is 1.045. The

b. Velocity at full flow.

The

only case where this method of using

velocity at full capacity is found by

figure 15 gives an incorrect answer is

entering the chart at the design

when the discharge ratio is 1.0.

slope.

Move up vertically until the

However, when this is the case the

design

pipe

size

is

intersected.

actual velocity must be equal to the

(This point of intersection is the

full flow velocity because the pipe is

same point found in a above.) Through

flowing full.

Since the velocities

this point draw a line parallel to the

must be equal, the velocity ratio is

velocity lines.

Knowing the velocity

1.

value of the line above and below, an

estimate of the velocity value of the

e. Actual

velocity.

The

new line can be made.

This is the

velocity

ratio

is

the

actual

velocity at full flow.

For the

velocity

divided

by

the

full

example above, the chart is entered at