Distribution Systems:
The three classes of mains are generally referred to as
primary feeders,
secondary feeders, and distribution mains.
Primary Feeders
Primary
feeders are large pipes that are used for moving water from the source of
supply or storage area to the secondary feeders. The size of the primary
feeders may vary from
48''
in larger cities to
12''
in smaller communities. If only one source of water is available, then a
minimum of 2 primary feeders should be used to transfer water from the
source of supply to the distribution system.
Secondary Feeders
Secondary
feeders are smaller than primary feeders, but larger than the distribution
mains used in the grid system. Secondary feeders tie the grid system to the
primary feeders so as to aid in the concentration of the needed fire flow at
any point within the grid network.
Whenever possible, secondary feeders should be looped so that water is
supplied to the distribution mains from two directions. Secondary feeders in
well-designed systems should not be spaced more than
3000'
apart.
Distribution Mains
12''
mains are recommended on all principle streets, with
8''
mains cross-connected every 600' in business districts. The recommended
minimum size for mains in residential area is
6''.
These should also be cross - connected at intervals of not more than 600'.
Hydrants
Standards
for hydrants are prepared by the AWWA. A standard hydrant is designed for a
working pressure of
150 psi
and is hydrostatically bench-tested at
300 psi.
An installed hydrant with a single 2 ½ inch outlet should be capable of a
flow of a minimum of
250 gpm
with a pressure loss of not more than 1 lb between the street main and
outlet. Those hydrants with two 2 ½ inch outlets should be capable of a flow
of a minimum of
500 gpm
with a pressure loss of not more than 2 lbs between the street main and the
outlet. 4'' outlets should be capable of a minimum flow of
1000 gpm
with a pressure loss of not more than 5 lbs between the street main and the
outlet. It is also good practice to maintain a clearance around hydrants of
at least 3 feet.
CALCULATING
ADDITIONAL
WATER AVAILABLE FROM
HYDRANTS
Percentage
Method
When a pumper is connected to a hydrant and not discharging water,
the pressure on the intake gauge is the
STATIC PRESSURE.
After pumping begins
discharging water, the intake gauge reading becomes the
FLOW PRESSURE.
The difference between the static and flow pressure is known as
RESIDUAL
PRESSURE. To
calculate how much water is available, use the following formula:
After
determining the percent of decrease of pressure, use the following
table:
% Decrease of Pressure
Additional water available
0-10%
3 times the original amount
11-15%
2 times
16-25%
1 time
>25%
less than original GPM
Anytime multiple lines are
used or a pressurized water supply is coming into the pump, the pressure relief
device MUST be set.
The amount of water still available from a hydrant
is determined by the difference between STATIC and
RESIDUAL pressures.
1. 0 to 10% drop = 3
more like amounts
2. 11 to 15% drop = 2
more like amounts
3. 16 to 25% drop = 1
like amount
FIRST DIGIT METHOD:
1. Find the difference
between static and residual pressures. (example: S = 90, R = 70, difference
= 20 REMEMBER this number)
2. Use :
and COMPARE
| Multiply first digit of Static
Pressure by 1
=9 |
3 more
like lines |
| Multiply first digit of Static Pressure by
2 =18 |
2
more like lines |
| Multiply first digit of static Pressure by
3 =27 |
1
more like lines |
If the
psi drop is equal to or less than the first digit of the static pressure
multiplied by (1), three additional lines of equal flow may be added.
If the
psi drop is equal to or less than the first digit of the static pressure
multiplied by two (2), two additional lines of equal flow may be added.
If the
psi drop is equal to or less than the first digit of the static pressure
multiplied by three (3), one additional line of equal flow may be added.
