MODULE FOUR
HYDRAULICS
To produce effective fire streams, it is necessary to know the nozzle pressure, the amount of
friction loss in the fire hose, and any pressure loss or gain due to elevation.
Total Pump Pressure Discharge = Nozzle pressure plus friction + elevation pressure.
loss.
Nozzle Pressure – the amount of pressure required at the nozzle to produce an effective fire
stream. Fog nozzle – 100 psi Straight Bore Master Streams – 80 psi.
Friction Loss – that part of the total pressure lost while forcing water through pipe, fittings, fire
hose and adapters.
The basis for fire hoseline friction loss calculations are the size of the fire hose, the amount of
water flowing, and the length of the hose lay. These three factors give rise to the formula for
computing friction loss.
FL = CQ2 L
Where:
FL = Friction loss in psi
C = Frictional loss coefficient
Q = Flow rate in hundreds of gpm
L = Hose length in hundreds of feet
Friction Loss Coefficients
1” = 150
1¾” = 15.5
2½” = 2
3” = .8
4” = .2
Q = gpm
100
Where:
q = hundreds of gpm
GPM = actual flow through the hose
100 = a constant
L = hose length
100
Where:
L = hose length
Hose length = actual length of the hose
100 = a constant
EXAMPLE: if 200 gpm is flowing from a nozzle, what is the total pressure loss due to friction for
100 feet of 2½” hose?
FL = CQ2 L
C = coefficient = 2
FL = CQ2 L
Q = gpm = 200 = 2
FL = (2) (2) squared 1
100 100
L = hose length = 100 = 1
FL = (2) (4) (1)
100
100
FL = 8
EXAMPLE: If 150 gpm is flowing from a nozzle, what is the total pressure loss due to friction
from 200 feet of 1 ¾” hose?
FL = CQ2 L
C = 15.5
Q = 1.5
L=2
FL = (15.5) (1.5) squared 2
FL = (15.5) (2.25) 2
FL = (15.5) (4.5)
FL = 69.75
What would total pump discharge pressure be with a fog nozzle?
NP = 100
FL = 70
Elevation = 0
Answer: 170 psi
ELEVATION PRESSURE
When a nozzle is operating at an elevation higher than the engine, this pressure is exerted back
against the pump. To compensate for this pressure “loss,” elevation pressure must be added.
Operating a nozzle lower than the pump results in pressure pushing against the nozzle. This
“gain” is compensated for by subtracting the elevation pressure.
When a hose lay results in an elevation change, use the following formula to determine the
elevation pressure.
Elevation Pressure = .5
.5 = is a constant
H = height in feet
EXAMPLE:
Calculate the total pressure loss due to elevation pressure for a hose line operating at the top of
a 100 foot hill.
.5 = Elevation Pressure
= (.5) (100)
= 50 psi
To determine the elevation pressure in a multi-storied building you should use the following
formula.
Elevation Pressure = .5 psi x (number of stories minus 1)
EXAMPLE:
A hose line operating on a ninth floor structure fire is connected to a building standpipe system.
What is the total pressure loss due to elevation?
Elevation Pressure = .5 psi x number of stories minus 1
Elevation Pressure = (.5) (8)
Elevation Pressure = 40 psi
Add 10 psi for flows greater than 350 gpm.
Calculating Additional Water Available from Hydrant:
When a pumper is connected to a hydrant and is not discharging water, the pressure shown on
the intake gauge is the static pressure. When the pumper is discharging water, the intake gauge
is the residual pressure. The difference between these two pressures is used to determine how
much more water is available. To use this method you must first calculate the drop in pressure
as a percentage. Use the following formula:
Percent Drop = Static minus residual (100)
Static
Water Available Table
Percent Decrease
Water Available
0 – 10
11 – 15
16 –25
25+
3 times amount being delivered
2 times amount being delivered
Same as being delivered, more
might be available but not as
much as is being delivered.
EXAMPLE:
A pumper is supplying one line with 250 gpm flowing. The static pressure was 70 psi and the
residual pressure reading 63 psi. Determine how many additional lines can be added.
Percent Drop = Static pressure minus residual (100)
Static
Percent Drop = (70 – 63) 100
70
Percent Drop = (7) (100) = 700
70
70
= 10% = 3 lines at 250 gpm or 750 gpm
Maximum Efficient Flows:
1”
1 ¾”
2½”
3”
4”
30 gpm
150 gpm
250 gpm
500 gpm
1000 gpm
STANDPIPES
When pumping to a standpipe system, don’t add any friction loss for the piping as it is
insignificant.
APPLIANCES
Fireground operations often require the use of hose line appliances. These appliances
include reducers, gates, wyes, manifolds, aerial apparatus. Appliance friction loss will
be insignificant in cases where the flow is less than 350 gpm. For fireground operations
you should add 10 psi for the appliance if the gpm is over 350.