Class A firefighting foam
is useful in fighting deep - seated fires, as well as structure fires, brush
fires, dumpster fires, etc. Class A firefighting foam has outstanding insulating
qualities and reduces surface tension, thus
providing better penetration of the water. Class A foam is specifically
designed to be used on Class A fires. It is NOT designed to be used on Class
B fires. Nozzles should be set at
95 GPM
for effectively using Class A foam.
Hose lays should
NOT
exceed 250'.
For
brush and dumpster fires,
a setting of
1/4
is most effective. For fires in structures in the
incipient stage,
a setting of
1/2
is most effective. This allows more foaming action than the 1/4 setting,
without producing full finished foam. When using this setting, allow the
foam to work by letting it penetrate into the material before applying more.
For
fully involved
fires, a setting of
1
is most effective.
Class B firefighting foam is used for fighting fires involving flammable and
combustible liquids. It is also used to suppress vapors from unignited
spills of these liquids. Nozzles should be preset to
95 GPM
and no more than
350'
of hoseline should
be used. Class B foam is used in concentrations of either
3% or 6%
solution. Both Class A and Class B foam should be supplied with
200 PSI inlet pressure.
Aqueous film forming foams
(AFFF)
are dual action synthetic firefighting foams. The first action of aqueous film forming foam (AFFF) is similar to
the detergent foam's air - entrapping action in forming a blanket of strong foam
that spreads over the burning liquid surface. This
blanket smothers the fire and retards vaporization of the flammable liquid
to below flammable limits. The second action is the controlled solution drain - age
from the foam blanket, which forms an aqueous solution film without bubbles
between the fuel surface and the blanket. The aqueous films self - healing, and
will recover open areas caused by agitation of the flammable liquid surface.
AFFF films are used on flammable liquid spills to prevent ignition of the
flammable liquid. AFFFs are marketed on
3%
and
6%
solutions, fresh or salt water, and have temperature characteristics like
alcohol foams.
High - Expansion Firefighting Foam
High-expansion foams are special purpose foams. Because they have a low
water content, they minimize water damage. The major uses for high-expansion
foam are in pesticide fires, vapor suppression for fuming acids, in
concealed
spaces such as basements, and in fixed extinguishing systems for specific
industrial uses. Expansion rates 300 : 1 to 1250 : 1 for high-expansion uses;
50 : 1 to 300 : 1 for medium-expansion uses. Ventilation is very important
when high-expansion foams are used. Foams conduct electricity.
FIREFIGHTING FOAM PROPORTIONERS
Most firefighting foam proportioners
are intended to be mixed with fresh or salt water in proportions from
1% to 6%. Most
alcohol-resistant firefighting foams are also effective in controlling hydrocarbon fires
when used at a 3%
concentration.
In-line eductors
is the most common type of portable foam proportioner used in fire service.
This eductor is designed to be directly attached to the pump panel
discharge or connected in the middle of a hose lay. When using an in-line
eductor, it is very important to follow the manufacturer’s instruction
about inlet pressure, maximum hose lay from the eductor, and appropriate
nozzle.
Eductors are relatively
tolerant of different operating conditions, but there are several very
important operating rules that must be observed. Failing to follow these
rules will lessen the eductor’s performance.
Rule 1.
The eductor must control the flow through the system.
Rule 2.
The pressure at the outlet of the eductor (also called back pressure) must
not exceed 65% to 70% of eductor inlet pressure.
Eductor back pressure is determined by the sum of the nozzle pressure,
friction loss in the hose between the eductor and the nozzle, and the
elevation pressure.
If back pressure is excessive, no
foam concentrate will be inducted into the water.
Rule
3. Foam solution concetration is only correct
at the rated INLET pressure of the eductor, usually 150 to 200 psi. Using
eductor inlet pressures lower than rated
pressure for the eductor will result in rich foam concetration, and vice - versa.
Rule 4. Eductors
must be properly maintained and flushed after each use.
Rule 5. Set metering
valve to match the foam concentrate percentage and the burning fuel.
Rule 6. The foam
concentrate inlet to the eductor should not be
more than 6 feet above the liquid surface of
the foam concentrate. If the inlet is too high, the foam concentration will be
very lean or foam may not be inducted at all.
Fire Hydraulics Foam Pumping Problem
A 95 GPM eductor is
operating at its rated 200 psi inlet pressure with a handline nozzle rated
at 95 GPM at a nozzle pressure of 100 psi. THERE IS NO ELEVATION DIFFERENCE.
What is the % of back pressure (BP) for a 1 ½ inch hoseline that is 100, 150, and 200
feet long?
Friction loss = CQ2L
C = 24 for 1
½ inch hose
Q =
.95 flow
in hundreds of gpm
L
=
1 hose
length in hundreds of feet
FL
=
(24)(0.95)2(1)
FL
=
22 PSI
NO
ELEVATION
20’ ELEVATION
Hose
length
100
150
200
100
150
200
Nozzle pressure
100
100
100
100
100
100
Friction loss
22
33
44
22
33
44
Elevation loss
0
0
0
8.7
8.7
8.7
Eductor
BP=NP+FL+E
122
133
144
130.7
141.7
152.2
Divide Eductor Back
Pressure and Inlet Pressure (200 psi) and you'll get following:
% of back pressure to inlet
press. 61%
66.5
72%
65%
71%
76%
After the calculations,
it is apparent that for the 150 and 200 foot hose
lengths, the back pressure EXCEEDS 70% of inlet pressure. Under these
conditions, the maximum length of
1 ½ hose should be 100 feet.
REMEMBER THAT BACK PRESSURE CANNOT EXCEED
70%
OF INLET PRESSURE!
1 3/4''
HOSE,
AUTOMATIC
NOZZLES,
AND AFFF
1 1/2'' HOSE:
55 PSI = EDUCTOR FRICTION LOSS (IN APPLIANCE ITSELF)
45 PSI = FRICTION LOSS FOR 150'' OF 1 1/2'' HOSE
100 PSI = NOZZLE PRESSURE
200 PSI = TOTAL INLET PRESSURE
1 3/4''
HOSE:
| FEET OF 1 3/4'' HOSE |
INLET PRESSURE |
| 100 |
169 PSI |
| 150 |
176 PSI |
| 200 |
182 PSI |
| 250 |
189 PSI |
| 300 |
196 PSI |
| 350 |
203 PSI |
| 400 |
209 PSI |
It is important that the pressures are calculated using
95 gpm as the flow rate.
When this is done, the automatic nozzles will deliver
95 gpm at the optimum
foam - making
nozzle pressure of 100 psi. It is also important that
the firefighter operates the automatic nozzle in the full open position so as not
to inadvertently reduce the flow.
The conventional constant gallonage nozzle may
also be used when set on
95 gpm.
If the flow rate is correct, a five gallon container of
3% AFFF
concentrate should be emptied in about
1 min and 45
seconds,
if set at
6% in about
45 seconds
If you want to solve more fire hydraulics pumping problems go to
fire hydraulics pumping test page.
