Chapter 8
8.1 PRINCIPAL EXTINGUISHING
AGENTS
8.1.1
Foam. Foam used for aircraft rescue and fire fighting is
primarily intended to provide an air‑excluding blanket, which prevents
volatile flammable vapours from mixing with air or oxygen. To perform this
function a foam must flow freely over the fuel surface, must resist disruption
due to wind or exposure to heat or flame and should be capable of resealing any
ruptures caused by the disturbance of an established blanket. Its water
retention properties will determine its resistance to thermal exposure and will
provide limited cooling to any elements of the aircraft structure to which it
adheres. There are several types of foam concentrate from which effective fire
fighting foams can be produced and these are described as follows:
(a) Protein foam. This consists primarily of protein
hydro listed, plus stabilizing additives and inhibitors to protect against freezing, to prevent
corrosion of equip mint and containers,
to resist bacterial decomposition, to control viscosity, and to otherwise
ensure readiness for use under emergency conditions. Current forum- Latinos
are used at recommended nominal concern- tractions of 3, 5 and 6 per cent by
volume of the water disc 1 hare. All of these can be used to produce a suitable
foam but the manufacturer of the foam‑making equip mint should be
consulted as to the correct concentrate to be used in any particular system
(the proportion installed must be properly designed and/or set for the
bconcentrate being used). Foam liquids of different types or different
manufacturers should not be mixed unless it is established that they are
completely interchangeable and compatible.
Where a dry chemical powder is to be used as the complementary agent in
conjunction with protein foam it is essential to determine the compete ability
of these agents for simultaneous application. Incompatibility will result in
the destruction of the foam blanket in areas where the two agents are in
contact. To ensure that the tank does not contain stale protein foam, the
entire contents should be discharged periodically and the entire system washed
through.
(b)Aqueous Film Forming Foam (AFFF). There are many
concentrates in this category all consisting basically of a fluorinated surfactant with foam stabilizer.
The concentrates may, according to the specification, be used in solutions from
1 per cent to 6 per cent, with appropriate proportioning systems, or in pre‑mixed
solutions. It is essential in selecting a concentrate to ensure that it is
suitable for use in the total system incorporated in a rescue and fire fighting
vehicle. It is also important to discuss with the manufacturer or supplier the
use of an AFFF concentrate in extremes of temperature or where salt or brackish
water may be used in the solution, with particular regard to any possibility of
interaction between the tank structure, any surface treatment or the associated
plumbing of the system. The foam produced acts to provide a barrier to exclude
air or oxygen and, by the drainage of a chemically impregnated fluid from the
foam, to provide a film on the fuel surface capable of containing fuel vapour.
The foam produced does not have the density and visual appearance of foams
produced from protein or fluoroprotein concentrates and training will be
necessary to accustom fire fighters to its effectiveness as a fire suppressant.
AFFF concentrates may be used in equipment normally used for protein or
fluoroprotein foam production but conversion should not be undertaken without
consultation with the manufacturer or supplier of the AFFF concentrate or
rescue and fire fighting vehicle. A thorough flushing of the foam tank and the
total foam‑making system will be necessary before the introduction of the
AFFF concentrate. Some changes in the foam‑making systems of vehicles,
particularly aspirating nozzles, where used, may be necessary to achieve the
optimum properties of AFFF foams. The AFFF foams are compatible with all
currently available dry chemical powder agents. Protein and fluoroprotein
concentrates are incompatible with AFFF concentrates and they should not be
mixed although foams produced from these concentrates, separately generated,
may be applied to a fire in sequence or simultaneously.
(c) Fluoroprotein foam (conventional). This
foam contains a concentration of synthetic fluorinated surfactant giving better
all around performance than ordinary protein foams, as well as providing
resistance to breakdown by chemical powders. Current formulations are used at
concentrations of 3 and 6 per cent by volume of the water discharge. The
manufacturer of the foammaking equipment should be consulted as to the correct
concentrate to be used in any particular system. (The proportioner used must be
properly designed and/or set for the concentrate being used.) Foam liquids of
different types or different manufacturer should not be mixed unless it is
established that they are completely interchangeable and compatible.
Compatibility of a foam produced by any proposed agent and system with a dry
chemical powder agent is essential and should be established by a test
programme although it is known that compatibility is a characteristic of most
fluoroprotein foams.
(d) Film forming fluoroprotein (FFFP) foams. Film forming fluoroprotein (FFFP) agents are composed of protein
together with film forming fluoronated surfactants, which make them capable of
forming water solution films on the surface of flammable liquids and of adding
oleophobic properties to the foam generated. This characteristic makes FFFP
particularly effective where the foam may be contaminated with fuel (such as
forceful application). Expanded foams generated from FFFP solutions have fast
spreading characteristics and act as surface barriers to exclude air and prevent
vaporization, thus suppressing combustible vapours. This film, which can spread
over fuel surfaces not covered with foam, is self‑sealing following
mechanical disruption and continues as long as there remains a reservoir of
foam for its production. To ensure extinction, however, an FFFP blanket should
cover the fuel surface, as is the practice with other foams. This foam is
highly effective on fuel spills because it is fluid, film forming and has oleo
phobic properties. Film forming fluoroprotein concentrates are available for
proportioning to a final concentration of 3 to 6 per cent by volume using
either fresh or sea water. They are compatible with dry chemical agents but
this should be confirmed by a test programme.
(e) Synthetic foams. This foam contains primarily petroleum
products ‑ alkyl sulphates, alkylsulphanates, alkylarylsulphanates, etc.
Synthetic foam forming substances also include stabilizers, anti‑corrosives,
and components to control viscosity, freezing temperature and bacteriological
decomposition. Concentrates of different types or from different manufacturers
should not be mixed in order to obtain extinguishing foam;
however, synthetic foams from different pieces of equipment are compatible and can be used one after the other or simultaneously to
extinguish a fire. The degree of compatibility between synthetic foams and dry
(powder) chemical substances should be determined prior to their intended use.
8.1.2 Methods
of foam production. Foam produced by most vehicles to be used for aircraft rescue and fire fighting
will utilize solutions, either in pre‑mixed forms or by the use of a
proportioning system, which are delivered at a predetermined pressure to
nozzles which induce air to aspirate the solution. The pressure may be created
by a pump or, with vehicles of smaller capacity, by a compressed gas source,
usually either dry nitrogen or dry air. In all cases the system will produce acceptable foam only if the solution is delivered in
the appropriate volume and in the correct pressure range to the aspirating
nozzle or nozzles. The operational advantage of aspirating nozzles lies in
their ability to produce foams of acceptable quality both at the vehicle
monitor and, where necessary, through extended hose‑lines, provided that
the pressure is adjusted to compensate for any friction and contour losses
contributed by the hose‑lines. This type of installation has largely
displaced earlier systems in which foam was produced within the vehicle and
distributed through nozzles. Aspiration of the solution within these systems
was by the induction or injection of air, by various methods, all of which
produced an effective foam. The disadvantages of these systems were that they
utilized large (
8.1.3 Quality of foams. The quality of foam produced by a
rescue and fire-fighting vehicle using any of the concentrate‑types
described in 8.1.1 will significantly affect the control and extinguishment
times of an aircraft fire. Functional fire tests are required to determine the
suitability of a foam concentrate in an
airport environment. Paragraph 8.1.5 below lists the minimum
specifications for foams produced from protein, synthetic, fluoroprotein, film
forming fluoroprotein and aqueous film forming concentrates. The
specifications include physical properties and the performance of the foams
under fire test conditions. Any foam concentrate to be used in aircraft rescue
and fire fighting vehicles should meet or exceed the criteria in these
specifications, so as to achieve the performance level A or B, as appropriate.
Further, to avail for a reduction in
the amounts of water for foam production (see Table 2‑2), the foam
concentrate should achieve the performance level B.
8.1.4 Where States or individual users do not have
the facilities for conducting the tests which will establish the specified
properties and performances, certification of the qualification of a
concentrate should be obtained from the manufacturer or supplier, based on the
local operating conditions.
8.1.5 Foam
specifications (See Table 8‑1)
pH value. pH value is a
measurement to express the acidity or alkaline properties of a liquid.
Therefore, in order to prevent the corrosion of plumbing or the foam tanks of a
rescue and fire fighting vehicle, the foam concentrate should be as neutral as
possible and should register between the values of 6 and 8.5.
Viscosity. The viscosity
of a foam concentrate is an indication of the resistance to flow of the liquid
in the plumbing of a rescue and fire fighting vehicle, and its consequential
entry into the water system. The viscosity measurement of a foam concentrate
when at its lowest temperature should not exceed 200 mm/s. Any higher registration
will restrict flow and retard its adequate blending into the water stream
unless special precautions are taken.
Sedimentation. Sediment
may form in foam which contains impurities or if it is subjected to adverse
storage, severe weather conditions and/or varying temperatures. The resultant
creation of sediment may affect the performance of a vehicle's foam
proportioning system or negate its fire fighting efficiency. When tested by the
centrifuge method, foams should contain no greater than 0.5 per cent of
sediment.
Objective: To evaluate the ability of a foam
concentrate to:
(a) extinguish
a fire of
(b) resist burn back due to exposure to fuel and heat.
Equipment:
(a) A
circular fire steel tray of
(b) Equipment or access to facilities to enable
accurate recordings of:
1) air temperature
2) water temperature; and
3) wind velocity;
Table 8‑1
Fire tests
|
Performance level A |
Performance level B |
|
1. Nozzle (air aspirated) |
|
|
|
a) Branch pipe |
"UNI foam nozzle (See Appendix 3) |
"UNI foam nozzle (See Appendix 3) |
|
b) Nozzle pressure |
700 kPa |
700 kPa |
|
c) Application rate |
4.1 L/min/m² |
2.5
L/ min / m² |
|
d) Discharge rate |
11.4 Umin |
11.4 Urnin |
|
2. Fire size |
≈2.8 M² (circular) |
≈4.5 M² |
|
|
(circular) |
|
|
3. Fuel (on water substrate) |
Kerosene |
Kerosene |
|
4. Preburn time |
60 s |
60 s |
|
5. Fire performance |
|
|
|
a) extinguishing time |
<60 s |
<60 s |
|
b) total application time |
120 s |
120 s |
|
c) 25 %reignition
time |
≥5 min |
≥5 min |
(c) Fuel:
(d) Branch
pipe, straight stream, air aspirating nozzle;
(e) Suitable stop watch; and
(f) Circular,
burn back pot, measuring
Preferable conditions:
a) Air temperature (°C)
≥ 15
b) Foam solution temperature (°C) ≥ 15
c) Wind velocity (m/s)
≤ 3
- Position the
chamber holding the premix foam upwind of the fire with the nozzle horizontal at a height of
- Test the foam
apparatus to ensure:
a)
nozzle
pressure; and
b) discharge rate.
- When testing
performance level B foam, place
-Ignite fuel
and allow 60 seconds preburn from full involvement.
-Apply foam
continuously while maintaining a nozzle pressure of 700 kPa for 120 seconds.
-Record extinction time.
-Place burn
back pot in centre of fire tray.
-Ignite burn
back pot 120 seconds after end of application of foam.
-Record when 25 per cent of the fuel
area is re‑involved with fire.
8.1.6 Operational
considerations. The quality of
foam produced by a vehicle system may be affected by the characteristics of the
local water supply. Adjustments to the solution strength may be necessary in
certain situations to achieve optimum foam quality. No corrosion inhibitors,
freezing point depressants or other additives should be used in the water
supply without prior consultation with, and the approval of, the foam
concentrate manufacturer.
8.1.7 Foam can be applied to fires in two distinct forms. Solid streams
are used where range of application is essential or where the stream may be
deflected from a solid object to distribute it in the fire area. Solid streams
must be employed with care at an aircraft accident where survivors are
evacuating the aircraft and escape slides may be in use. Dispersed patterns may
be employed to deliver foam at shorter ranges to a fire area, combining greater
coverage with the more effective surface application of the foam. Dispersed
patterns are particularly valuable in protecting fire fighters from radiated
heat. In some vehicles standard water nozzles are employed to produce "fogfoam",
mainly from sideline deliveries. While these nozzles are effective in achieving
rapid knockdown they do not produce foams of the specified qualities and these
may not have the degree of permanence associated with fully aspirated foams.
8.2 COMPLEMENTARY AGENTS
8.2.1 Complementary agents do not generally have any substantial cooling
effect on liquids or materials involved in fire. In a major fire situation
extinguishment achieved by complementary agents may well only be transient and
danger of ‑flashback‑ or reignition may occur when foam is not
available to secure a fire. They are particularly effective on concealed fires
(e.g. engine fires) in aircraft freight holds and beneath wings, where foams
may not penetrate and on running fuel fire situations, on which foams are
ineffective. They are known as complementary agents because while they may have
the capability of rapid fire suppression (when applied at a sufficient rate),
it is generally necessary to apply a principal agent simultaneously or at least
before flashback can occur in order to achieve permanent control. Considerably
improved complementary agents have become available in recent years, and
constant studies are still being made in both the dry chemical and halocarbon
fields.
8.2.2 Due regard must be made to the problems which may arise when large
quantities of complementary agents are discharged rapidly. A dense cloud of the
agent may impede aircraft evacuation or rescue operations by limiting the
visibility and affecting the respiration of those exposed to the effects.
8.2.3 Replacement of water for foam production by complementary agents. Paragraph 2.3.1 defines
conditions in which water for foam production may be replaced by complementary
agents. Paragraph 2.3.8 provides the
substitution ratios for each of the complementary agents in these calculations.
8.2.4 Dry chemical powders. These are available in a number of formulations, each
consisting of finely divided chemical products which are combined with
additives to improve their performance. The dry chemical powders normally
provided for aircraft rescue and fire fighting applications are not
specifically designed or intended for use on flammable metal fires, which
require specialized agents (see 12.2.17).
In aircraft rescue and fire fighting operations dry chemical powders are
normally of the "BC" type, indicating their effectiveness against
fires involving flammable liquids and those of an electrical origin.
Operational applications are usually in one of the following ways:
(a) as
a medium when fires are in an incipient stage, particularly fires involving
undercarriage assemblies. They are also effective against fires in concealed or
inaccessible locations and for running fuel fires, where foams are largely
ineffective;
(b) at a high rate of application in
the role of a principal agent, which may well be an acceptable practice at
airports with extreme climatic conditions. Details of the equivalents for the
substitution of dry chemical for water for foam production may be found in 2.3.8. In addition to the problems
described in 8.2.2, when large
quantities of dry chemical powders are discharged rapidly limited visibility
will also reduce the effective placement of foam in a dual‑agent attack
to those areas where the dry chemical powder has achieved
"knockdown".
8.2.5 As with all
complementary agents, the successful use of dry chemical powders is strongly
dependent on the technique of its application. When used with foam in dual
agent attack it can provide rapid knockdown of flammable liquid fires and some
protection to operatives from radiated heat when delivered at suitable rates.
Rates of 3 kg/s are about the limit
for handline operatives in fireground conditions but much higher discharge
rates are available when dry chemical powder monitors are provided. Operatives
must appreciate the limited cooling effect of dry chemical powders, which means
that liquid fuel fires may be extinguished without any corresponding reduction
in the temperature of metal components in the fire area. Reignition will be a
constant hazard in these circumstances. The application of dry chemical powders
is also significantly affected by wind speed but use may be made of the wind to augment the range of a powder stream and to
influence its pattern of distribution. Any dry chemical powder intended for use
in a dual‑agent attack with a foam must be tested for its compatibility
with that foam. (See also 8.1.1.) In
addition, dry chemical powders should comply with the specifications of the
International Organization for Standardization (ISO 7202).
8.2.6 Halogenated hydrocarbons. These agents, also known as halons, have been
available as fire extinguishing agents for many years but the early compounds
produced vapours which had unacceptable levels of toxicity, either in their
natural state or after exposure to heat. More recent products have more
acceptable toxicity factors and have gained wide acceptance in aircraft rescue
and fire fighting applications. The agents have complex chemical names and to
simplify references to them a "numbering system" was devised by the
United States Corps of Engineers. The figures, from left to right, represent
the numbers of carbon, fluorine, chlorine and bromine atoms in the described
compound. Thus a compound having the chemical name of bromochlorodifluoromethane,
and the formula CF2 Cl Br, is known as Halon 1211. Similarly, bromotrifluoromethane, CF, Br, is known as Halon 1301. These two agents are currently in
use in fire fighting systems but differences in their physical properties have
tended to direct them into fields where these characteristics can be directed
to produce maximum operational advantages with the minimum of installation
problems.
8.2.7 Halogenated
hydrocarbons should comply with the specifications of the International Organization
for Standardization (ISO 7201). Halon
1211, with its lower vapour pressure,
230 kPa at 2WC, can be contained in lighter pressure
vessels than Halon 1301, which has a
vapour pressure of 1 430 kPa at 2WC. The higher boiling point of Halon 1211 (‑4°C) ensures that more of the discharge from a system
reaches the fire in liquid droplets than would be the case with Halon 1301, which boils at ‑
8.2.8 For the fire protection of delicate equipment within buildings,
where toxicity factors may be more significant due to the size of the protected
compartment, Halon 1301 installations are more frequently the choice, due to
the acceptability of slightly higher concentrations of this agent. Range of
application and total system weight are of less consequence in these
installations.
8.2.9 Halon installations on aircraft rescue and fire fighting vehicles
consist of one or more pressure vessels, in individual capacities from
8.2.10 Operators must be trained to deliver halon agents in a series of
brief discharges, interspersed with observations of the degree of fire control
achieved. Use may be made of wind‑effect to increase range of application
and where a jet/spray applicator is available, transition to the spray pattern
should be made as the operator is able to close with the fire. These tactics
are particularly important in dealing with fire situations involving aircraft
undercarriage assemblies. (See 12.2.3)
8.2.11 The availability of equipment to permit the recharging of the pressure
vessels containing Halon 1211 at the airport of use has greatly eased the
operational management problems associated with the early installations. The
total equipment requirement consists of a bulk supply of the agent, a cylinder
of compressed nitrogen and a filling rig, usually in a container. The filling
rig consists of a series of flexible pipes to deliver the halon and its
expellant to the pressure vessel, pressure gauges to ensure correct
pressurization, a safety valve to protect all equipment and the operatives and
a range of adaptors to accommodate pressure vessels in sizes from those of
portable extinguishers to the larger units on trolleys or vehicles. Personnel
appointed to perform refilling operations will require only minimal initial
training to ensure the correct sequence of operations and the observation of
safety precautions.
8.2.12 Carbon dioxide (C02). Carbon dioxide is traditionally used in
aircraft rescue and fire fighting operations in two ways:
(a) as
a means of rapid knockdown for small fires or as a flooding agent in reaching
concealed fires in areas inaccessible to foam. It should not be used on fires
involving flammable metals; and
(b) as the complementary agent in a
dual‑agent attack with a foam. In this form of application the C02 is
most
effective at high rates of delivery,
achieved through ‑‑‑lowpressure" systems.
8.2.13 Carbon dioxide installations in aircraft rescue and fire fighting
vehicles were originally of two types:
(a) "high
pressure" systems, consisting of a series of cylinders, manifolded
together, containing C02 gas at 5 900 kPa pressure at an ambient
temperature of 21º C; and
(b) "low
pressure‑ systems, where the carbon dioxide is contained in an insulated
pressure vessel at a controlled low temperature, usually ‑ 18ºC. At this temperature the storage pressure is 2 100
kPa and the delivery systems can provide discharge rates up to 1 100 kg/min, giving a long throw to a great volume of gas. Equipment of this
type is not now in regular production.
8.2.14 CO2, gas is only
1.5 times the weight of air and is therefore seriously affected in outdoor
applications by the wind and the convection currents associated with a fire.
The availability of alternative complementary agents has provided the
opportunity for the replacement of CO2, gas in vehicle installations.
8.2.15 Carbon dioxide should comply with the specifications of the
International Organization for Standardization (ISO 5923).
8.3 CONDITIONS OF
STORAGE OF EXTINGUISHING AGENTS
Paragraph 2.6.1
proposes that a reserve supply of foam concentrate and complementary agents
should be maintained on the airport, equivalent to 200 per cent of the
quantities carried in the vehicles. Paragraph 9.3.5 suggests that this reserve
of agents shall be stored in the fire station(s). The conditions of storage are
frequently specified by manufacturers or suppliers but in general terms the aim
should be:
(a) foam concentrate. Avoid
extremes of temperature. Use stocks in order of receipt. Keep concentrate in
manufacturers' containers until required for use. Replace and seal the caps of
any partly‑used containers;
(b) dry chemical powders. Use
stocks in order of receipt. Replace and seal the caps of any partly‑used
containers; and
(c) halogenated hydro7carbon agents. Avoid direct exposure to sunlight and high
temperatures, even where pressure vessels are filled to tropical levels. Use
pressure‑relief valve, where fitted, to reduce overpressure in accordance
with manufacturers' instructions.