Medium expansion foams are aggregations of bubbles that are mechanically generated by the passage of air or other gases through a net, screen, or other porous medium that is wetted by an aqueous solution of surface active foaming agents. Under proper conditions, fire-fighting foams of expansions from 20:1 to 200:1 can be generated.
These foams provide a unique agent for transporting water to inaccessible places; for total flooding of confined spaces; and for volumetric displacement of vapour, heat, and smoke. Optimum efficiency in any type of hazard depends to some extent on the rate of application and the foam expansion and stability.
Medium expansion foam can be used on solid fuel and liquid fuel fires where some degree of in-depth coverage is necessary for example, for the total flooding of small enclosed or partially enclosed volumes such as engine test cells and transformer rooms.
Medium-expansion foam can provide quick and effective coverage of flammable liquid spill fires or some toxic liquid spills where rapid vapour suppression is essential. It is effective both indoors and outdoors.
Medium expansion foam has the following effects on fires:
1) Where generated in sufficient volume, medium expansion foam can prevent the free movement of air, which is necessary for continued combustion.
2) Where forced into the heat of a fire, the water in the foam is converted to steam, thus reducing the oxygen concentration by dilution of the air.
3) The conversion of the water to steam absorbs heat from the burning fuel. Any hot object exposed to the foam will continue the process of breaking the foam, converting the water to steam, and cooling.
4) Because of its relatively low surface tension, solution from the foam that is not converted to steam will tend to penetrate Class A materials. However, deep-seated fires might require overhaul.
5) Where accumulated in depth, medium expansion foam can provide an insulating barrier for protection of exposed materials or structures not involved in a fire and can thus prevent fire spread.
6) Class A fires are controlled when the foam completely covers the fire and burning material. If the foam is sufficiently wet and is maintained long enough, the fire can be extinguished.
7) Class B fires involving high-flash-point liquids can be extinguished when the surface is cooled below the flash point. Class B fires involving low- flash-point liquids can be extinguished when a foam blanket of sufficient depth is established over the liquid surface.
Under certain circumstances, it might be possible to utilize medium expansion foam systems for control of fires involving flammable liquids or gases issuing under pressure, but no general recommendations can be made in this standard due to the large variety of particular situations that can be encountered in actual practice.
Ability to control or extinguish a fire in a given hazard might depend on such factors as expansion, drainage, and fluidity. These factors will vary with the concentrate, equipment, water supply, and air supply.
The discharge of large amounts of medium expansion foam in contact with people can block vision, make hearing difficult, create some discomfort in breathing, and cause spatial disorientation
Mechanisms of extinguishment
Medium expansion foam extinguishes fire by reducing the concentration of oxygen at the seat of the fire, by cooling, by halting convection and radiation, by excluding additional air, and by retarding flammable vapour release.
• Tank bunds
• Collecting area
• Process area
• Aircraft hanger (type 3 only)
• Fuel transfer area
• Plastic packaging and storage • Plastic recycling
• Marine jetties
• LPG (Liquefied Petroleum Gas)
Use and limitations
Medium expansion foams are finding application for a broad range of fire- fighting scenarios, each type of hazard shall be specifically evaluated to verify the applicability of medium expansion foam as a fire control agent. Some important types of hazards that medium expansion foam systems shall be permitted to protect include the following:
1) Ordinary combustibles
2) Flammable and combustible liquids
3) Combinations of (1) and (2)
Susceptibility of the protected hazard to water damage shall be evaluated.
Medium expansion foam systems shall not be used on fires in the following hazards unless competent evaluation, including tests, indicates acceptability:
1) Chemicals, such as cellulose nitrate, that release sufficient oxygen or other oxidizing agents to sustain combustion
2) Energized unenclosed electrical equipment
3) Water-reactive metals such as sodium, potassium, and NaK (sodium- potassium alloys)
4) Hazardous water-reactive materials, such as triethylaluminum and phosphorus pentoxide
5) Liquefied flammable gas
Can be thrown reasonable distance. Give reasonable foam depth to provide good vapour suppression.
Limited volume. Water content can still be too damaging to some risk.
Medium expansion foam fixed system
In general, generator can provide a much more effective and evenly distributed foam blanket than a low expansion system.
With this type of system, a number of generators are located around the bund walls at regular intervals to obtain a homogeneous distribution.
In some cases, generator can also be installed within the bund area, itself in order to give better distribution. In this circumstance it is vital that system actuation is very fast or that the other measures are taken to safeguard the equipment. For this specify use, Sabo Española has planned a typology of foam branch pipe strengthened for withstanding to a possible pressure wave (lateral).
The reference rules on the medium expansion foam system, for example NFPA-11 or the EN 13545-2 do not provide specification on the installation of this type of generators, but, typically, maximum area protected by one device would be 450-500 m2.
Portable foam equipment – Foam application
There are basic foam application rules which should always be remembered when using portable foam equipment:
A) The nozzle should be turned away from the fire area until foam is flowing. Water will flow initially until the foam is proportioned. This can continue for some time. Applying water onto a flammable liquid fire can lead to flare-up or fire spread.
B) If possible the foam stream should be directed at any banking, wall or structure on the edge of the fire area. This will reduce plunging and allow the foam to flow smoothly over the liquid surface.
C) If there is no means of deflecting the foam stream, it is best to point the stream at the front of the edge of the liquid surface and allow the foam to flow gently forward over the surface.
D) Never direct the foam stream into the centre of the fire area. This will hinder the formation of a foam blanket since the foam will be plugged under the liquid surface and will cause the foam to be polluted by fuel or destroyed.
E) Do not move into the foamed area over the flammable liquid surface even the fire is extinguished. This will disturb the foam blanket and may lead to reignition of the fuel.
F) During fires which require the use of water cooling stream in tandem with foam application, care must be exercised to ensure the cooling water does not dilute or break up the foam blanket.
G) Do not mix different concentrate together. AFFF mixed with Fluoroprotein, or multipurpose foam mixed with other concentrate will destroy the foam- making capability of the concentrate.
H) Pressure fire or jet fires cannot be extinguished by foam although applying foam onto such fire gives a measure of control since the foam will flow with the fuel until is retained and will then blanket the surface in the area where the fuel stops.
I) Foam and dry chemical powders are often used in combination or as complements to each other. Whenever the choice in dual application of these two extinguishants is taken, the recommendation of the chemical’s manufacturers at to their compatibility should be closely adhered to.
Maintenance and instructions
Inspection and Tests.
At least annually, all medium foam systems shall be thoroughly inspected and checked for proper operation by a competent engineer or inspector.
This inspection shall include determination of any changes in physical properties of the foam concentrate that indicate any deterioration in quality.
The goal of this inspection and testing shall be to ensure that the system is in full operating condition and to indicate the probable continuance of that condition until the next inspection.
Medium expansion foam systems shall be maintained in full operating condition at all times. Use, impairment, and restoration of this protection shall be reported promptly to the authority having jurisdiction. Any troubles or impairments shall be corrected at once by competent personnel.
All people who might be expected to inspect, test, maintain, or operate foam- generating apparatus shall be thoroughly trained and kept thoroughly trained in the functions they are expected to perform.
Training programs approved by the authority having jurisdiction shall be established.
Operating instructions shall be posted at control stations
Availability and service
The Sabo Española medium expansion foam system is available through a network of domestic and international distributors.
(To find a distributor near you contact Sabo Española).
For details of warranty contact Sabo Española directly.