The LWF Blog
Fire Engineering Design and Risk Assessment – Foam Systems – Part 3
May 17, 2018 1:29 pmIn LWF’s Fire Engineering blog series for Architects and others in the building design business, we have been looking at fire suppression systems and, in particular, foam concentrate systems. In part 2, we began an overview of the different types of foam concentrate and part 3 will continue from that point before discussing foam proportioning.
Synthetic, or high expansion foam is produced from detergents and, as a result, can produce great quantities of ‘bubbles’ making it particularly useful in high-expansion foam systems.
A Class A foam is commonly used as a wetting agent to reduce the surface tension of the water, allowing increased penetration which offers greater effectiveness on fires involving ordinary combustibles. Class A foams can also be aspirated for use which enables their use as a surface fire barrier. The foam comes in various strengths, usually 3% or 6%. The percentage indicates the proportion of foam to water, i.e. 3% is 3 parts foam concentrate to 97 parts water. Aqueous film-forming foam is also available at 1%. It is possible to obtain Class A foam to work at lower temperatures, including a frost-protected version. This allows the use of the foam in conditions as low as -18 °C, where normal foams would freeze at around 2 °C.
Fluorine-free foams are a relatively new addition to the types of foam concentrate available. As the name might indicate, they do not contain fluorine-based surfactants. There is some controversy about the performance of such concentrates. While the aim of this foam is to provide a less environmentally damaging substance with which to fight fire, fluorine-free foams do have an impact on the environment themselves and the relatively poor fire-fighting performance offered means that more would have to be used to be as effective as some of the other type of foam concentrate. If the use of this foam is being considered, it would be prudent to check BS EN 1568-3:2018 Fire extinguishing media. Foam concentrates. Specification for low expansion foam concentrates for surface application to water-immiscible liquids.
Foam Proportioning
The process of foam proportioning is the method of mixing foam concentrate with water at the necessary ratio for it to be effective. There are various options, the first of which is inductors.
An inductor is usually a Venturi device, named after the Italian physicist, which produces a reduction in liquid pressure when it is drawn through a constricted section of a pipe. The general principle is based on the inlet water creating a velocity across an orifice, which in turn generates a pressure differential, i.e. high pressure to low pressure. This differential creates a vacuum, drawing the foam concentrate into the mix through an appropriately sized orifice.
Inductors are single-flow single-pressure devices that need an inlet pressure of at least 5 bar and incur a 35% pressure drop across the device. They commonly draw foam concentrate from an external storage vessel which is not typically pressurised, i.e. is held at atmospheric pressure. As discussed above, the creation of a pressure differential creates a vacuum, with the foam moving from an area of low pressure (atmospheric) to high pressure across the orifice/commonly referenced ‘pick up tube’.
In part 4 of this series, LWF will continue looking at methods of foam proportioning with Bladder Tanks. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.