The LWF Blog
Fire Engineering Design and Risk Assessment – Foam Proportioning – Part 4May 24, 2018 12:55 pm
In 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 systems. In part 3, it was ascertained that foam proportioning is the means by which foam concentrate is mixed with water at the necessary ratio. An overview of inductors (also known as line proportioners) was given. In part 4, we will continue looking at methods of foam proportioning, beginning with bladder tanks.
Bladder tank proportioning systems are made up of a pressure vessel with a rubber bladder of foam concentrate within. The system is primed by water being fed through from the inlet supply into the body of the vessel, which results in pressurisation between the inside wall of the vessel and the bladder containing the foam concentrate. The pressure created then squeezes the foam out, through a pipe to a foam proportioner.
The proportioner is sited in the pipework to the foam system and its role is to mix the foam concentrate with water at the required ratio.
Bladder tank proportioning systems are able to operate at a wide range of flows and pressures and the range itself depends on the proportioner and foam concentrate used.
Balanced Pressure Proportioning
A balanced pressure proportioning system will also operate at a wide range of flows and pressures. Such systems utilise a foam concentrate pump which draws from an atmospheric storage tank. The pump delivers foam concentrate through a pressure balancing valve into a proportioner. The pressure balancing valve’s job is to sense the pressure of both the foam and the water in order to regulate the foam pressure to match the water pressure.
Water Driven Foam Metering Pumps
Water driven foam metering pumps incorporate a water motor which is sited within the water line to the foam system and drives a foam pump, drawing foam from an atmospheric storage tank. The flow through the water motor is matched by the flow through the foam pump and is able to accurately provide the correct amount of foam concentrate into the water downstream. While such systems provide proportional accuracy, this is over a lesser range than either bladder tank proportioning systems or balanced pressure proportioning systems.
Premix Foam Units
A premix foam unit is a pressurised vessel charged with a pre-measured combination of foam concentrate and water, in the same way as a fire extinguisher is. The units are linked to a gas supply, most commonly CO2, which actuates the vessel and releases the foam solution from the vessel into the system. As premix foam units have a finite capacity, they are best used for small areas of hazard.
In part 5 of this series, LWF will discuss the types of foam system available, beginning with low-expansion foam systems. 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.