The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & External Water Supplies – Part 28

February 18, 2019 2:19 pm

In LWF’s fire engineering blog series for Architects and others in the building design business, we have been talking about firefighting and the necessary provisions that should be made to support the Fire Service upon their attendance at a fire. In part 27, we looked at the provision of sufficient additional water for use after the on-board supply in the pump was exhausted. In part 28, the necessity to assess the required flow from water hydrants is discussed.


Provision of a fire hydrant in a suitable position at a site is important and the flow rate of the water must be sufficient for Fire Service use.  The current UK Building Regulations indicate that the maximum flow rate recommended is equivalent to 4500 litres/minute.


The NFPA indicate typical flow rates of 1893 litres/minute for the first rising main and 946 litres/minute for each additional rising main up to a maximum of 4731 litres/minute. NFPA 14 also requires that the water supply be sufficient to provide the minimum flow rate for 30 minutes. The duration is intended to apply to automatic and semi-automatic supplies and it is assumed that the Fire Department runs on a separate water source to supply the system using fire engine pumps within 30 minutes.


In the 1950s, Keith Royer and Floyd W. Nelson began working for Iowa State University and became involved in the Exploratory Committee on the Application of Water, the main work of which was to investigate the use of water fog for firefighting purposes. The two men worked on a critical question – How much water is required to control a fire with water fog? In Nelson’s 1989 book, Qualitative Fire Behavior, he said, ‘In principle, firefighting is very simple. All one needs to do is put the right amount of water in the right place and the fire is controlled’.


Royer and Nelson saw that the heat release from a fire must be balanced by the energy necessary to heat water to its boiling point and convert it into steam. They discovered that too little water or too much water was much less effective than the right amount.


The resulting formula, known as the Iowa rate-of-flow formula, showed that the flow required in litres to control a fully-developed compartment fire is four-thirds of the volume of the compartment in cubic metres. For example, a compartment of 900 m3 would require a flow of water equalling 1200 litres/minute.


In part 29 of this series, LWF will continue looking at how to assess required flow from fire hydrants. 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.


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