The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & External Water Supplies – Part 31

March 11, 2019 3:25 pm

In LWF’s Fire Engineering blog series for Architects and others in the building design business, we have been looking at the subject of fire safety engineering and most recently, firefighting. In part 30, we considered additional external water supplies which may be used instead of hydrants. In part 31, we discuss capacities and water relay.

The BDAG report – Effect of reduced pressures on performance of firefighting branches in tall buildings – Aspects of high-rise firefighting. Fire Research Technical Report 3/2005. (PDF) provided flow rates which can be used to estimate necessary capacities. For instance, three firefighting jets at a flow rate of 500 litres/minute per jet would empty a 113,550-litre capacity tank in 75 minutes. A 45,000-litre tank would last for 30 minutes.

Previous fire statistics have shown that most fires in the UK attended by the Fire Service are extinguished using a single hose line. It was estimated in CIBSE Guide E that 97% of fires can be dealt with by eight people on two pumps and 90% of fires can be dealt with by a single hose line or water from a maximum of two hose reels. 

Taking into account the available data, it would seem that where no piped water supply is available at the premises, or where the provided water supply has insufficient water pressure and flow rate to be used for firefighting purposes, a 45,000-litre tank as recommended in subsection 15.8 of Approved Document B (quoted in part 30 of this blog series) is sufficient and reasonable provision for normal risks. 

It may be, however, that a particular building should be assessed for additional risks in case the provision of water should be greater to ensure fire safety. The design team along with consultation with the Fire Service, fire engineers and insurers should consider such matters prior to a design decision being made.

Where there is no water supply near the source of the fire, or the nearest supply has been depleted, the Fire Service may resort to using hoses and pumps to relay water from one location to the seat of the fire. 

It is possible for the total water requirement to be calculated and provided from several sources. For instance, initially, hydrant use for stopping jets upon arrival, a static tank or other on-site supply for use by the next attending appliances, which would allow sufficient time for a water relay to be established in order to provide larger quantities of water.

Such arrangements cannot be considered ideal, as relaying water is time-consuming and puts additional pressure on the Fire Service personnel and equipment.

In part 32 of this series on Firefighting, LWF will begin to look at internal water supplies for firefighting. 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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