The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & External Water Supplies – Part 27

February 12, 2019 11:32 am

In LWF’s Fire Engineering blog series for Architects and others in the building design business, we have been looking at fire safety engineering. In part 26, we looked at how the choice of fire hydrant can affect the efficiency of delivery and by working out the additional time required to prime an underground hydrant when compared to a pillar hydrant with instantaneous couplings, it was established there could be as much as 2 minutes delay on the former. In part 27, LWF will continue to discuss the provision and placing of hydrants for firefighting purposes and the importance of water pressure to firefighting.

A formula can be used to work out the time taken to get additional water supplies, as follows:

t – (Wp – Wh) / D

t is the time available to obtain additional water supplies in minutes, Wp is the quantity of water in the pumping appliance tank in litres, Wh is the quantity of water in the hose lines from pump to branch in litres and D is the discharge rate from a nozzle in 1/min.

The standards relating to hydrant positioning have changed over the years, with requirements made depending on the risk and fire-fighting requirements of a building. For example, a building fitted with a rising main requires a fire hydrant within 90 m of the parking position of the fire appliance. The fire appliance must be able to park within 18 m of the rising main inlet point. These distances are based on standard firefighting hose lengths, i.e. 25 m, so based on the above, it is designed that a fire appliance can park, use one length of hose to charge the rising main, with 4 lengths used to replenish the fire appliance. This would leave remaining lengths of hose to be used for internal fire- fighting. Hydrant coverage should be reviewed and considered as part of the buildings fire risk assessment. 

Some locations in the UK are subject to the installation of pressure-reducing valves meaning that the water pressure available is insufficient for firefighting purposes. There are currently two methods used by the Fire Service to overcome these limitations. The first is that they are able to identify upstream hydrants which produce sufficient water pressure and relay the water to the site of the fire with the use of additional pumping appliances. 

The second method of overcoming the pressure-reducing valves which lower the water pressure is that the Fire Service request the attendance of a turncock from the water supply company which opens the valves to increase flow. 

When neither of these options are possible, the Fire Service may resort to collector pumping and large-scale water relay using hoses. 

In part 28 of this series, LWF will look at how to assess the 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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