Lawrence Webster Forrest
Legion House
Kenley
Surrey
CR8 5NH

Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
E-mail: fire@lwf.co.uk

Fire Engineering Design and Risk Assessment - Sprinkler Installation Design - Part 36

Posted by LWF: 08/03/2018 11:20

In LWF’s fire engineering blog series for Architects and others in the building design business, we have been looking at what must be taken into consideration when designing a sprinkler installation. In part 35, water supply to the sprinkler system was discussed and in part 36, we continue on that subject, starting with ‘high-rise’ systems.

In the UK, a building containing a sprinkler system where the distance between the highest and lowest sprinklers exceed 45m is classed as ‘high rise’ and certain conditions must be fulfilled. The system which exceeds 45m must be subdivided and each section must have a highest to lowest differential which does not exceed 45m.

In order to facilitate a constant water supply, each of the sub-divided sections must have its own separate set of pumps or be fed from a separate stage of a multi-stage pump. Multiple pumps may feed from the same water source, which must be sufficient to supply the sprinklers at highest demand.

The use of hydraulic calculations to ascertain the requirements of the sprinkler system was mentioned in previous blogs of this series and in circumstances where this method has been utilised, those calculations will indicate the necessary flow/pressure characteristics of the pumps and size of the storage tanks.

The figures for the most favourable and unfavourable locations, hydraulically speaking, can be plotted on a graph where the linear scale indicates pressure and the square-law scale for water flow. The results will provide a system demand curve which will appear as a practically straight line on the graph. The next stage is to ascertain the figures necessary to plot the design site performance curve for the pump in two different states – when the water tank is full and when the water tank is at its lowest viable level.

The aim of the graph illustration is to ensure that the installation demand points are all covered by the pump curve when it is at its lowest operational level to ensure that the design flow rate is available as required when the sprinklers are in use.

In addition, consideration must be given to the circumstances of a full design size fire in the area of the most hydraulically favourable sprinkler location. This situation would result in an increased flow rate and must be provided for in terms of pump driver power and tank capacity. The demand curve in this instance should be extended on the graph and the point where it intercepts the pump curve at its highest point is known as Qmax and is used to work out the necessary tank size and pump duty.

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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