The LWF Blog

Facilities Management & Fire Safety – Sprinkler System Design Principles – Part 7

May 10, 2017 2:30 pm

In this blog series for those who work in Facilities Management and who have a responsibility for or interest in fire safety, we have been looking at the use of fixed fire-fighting systems in buildings. In Part 7, we’re going to discuss the design principles for sprinkler installation.


When approaching the design for sprinkler installation, firstly the occupancy under consideration must be allocated into one of three hazard categories, depending upon the nature of the activities which will take place within, and also, considering the nature of any combustible materials which may be involved in the activities or stored on the premises. These categories are: Light hazard, Ordinary hazard and High hazard.


As you might expect, the higher the hazard category, the more likely that any fire which ensues will be of a great size and/or high rate of development. For this reason, a different density of water discharge is necessary for each of the categories. The density of the water discharge is measured in the same way rainfall is – millimetres per minute.


Each of the hazard levels also has a different assumed maximum area of operation (AMAO), because when a fire starts, only those heads which are in the immediate area of the fire will become operational.


A light hazard installation is not commonly seen because of the circumstances of the building it must be installed into. They are used in non-industrial premises where the combustible materials are very limited and the building is subject to fire-resistant compartmentation of limited sizes. This classification might apply to an office block where individual offices are compartmentalised and the likely size of fire and growth rate would be small.  Other situations in which you might find a light hazard installation include hospitals, hotels and museums, although many of these would not use a sprinkler installation as a fire protection measure.


Once a light hazard sprinkler installation is operational in a building, it means that functional changes to the building are impractical without having to completely change the sprinkler system for one which is appropriate to the new layout. This means that individual offices, for example, could not be knocked through into one large open plan office under the light hazard sprinkler system.


The minimum design density of a light hazard installation must be 2.25mm/minute with an AMAO of 84m2.


In Part 8 of this series, we will give our attention to ordinary and high hazard categories and look at the design requirements as well as giving some information on the likely practical uses. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact Peter Gyere in the first instance on 0208 668 8663.


Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.


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