The LWF Blog

Facilities Management & Fire Safety – Sprinkler System Design Principles – Part 8

May 18, 2017 12:14 pm

In this Fire Safety blog series for those who work in Facilities Management, we have been looking at the design principles of Sprinkler Systems and how they can be divided into hazard categories – light hazard, ordinary hazard and high hazard. The higher the hazard category, the greater the size and rate of development of any potential fire. In Part 7 of this series, we looked at light hazard installations and today we will discuss ordinary and high hazard installations.


Unlike light installations, ordinary and high hazard occupancies can be further subdivided, as per the breakdown given in BS EN 12845. Ordinary hazard installations are divided into sub-category hazard groups – Group I, Group II, Group III and Group IV. In these cases, the minimum design density in the most hydraulically unfavourable area is the same at 5mm/minute, but the AMAO (Assumed Maximum Area of Operation) ranges from 72m2 to 360m2 respectively.


When considering the types of building and occupation which are most likely to fall under the heading of ‘Ordinary Hazard’, they are commonly industrial and commercial premises where there is unlikely to be rapid fire development due to the methods of storage used for any potential fire load. Some office buildings, except from high rise developments, can fall into ordinary hazard group I, provided that there are no major storage areas which would provide a large fire load. A department store might be classed as hazard group III and a theatre perhaps as group IV. BS EN 12845 offers typical grouping types, but ‘on the ground’ classification can vary significantly, with different insurance companies having variable opinions on risk classification. It is important that the insurer is consulted early in the sprinkler design process to avoid issues later in the process.


High hazard premises are those properties which may have more complex sprinkler system needs because they are aiming to store a large amount of combustibles, or highly-combustible items or are designed in such a way that there is more required of the installed sprinkler system. High hazard risks can also be further broken down into four areas – Process hazards, high-piled storage hazards, potable spirit storage hazards, oil and flammable liquid hazards, the latter of which would require special protection in any case.


Looking at the first high hazard risk area, process hazards, there are four different types and the densities of each vary between 7.5mm/minute to 12.5mm/minute. The first three types of process hazard has the same AMAO with only the minimum density of the discharge varies. The fourth type, however, is rare and includes firework manufacturing. For this category of hazard, BS EN 12845 states that it is their recommendation that each building have complete ‘deluge’ protection. This means that upon triggering of the detector, open heads will discharge water over the entire area.


In Part 9 of this series, we will continue looking at high hazard installations and their 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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