Automatic sprinklers — principles of design
1.1 General
Since a most important principle of successful fire extinction is to attack an outbreak immediately, it follows that any device which can detect a fire automatically and then control or extinguish it with the minimum loss, must he of great value. Automatic sprinkler systems using water as
the extinguishing medium have been universally adopted as one means of achieving this purpose.
Basically an automatic sprinkler installation comprises of a system of pipes erected at, or near, the ceiling on each floor of a building and connected, through controlling valves, to one or more water supplies. At intervals on the pipework are sealed outlets called sprinkler heads. These incorporate a device whereby a rise in temperature to a predetermined limit causes the sprinkler to open and water to be discharged in the form of a spray over an area of the floor below. The sprinklers are so spaced that the extremities of the discharge pattern from any two sprinklers overlap, leaving no part of the floor unprotected.
The operation of the sprinkler leads to the opening of a valve, which causes an alarm bell to ring. The layout of a typical sprinkler system is shown in Figure 1.1. 1.2 Historical
The first automatic sprinklers were invented in the mid-l9th century. Various types were produced, one
of the most successful by Grinnell in 1882. This incorporated a fusible soldered link, which
Figure 1.1 Layout of a
Typical sprinkler system
melted when heated and released water through the sprinkler head. From this design many modem solder-type heads have been developed.
Shortly after the First World War, two major developments took place: the introduction of the glass bulb sprinkler head and the multiplecontrol system.
A more recent innovation is the fast response sprinkler head. Here, the heat sensitive device is much more responsive to heat and will cause the sprinkler to operate much earlier than with the standard sprinkler head. Such systems can fulfil the function of both automatic fire
detectionand alarm and fire suppression.
1.3 Installation and design requirements
In England and Wales the Approved Document B states that retail buildings over 2000m should be
sprinklered, compartmented or designed using another acceptable method. Technical Standards in
Scotland differ from those in England and Wales. Fire authorities can require maintenance and testing
of sprinkler installations where the system is taken into account when granting a fire certificate under
the Fire Precautions Act, 1971.
Where sprinkler systems are installed as a requirement/recommendation by the fire service, then the
requirement/recommendation must satisfy that the system to be installed is fit for purpose, that is, that a
correctly designed system is installed taking account of the hazard.
Insurance companies encourage the installation of sprinkler systems, specifically where arson is
considered within a fire risk analysis. Buildings fitted with sprinkler systems to an approved standard
sometimes qualify for substantial reductions in premiums for property so protected. They, through the
Loss Prevention Council (LPC), lay down the minimum standards necessary.
Sprinkler systems are designed in accordance with:
(I) British Standard 5306: Part 2 (1990).
(II) LPC rules for Automatic Sprinkler Installations.
These two documents are now synonymous and any amendment to one is reflected in the other.
Further reference in this Manual will be to ‘the BS/LPC Rules’.
The BS/LCP Rules detail standards for sprinkler systems designed for life safety. Sprinkler systems fall
into various hazard groups according to the use of the building. Each class of system is designed to
produce a certain density of water discharge over a predetermined area for a given period of time
depending on the expected area of fire development in that particular occupancy (see Section 6).
Sprinkler installations are used to protect a very wide range of premises and there are very few
buildings which are totally unsuitable for sprinklers. Where parts of a building contain materials or
processes for which water would be unsuitable as an extinguishing medium, these areas can be isolated
by fire resisting construction and the remainder protected by sprinklers.
The terms (a) ‘high rise’ and (b) ‘low rise’ systems are used to describe systems where:
(a) The highest sprinkler is more than 45m above the lowest sprinkler or the sprinkler pumps,
whichever is the lowest and;
(b) The highest sprinkler is not more than 45m above ground level or the sprinkler pump.
1.4 Risk categories
The BS/LPC Rules arranges occupancies into risk categories each having an accepted abbreviation.
These are shown in Table 1.1.
Table 1.1 BSILPC risk categories__________________
Category__________________________Abbreviations
Light Hazard LH
Ordinary Hazard: Group I 0H1
Ordinary Hazard:Group II 0H2
Ordinary Hazard: Group III 0H3
Ordinary Hazard: Group HIS (Special ) 0H3(S)
High Hazard HH
1.4.1 Light hazard
These are non-industrial type premises of not more than l26m between construction of not less than
half-hour fire-resistance, e.g. offices, libraries, hospitals where the amount and combustibility of the
contents is low.
1.4.2 Ordinary hazards: Group I, II, III and IIIS
Ordinary hazards are commercial and industrial premises involving the handling, processing and
storage of a very wide range of mainly combustible materials, which are unlikely to burn intensely in
the early stages of a fire. It has been found necessary to sub-divide them into four groups as below:
Group Examples
Group I Breweries, dairies and restaurants
Group II Engineering works, garages, medium size retail shops
Group Ill Soap factories, sugar refineries, air craft factories
Group Ill Film and television studios, cotton mills, match factories.
1.4.3 High Hazard
This category covers commercial and industrial occupancies having abnormal fire loads:
(II) Where materials handled or processed are mainly of an extra hazardous nature likely to
develop rapid and intensely-burning fires.
(II) Those involving high-piled storage.
According to the hazardous nature of the stock and the height of the storage, those included in (II)
above are sub-divided into four categories:
Category I Process high hazards
Category II High-piled storage hazards
Category Ill Potable spirit storage hazards
Category TV Oil and flammable liquids hazards
The term ‘storage’ includes the warehousing or temporary depositing of goods or materials.
1.5 Classes of system
Three classes of sprinkler system have been developed to suit the above risk categories:
(I) Light Hazard system
(II) Ordinary Hazard system
(III) High Hazard system.
Pipework for two or more different types of hazard system may be connected to a common set of
control valves, provided the total number of sprinklers does not exceed the permitted maximum. Each
of these systems is designed to give the appropriate density of discharge over an assumed area of
maximum’ operation (AMAO) in the highest and most hydraulically remote parts of a protected
building.
1.6 Design density and assumed area of maximum operation
The amount of water required to control or extinguish a fire is called the minimum design density and
will depend, among other criteria, on the type of hazard involved. Minimum design density is pre-set
according to the recommendations of the BS/LPC Rules and is specified for each hazard class.
The standard requires that the minimum design density of discharge of water in mm/min from a
particular group of sprinklers be not less than a given value (see Table 1.2). This group of sprinklers -
usually numbering four or more - is that which is most hydraulically remote from the water supply and
constitutes part of a ‘larger group’ of sprinklers discharging simultaneously.
The ‘larger group’ forms the ‘area of assumed maximum operation’ (AMAO). This is the maxi mum
area over which it is assumed, for design purposes, sprinklers will operate in a fire. The hydraulically
most removed AMAO is used to calculate design density.
1.7 Life safety systems
The use of sprinkler installations in the saving of lives by preventing the development of fire is well
understood. Today, sprinkler systems have played an increasing role in life safety fire precautions.
Classification of fire hazards in the current BS/LPC rules considers systems which, addition ally, serve
for the protection of life, where they are designed to restrict fire spread to predetermined limits.
Buildings employing fire safety engineering principles, as defined in ES 7529, often rely on sprinklers
as part of a package of measures to create safe egress from large or complex structures when fire
occurs.
1.7.1 General
Life safety systems are basic sprinkler systems enhanced to give a higher standard of reliability and
continuity of service. To ensure the system is ‘live’ during servicing, two main valves are normally
provided, one either side of the alarm valve. This allows the alarm valve to be serviced whilst
maintaining the system through a by-pass valve. This valve maintains water pressure to the system via
a by-pass connection.
1.7.2 Requirements
The ‘Life Safety’ requirements may include the following
(I) The system should he of a wet type.
(II) The system should be zoned, each zone being controlled by a separate stop valve and having a
maximum of 200 heads.
(III) A zone may require the installation control valve-set to be duplicated so that one set of valves
can be serviced whilst the system is operational.
(IV) No zone shall extend to an area of the building under separate ownership.
(V) No zone shall extend to more than one floor level but a zone may include a mezzanine floor of
not more lOOm²
(VI) Stop valves shall be accessible at the floor level of the zone they control.
(VII) Only one zone of a multi-zone installation shall be shut down at any one time. The fire
authority is to be advised of the intention and should have to approve.
(VIII) All stop valves and tamper-proof electrical switches indicating that the valves are in the
correct operating mode shall monitor alarm valves.
(IX) All practical steps shall be taken to ensure continuity and reliability of water supplies.
(X) Means shall be provided to initiate visual and audible warnings to an area with responsible
manning when the pressure in the sprinkler trunk falls to the point at which the pump should
start. These warnings shall ‘latch in’ and only be capable of manual cancellation.
(XI) On indicator panels, audible alarms may be silenced after the system has operated, but the
visual alarm signal shall remain until the installation has been reset to its normal operational
position.
In theatres and similar buildings, where a fire break curtain is protected by open drenchers or sprinklers
operated by a quick opening valve, the water supply to these should not be taken from that supplying
the automatic sprinkler installation
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