Civil Engineering Reference
In-Depth Information
fl ow of water through the system. Most control valves are
designed in such a way that a small amount of pressure
in the range pipes will hold back a greater amount of
pressure in the supply pipe. This is simply achieved with
the use of a differential valve which has the range pipe
side much bigger than the water supply side.
Alarm and monitoring unit
- this unit is fi tted
immediately above the control valve and monitors the
pressure in both sides of the control valve. An alarm is
activated if the pressure in either the supply pipes or the
range pipes drops or a fl ow of water is detected in the
system, which indicates the operation of a sprinkler head.
Local/external alarm
- once the alarm monitor-
ing unit has detected a situation that triggers an alarm
it may activate a local and external alarm. The local
alarm which indicates that there is a fl ow in the system
is commonly achieved by the use of a Pelton wheel
where a small quantity of water that is fl owing through
the range pipes is diverted and turns a mechanical bell
situated near the sprinkler control valve location. The
alarm can be raised externally through an automatic link
to a commercial, alarm receiving centre (ARC).
Sprinkler heads
- sprinkler heads have two main
components, the head itself and a fusible link. When a
head has operated the water from the range pipe hits a
defl ector plate which causes the water to be sprayed in
any number of different patterns and/or directions. The
shape of the head and the size of the hole for the water is
dependent upon the location of the sprinkler head and fi re
risk being protected. The most common type of fusible
link is a glass bulb which is fi lled with a liquid. Each glass
bulb has a small air bubble to allow for the normal range
of temperature rises in the atmosphere, i.e. when there
is a temperature increase the liquid in the glass bulb will
expand and compress the air bubble. However, when
there is a signifi cant rise in the temperature within the
bulb the air bubble will compress as the liquid continues
to expand and will fracture the glass bulb itself thereby
allowing water to fl ow and thus operating the entire
system. Glass bulbs are colour coded in relation to their
operating temperature, see Table 9.12.
Figure 9.62
A variety of sprinkler heads in use
To ensure effective operation, sprinkler heads must
always be installed in the condition in which they are
supplied by the manufacturer; they must not be painted
or decorated in any way. The choice of sprinkler head
will depend upon:
➤
Sprinkler head pattern
➤
Sprinkler head spacing
➤
Location of thermally sensitive element relative to
the ceiling
➤
Clearance below the sprinkler
➤
Potential for a shielded fi re to develop.
Types of automatic water sprinkler systems
Wet, dry and alternate
- automatic water sprinkler sys-
tems can be designed to be wet, dry or alternate. The
range pipes of wet systems are always charged with
water and therefore will provide the most rapid response
to a fi re. However, wet systems can only be provided in
locations that are not subjected to freezing temperatures.
Sprinkler systems in shopping malls and offi ces are typ-
ically wet systems. In contrast a dry system is fi lled with
pressurised air. Dry systems are therefore appropriate
where the temperature is likely to fall below freezing, for
example in multi-storey car parks. The disadvantage of
a dry system is that there can be a delay in reacting to a
fi re situation while the air is driven out of the range pipes
by the advancing water. To minimise this disadvantage an
alternate system may be used on those occasions where,
in the winter, the system may be exposed to freezing
temperatures but in the summer in a temperate climate
the range pipes can be fi lled and thereby achieve a more
rapid response time.
Pre-action sprinkler system
- this is a combina-
tion of a standard sprinkler system and an independent
approved system of heat or smoke detectors installed in
the same area as the sprinklers. Like dry pipe systems
the pipes are fi lled with air but water is only let into the
pipes when the detector operates. Pre-action systems
Table 9.12
Colours and operating temperatures of sprink-
ler heads
Colour
Temperature at which
it will activate ºC
Orange
57
Red
68
Yellow
79
