Automatic sprinklers systems Flashcards
According to the BS/LPC Rules a sprinkler installation
should be one of the following
main types ?
Wet pipe system n Dry pipe n Alternate (wet and dry pipe) n Life safety n Pre-action n Recycling n Deluge may also include extensions of an additional type as: n Tail-end alternate n Tail-end dry type
Describe a wet system and where it would be used
In this type of system, all the pipes that lead from
the water supplies through the various controlling
valves to the sprinkler heads throughout the building
are kept permanently filled with water. Wet pipe
systems are installed in premises where there is no
danger, at any time, of the water in the pipes freezing.
The principle controls of a wet system are :
(i) A stop valve on each separate source of supply.
(ii) A non-return valve on each source of supply,
(iii) An installation main stop valve to cut off the
flow of water to the system after a head has
opened and the fire has been extinguished,
(iv) The alarm valve has two junctions,
(a) on actuation of a sprinkler head it allows
the valve to open, due to a drop in pressure,
and lets water flow into the system
from the main supply,
(b)a flow is permitted to the water flow
alarm to indicate the system has activated.
(v) A test and drain valve, used for testing the
water flow of the installation and to empty
the system when necessary. The size of this
valve is 40mm in low hazard installations
and 50mm in both ordinary hazard and high
hazard installations.
draw and label a typical wet system
In Figure 3.1, page 18, a typical wet pipe installation is
shown and it can be seen that there are two sources
of supply, one from the town main (1) and the
other from a secondary source supply (2). Both are
fitted with stop valves and non-return valves to
ensure that water from either supply will not flow
into the other. These supplies unite in the main
supply pipe (3), which is fitted with a main stop
valve (4).
Above the main stop valve is an alarm valve (5)
from which a pipe is led off to the alarm motor and
gong (6). When the alarm valve functions some
water passes through the annular groove in the
alarm valve seating to the water turbine causing it
to rotate and the clapper to strike the gong.
Adjacent to the alarm valve there is a test and
drainpipe (7) and the discharge from the pipe is
controlled by a test and drain valve (8).
What are the three gauges on a wet system
(i) Gauge (9) showing the pressure in the installation
above the main stop and alarm valves.
(ii) Gauge (10) showing the pressure of the supply
below the main stop valves.
(iii) Gauge (11), which shows pressure in the
town main.
Describe a dry system
Dry pipe installations are installed where the temp
conditions are artificially maintained close to, or below freezing point - e.g. cold stores,or where the temperature is maintained above 70°C. The pipes are, at all times, kept charged with air under sufficient pressure to hold back the water supply. Only upright or dry pendant sprinklers are fitted in this type of system
What is the differential air valve /
A valve that uses air pressure to keep the water back in a dry system.
Describe a differential air valve and its operation
A differential air valve system (Figure 3.4) consists
of two valves, one large and one small. The
upper valve is eight times as large as the lower
valve and is held in position by air pressure and a water seal. In theory, the air pressure acting on the
upper valve is capable of holding back a water
pressure eight times as great, but in practice it is a
little less. The area between the two sections of the
valve is subject to atmospheric pressure.
When a sprinkler head opens, the compressed air
escapes reducing the pressure on the upper valve,
allowing the lower valve to open and water to
enter the system and emerge at the open sprinkler
head.
Ina dry system what is an accelerator
There is some delay before the water reaches
the sprinkler head because of the time required
to release sufficient air from the system to allow
the valve to open and water to enter and travel up
the pipe to the open sprinkler head.
A device, known as an accelerator, is therefore
normally fitted or a special type of valve is incorporated.
The function of both is to speed up the
entry of water into the system.
What is a disadvantage of a dry system and what can be done to mitigate it
There is some delay before the water reaches
the sprinkler head because of the time required
to release sufficient air from the system to allow
the valve to open and water to enter and travel up
the pipe to the open sprinkler head.
Accelerator fitted
Describe an alternate wet and dry system
This system is usually installed in premises that
are without adequate artificial heating and where
water in a wet system would be liable to freeze
during cold weather. The system usually operates
on a wet principle in the summer months and dry
principle in the winter. When functioning on the
wet system, the dry valve is either changed over or
placed out of commission and the system functions
as a wet system
describe a tail end system (dry pipe or alternate )
These systems are essentially similar to those previously
described, except that they are of comparatively
small extent and form extensions to
standard sprinkler installations.
Where are tail end systems ( dry pipe or alternate) permitted
(a) As extensions to a wet pipe system in comparatively
small areas
(i) where there is possible
frost danger in an otherwise adequately
heated building
(ii) in high temperature
areas or stoves. The tail-end would be on the
alternate wet and dry principle in the case of
(i) and on the dry pipe principle for (ii).
(b) As extensions to an alternate wet and dry
system in high temperature areas or stoves,
when tail-end systems would be on the dry
pipe principle.
Describe a pre action sprinkler system
A pre-action system is a combination of a standard
sprinkler system and an independent, approved
system of heat or smoke detectors installed in the
same area as the sprinklers. Heat and smoke detectors
will, generally, operate prior to sprinklers and
open a ‘pre-action valve’ to allow water to flow
into a ‘dry’ system before the first sprinkler operates.
What are the max sprinkler heads permitted in a pre action sprinkler system
The maximum number of sprinklers controlled by
a pre-action valve, whether it is in a heated or
unheated building, is 500 for low hazard and 1,000
for ordinary hazard and high hazard systems.
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