valve (fig. 8-10). The water deluge valve remains
closed until a fire is detected by a heat-actuated
device that in turn causes the valve to open. Heat-
actuated devices (H.A.D.) can be either mechanical
or electrical in operation. They are discussed in
further detail later in this chapter.
The deluge system has a time delay between
detection of a fire and the discharge of water at the
sprinkler heads. This delay is due to the time
required to operate the valve and fill the piping
network with water, similar to the dry pipe system.
To reduce the delay, the deluge system may be pre-
primed by filling the piping network with water
downstream from the deluge valve. To prevent
water from escaping from the sprinklers, pre-prime
plugs (fig. 8-11) are placed on the sprinklers. These
plugs blow out of the sprinklers at approximately
20 psi water pressure.
A pre-action system differs from a deluge
system only in that it has normally closed
automatic sprinklers. When the fire detecting
device is actuated, the water control valve opens
and admits water into the piping system. The
system then acts the same as a wet pipe system.
Individual sprinklers are opened by the heat of the
fire. The advantage of the pre-action system is that
the probability of inadvertent water discharge is
minimized because operation of both the detection
system and automatic sprinklers is necessary for
discharge of extinguishing water.
It is incorrect to refer to pre-action systems as
dry pipe sprinkler systems. It is true that the pre-
action system piping does not contain water.
However, the term dry pipe system refers to the
type of sprinkler system and the type of water
control valve that operates the system.
There are two types of pre-action systems.
First is the supervised system, which has air
introduced into the system piping at a pressure of
approximately 5 psi. This air pressure supervises
the piping to detect leaks. The pressure switches
used for detection of low air pressure on
Figure 8-11.Sprinkler pre-prime plugs.
Figure 8-10.Deluge valve.