Pre-Action System

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 preaction 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.

the supervised system should record in inches of water rather than pounds per square inch. The second system is the unsupervised pre-action system. It has no means of continuous monitoring.

Combined System

A combined system (fig. 8-12) is a special purpose arrangement using two modified dry pipe valves connected to tripping devices and piped in parallel to supply water to the same sprinkler system. The piping net work is filled with air under pressure. When a fire is detected, an exhauster at the end of the system opens and releases the air within the system. The system then operates the same as a pre-action system. However, if the detection system fails, the combined system acts the same as a dry pipe system and allows water to be admitted to the system when the sprinklers open, discharging the air from the piping network.

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