Wet sprinkler systems are the most common type of sprinkler system installed. A wet pipe system has water in the pipes in the ambient or normal condition and has heat responsive elements on all sprinklers. Thus, water is instantaneously discharged from a sprinkler when it actuates.
A dry sprjnkler system is intended to be used In areas where low temperatures could cause a wet pipe system to freeze. Dry pipe systems are pressurized with air in the ambient condition and experience an inherent delay in the discharge of water to allow the pressurized air in the system to escape. When a sprinkler actuates, air is released through the sprinkler, allowing water to flow into the piping system through the dry pipe valve. NFPA 13 mandates that the time for the water to reach the most remote sprinkler be no longer than 60 seconds. This time delay allows the fire to grow larger than it would with a wet pipe system of similar design, and the larger fire size results in more sprinklers in the fire area actuating.
Pre-Action and Deluge
Pre-action and deluge systems require fire detectors (smoke, heat, etc.) to actuate.
A deluge system uses open sprinklers or nozzles to allow flow water to be discharged when the deluge valve actuates. Deluge systems can be used for occupancies where the hazard is considered severe, such as with flammable liquid hazards where the fire could spread over a large floor area.
Pre-action systems have closed heads and pipes filled with pressurized air that supervise a piping system, and can be considered for the protection of valuable assets or irreplaceable property. The detection system for a pre-action system can be designed to prevent water discharge in cases of a false alarm from the detection system, or in case of a sprinkler whose element has encountered mechanical damage.
The detection system on a pre-action system can be designed with a pre-action logic capable of meeting one of the following objectives:
- Actuation of a fire detector trips a deluge valve to admit water into the sprinkler piping to await the actuation of a sprinkler.
- Actuation of a fire detector or actuation of a heat-responsive element on a sprinkler trips a deluge valve to allow water into the sprinkler piping.
- Actuation of a fire detector and actuation of a heat-responsive element on a sprinkler trips a deluge valve to allow water into the sprinkler piping
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