A water spray system is a fixed-piping system connected to a reliable source of firewater. Such a system is hydraulically designed with water spray nozzles to achieve specific water discharge and distribution on the surface or area to be covered. The piping system is connected to the water supply through a manually or automatically actuated valve that initiates the flow of water. An automatic valve is actuated by a detection system installed in the same area as the water spray nozzles. NFPA 15 provides detailed descriptions of water spray systems.
Fixed water spray systems are designed to provide fire exposure protection, control of burning, extinguishment, or egress protection. They can be independent of other forms of protection, or they may supplement them. Gas fires should be extinguished by isolation. Water spray systems are neither intended nor suitable for extinguishment of pressurized jet fires.
The selection of specific spray involves consideration of the following factors:
Nozzles: Nozzles should be of a type that has been tested and listed for use in water spray systems
Piping and fittings: pipe fittings, hangers and other pipe support methods should be as recommended by NFPA 15
System actuation Valves: Each water spray system should be provided with a spray system actuation valve, which controls the flow of water to the spray nozzles
Strainers: Strainers shall be provided for any system or group of systems that uses nozzles with waterways less than 3/8-in
Pressure gauges: Pressure gauges are generally provided to indicate the status of the system, and are utilized to monitor functionality during system operation and testing
Alarm Control and Detection: Water spray systems (manual or automatic) may be provided with an audible alarm at the protected premises to indicate that the system has been actuated
Design Objectives
Three specific objectives of water spray systems design can include
- Exposure protection
- Controlof burning
- Extinguishing
Exposure Protection
The most common objective of a water spray system is exposure protection; protecting equipment and structures from heat stress caused by exposure to radiant and convective heat and preventing ignition of combustible components. One factor to consider when designing a system for exposure protection is the allowable temperature a structure could safely sustain before significant damage or failure occurs. The purpose of these systems is to absorb heat and reduce temperatures. A continuous water film from sprays will theoretically limit the surface temperature to the boiling point of water, 212 °F (100 °C). Much heat can be absorbed, and damage can be reduced through exposure protection; however, systems designed for exposure control are not intended to extinguish fires or protect against direct jet fire impingement.
Control of Burning
Water spray systems can be used to control the rate of burning. This is achieved by applying water to the flame, or burning surface, to absorb heat near its source, reduce vapor generation and flame intensity, and limit the amount of heat released to expose the surrounding environment while the fuel is consumed.
Extinguishing
The physical properties of the fuel involved in a fire will determine whether extinguishment by water spray is possible. In some scenarios, extinguishment can be accomplished by surface cooling, emulsification, or dilution (all of which reduce vapor release) or smothering by the steam produced. Extinguishment by water spray is generally most effective where the fuel is a combustible solid, water-soluble liquid, or high flash point liquid. However, the risks associated with extinguishing certain fires should be carefully evaluated. If significant quantities of flammable gases or vapours are released, a more hazardous condition with potential for explosive re-ignition can be created by extinguishing such fires instead of allowing them to burn at a controlled rate with appropriate surveillance and protection of surrounding equipment.
