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What are the different types of watermist system operation?

Wet pipe system (automatic nozzles)

This water mist system uses automatic, heat-sensitive, nozzles fitted into distribution pipework that is permanently pressurised with water. 

Wet pipe systems are the most common water-based suppression configuration and are typically used to protect areas where temperatures are above 4oC and below 95oC or are unlikely to fall below freezing point, for example: machinery spaces or in rooms, corridors and spaces where light-to-moderate fire loads of ordinary combustibles exist, such as offices and residential.  

Dry pipe system (automatic nozzles)

This water mist system uses automatic nozzles fitted into distribution pipework that are permanently pressurised with air, nitrogen or other inert gas. In the event of a fire, when a nozzle operates, the pressure drop within the distribution pipework activates the system's control valve to release water into the pipework where it is discharged through the nozzle.

These systems are typically used in the same applications as wet pipe systems where the key constraint is the need to avoid freezing of water in the pipes. There is naturally a delay in the flow of water because of the time it takes to fill the distribution network with water, which increases significantly with the size of the area being protected. Because of this delay, such systems should not normally be considered for applications where there is a need for the protection of life. 

Deluge system (open nozzle)

A deluge system is designed to bring several open nozzles into action simultaneously in the event of a fire. This is achieved when a separate fire detection system is actuated. This in turn operates the main control unit to release the water through all the open water mist nozzles with an open path to the pump to effect rapid control and/or extinguishment of the fire.  Fire detection systems can be electrical, electronic or pneumatic and should comply with BS EN BS 7273-3, Code of Practice for the operation of fire protection measures. Electrical actuation of pre-action water mist and sprinkler systems or BS 7273-5, Code of practice for the operation of fire protection measures. Electrical actuation of water mist systems (except pre-action systems)  
Because the nozzles are open, the water distribution system is kept dry and unpressurised (no leaks, freezing risk) but as a result, there is also a delay while the water is being driven through the distribution system. However, in many cases, such as in local applications, the activation of the fire detection system (where flame, IR or UV detectors can be employed) is so much more sensitive than the frangible bulb (which has thermal inertia) that the overall time for discharge may be greater or smaller than from an automatic nozzle setup.  This depends on the type of detection, the length of the distribution piping and the distance of the operating nozzle from the water supply.  However such a configuration brings some certainty in the calculation of water supply demand as the number of nozzles that will be discharging is known,

A deluge system may also have sections where only the nozzles in that zone where the fire was detected activate. This allows for several areas to be protected but permits a discharge in only one area (zone).

These systems are most frequently used in high fire load applications, such as within recycling plants, where a delay in water flow through progressive nozzle activations could result in too little water discharge, too late, to successfully control the fire.  

Pre-action system

Such systems comprise a water mist installation plus an independent system of heat or smoke detectors installed in the same areas as the automatic nozzles. On receipt of a signal from two or more detectors, the main control panel automatically opens the control valves, allowing water to flow into the distribution pipework in readiness for the first water mist nozzle to operate due to the breakage of the frangible bulb.  

These systems are commonly used for the protection of high-value, ‘mission-critical’ areas such as data centres, server rooms or communications switches or where sensitive electrical equipment and goods are stored. They provide prior warning of system discharge (through the sounding of the alarm system) and prevent system discharge caused by accidental damage to a water mist nozzle’s thermal element or system pipework (by having no pressurised water in the distribution system).  

Electronically Controlled

An electronically controlled system consists of a nozzle which incorporates a valve which is commanded by a controller to discharge or shut off flow.  This allows for a pre-determined number of nozzles to operate. The operation will depend on the determination of the fire location by the controller.  This may be via the use of a detection system or other sensing method.  There are also domestic systems which utilise this principle to activate sooner, and more accurately targeted than an automatic nozzle, thus requiring less water flow to fight the fire.

Read more - What are electronically controlled water mist nozzles for fire protection?