Fine Water Spray / Water Mist Systems
NFPA 750:2000 classifies 3 types of protection for fine water spray systems:
- Fire Control
- Fire Suppression
- Fire Extinguisent
If a manufacturer of a fire suppression system claim the product is a fire extinguisher in order to reduce the period of discharge, conclusive testing information should be produced by the manufacturer to confirm the viability of the proposals.
In order to comply with the requirements of current standards, consideration must be given to the period of the discharge which may be 30 minutes or more.
Pneumatic powered systems are unlikely to be capable of lasting an adequate time and providing continuous pressure, in which case pump power is more appropriate.
There are two basic categories of fine water spray systems: Single fluid systems and dual systems.
Single fluid systems can use either water stored at pressures varying from 12.1 bar or less to 34.5 bar or greater, or by the use of a pump. These systems discharge through nozzles that produce droplets from small than 400 microns up 1000 microns.
Dual Systems incorporate a gas such as air or nitrogen to atomize the water at the point of discharge.
Both systems are not particularly good at penetrating directly into shielded area as would a gas system, so careful thought needs to be given to the selection and location of the discharge nozzles. Therefore the design and engineering aspects of these systems are complicated in comparison to many other systems and each case must be individually considered.
As these systems are capable of using de-ionized water it is feasible to use the medium on live electrical equipment. However, as with foam systems, these systems should not be used on fires where there is a likelihood of a violent reaction with water.
One of the main benefits of fine water spray/water mist systems is the greatly reduced quantity of water used to extinguish a fire, resulting in less potential water damage and lower environmental issues caused by water run off.
Typical applications for fine water spray/water mist systems are:
- Transformer and switchgear rooms
- Flammable liquid hazards
- Shipboard accommodation, storage machinery spaces
- Combustion Turbine enclosures
- Rotating electrical equipment
- Cable tunnels and voids
