Water mist systems

Water mist systems are intended for rapid suppression of fires using water discharged into completely enclosed limited volume spaces. Water mist systems are desirable for spaces where the amount of water that can be stored or that can be discharged is limited. In addition, their application and effectiveness for flammable liquid storage facilities and electrical equipment spaces continues to be investigated with optimistic results. Water mist systems are also used for gas turbine enclosure protection. 

Properly designed water mist systems can be effective on both liquid fuel (Class B) and solid fuel (Class A) fires. Research indicates that fine (i.e., smaller than 400 microns) droplets are essential for extinguishing Class B fires, although 

A significant challenge with using mist systems is determining whether the conditions of a particular manufacturer s limitation are representative of the actual conditions in which the system will be used. An engineering analysis should be performed to evaluate the significance of any variations between the actual fire hazard and the known performance capabilities of the water mist system. It is important that the test protocol contain these factors  

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A water mist system is a proprietary fire protection system using very fine water sprays. The very small water droplets allow the water mist to control or extinguish fires by cooling of the flame and fire plume, oxygen displacement by water vapor, and radiant heat attenuation. These systems are single shot systems. A water mist system and nozzles are shown in Figure 7-28. 

There has been a reluctance to provide water spray on compressors or turbines that typically tend to run "hot," with respect to the risk that a sudden deluge of cold water will result in severe thermal shock and cause cracking of the compressor case. However, studies indicate that this hypothesis is not valid (EPRI, 1985). These studies support the use of water spray protection where the fire hazard so warrants. Water mist systems are now being approved for gas turbines and compressors. 

Water-mist systems (with droplet sizes less than 100 pm) have been considered as replacements for Halon 1301. Phase Doppler interferometry has been used to characterize various surrogate fluids in misted liquid sprays that are generated by different screens [7, 28, 29]. The effect of mist droplet diameter (< 200 pm), density, and injection angle have shown that when the mist and flame are aligned spatially, flame suppression is enhanced with increased density, decreased velocity, and smaller size droplets [30]. Larger droplets and injection angles become important for asymmetric mist/flame alignment. Efforts to decouple the physical (suppression due to droplet size and dynamics), thermal (due to heat capacity and... 

The fire safety of equipment enclosed in cabinets could be controlled by consideration of the components and cables inside or by designing the cabinet in an appropriate way. In some cases active measures like detectors or water mist systems might be included. 

A development to replace halon systems has been water mist systems in which a system of open discharge heads is used. The system is actuated by smoke detectors located witiiin the area to be protected. Operation of one detector raises the alarm and it requires a second detector to be actuated before the water is released into the discharge pipework. Water for the system is stored in cylinders at high pressure and the discharge heads are designed to produce a fine water mist. Water mist systems use significantly less water than a sprinkler system and extinguish fires by ... 

Fiooding and inerting systems >- Water mist systems. 

Fixed water mist systems have begun to be introduced in recent years. They can quickly extinguish large fires in enclosed spaces because the heat that is generated by the fire rapidly vaporises the water mist spray droplets into steam. However, small fires that have not yet heated an enclosure or are in large open areas cannot be easily extinguished by water mist sprays unless the fire is literally within the range of the spray. 

Figure 9.67 High pressure water mist system in operation...

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