Water mist fire extinguishing systems are fixed automatic fire protection systems. These include components for detection, activation, water supply and water atomization. The purpose of water mist systems is to generate, deliver and maintain an appropriate concentration (density) of water droplets sufficient to prevent from the relevant fire risk.
Water mist systems provide „three-dimensional” fire protection – an incomparably improved feature compared to traditional sprinkler technology.
Once the system is activated, the water mist removes heat and oxygen from the burning zone. The result is control, suppression and extinguishing of the fire. Lowering of the temperature can be observed in the area of water mist interaction with fire, reducing the radiated heat and reducing the oxygen concentration to such an extent that the further fire burning is no longer possible.
The phenomena underlying the operation of water mist systems can be defined as a „three-dimensional” protection against fire, as follows:
1) Fire extinguishing
A sharp decrease in the rate of heat release, which leads to the total elimination of any type of fire – flaming or smouldering.
2) Fire suppression
A small but stable and constant reduction of the rate of heat release, resulting in controlling the degree of combustion.
3) Fire control
Limiting the expansion of the fire and protecting the structure (by cooling of the objects, absorbing the exhaust gases during combustion and/or pre-wetting of the adjacent flammable materials).
Due to these unique characteristics of the technology, there are two types of fire protection strategies through water mist systems, which can be defined, as follows:
1) Water mist systems for local application or systems for protection of specific facilities or areas.
These systems are designed to directly affect the protected object or area. They can be installed in closed facilities, separate objects or specific areas in buildings or in open spaces. (A typical example are generators in power plants, machine spaces, fryers in public kitchens).
2) Water mist systems for general use or protection in the entire volume.
These systems are designed for total fire protection in enclosed spaces, usually pre-defined rooms or premises. (A typical example are production facilities in chemical plants, control rooms in power plants, server rooms, etc.).
Another categorization of water mist systems can be made based on the components used, as follows: Extinguishing agent: Drinking water, sea water, deionized water, water with antifreeze, water with additives, water with inert gas; Detection: Automatic nozzles with thermosensitive element, detectors for very early fire detection, actuators, control panels; Atomization: Nozzles and orifices with single and double fluid system for low, medium and high pressure depending on the various established criteria; Water supply: Wet pipe systems, dry pipe systems, pre-action systems, flooding systems, stainless steel or plastic pipes; Water delivery: Storage container (tank or bottles), pumps.
The design of water mist systems requires determination of flow rate, nozzle pressure, pipe size, area or volume covered by each nozzle, density of water mist re-
leased, number and type of nozzles and nozzle’s location at the specific sitе.
Depending on the pressure at which the systems operate, we can determine three main types of water mist systems:
- Low pressure (up to 12,5 bar)
- Medium pressure (from 12,5 to 35 bar)
- High pressure (over 35 up to 200 bar)
Each of these systems covers different fire hazards and each of them requires specific design and calculation criteria. Each of the three systems works with various specially designed nozzles that meet the relevant pressure criteria. By increasing pressure in the system, the size of the droplets decreases. The high pressure system produces much finer droplets of a size less than 50 microns and uses smaller pipe dimensions in comparison to low and medium pressure systems. Different configurations can be applied to each of the three categories of water mist systems depending on the fire risk covered:
- Automatic water mist systems
These systems use nozzles with a thermal element that prevents water release prior the activation of the system. The nozzles are mounted onto a pipe network connected to a pressurized water supply. When exposed to a certain high temperature, resulting in a fire, for a certain period of time, the thermal element is activated and allows the water to flow through the nozzle. The nozzles may be mounted onto a „wet pipe” or „dry pipe” type of system. In one case, the nozzles are mounted onto a pipe network containing water, and in the other case – to a pipe network filled with compressed air. The „dry pipe” system can be pre-action and, in this case, an additional fire detection system detecting smoke or flame is installed. Activation of the detection system will allow the water to fill the pipe network. The water is discharged only through the nozzles with activated thermosensitive element.
- Deluge water mist systems
These systems use open discharge nozzles (without a thermal element) mounted onto a pipe network that is connected via a valve to a pressurized water supply. The valve is controlled mechanically or by an automatic fire detection system. Once the detection system is activated, it sends an electronic signal to the control panel, which in turn sends a signal to open the discharge valve. Once the valve is open, water enters the system and provides water to all nozzles connected to the valve. This protects the area and is most commonly used in enclosed spaces with machines, turbines and other equipment. The name of this type of systems is systems with general or „volume application”. When the room has such a volume that does not allow a complete solution, a system with „local application” is used.
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