What is a water mist fire suppression system?
The water mist system is a one-of-a-kind fire extinguisher. An extremely fine mist is formed by pumping water at high pressure via nozzles. A pump unit is used to provide water. All water mist applications can be supplied by the pump unit.
Why is water mist used as a fire extinguisher?
Because of its non-toxic nature and great effectiveness in the suppression of a wide range of fires, water mist fire suppression technology is gaining popularity in the field of fire protection services such as fire safety for buildings, ships, spacecraft, libraries, and museums.
Although halogen-based chemicals are good fire suppressants, they have been restricted from further manufacturing since the last century due to their negative impacts on the ozone layer. There has been a significantly increased interest in the use of CO2, inert gas, and water sprinkler as alternatives to discover a suitable replacement. Because of several limitations, the mission remains challenging to attain.
For instance, to extinguish the fire, CO2 will displace oxygen in the air, posing a greater risk to human health. As a result, fire suppression technology has become a hot topic in the field of fire protection. Water mist fire suppression technology has been developed in recent years and is being regarded as a promising replacement agent due to the speed with which it suppresses flames while requiring less water and creating no environmental harm. The limitations of traditional sprinklers and inert gases pushed the creation of today's water mist fire suppression technology.
Since 1990, research into water mist technology has advanced by leaps and bounds, making water mist the most acceptable alternative solution for a wide range of applications.
What is the Mechanism of water mist suppression?
Water mist systems are excellent firefighting systems that deliver a fine water spray for controlling, suppressing, or extinguishing a wide range of fires. Water mist systems, unlike traditional sprinkler systems, utilize less water, which is ideal in regions that are prone to water damage.
Water mist is used to extinguish the fire in a variety of ways. In enclosed locations, the atomized water droplets, coupled with ambient air, are rapidly drawn into the fire's base. By displacing the oxygen required for burning, the water swiftly evaporates into steam. When properly constructed, this motion makes water mist extremely effective, especially when combined with the cooling and air scrubbing effects of the system. We must keep in mind that the water mist system is less sensitive to enclosure openings when designing it. As a result, a finer water droplet is necessary for an effective system that blocks radiant heat and prevents the ignition of nearby combustible materials.
1. Operating Pressure
The three pressure distributions of the system pipes are described by the NFPA 750 water mist standards: low pressure, intermediate pressure, and high pressure.
Water mist system with low pressure
The Water Mist system uses a low pressure of less than or equal to 12.1 bar (175 psi) for standard pipes, as well as the connection to the water plant and air supply.
It consumes less water per discharge than traditional sprinklers, allowing for infinite protection timing and eliminating the need for water tanks.
As a result, the cleaning time after discharge is small, reducing the impact of damages and losses.
Advantages- their installation and maintenance cost are less, minimal risk of clogging, possess low power consumption requirements and consequently saves generator cost.
Water mist system with Intermediate pressure
These can operate at a pressure greater than 12.1 bar (175 psi) but less than 34.5 bar (500 psi)
The intermediate pressure water systems' limited water supply is usually limited to a risk volume of 1000 cubic meters.
Machinery spaces, hydraulic pump rooms, turbine enclosures, and other areas are some of the places where this type of water mist system is used.
Water mist system with High pressure
These can operate at 34.5 bar (500 psi) or higher. At roughly 120 bar pressure, a high-pressure water mist frequently produces a thin water mist with visible nozzles.
Water supplied by this high-pressure pump or nitrogen cylinders (200 bar) is either connected to a continuous water tank or powered by several individual water containers.
Turbines, volatile liquids, archives and museums, clean rooms, and others are applications for high-pressure fine spray systems The nozzle type and droplet distribution are determined separately for each application, which also determines the system's efficacy and optimal protection.
2. Components of water mist system
Nozzles - are classified as open (deluge) or closed (sprinkler) in the same way that sprinklers are. Depending on the parameters of the room, each nozzle consumes between 2.8 and 25 litres (0.74 and 6.6 gallons) of water per minute. When compared to a traditional sprinkler that produces 80-250 litres (21-66 gallons) per minute, this is a significant reduction.
Piping system - This system comprises a stainless-steel main pipe that is positioned in the centre of a frost-free environment. Depending on the scenario, different pipes often lead to each designated fire section, with a shut-off valve at the front end of each pipe.
Automizing medium - In twin fluid systems, an atomizing medium made of compressed air or nitrogen is frequently used to distribute the water supply.
Pump - To activate the system, the high-pressure pump is triggered. The centrifugal or positive displacement pump also referred to electric motor-driven pump is used to convey the required water flow and pressure in the fluid systems
Water supply - The water supply can either be public or private supply. The dedicated water tank, which must be dependable and similar to potable water or natural saltwater.
3. Working principle
The water mist fire suppression procedure has three stages of operation: cooling, oxygen depletion, and finally separation and shielding effect.
When the suppression water system is broken into droplets during the cooling process, a surface reaction is triggered, which absorbs the heat from the fire.
When the mean droplet size is small, and the smaller droplets descend more slowly than the bigger droplets, the cooling action is considered to be efficient.
This means that the air and gases around the fire were cooled rather than the fire load itself. As a result, water mist systems are referred to as the heat absorption medium with the maximum capacity.
Oxygen depletion process:
Evaporation increases the water level by 1640 times during the oxygen depletion process stage, resulting in a loss of oxygen in the air at the fire source.
As a result of the lack of sufficient oxygen essential for combustion, the fire is suffocated. This procedure occurs in the very centre, where the temperature of the vehement flame is quite high, allowing the occupants to flee through other exits.
The separation effect occurs when water droplets between the flame and the fuel surface diminish thermal radiation by the fuel surface, causing heat to be effectively reflected.
At the same time, the burning rate is drastically lowered, and the thermal radiation induced by any nearby fire loads is lowered, reducing the likelihood of flame spread owing to this phenomenon.
The shielding effect will help to prevent the fire from spreading by lowering thermal radiation from nearby objects, safeguarding inhabitants by escaping the fire.
4. Performance objective
Fire control- Spraying water on the fire to limit the heat circulation rate and pre-wet the flammable items nearby, as well as ensuring that the temperatures of the ceiling gas are managed to avoid damaging the structure.
Fire suppression- Is a sudden reduction in the rate of heat emission and the prevention from regrouping.
Fire extinguishment- refers to fighting a fire completely until there are no combustibles left to burn.
What are the applications of water mist system as fire extinguisher?
With their cooling mechanism and lack of toxicity to the environment, they have proved their ability to effectively extinguish Class A, B, C, E, and F fires.
As a result, they've found employment in a variety of settings, including gas turbine enclosures, ships, houses, public spaces, pump rooms, electrical/electronic rooms, heritage structures, machinery protection, and so on.
The spray properties, the configuration and size, the piping system, and the operating pressures all have a role in their effectiveness.
Over the last few decades, remarkable progress has been achieved in enhancing the effectiveness of water mist.
New approaches such as combining entire flooding and local applications, cyclic water mist discharge, hybrid water mist, and intelligent water mist, which combines zonal water mist with intelligent detection, have been developed.
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