Water-Mist Fire Protection: a Guide for the Responsible Person
The recent rise in the interest and use of water mist fire suppression systems in the UK is due to the many advantages over other fire suppression systems.
The advantages of water mist include:
No chemicals used
No risk of asphyxiation as with gas fire suppression
Less damage to property than sprinklers and easier to clean up
So what is water mist?
A water mist system is a fixed fire protection system that discharges a fine spray of small water droplets. Water mist systems propel water through a specially designed nozzle, breaking the water up into hundreds of micro droplets which produce the mist.
In the event of a fire, the water droplets will the remove heat and displace oxygen from the affected area, resulting in the fire being extinguished. The mist lowers the temperature, lessens the radiated heat and reduces the oxygen concentration to the extent that combustion can no longer be maintained.
Sprinkler Systems Vs Water Mist Systems
So what are the differences between sprinkler systems and water mist suppression systems?
Water mist systems discharge less water than sprinkler systems.
Because less water is used, clean up and damage to property is less with water mist suppression
The average water mist droplet has a total surface area at least 100 times greater than the volume of water from a sprinkler system, meaning more effective suppression.
Due to the large volumes of water required with sprinklers then a large tank or even pumphouse as is required whereas cylinders or small standalone tanks can be used for water mist.
Water mist enables ‘smoke washing’, whereby the smoke particles attach to the droplets and fall to the floor, eliminating the spread of smoke throughout the building.
However, water mist suppression is more effective in compartmentalised situations and may not be suitable for larger areas.
Water mist droplets can be blown away under certain conditions such as air conditioning and open doors reducing effectiveness.
The major difference however, is that sprinkler systems have standards and approvals for both the components of the system and the design and installation. At the moment, water mist suppression systems don’t.
Currently there is no published British Standard for water mist systems or the components.
Existing water mist standards such as the National Fire Protection Association (NFPA) 750 Standard on Water Mist Fire Protection Systems or International Maritime Organisation (IMO) Standards are not directly applicable to UK land-based applications. A European water mist technical specification (CEN TS 14972) has been published but has not been adopted in the UK as its suitability for UK application is uncertain.
However, two new Drafts for Development have been issued:
DD 8489: Fixed fire protection systems – Industrial and commercial water mist systems
DD 8458: Fixed fire protection systems – Residential and domestic water mist systems
These haven’t been published as full British Standards as there is currently insufficient technical data to support them. These documents try to define the testing protocols for water mist systems in various different applications, but as yet they don’t cover all water mist scenarios. Tests for further applications should be developed going forward and published in additional sections.
Should you Choose Water Mist Fire Suppression?
In the absence of a generalised design and installation standard, the suitability of water mist fire suppression for your building depends on a number of factors such as room size, ventilation and insurer’s requirements.
Water mist systems should be considered on a case-by-case basis to determine whether a proposed system will meet the fire protection objectives, which need to be clearly stated by the supplier.