How closed head water mist systems work

A closed head (bulb) water mist system is a fixed fire protection system that uses fine water droplets to control, suppress or extinguish a fire. The system consists of automatic nozzles attached to a hydraulically calculated piping system containing water and connected to a water supply.  

Water mist nozzles release a very fine spray containing small water droplets that fill the compartment with a mist. An effective water mist system should generate, distribute, and maintain a concentration of small water droplets designed specifically for the protection of the fire risk for the required duration for the standard adopted. The smaller the droplet size, the more efficient the use of water. 

Fundamental to achieving this is:  

  • Maintaining the correct pressure/flow characteristics at the nozzle for sufficient time
  • Careful placement of the nozzles to ensure full coverage of the space and minimal water wastage due to collisions of the spray with solid surfaces
  • Due diligence on selecting the correct water mist system to meet the requirements of the fire hazard and the correct compartment conditions to allow the system to operate effectively

Why is water mist so effective?  

Water mist systems are very effective at extinguishing fires as they provide thermodynamic cooling through evaporation and oxygen reduction through steam displacement. However, like traditional sprinkler systems, they must be designed to an applicable standard and within the correct environment.

The design of each water mist system is almost entirely unique to the manufacturer. Each water mist system should be tested to recognised and established international test protocols for conformity prior to being specified and installed.  

Advantages of a water mist system: 

  • Very effective fire suppression in enclosed environments 
  • Excellent environmental cooling properties 
  • Stand alone local application systems 
  • Design and installation based on fire test scenarios 
  • Reduced size water supplies (water storage tank capacity) 
  • Gas cooling properties 
  • Liquid fuel fires 
  • High heat output fuels
  • Potential to minimise water damage 

Disadvantages of a water mist system: 

  • Ventilated spaces 
  • Ceiling height limit 
  • Small fires 
  • No design solutions for common issues in commercial premises 
  • Ductwork and cable tray 
  • Deep beams and bay construction 
  • Open areas between floors 
  • No third-party approval scheme for designers or components 
  • Little benefit once water exhausted 
  • Deep seated fires
  • Class A fuels 

Reasons for considering water mist systems:  

  • Life safety benefit additional to the current provisions of the Building Regulations  
  • Life safety compensatory feature to meet current provisions of the Building Regulations  
  • Spatial limitations and retro fitting benefits 

Testing and British Standards 

Water mist systems must be third party tested to be considered credible. Testing conducted by a UKAS accredited laboratory provides evidence of the effectiveness of your system, via an industry method. If you are looking to build trust with your customers and demonstrate they are investing in a reliable system, third party testing is the only way to achieve this. 

British Standard 8458 (BS:8458) is the current standard for compliance domestic and residential water mist fire suppression systems. Fire suppression systems are requested in most cases to satisfy building control regulations. Meeting the regulations requirements gives the project a better opportunity for full approval and compliant certification.

Where the fire suppression system does not comply with the standard, BS:8458, this would render approval and certification unobtainable. More importantly, the system may fail to protect your property when it is most needed.

The Fire Protection Association offer ‘UKAS’ approved testing of residential and domestic water mist systems to BS 8458.