WHAT IS WATER MIST?
The water-mist system is defined as: ‘distribution system of a fire-fighting system connected to a water supply equipped with one or more water mist nozzles capable of delivering water mist intended to the control, suppression or extinguish fires’ (definition from standard UNI EN 14972-1:2021)
A BIT OF HISTORY
Water-mist systems were introduced in the 1940s and were initially used to protect specific environments inside passenger ships.
In the 1990s, they began to spread on the market for other types of applications as well.
Renewed interest in such systems arose following the Montreal Protocol (signed in 1987 and entered into force in 1989) and thus the banning of halon, as water-mist systems were identified as safe and suitable for many types of spaces even where the quantities of water to be discharged and stored are limited.
HOW THE WATER MIST WORKS
Water-mist systems use water as a means of extinguishing, suppressing or controlling fire more efficiently than conventional systems: a small amount of water is ‘sprayed’ onto the fire to create a real mist.
The water droplets released have an average diameter in the order of microns, compared to the average diameter in the order of millimetres of normal sprinkler systems.
The small size of the individual droplet allows the mist to be transformed very quickly into water vapour.
The operation of water mist systems is based on 3 principles:
COOLING
Strong and immediate removal of heat from the fire bed.
- The jet of the small water droplets produced absorbs considerable amounts of heat from the fire
- Water absorbs more heat to become vapour than other extinguishing agent (> 2MJ/kg)
- Small droplets have a high S/V (surface area/volume) ratio due to the large surface area of heat exposure, which produces rapid heat absorption and a sudden change of state
OXYGEN SEPARATION
Change of state and immediate increase in volume of the individual droplet, resulting in a rarefaction of oxygen molecules per cubic centimetre
- Smaller droplets vaporise faster than larger ones
- Water droplets turn into vapour and expand about 1760 times in the change of state.
- The vaporisation pulls the oxygen molecules away from the base of the fire, thus effectively smothering the fire.
RADIANT HEAT ATTENUATION
Creating a barrier to heat radiation to the outside of the fire.
- The high density of the fog, due to the large number of droplets present, creates a real barrier that prevents radiant heat from propagating
- Blocking and absorbing radiant heat prevents the spread of fire
- It improves the possibility of rapid approach to the fire by emergency crews
EXAMPLE EXTINGUISHING TESTS AT THE PETRUZZELLI THEATRE OF TEMA SISTEMI’S AQUATECH® WATER MIST SYSTEM
FIRE SCENARIO
WOODEN CRIB CONSISTING OF ALTERNATING LAYERS WITH A SQUARE CROSS-SECTION SUPPORTED BY A GRID UNDER WHICH A TANK CONTAINING FUEL WAS PLACED.
FIRE LOAD
36KG WOOD/MQ
STAGES OF THE FIRE AFTER THE SYSTEM WAS TRIGGERED
TYPES OF WATER MIST SYSTEMS
Low pressure systems p ≤ 12.1 bar
Medium oressure systems 12.1 < p < 34.5 bar
High pressure systems p ≥ 34.5 bar
REFERENCE STANDARDS IN EUROPE
UNI EN 14972-1:2021
On 23 December 2020, the European standard on water mist systems was published (EN 14972-1:2020), which replaces the European technical specification in force until then (CEN/TS 14972:2011). UNI has fully implemented this European standard and from 25 February 2021 it became part of the Italian standard body (UNI EN 14972-1:2021).
The standard is structured in 17 parts, of which part 1 specifies the requirements and provides information on the design, installation, controls and maintenance of all types of water mist systems (cylinder or with pumping unit, with automatic or open nozzles).
Parts 2 to 17 represent the fire test protocols for which the standard applies.
SYSTEM COMPONENTS
Water mist nozzles are the heart of the system; they are able to generate a mist of water that does not damage furniture and equipment.
The nozzles can be:
- Non-automatic (open type) -> contain open holes and an independent detection system activates the flow of water into the nozzles
- Automatic (with temperature-sensitive bulb) -> operate independently of other nozzles, are triggered by detection/activation devices (temperature-sensitive bulbs)
- Hybrid -> are capable of operating with both automatic and non-automatic devices. The activation of a water spray nozzle can also take place via a detection and activation device.
Valves, filters, piping, fittings and brackets. Components to be installed in the system in accordance with current regulations; they must guarantee durability, resistance to high operating pressures (up to 200 bar) and meet the required standards;
Valves
Filter
Piping, fittings and brackets
Two types of pressurisation systems are used:
- High-pressure piston pumps
2. Batteries of water cylinders pressurised with nitrogen;
APPLICATIONS
Carefully designed Water Mist systems are effective on all fire classes except Class D fires.
Here are the main areas of use
Civil & Cultural Heritage: Hotels, Shopping Centres, Restaurants & Bars, Hospitals, Offices, Churches & Cathedrals, Theatres & Auditoriums, Libraries & Archives, Warehouses & Depots, Car Silos, Saunas & Hospitality Facilities, Kitchens, Offices, Rooms, Cellars, Data Centres & Technical Rooms
Oil & Gas and Energy: Steam and gas turbines, Diesel generators, Hydraulic systems, Transformers, Cable tunnels, Electrical rooms, Transportation systems, Offices, Data centres
Industry: Onshore and Offshore oil environments, Petrochemical environments, Turbines, Transformers and electrical substations, Telecommunication, IT, Electrical rooms, Machine rooms, Industrial spaces, Production areas, Petroleum product tanks
Naval and Offshore: Engine Room, Accommodation, Ro-Ro Deck, Kitchens, Balconies, PSV Fire Protection
Transport and Infrastructure: Tunnels/Tunnels, Hangars, Stations and Subways, Airports
Luxury and Valuable Environments: Buildings of Historical and Cultural Value, Churches and Cathedrals, Museums and Art Galleries, Shopping Malls, Hotels and Resorts, Boutiques and Showrooms, Yachts and Cruise Ships, Historical Palaces and Tourist Sites