Water mist breaks water into very small droplets, creating a large evaporating surface and aiming for a strong cooling effect with little water. Low water use, limited water damage and compact pipework are attractive; yet the strength and the weakness of water mist are in the same place: its behaviour is tightly tied to the scenario in which it was tested. FM Global DS 4-2 Water Mist Systems sets out where FM accepts water mist and which limits the designer must not step outside.
Scope and Definitions
The data sheet covers the use, design limits, installation and maintenance of fixed water mist systems. Water mist is usually defined by droplet size; NFPA 750 takes as its definition a spray in which 99 per cent of the water volume, at the minimum operating pressure, is in droplets smaller than 1000 microns. Systems are also grouped by operating pressure:
| Class (NFPA 750) | Operating pressure | Typical feature |
|---|---|---|
| Low pressure | 12.1 bar (175 psi) or less | Hardware close to standard sprinkler pipework |
| Intermediate pressure | 12.1–34.5 bar (175–500 psi) | Finer droplets, dedicated pump |
| High pressure | 34.5 bar (500 psi) or more | Stainless pipe, positive displacement pump or gas-driven cylinders |
By application there are three basic types: total compartment systems protecting a whole enclosed volume, local application systems aimed at a specific item of equipment, and area protection systems using automatic nozzles in a sprinkler-like arrangement.
How It Extinguishes
Water mist works through three effects: rapid evaporation cools the flame and the hot gases, the resulting steam locally dilutes oxygen in the fire zone, and the droplet cloud attenuates radiant heat. These effects are strongest in small enclosures and with larger fires. A small fire produces little steam, and in an open, ventilated space steam and droplets disperse easily. That is why mist performance cannot be predicted by intuition; it has to be demonstrated by test.
Key Loss-Prevention Principles
Stay inside the approval test scenario
For FM, a water mist system must be FM Approved and used only for the application defined in its approval. The approval rests on fire tests under FM Approval Standard 5560. The enclosure volume tested, ceiling height, ventilation openings, nozzle spacing, fuel type and fire size are the limits of validity. A larger enclosure, a more open space or a different fuel falls outside the approval even if it looks "similar".
Enclosure integrity and interlocks
For total compartment systems the state of doors, louvres and ventilation is critical. Where ventilation must stop and openings must close on discharge, those interlocks are designed and tested as part of the system. Shutting off the fuel supply prevents re-ignition, particularly in oil systems.
Water quality and pipe material
Small nozzle orifices are sensitive to blockage. Stainless steel pipe, filtration and a clean water source are therefore the basis of reliability. The water quantity and discharge duration the system needs are stated in the approved design manual, and the supply is sized accordingly.
Relationship with sprinklers
In some approved scenarios water mist can replace sprinklers, but only where the approval explicitly covers that occupancy and ceiling height. In storage areas in particular, water mist does not generally take the place of the sprinkler criteria. DS 4-2 and the occupancy data sheet for the area should be read together.
Questions to Ask When Reviewing a Proposal
Water mist proposals usually lead with "less water, less damage, easier installation". When comparing offers, the following questions quickly show whether the proposal really fits the project:
- Which application and which fire test protocol is the system approved for? What volume and ceiling height limits does the approval state?
- Are the project's volume, openings and ventilation inside those limits, or does it rely on "engineering judgement"?
- What are the discharge duration and water quantity, and how are supply and pump redundancy provided?
- How does the system behave on loss of power; is the pump fed from a fire-rated supply?
- What is the maintenance interval for nozzles and filters, and are spare parts available locally?
- If omitting sprinklers is proposed, is there written insurer acceptance?
If any of these cannot be answered clearly, the economic advantage of water mist can evaporate over the life of the project.
How It Differs from NFPA 750 and EN 14972
NFPA 750 and the European EN 14972 series also accept that water mist is test-based; the EN 14972 series sets out fire test protocols for different applications in separate parts. The FM difference is that it treats its own approval tests and its own list of approved products as the only valid reference. A system approved to another body's test protocol may not be accepted automatically at an FM-insured site. When water mist is offered on a project, the first question should therefore be "approved to which protocol, for which application?"
Caution: water mist systems are manufacturer-specific. Nozzles, pumps, valves and pipe are approved as a package; using one manufacturer's nozzles with another's pump unit invalidates the approval. Spare parts and service continuity should be secured in the contract for the same reason.
Practical Checklist
- The system is FM Approved for the protected application and the approval scope (volume, height, fuel) is documented.
- The enclosure size, openings and ventilation are within the approval limits.
- Ventilation shutdown and fuel isolation interlocks are designed and tested.
- Pipe material, filters and water quality meet the manufacturer's requirements.
- Water supply and discharge duration are sized to the approved manual.
- Whether sprinklers are omitted has been decided together with the occupancy data sheet.
- Maintenance is carried out by manufacturer-trained personnel to the manufacturer's schedule.
- Spare parts and service continuity are secured by contract.
Frequently Asked Questions
Can water mist replace sprinklers?
Only where the approval explicitly covers the occupancy and the geometry. In high fire load areas such as storage, water mist generally does not replace the sprinkler criteria.
Why can it not be used in a 'similar' enclosure?
Mist performance depends strongly on enclosure size, openings, ventilation and fire size. In an untested geometry its performance cannot be predicted.
Is water mist safe on electrical equipment?
Fine droplets and low water quantities help, but each application needs manufacturer approval and a separate electrical safety assessment. A blanket 'safe' assumption should not be made.
Where is water mist commonly used?
Gas and steam turbine enclosures, diesel generator and machinery rooms, oil hazards such as industrial fryers, and some cable and equipment spaces are typical. Each needs its own approval scope.
Can a system that is not FM Approved be used at an FM-insured site?
It is generally not accepted. For systems approved under another protocol, obtain the insurer's written position first.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet 4-2, Water Mist Systems; FM Approval Standard 5560; NFPA 750, Standard on Water Mist Fire Protection Systems; EN 14972 series. This page is a summary in our own words, not the data sheet text. Download and design to the current revision from fmglobal.com.