Water mist systems use a small quantity of water broken into very fine droplets. The droplets absorb fire heat quickly and reduce oxygen close to the flame. That is why water mist is proposed where large volumes of water are a concern, such as machinery spaces, turbine enclosures and some heritage buildings.
But water mist is not a smaller sprinkler. Its performance is tied to a specific configuration that has passed a real fire test, and it cannot simply be moved to a different hazard. This article explains how it works, the pressure classes, applications and limits, the role of NFPA 750, and how to select a system on an Egyptian project.
Key takeaways
- NFPA 750 defines water mist as a spray in which 99% of the water volume is in droplets smaller than 1,000 microns at the minimum design operating pressure.
- Water mist works by cooling, local oxygen displacement by steam, and blocking radiant heat.
- Systems are classed by pressure: low up to 12.1 bar, intermediate, and high at 34.5 bar or more.
- An approval covers an application and configuration that were actually tested, for example under FM 5560 protocols or the EN 14972 series. It does not extend to other hazards.
- In Egypt, the binding reference is the Egyptian fire code (2026 edition) and Civil Protection approval. NFPA 750 is a technical reference.
In this article
- What is a water mist system?
- How does water mist put out a fire?
- Low, intermediate and high pressure
- System components
- Typical applications
- Limitations: when water mist is not the answer
- NFPA 750 and hazard-specific approval
- Water mist on Egyptian projects
- How to select a water mist system
- Common mistakes
- How AZEIDK Egypt can help
- Frequently asked questions
- Conclusion
What is a water mist system?
NFPA 750 (Standard on Water Mist Fire Protection Systems) defines water mist as a water spray in which the droplet diameter covering 99% of the total volume (Dv0.99) is below 1,000 microns (1 mm) at the nozzle’s minimum design operating pressure. The standard sets minimum requirements for design, installation, maintenance and testing. It does not set final performance criteria or general design methods for every fire. Instead, it relies on listed equipment that has proved its performance in fire tests. This article is based on the 2023 edition.
The practical difference from sprinklers is that small droplets alone do not guarantee extinguishment. Spray pattern, density, direction, velocity and nozzle position relative to the fire all affect the result, so every system is designed on the basis of what was proven in testing.
How does water mist put out a fire?
- Cooling: small droplets have a large surface area for their volume, so they absorb heat quickly from fire gases and the fuel surface, and less heat feeds back to the fuel.
- Oxygen displacement: as droplets evaporate, the steam expands and pushes air away from the flame, lowering oxygen locally. This is a local effect near the fire, not total flooding of the room as with gas systems.
- Radiant heat attenuation: the droplet cloud absorbs part of the radiant heat, protecting nearby surfaces and materials and limiting fire spread.
- Fuel vapour dilution: steam and the air drawn in with the spray dilute flammable vapours.
One consequence: water mist needs a fire hot enough to evaporate the droplets. A very small fire in a large space may not be extinguished as quickly as a larger one.
Low, intermediate and high pressure
NFPA 750 classes systems by operating pressure in the distribution piping:
| Class | Operating pressure | Typical pressure source | Notes |
|---|---|---|---|
| Low pressure | 12.1 bar (175 psi) or less | Pumps or a water supply at pressures close to sprinkler systems | Relatively larger droplets and larger pipe sizes |
| Intermediate pressure | Above 12.1 and below 34.5 bar | Pumps or cylinders | Depends on the manufacturer’s design |
| High pressure | 34.5 bar (500 psi) or more | Positive displacement pump skid, or water and nitrogen cylinders | Finer droplets, small-bore high-pressure piping |
Systems are also single-fluid (water only, one pipe network) or twin-fluid, with a water network and a compressed air or inert gas network that atomises the droplets. Higher pressure is not better in every case. The fire test on the specific hazard decides.
System components
- Water and pressure source: a clean water tank with pumps, or water cylinders driven by compressed gas.
- Nozzles: closed nozzles with a heat-sensitive element that work like sprinklers, or open nozzles that discharge as a deluge system on a signal from a detection system.
- Strainers: protect the fine nozzle orifices from clogging.
- Piping and fittings: usually corrosion-resistant, such as stainless steel or copper. Galvanised pipe is avoided because flakes can block nozzles. The manufacturer’s manual is the final word.
- Section valves and the detection and control panel: for open-nozzle or zoned systems.
Water quality is an operating requirement, and the manufacturer defines it in the design and installation manual.
Typical applications
Water mist applications are defined by the fire test protocols available. FM Approvals standard FM 5560, for example, includes protocols for:
- Machinery spaces and special hazard machinery spaces, up to large volumes.
- Combustion turbine enclosures.
- Industrial oil cookers.
- Light hazard occupancies such as offices, schools, hotels and museums.
- Wet benches in precision manufacturing.
- Local application, and computer room subfloors.
In 2022 FM Approvals added two protocols for non-storage occupancies: one covering areas such as machine shops, electronics assembly and retail, and one for heavier loads such as plastics processing and parking garages. Water mist is also often proposed for heritage buildings and museums because it uses less water than sprinklers.
Limitations: when water mist is not the answer
- Water-reactive materials: not for direct application on reactive metals such as lithium and sodium, or on cryogenic liquefied gases such as LNG.
- Strong ventilation: strong air currents can stop the mist reaching the fire.
- Shielded fires: obstructions reduce how much mist reaches the fire.
- Untested hazards: it replaces sprinklers only where large-scale fire tests have proven it.
- Not a general substitute for gas systems: each has its own field; compare them on the same hazard.
- Design sensitivity: the design cannot be extrapolated beyond the tested limits, such as a higher ceiling or larger room than was tested.
NFPA 750 and hazard-specific approval
The key point with water mist is that approval covers a specific application. The main references are:
- FM 5560: the FM Approvals standard for land-based water mist systems. Each system is tested to the protocol for its intended application.
- UL 2167: the UL standard for water mist nozzles for fire protection service.
- EN 14972-1:2020 (with amendment A1:2025): the European standard for design, installation, inspection and maintenance of land-based systems. Its scope covers only applications included in the fire test protocols of the EN 14972 series.
In practice, the submittal must include the manufacturer’s design and installation manual, fire test reports for the specific hazard, a datasheet for each component, and the limits of the tested configuration: room volume, ceiling height, ventilation, nozzle spacing, operating pressure and discharge duration. Design parameters proven in the test are not changed.
Water mist on Egyptian projects
The legally binding reference is the Egyptian Code for the Design and Implementation of Fire Protection for Buildings — 2026 edition (Ministerial Decree No. 355 of 2026). The General Directorate of Civil Protection (Ministry of Interior) reviews drawings and inspects systems before operation. NFPA 750, FM 5560 and EN 14972 are international technical references used by the designer.
So if a designer proposes water mist as an alternative to another system, or for a special hazard, it is wise to confirm early that Civil Protection will accept it on the project drawings, and to submit the approval documents and test reports for that exact application. Do not assume a system is accepted just because an international standard covers it.
How to select a water mist system
- Define the hazard precisely: fuel type, room volume, ceiling height, ventilation and obstructions.
- Match the hazard to a test protocol: is there a listed or approved system for this exact application?
- Set the objective: extinguishment, control, or exposure protection. The objective sets the water quantity and discharge duration.
- Compare alternatives: sprinklers, clean agent or water mist, based on the hazard, the space and the impact of water.
- Check water supply and space: location of the pump skid or cylinders, water quality and pipe routing.
- Plan acceptance and maintenance: flow testing, strainer cleaning and nozzle inspection, per NFPA 750 and the manufacturer.
Common mistakes
- Choosing a system because its droplets are finer, without checking it was tested on the same hazard.
- Copying a design tested for one room size to a larger room or higher ceiling.
- Ignoring ventilation openings or doors left open during operation.
- Using pipes or fittings other than those the manufacturer’s manual specifies.
- Neglecting water quality and strainers, so nozzles clog.
- Assuming Civil Protection acceptance without presenting the system and its documents early.
How AZEIDK Egypt can help
AZEIDK Egypt does not list water mist systems in its current product range. Our technical team can review the hazard with you and compare the available options, including clean agent systems within our special fire suppression solutions. For detailed comparisons, see our clean agent systems guide, automatic sprinkler systems guide and special hazard suppression comparison.
Frequently asked questions
Can water mist replace sprinklers?
Only where a listed or approved system exists for that application after large-scale fire tests, and after Civil Protection accepts it on the project drawings. It is not a general substitute in large open areas or storage.
Is water mist safe in electrical equipment rooms?
It is used in some electrical equipment applications it has been tested for, but safety depends on the approved application and the manufacturer’s instructions, not on the system type in general.
What is the difference between low and high pressure?
Under NFPA 750, a low-pressure system operates at 12.1 bar or less and a high-pressure system at 34.5 bar or more. High pressure usually produces finer droplets, but the choice is decided by testing on the hazard.
Is NFPA 750 mandatory in Egypt?
No. The binding reference is the Egyptian fire code and what Civil Protection approves. NFPA 750 is used as a technical reference for design, installation and maintenance.
Conclusion
Water mist is effective when used on a hazard it was tested for, in the same configuration that passed the test. Selection starts with defining the hazard, then finding a system approved for that application, then confirming acceptance with Civil Protection. Where that match does not exist, sprinklers or clean agent are usually the clearer choice.
Looking at protection for a machinery space or a heritage building? Send us the hazard description and room dimensions, and we will review the suitable options and the approval documents you will need. Talk to the AZEIDK Egypt team
