The LWF Blog

Fire Safety Engineering for Design – Types of Water Mist System – Part 299

August 10, 2026 10:29 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 298, LWF discussed fire test protocols for water mist systems. In part 299, we begin to explore the types of water mist system.

Pressure is one way to classify a water mist system, but it is not the factor that determines whether the system will extinguish a fire effectively. Performance depends on the size and distribution of the water droplets, the design of the discharge nozzles, and successful fire testing against the hazards for which the system is intended. The international definition of water mist is based on droplet size rather than operating pressure, a distinction reflected in standards such as NFPA 750: Standard on Water Mist Fire Protection Systems, which provides the principal design framework for water mist technology.

NFPA 750 introduced pressure categories to distinguish between the engineering characteristics of different systems rather than to rank their effectiveness. The familiar terms ‘low pressure’, ‘intermediate pressure’, and ‘high pressure’ describe operating ranges that influence component selection, pipework design, and methods of generating pressure. Over time, these labels have sometimes been interpreted as indicators of superior fire performance, despite there being no direct relationship between higher operating pressure and better fire suppression. A system operating at relatively low pressure can perform as well as a high-pressure system if it produces the required droplet characteristics and has been successfully tested for the intended application as per NFPA 750.

In practical terms, water mist systems are often divided into two broad groups: those operating below 16 bar and those operating above 16 bar. Most approved commercial installations fall at either end of this range, with systems commonly operating at 12 bar or less or at 70 bar and above. Systems within the intermediate range also exist, although they represent a smaller share of the market. These pressure ranges are significant because they influence the mechanical design of pumps, valves, fittings, and pipework rather than the extinguishing capability of the water mist itself.

Another important distinction lies in the method used to generate and deliver the mist. Single-fluid systems rely solely on water. Pressure is created by a pump or another hydraulic source, forcing water through specially designed nozzles that produce the required droplet spectrum. Twin-fluid systems use both water and a compressed gas, most commonly nitrogen. The gas assists with atomisation, allowing fine droplets to be produced without relying entirely on very high water pressure. This approach can offer greater flexibility in certain applications, although it introduces additional components that require inspection and maintenance. Guidance on these system configurations is also provided in BS EN 14972-1: Fixed firefighting systems – Water mist systems.

Water supply arrangements provide another useful way of classifying water mist systems. Many installations use a dedicated water storage tank and pump set, closely resembling the arrangement found in conventional sprinkler systems. Others store water in cylinders, along with a separate supply of propellant gas. Each approach has advantages depending on the protected hazard, available space, and required system response.

The choice of water supply also affects discharge duration. Tank-fed systems can often sustain operation for longer periods, provided sufficient stored water is available. Cylinder-based systems, by contrast, have a finite quantity of both water and propellant gas. Once either supply is exhausted, the system can no longer discharge. For that reason, designers must ensure that the available discharge time satisfies the requirements established during fire testing and complies with the relevant installation standard. Both NFPA 750 and BS EN 14972 emphasise that water mist systems should be selected and designed according to verified fire test data for the specific hazard being protected rather than by operating pressure alone.

In part 300 of LWF’s series on fire engineering we will continue looking at the types of water mist system. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.