The LWF Blog

Fire Safety Engineering for Design – Water Mist Components – Part 307

October 6, 2026 9:38 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 306, LWF continued discussing water mist components by looking at nozzles and valves. In part 307, we talk about pipes and pipework.

Pipework is a critical part of any water mist fire protection system. Unlike conventional sprinkler systems, water mist installations can use very small nozzle orifice sizes, making the cleanliness and condition of the distribution pipework particularly important. Suitable pipe must withstand the system’s operating pressure, maintain efficient water flow and resist corrosion or deterioration that could introduce debris into the system.

Material selection therefore goes beyond choosing a pipe with an adequate pressure rating. Internal surfaces should be smooth, and the material must be compatible with the system water and associated fittings. Corrosion products, scale, coating flakes or installation debris can obstruct small nozzle openings and affect discharge performance. BS 8663-1:2019 covers the specification and testing of water mist nozzles and recognises the importance of pressure conditions within these systems.

Stainless steel is widely used for water mist pipework, with grades such as 304 and 316 commonly specified. The required wall thickness depends on the system pressure and the pipe or tube construction. Thin-wall press or crimp systems may be suitable for lower-pressure applications, while higher-pressure systems require pipe or tubing designed for the greater mechanical loads involved. The connection method must be considered alongside the pipe itself, since a connection that is unsuitable for the system pressure can become a point of failure.

Pressure surges also need consideration. Rapid changes in pressure or flow can create water hammer, imposing additional loads on pipework and connections. For higher-pressure installations, the use of specialist fittings or connection systems designed for the application can reduce the risk of joint failure. The precise arrangement should follow the water mist system manufacturer’s approved design rather than relying on a generic pipe specification.

FM Property Loss Prevention Data Sheet 4-2, Water Mist Systems, provides specific guidance on distribution systems. It states that pipe and tubing selection should follow the manufacturer’s FM-approved design, installation, operation and maintenance documentation. The data sheet also addresses materials that could contribute to corrosion or blockage, particularly where small-orifice nozzles are used.

Galvanised steel warrants particular care. FM’s water mist guidance does not recommend galvanised steel pipe and fittings for water supply lines because corrosion, water quality and deterioration of internal coatings can create particles capable of blocking small orifices. Compatible pipe materials should also be selected to limit galvanic corrosion between dissimilar metals.

Cleanliness during installation is just as important as material selection. FM guidance requires pipe or tube sections to be internally clean and free from particulate matter and oil residue before assembly. Jointing compounds, sealants and lubricants must also be prevented from entering the pipe network, since even small quantities can reach control devices or nozzles.

Pipework selection should therefore be treated as part of the complete water mist system design. Pressure rating, wall thickness, joining method, corrosion resistance, water quality, cleanliness and component approval all need to work together. Selecting pipe solely by diameter or pressure rating can overlook the factors that determine whether the system will deliver water mist reliably when it is needed.

In part 308 of LWF’s series on fire engineering we will discuss the approvals system for water mist systems. 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.