The LWF Blog

Fire Safety Engineering for Design – Water Mist Components – Part 305

September 21, 2026 8:55 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 304, LWF continued the discussion of water mist system configurations by talking about automatic water systems. In part 305, we begin to look at water mist components, starting with pumps.

Pumps are a critical part of a water mist fire protection system because they provide the pressure and flow needed to deliver water through the system pipework and discharge devices. The pump must be selected around the hydraulic demands of the installation, with the required pressure and flow determined by the system design.

Water mist systems can use pneumatic, electric or diesel-driven pumps. The two principal pump technologies are centrifugal and positive displacement pumps. Each operates differently, so understanding the distinction is essential when selecting equipment for a particular application.

Centrifugal pumps are widely used in fire protection systems. They generate pressure by transferring energy from rotating impellers to the water. Multiple impellers can be arranged in stages where higher pressures are required. A defining characteristic of a centrifugal pump is that its pressure generally decreases as flow increases. The relationship between pressure and flow is represented by the pump’s performance curve, which allows designers to establish whether the pump can meet the system’s hydraulic requirements.

Positive displacement pumps work differently. Rather than relying on a pressure-generating impeller arrangement, they move a defined volume of water during each operating cycle. Their flow remains relatively stable as discharge pressure changes, within the pump’s operating range. NFPA documentation concerning water mist pumping units recognises this distinction, noting that positive displacement pumps maintain volumetric flow largely independently of discharge pressure within their design limits. NFPA Technical Committee material – NFPA 750 Standard on Water Mist Fire Protection Systems – Technical Committee Comments to NFPA 25 Technical Committee states that an NFPA 750 Task Group worked with the NFPA 20 Technical Committee on performance and maintenance requirements for positive displacement pumps. (PDF)

The characteristics of positive displacement pumps make pressure management particularly important. Because the pump can continue producing flow as pressure rises, a relief or unloader valve may be required to provide a safe path for unused water to return to the supply tank. The arrangement prevents the pump from operating against an unsuitable closed discharge condition.

Pump selection also affects power requirements. At pressures above approximately 50 bar, positive displacement pumps can require substantially more power for a given flow than comparable centrifugal arrangements. The actual choice should therefore be based on the complete system duty rather than pressure alone. Factors such as required flow, operating pressure, available power, water supply and the configuration of the water mist system all influence the final selection.

Fire protection standards distinguish between the requirements associated with different pump types. NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, contains separate provisions for centrifugal and positive displacement pumps, reflecting their different operating characteristics.

Water mist systems also have their own dedicated standard. NFPA 750, Standard on Water Mist Fire Protection Systems, addresses water mist system design and related equipment requirements. Its technical committee has worked with NFPA 20 on requirements covering positive displacement pumping units, including inspection and testing considerations.

Selecting a pump is not simply a matter of choosing the highest available pressure. The pump must deliver the required performance at the system’s design operating point and work reliably with the rest of the water mist installation. Understanding the difference between centrifugal and positive displacement operation provides a sound starting point for making that selection.

In part 306 of LWF’s series on fire engineering we will continue discussing water mist components by looking at nozzles and valves. 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.