Water Mist System

What sets water mist apart from other fixed suppression

Where a CO2 flooding system displaces the oxygen in an entire compartment and a foam system blankets a surface, a water mist system atomises water into droplets small enough, typically under 200 microns, to absorb heat by rapid evaporation and locally starve the fire of oxygen as steam expands, while still being water that a crew can safely walk through, unlike CO2. High-pressure systems such as HI-FOG use pump units at 80-140 bar; low-pressure systems run closer to 10-15 bar with larger droplets and rely more on cooling than on oxygen displacement. The choice between them shapes pipe sizing, pump selection and nozzle spacing throughout the design.

Main components

Pump unit

Electrically or diesel driven positive-displacement pump raising fresh or sea water to system working pressure, usually duplicated for redundancy with an emergency power connection.

Water supply and tank

A dedicated fresh water tank sized to the design duration, commonly 30 minutes for machinery spaces, sometimes topped up from a sea water connection for extended discharge.

Section valves and distribution piping

Each protected zone has its own section valve, so a fire in one machinery space or cabin zone triggers only that section rather than the whole ship.

Nozzles

Open or closed sprinkler-type nozzles depending on application; engine rooms typically use open nozzles on a deluge principle triggered by the detection system, while accommodation zones often use closed, heat-activated nozzles similar to a conventional sprinkler.

Detection and control panel

Linked to the ship's fire detection system so that a confirmed alarm in a zone automatically opens that zone's section valve and starts the pump.

Selection and sizing

System choice depends on the protected space: engine rooms and pump rooms with three-dimensional fire risk favour high-pressure total flooding mist; accommodation and cabin areas are commonly protected as an alternative to conventional sprinklers under SOLAS II-2/10, using the same detection zoning as the fire alarm system. Nozzle density and section valve zoning are set by the system's type approval test protocol for the specific hazard category, not by a generic rule of thumb.

Regulations and class

Water mist systems used as an alternative to fixed sprinkler or CO2 systems must meet the performance standard in IMO MSC/Circ.1165, or the equivalent revised guidelines, and are approved against fire test protocols rather than prescriptive pipe-sizing rules, since droplet size and pressure vary by manufacturer. FSS Code Chapter 8 covers fixed water-based systems generally, with SOLAS II-2/10 setting out where a fixed system is required by ship type and space. Annual servicing and a manufacturer-specified overhaul of the pump unit and nozzles is typical under class-approved maintenance schemes.

Typical faults

• Nozzle blockage from scale or paint overspray during maintenance: reduced or absent discharge from that nozzle in a real fire, often undetected until the next test.
• Section valve seized closed: a whole zone loses protection even though the pump and panel test normally.
• Pump unit accumulator or pressure switch drift: delayed pump start, slower buildup to design pressure after activation.
• Fresh water tank low or contaminated: insufficient discharge duration, or nozzle fouling from sediment.
• Detection zone mismatch after a space reconfiguration: new equipment or bulkheads added without updating nozzle coverage.

What to look for in a supplier

• Type approval certificate referencing the specific hazard category, machinery space, accommodation or galley duct, the ship needs protected.
• Documented fire test basis for the nozzle and pressure combination, since water mist performance is not interchangeable between manufacturers.
• Spare nozzle and section valve availability matched to the installed system, not a generic replacement.
• Service network capable of the pump unit's specified overhaul interval without shipping the whole unit ashore.

Check nozzle orifices for paint or scale every time the space is worked on, not just at the annual service - a partially blocked nozzle looks fine on a pressure test but delivers almost nothing in a real discharge.