Low-Pressure Water Mist Fire Protection for Ships

Low-pressure water mist puts out fires with a fraction of the water a sprinkler or deluge system discharges, using standard-pressure pipework and pumps rather than high-pressure tube and piston pumps. SATU Innovative supplies low-pressure water mist systems made by Savemist for machinery spaces, local protection of engines, accommodation and enclosed vehicle spaces, and helps owners, shipyards and designers specify them against the right IMO approval route.

What counts as water mist

NFPA 750 (2022 edition) defines water mist as a water spray for which Dv0.99 — the droplet diameter below which 99 % of the sprayed water volume lies — is less than 1,000 µm at the minimum design operating pressure of the nozzle. Fine droplets have far more surface area than sprinkler drops, so they evaporate quickly, and that works in three ways:

Cooling. Evaporation absorbs heat from the flame and the hot gases around it.
Local oxygen displacement. Water expands roughly 1,700 times as it turns to steam, which dilutes the oxygen at the flame front. This effect needs enclosure: it carries real weight in a closed machinery space and almost none on an open, windy deck.
Blocking radiant heat. The mist cloud absorbs radiant heat, which protects people, structure and nearby equipment and helps prevent re-ignition.

What “low pressure” actually means

IMO does not define pressure classes. They come from NFPA 750, which sets them by the pressure in the distribution piping:

Class	NFPA 750 boundary
Low pressure	12.1 bar (175 psi) or less
Intermediate pressure	Above 12.1 bar (175 psi) but below 34.5 bar (500 psi)
High pressure	34.5 bar (500 psi) and above

Manufacturers use the label more loosely. Savemist’s own comparison shows its low-pressure system working at about 16 bar, against about 120 bar for high-pressure mist and 6 bar for a sprinkler system. On NFPA 750’s boundaries 16 bar sits in the intermediate band. The name of the class matters less than the tested pressure range written on the approval certificate, so specify by that.

Sprinkler, high-pressure mist and low-pressure mist compared

Illustration of a low-pressure water mist pump skid with two pumps — Low-pressure pump skid. Illustration: Savemist

The comparison below is Savemist’s, shared with us in October 2026. Water volume is on a relative scale where a sprinkler system is 10.

High-pressure mist uses the least water. Low-pressure mist trades a little more water for standard Class III piping, larger nozzle orifices that are less prone to clogging, a smaller emergency generator, and equipment that ordinary marine engineers can service. Real costs should be priced on your own project rather than taken from any chart, including this one.

Cost element	Sprinkler system	High-pressure water mist	Low-pressure water mist
Typical working pressure	6 bar	120 bar	16 bar
Relative water volume (sprinkler = 10)	10	1	1.5
Equipment cost	Medium	High	Low
Installation	Low cost; Class III piping	Medium cost; Class I piping, low piping weight, wet system	Medium cost; Class III piping, low piping weight, dry system
Nozzle clogging risk	Bigger nozzle orifice	Smaller orifice, risk of clogging	Bigger orifice, less risk of clogging
Emergency power demand	Low	High: large emergency generator	Low: small emergency generator, about 40 % of high-pressure
Operation and service	Low-pressure equipment, medium-skilled service	High-pressure equipment, highly skilled service	Low-pressure equipment, medium-skilled service
Fresh water for 30 minutes of operation	High	Low: about 50 % of low-pressure	Medium

Source: Savemist, “Water based system technologies overview”, October 2026.

Where it is used on board, and the approval route for each

Each space on a ship has its own IMO approval guideline, and an approval in one space does not carry over to another. A certificate is tied to the nozzle model, spacing, mounting height and pressure range that were tested.

Protected space	Protection type	IMO approval guideline
Category A machinery spaces and cargo pump rooms	Total protection of the whole space	MSC/Circ.1165, as amended
Fire-hazard parts of engines, generators, boilers and purifiers inside a category A machinery space	Local application — protects the object, not the whole space	MSC.1/Circ.1387
Enclosed vehicle, special category and ro-ro spaces	Fixed water-based system	MSC.1/Circ.1430
Accommodation, public spaces and service spaces	Automatic water mist	Res. A.800(19), as amended by Res. MSC.265(84)

MSC.1/Circ.1387 is local protection. It covers systems that protect specific fire hazards inside a machinery space — engines, boiler fronts, incinerators and purifiers. It does not cover the machinery space as a whole; that is total flooding under MSC/Circ.1165.
Open weather decks carrying vehicles are not a water mist duty. SOLAS II-2/20.6.2 and FSS Code 7/2.5 call for fixed monitors. See the Savemist Weather Water Monitor System and our article on the weather deck requirement.

Electrical equipment and water damage

Water mist discharges water, not a clean agent, but far less of it than a sprinkler or deluge system. Savemist advises that IP44-rated electrical equipment is sufficient in spaces protected by its low-pressure system, and that high-pressure mist has a greater water-damage impact on electrical equipment. For rooms with sensitive electrical or IT equipment, check the equipment’s water tolerance, the nozzle positions relative to it and the room’s drainage at design stage.

Design limits to check early

Ceiling height. For full protection, Savemist’s limits are 11 m in large volumes and 5 m in small volumes. Its accommodation system has been fire-tested at ceiling heights of 2.5 m and 5 m.
Nozzles. K-factors range from 3.85 to 23, chosen for the application.
Design density, discharge time, water volume and pump capacity have no single answer. They follow from the system arrangement and from what the approval for that space type was tested at.

Ashore

The same principle is used ashore in turbine halls, generator rooms, transformer and switchgear rooms, battery rooms and cable tunnels. On land the reference standard is NFPA 750, and acceptance comes from the listing for that application and from the authority having jurisdiction or the insurer. A marine certificate does not transfer automatically. Ask us about land projects case by case.

Frequently asked questions

What is the difference between low-pressure and high-pressure water mist?

High-pressure systems work at around 100 bar or more and make the finest droplets, so they use the least water, but they need thick-wall stainless tube, high-pressure pumps and a large emergency generator. Low-pressure systems work at around 16 bar with standard marine pipe, fittings and pumps, larger nozzle orifices that clog less, and a smaller emergency generator, at the cost of somewhat more water. Both use far less water than a sprinkler or deluge system.

Does IMO require a particular pressure class?

No. The IMO approval guidelines — MSC/Circ.1165, MSC.1/Circ.1387, MSC.1/Circ.1430 and Res. A.800(19) as amended — are performance-based and rest on fire tests. Pressure classes come from NFPA 750. What matters is the pressure range written on the approval certificate for your space.

Can water mist protect an open weather deck carrying vehicles?

Not normally. SOLAS II-2/20.6.2 and FSS Code 7/2.5 call for fixed monitors delivering a stated flow and throw distance. Water mist is the answer for the enclosed spaces on the same ship: machinery spaces, accommodation and closed vehicle decks.

Is low-pressure water mist safe for electrical equipment?

Savemist advises that IP44-rated electrical equipment is sufficient in spaces protected by its low-pressure system, and that high-pressure mist has a greater water-damage impact on electrical equipment. The equipment’s water tolerance, nozzle positions and drainage should still be checked at design stage.

Send us your project

Send us the space type and volume, the ceiling height, the equipment to be protected, the class society and whether it is a newbuild or a retrofit, and we will come back with a configuration to discuss. Contact SATU Innovative.