How water mist extinguishes fire, and which parts of a ship it suits
Two things have to be separated before anything else, because marketing material in this market routinely runs them together.
The weather deck requirement is a monitor requirement, not a water mist requirement. FSS Code 7/2.5 calls for fixed monitors delivering a stated flow. It does not call for water mist technology, and it does not prohibit it either. What has to be demonstrated to a classification society is flow rate, throw distance and uniform distribution — not the name of a technology. So the answer on an open weather deck is monitors. Low-pressure water mist is the answer for the enclosed spaces on the same ship — machinery spaces, closed vehicle decks, accommodation — which run through entirely different approval routes.
With that separated, it becomes possible to say plainly what water mist is actually good for.
The extinguishing mechanism
A water mist system differs from a sprinkler or deluge system principally in droplet size. Breaking the same volume of water into smaller droplets increases the total surface area enormously, and three things follow from that.
- Heat absorption. Smaller droplets evaporate faster at flame temperature. The change of state from liquid to vapour draws latent heat out of the flame region and off the surrounding surfaces.
- Oxygen dilution. The steam produced occupies many times the volume of the liquid water it came from, so it displaces air at the base of the flame and reduces the local oxygen fraction. This effect is pronounced in enclosed or semi-enclosed spaces and falls away sharply in open, windy areas.
- Surface cooling, which is the mechanism that reduces the chance of re-ignition once the flame is out.
Manufacturers describe water mist as removing both heat and oxygen while a conventional sprinkler reduces only heat. That is a statement of principle, and it should be read as a comparison of the proportions in which the mechanisms act, not as an absolute. The real proportions depend on the size of the space, how enclosed it is, the ventilation rate and the fuel involved. This page gives no numeric steam expansion ratio, because that figure depends on the temperature and pressure at the point of evaporation and no single value holds for every case.
The advantage you can verify, and the limits you have to accept
The advantage that can be checked against the system’s own design figures is a lower water volume per unit area than a traditional total-flooding system. On a ship that feeds directly into three things: the size of the pumps and pipework, the load on the drainage system, and water damage to cargo and equipment. All three are measurable at design stage. None of them requires taking a brochure on trust.
Three limits have to be accepted just as plainly.
- Wind destroys the effectiveness of small droplets. Finer droplets are carried off target more easily. This is the physical reason the regulation requires nozzles on an open weather deck to be designed taking weather conditions into account, and the reason open-deck work uses monitors that project a coherent stream rather than pure mist nozzles.
- The oxygen dilution effect needs enclosure. In a sealed machinery space it carries real weight. On an open weather deck it carries almost none.
- Water mist is not the answer for every fuel and every geometry. The envelope in which it genuinely works is set by the fire test results for each specific nozzle set in each space type — not by the general principle.
Low, intermediate and high pressure — the classes come from NFPA 750, not from IMO
The terms low pressure, intermediate pressure and high pressure are used as though they were legal categories. In fact IMO does not define these pressure classes at all. Nothing in MSC/Circ.1165, MSC.1/Circ.1387, MSC.1/Circ.1430/Rev.3 or Res. A.800(19) defines, mandates or excludes any pressure class.
The classes as NFPA 750 defines them
The standard that actually defines water mist pressure classes is NFPA 750, and it defines them by the pressure at the system 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 |
Why this affects price and installation work
What carries real weight in marine work is not the name of the class but what follows from the working pressure.
- Pipe and fitting pressure class. A high-pressure system needs thick-wall stainless tube, high-pressure fittings, and welding or forming work that requires specialist labour and specialist equipment. A low-pressure system uses pipe and fittings of the standard pressure classes already common in marine work.
- The power unit. High-pressure systems generally need a high-pressure piston pump set or a gas-cylinder driven unit, which takes space and weight and carries its own maintenance schedule. Low-pressure systems generally use a pump set close to ordinary shipboard firewater practice.
- The skills of the ship’s engineers and of yards in the region, which is a whole-life cost that does not appear in the first quotation but does appear in repair bills and off-hire time.
Manufacturers state that the installation and maintenance cost advantage of low-pressure systems comes from using standard-pressure-class nozzles and pipe fittings. That is a manufacturer’s claim, and it should be proved by comparing real material and labour prices on your own project rather than accepted as a finished conclusion.
High-pressure systems have their own genuine advantages, which should be acknowledged just as directly: a smaller droplet size at the same nozzle type and a water volume per unit area that is usually lower — both of which matter a great deal in small enclosed spaces where water quantity and the weight of stored water are the binding constraints.
An honest comparison: deluge, low-pressure mist and high-pressure mist
This table compares system types, not brands. It is meant to help decide which space suits which system, not to conclude that one system is better overall, because none of them wins every row.
| Criterion | Deluge / traditional water spray | Low-pressure water mist | High-pressure water mist |
|---|---|---|---|
| Principal mechanism | Cooling by water volume | Cooling plus local oxygen dilution | Cooling plus local oxygen dilution, more pronounced in small enclosed spaces |
| Typical working pressure | Low | Low (NFPA 750: 12.1 bar or less) | High (NFPA 750: 34.5 bar and above) |
| Water volume per unit area | Highest of the three | Significantly lower than deluge | Usually the lowest |
| Pipe and fittings | Large bore, standard pressure class | Standard pressure class, smaller bore | Thick-wall stainless, high-pressure fittings, specialist work |
| Load on shipboard drainage | Heaviest | Lighter in proportion to water volume | Lightest |
| Tolerance of wind and open areas | Good with high-velocity nozzles | Moderate, depending on nozzle type | Most sensitive to wind |
| Long throw | Achievable with monitors | Achievable with monitors | Not a strength |
| Suits | Open weather decks, outdoor areas, boundary cooling | Enclosed and semi-enclosed shipboard spaces: machinery spaces, closed vehicle decks, accommodation, and work where water weight is a constraint | Small to medium enclosed spaces where water and weight constraints are severe |
How to read this table usefully
- On an open weather deck carrying vehicles, the regulation requires monitors delivering a stated flow. That job is not a choice between deluge and mist; it is a choice of monitor whose throw distance and flow rate can be evidenced, positioned to satisfy the 75 % rule.
- For areas of the same deck that monitors cannot reach, the regulation requires nozzles at 5.0 L/min per square metre, designed taking weather conditions into account. This is where nozzle type must be chosen carefully, because droplets that are too fine will be blown off target.
- Machinery spaces and enclosed spaces below deck are where water mist has its clearest physical advantage, because the enclosure is what gives the steam effect weight.
- Choosing between low and high pressure should be decided from the constraints of that particular ship — space for the power unit, acceptable weight, the skills of the people who will maintain it over its life, and the approval scope that actually exists for that space type — not from the pressure figure alone.
What this table does not tell you is which system is actually approved for your space. Approval attaches to a specific nozzle set, spacing, mounting height and tested pressure range. It is particular, and it does not attach to a pressure class or to a technology.
Other shipboard duties low-pressure water mist can cover, and the approval route for each
Beyond the weather deck, low-pressure water mist is used in other spaces on the same ship. But each space runs through a different approval route, and approval in one space does not carry over to another. The table below is therefore a table of approval routes, not a table of products.
| Shipboard duty | The correct reference document | Type of document |
|---|---|---|
| Total flooding for category A machinery spaces and cargo pump rooms | MSC/Circ.1165, as amended by MSC.1/Circ.1237, MSC.1/Circ.1269 and MSC.1/Circ.1386 | Approval guidance and fire test protocol |
| Local application protection in category A machinery spaces | MSC.1/Circ.1387 and MSC.1/Circ.1387/Corr.1 (8 September 2022) | Approval guidance and fire test protocol |
| Accommodation, corridors, public spaces and service spaces | Res. A.800(19) as amended by Res. MSC.265(84), together with FSS Code chapter 8 | Approval guidance and fire test protocol |
| Closed vehicle decks, ro-ro spaces and special category spaces | MSC.1/Circ.1430/Rev.3 (adopted by MSC 107, June 2023) | A document for both design and approval |
| Open weather decks carrying vehicles | SOLAS II-2/20.6.2 (Res. MSC.550(108)) and FSS Code 7/2.5 (Res. MSC.555(108)) | Regulation |
The distinction that changes how a specification is written: “tested and approved to” is not “designed to”
Of the documents above, only MSC.1/Circ.1430/Rev.3 is also a design document. Its clause 1.1 opens by saying the guidelines and fire tests are for design and approval. MSC/Circ.1165, MSC.1/Circ.1387 and Res. MSC.265(84) are approval and fire test protocols only. A system is *tested and approved to* those documents; it is not *designed to* them or *compliant with* them in the sense that phrase carries for a design standard.
This is not word-play. MSC.1/Circ.1387 clause 3.1.2 states explicitly that the fire test results determine the design and installation criteria — maximum nozzle spacing, minimum and maximum distance from nozzle to the protected object, whether nozzles must be placed outside the protected area, and minimum working pressure. In other words, the design envelope is an output of the testing, not a rule written in advance inside that document. Writing in a TOR that “the system shall comply with MSC.1/Circ.1387” therefore specifies nothing that can be verified on acceptance. Writing that “the bidder shall produce a certificate stating that this nozzle set was tested to MSC.1/Circ.1387 for this space type, at the proposed spacing and working pressure” is something that can be checked.
Pressure class is not a condition of approval
This is the most important technical answer on this page. None of the documents in the table above defines, mandates or excludes any pressure class. Every one of them is performance-based and rests on fire test results, and pressure enters only as a parameter that was tested. MSC.1/Circ.1430/Rev.2 handles pressure through the K-factor relationship, Q = k multiplied by the square root of P, with P in bar. MSC/Circ.1165 states that nozzle characteristics should stay within the range that was tested, and requires pipe sizing by hydraulic calculation.
The consequence is that a low-pressure system has equal access to every approval route above. But each certificate is tied to the values actually tested — nozzle model, nozzle spacing, mounting height and working pressure range — and to one space type only. A machinery space certificate has no effect whatever on a vehicle deck, and none on accommodation.
Methanol-fuelled machinery spaces — what can and cannot be said
This subject is drawing a great deal of interest as fleets change fuel, and it is the one where overclaiming is easiest.
What cannot be said: it cannot be claimed that MSC.1/Circ.1387 covers methanol-fuelled machinery spaces. The word *methanol* does not appear once in that document. Its fire test protocol uses light diesel oil spray fires only, at nominal 1 MW and 6 MW.
What can be said: MSC.1/Circ.1387 is the approval route for local application protection in category A machinery spaces. Capability against methanol is a separate manufacturer’s claim. Anyone considering a methanol-fuelled vessel should ask for a certificate stating the fuel type tested, the fire size tested and the coverage limits established, and treat it as evidence to be examined rather than accepting a statement in marketing material.
Land-based duties that use the same principle, under a different standard
The water mist principle is not limited to marine work. Ashore it is used in turbine halls and generator rooms, transformer and switchgear rooms, battery and UPS rooms, cable tunnels, and enclosed machinery.
What changes when you come ashore. The reference standard changes from IMO resolutions and circulars to NFPA 750, the standard for water mist fire protection systems, and the approval route changes with it. Marine work is proved by type approval against a circular. Land work is proved by testing against the application envelope for which the manufacturer holds a listing, and by acceptance from the authority having jurisdiction or the insurer. A marine certificate does not transfer automatically.
The questions to ask before choosing water mist for a land duty. Are the room volume and ceiling height within the envelope the manufacturer actually tested? Is the room tight enough? Will ventilation be shut down on activation? How much water can the protected equipment tolerate, and is there drainage to take it? Answer those five first, then compare prices.
SATU Innovative and water mist work
This page explains the principle and the approval routes. It is not a product page and it names no brand.
SATU Innovative works on the engineering and procurement side: defining a scope that matches the application envelope a manufacturer actually holds a listing for, checking whether the certificates a bidder submits genuinely cover the room volume, ceiling height and nozzle arrangement being proposed, and writing the checklist for the acceptance test.
We do not claim to be the distributor of any water mist brand on this page. When verifiable manufacturer data becomes available, we will publish it with its source stated. Talk to our engineers.
Frequently asked questions
How does water mist differ from a sprinkler or deluge system?
Principally in droplet size, not in pressure. Smaller droplets give a much greater total surface area for the same volume of water, so they evaporate faster and absorb more latent heat. The steam produced also dilutes oxygen locally at the base of the flame, though that second effect needs enclosure to carry weight and does almost nothing in an open, windy area. Sprinklers and deluge systems work mainly through water volume and cooling. The practical consequence is that water mist uses less water per unit area, which feeds into pump size, pipe size, drainage load and water damage.
Does IMO require low pressure or high pressure?
Neither. Nothing in MSC/Circ.1165, MSC.1/Circ.1387, MSC.1/Circ.1430/Rev.3 or Res. A.800(19) defines, mandates or excludes any pressure class. The classes come from NFPA 750, which sets low pressure at 12.1 bar or less, intermediate above 12.1 but below 34.5 bar, and high pressure at 34.5 bar and above. The IMO documents are performance-based and rest on fire test results; pressure enters only as a parameter that was tested.
Which document applies to total flooding of a category A machinery space?
MSC/Circ.1165, as amended by MSC.1/Circ.1237, MSC.1/Circ.1269 and MSC.1/Circ.1386. It is an approval guidance and fire test protocol, not a design standard. A system is tested and approved to it. The design criteria — nozzle spacing, minimum working pressure and so on — are outputs of that testing rather than rules written in advance in the document.
Does a machinery space certificate also cover a closed vehicle deck?
No. Approval attaches to one space type and to the values actually tested: nozzle model, nozzle spacing, mounting height and working pressure range. A machinery space certificate under MSC/Circ.1165 has no effect on a closed vehicle deck, which runs under MSC.1/Circ.1430/Rev.3, and none on accommodation, which runs under Res. A.800(19) as amended by Res. MSC.265(84). When reading a bid, check that the certificate submitted names the same space type as your duty.
Does water mist damage the equipment it protects?
Water volume per unit area is significantly lower than a traditional total-flooding system, and that is an advantage you can verify from the design figures rather than from advertising. But the system still discharges water, not a clean agent. For rooms holding sensitive electrical or IT equipment, the equipment’s water tolerance, the nozzle arrangement relative to it, and the room’s drainage all have to be considered at design stage.
Can the same system be used ashore?
The principle transfers, and it is used in turbine halls, generator rooms, transformer and switchgear rooms, battery and UPS rooms, cable tunnels and enclosed machinery. But the reference standard changes from the IMO circulars to NFPA 750, and the approval route changes with it. A marine certificate does not transfer automatically. Check that room volume and ceiling height fall inside the envelope the manufacturer actually tested.
Is low-pressure water mist approved for methanol-fuelled machinery spaces?
Two things have to be separated. The approval route for local application protection in a category A machinery space is MSC.1/Circ.1387 — but that document’s fire test protocol uses light diesel oil spray fires only, at nominal 1 MW and 6 MW, and the word methanol does not appear in it once. So it cannot be said that MSC.1/Circ.1387 covers methanol. Capability against methanol is a separate manufacturer’s claim, and it requires a certificate stating the fuel type tested, the fire size tested and the coverage limits established.