Issues and Future Directions for Water Mist Fire Protection Systems

Issues and Future Directions for Water Mist Fire Protection Systems

“Watermist” only became widespread as a formalized fire protection technology in the early 1990s. Initially, it was viewed as an alternative to gaseous fire extinguishing agents, for example the ozone-depleting halocarbon gases such as Halon 1301. At the same time (1990 to 1995), the International Maritime Organization (IMO) mandated the installation of sprinkler systems on passenger ships capable of carrying more than 36 overnight passengers.

This mandate created a second innovation motivator: to develop a new form of sprinkler system that would require less water and weigh less than conventional sprinkler systems of the day. Thus, in the marine sector, water mist systems were simultaneouly developed for deluge-type, total-flooding fire protection systems for machinery spaces, and “water mist sprinkler systems” that were recognized by maritime authorities to be equivalent to automatic sprinkler systems.1,2

Creating and delivering water mist as an effective fire suppression agent demanded different types of hardware than traditional fire sprinkler equipment. The first decade of development saw the adoption of innovative ideas and hardware from non-fire related industries, such as positive displacement pumps from the hydraulics (machinery) field, and the use of compressed gas as an energy source.

By the end of the 1990s there were several distinct types of water mist systems on the world market: low pressure systems operating within the pressure range of conventional sprinkler pumps and fittings; intermediate pressure systems requiring slightly higher pressure than conventional sprinklers, and high pressure systems operating at pressures much higher than conventional sprinklers. The types of nozzles differed greatly, as different manufacturers attempted to “stake out” and patent their preferred atomization methods and lines of equipment.

Along with the innovations in hardware, the regulatory bodies and listing agencies such as IMO,1,2 Underwriters Laboratories, Inc. (UL)3 and FM Approvals (FM)4 began to develop test protocols to confirm the performance of water mist systems and to evaluate the reliability of components used in the systems. Component testing protocols were developed by IMO to test the corrosion resistance, serviceability and reliability of the new types of components being introduced.

Hardware such as positive displacement pumps and pneumatically-released deluge valves came from industries that did not have or need to obtain UL Listings or FM Approval. Therefore, the water mist approval protocols were designed to evaluate an assembly of components as a whole. Even if the individual valves/switches/motors contained in the assembly were not “listed” or “approved,” the assembly could be employed in a fire protection system if it passed the comprehensive performance tests.

A number of European testing laboratories became involved in the development of fire tests for water mist systems. Fire testing programs were developed at the Swedish National Testing Laboratory (SP); the Norwegian national fire laboratory (SINTEF); VTT Technical Research Centre and Verbandder Schadenversichen (VdS) to show the performance of water mist against hydrocarbon fires in machinery rooms.2

Fire tests were also developed for Class A combustibles typical of accommodation spaces and shopping areas on cruise ships. The fire test protocols were discussed at meetings of the IMO fire protection committee over a number of years, and finally accepted as formal test protocols described in the Safety of Life at Sea (SOLAS) fire testing document.1

In North America, FM and UL initially borrowed both the component testing and fire test protocols developed for the marine sector, and began to modify them to reflect their own fire safety objectives for land-based applications. By 2005, FM Approvals had developed a water mist approval guide,4 which contains component testing requirements and fire test protocols for a variety of applications, including turbine enclosures, machinery spaces, industrial cookers, light hazard occupancies, wet benches and computer room subfloors.

The NFPA 750 committee was formed in the early 1990s and asked to write an installation standard for water mist systems. At first it was thought that the NFPA 750 document could be modeled on NFPA 13.5 Throughout North America and in several other parts of the world, NFPA 13 is used by all parties involved in manufacturing, designing, installing, approving and testing of sprinklers and sprinkler system components. Along with specifying materials and methods for pipe, fittings and hangers, NFPA 13 provides the design criteria needed to match the sprinkler system to the various occupancy classifications.

A major advantage of NFPA 13 is that it is an easily accessible installation standard. All manufacturers produce equipment that operates within its pressure limits and types of hardware; all engineers and designers refer to it for design criteria; Authorities Having Jurisdiction (AHJs) require compliance with it as the basis for their approval; and installers and maintainers are familiar with the installation and maintenance requirements.

The NFPA 750 committee observed that, in spite of its advantages, opportunities for increasing efficiency through innovation were limited by NFPA 13. If NFPA 13 requires a minimum density of, say, 8 mm/min (0.2 gpm/ft2), to control fire in a specific occupancy, any new technology utilizing less water density must utilize the equivalency or new technology generic sections to be accepted.

NFPA 7506 was written with the purpose of allowing innovative ideas for increasing the efficiency of water-based fire protection systems. It was founded on the premise of performance-based design. NFPA 750 mandates that the application density for each type of water mist system be individually determined by fire testing to a comprehensive, unbiased fire test protocol.

Whereas NFPA 13 dictates that all pumps, pipes and fittings be of a certain type and pressure rating, NFPA 750 allows for a range of pressure regimes, energy sources and piping technologies. To support the use of non-traditional materials and piping methods, NFPA 750 sets only generic requirements, such as those needed for corrosion control and mechanical strength.

The details of how to properly select and install water mist piping and hardware are to be included in a Design, Installation, Operationand Maintenance (DIOM) Manual written by people who understand the cross-over equipment or materials better than NFPA 750 committee members. The DIOM manual is supposed to be reviewed and approved by the same listing or approval agency that conducted the fire tests and component evaluations.

NFPA 750 establishes general design factors applicable to all water mist systems, such as hydraulic calculation procedures, acceptance testing, water quality and duration of protection. It also provides guidance to the testing agencies on what constitutes an adequate fire test protocol – e.g., it explains application parameters that must be taken into account in design of the fire test protocol, such as the geometry and ventilation conditions of the protected space. It relies on the manufacturers and the listing or approval agencies to have the expertise to generate the details needed to provide the design, installation, operation and maintenance requirements unique to the particular technology.


The basic model of NFPA 750 is supportive of performance-based design and allows innovation in the design of water-based fire protection systems. However, some potential end-users and manufacturers believe that the document is not an “installation standard” in the same mold as NFPA 13. NFPA 13 contains sufficient information to provide instruction on design criteria and detailed installation instructions for the hardware, that is, the pumps, pipe, fittings and hangers.

The designer may reference additional data sheets associated with special listed items, for example, special application sprinklers, but the majority of the technology associated with conventional sprinklers is contained in the standard itself or its appendices. The NFPA 750 approach is similar in principle, except that, because the content of technology utilized by different water mist equipment manufacturers may be new to the fire protection world, reliance on the external listing data sheet and DIOM manual is greater than with NFPA 13. Information that is necessary to accomplish a design and install the hardware is found only in the manufacturer’s proprietary DIOM manual. Water mist DIOM manuals may include instructions on piping or components that require special training by the manufacturer before a third-party engineer could specify a design. Therefore, although the NFPA 750 model is similar to current conventional practice with NFPA 13, the “technology transfer” experience with water mist technology is more involved, which appears to slow the acceptance of water mist systems.

There is interest among proponents of water mist systems in making NFPA 750 a more functional installation standard. Whatever change proposals are made, it is important not to lose sight of the original philosophy of allowing innovative technologies to be used to improve the efficiency of fire suppression systems. It would be a challenge, but not impossible, to incorporate critical design information for specific manufacturers into the document or as annexes. The document would be longer, but more readily useful to designers, end-users, authorities and installers.

A second barrier to the acceptance of water mist systems in land-based applications is that there are not enough approvals for the range of fire hazards encountered in buildings. Water mist sprinkler systems are already installed in accommodation spaces, shopping areas and public areas throughout passenger ships, and are approved under IMO as fully equivalent to sprinkler systems. However, there is limited comparable recognition for water mist sprinkler systems in land-based buildings.

There is growing interest in North America and Europe to have water mist systems installed throughout buildings and to be granted the same recognition under the building code as conventional automatic sprinkler systems. This objective is revealed in a recently released draft of proposed revisions to the European water mist standard, CEN/TS 14972.7 The CEN revision task group notes that in Europe the sales of land-based water mist sprinkler systems, similar in protection purposes to automatic sprinklers, now exceed the sales of machinery space systems, yet “sprinkler equivalent water mist systems” are not clearly identified as a distinct application in either CEN or NFPA water mist documents.

Both in the USA and in Europe there are water mist approvals for “light hazard” and “ordinary hazard” applications in buildings. Buildings potentially contain spaces such as extra hazard occupancies. Because of the limited approvals, the use of water mist to provide protection throughout some buildings may be limited.

Building codes have long recognized the increased safety provided by conventional sprinklers and provide various “trade-off” benefits in the construction of the building. Water mist systems are evaluated through performance-based testing, and have been shown to perform as well if not better than conventional sprinklers in fire tests. If a well designed water mist sprinkler system is installed throughout a building, and measures to ensure equivalent reliability to sprinkler systems are incorporated into the approval, the same building code recognition as conventional sprinklers should be allowed.

Gaining support for sprinkler equivalent water mist systems is likely to be the focus for water mist proponents for the next few years. Manufacturers must obtain approvals for the range of fire hazards in buildings, and they need to convince building and fire officials to grant appropriate trade-off recognition to water mist sprinkler systems as currently exists for conventional sprinklers. In addition, water mist may not be appropriate for all applications without further testing, just as sprinklers must be tested and listed for specific hazards. Examples include window or glazing sprinklers, and attic sprinklers.

On the other hand, sprinklers are sometimes granted a specific trade-off based on “tradition” rather than actual testing. It has occurred that when sprinklers were tested to the test standard to which water mist nozzles are tested, the sprinklers failed to meet the performance criteria demanded of the water mist system. Efforts to create a level playing field for comparison of the performance of conventional sprinklers and water mist systems are underway for the current NFPA 750 change cycle.

There is a further problem with NFPA 750’s deference to external fire test protocols for design criteria, and that is the matter of differences between the fire test protocols produced by different authorities for particular hazards. To give anexample, IMO accreditation for total compartment flooding water mist systems for marine machinery spaces is based on the IMO published test protocol.2 In addition, FM grants an approval for water mist systems for machinery spaces, special hazard machinery spaces and combustion turbine enclosures, based on test protocols in the FM 5560 document.3 These two apparently similar approvals for “machinery spaces” are not equal in terms of performance. Explaining the significance of the differences to end-users and to the AHJ potentially contributes to doubts about the technology. It would better serve the users of NFPA 750 if it could provide enforceable language to resolve uncertainties created by inconsistencies between approvals from different organizations.


Widespread acceptance of protecting buildings entirely by water mist systems is potentially limited by people’s perceptions of the capabilities of the systems. Many engineers who do not have a full appreciation for the capability of water mist believe that water mist systems rely on sealed enclosures to extinguish fires. In fact, systems that rely on “enclosure effects” are only one category of water mist application.8

Such systems are defined as “total-compartment flooding type” in NFPA 750 and are suitable for Class B hydrocarbon fires in machinery spaces and turbine enclosures. However, water mist also works to extinguish or control fires in Class A combustibles in fully open compartments. The performance of these systems depends on pre-wetting of combustibles and cooling of hot gases, the same control mechanisms as conventional sprinklers, and they do not require sealed enclosures. This is the basis of performance for water mist sprinkler systems approved for accommodation spaces and public spaces on passenger ships.1, 7 Water mist sprinkler systems are tested in environments identical to automatic sprinkler testing and have been found to achieve equal or superior performance using significantly less water than conventional sprinklers.

In the other extreme, manufacturers or vendors who are enthusiastic about their water mist system may exaggerate its applicability. For example, the use of water mist as an alternative to gaseous extinguishing agents in computer rooms is subject to debate. For a computer room containing electronic equipment, there are many factors to consider in selecting an appropriate fire suppression agent. These factors potentially include the user’s objectives, the potential for certain types of electronic circuits to be irreversibly damaged by wetting or by corrosive products of combustion, the capability to extinguish a fire concealed in a computer cabinet, and both capital and life-cycle costs.

Water mist systems have been approved by FM Approvals for computer room subfloors and inside electronic cabinets, which may cause no more collateral damage than a gaseous clean agent. However, a different type of water mist system (i.e., sprinkler equivalent) will be required for general computer room protection. The use of water mist as an appropriate technology for protection of electronics is not clearly established in either NFPA 750 or the CEN standard. Further open discussion on this application, with input from approvals agencies such as FM Approvals, is needed.

It is in the interest of proponents of water mist systems to prevent harm being done to the credibility of the technology by spread of misinformation and inappropriate performance claims of water mist. Reported successes and demonstrated capabilities provide a foundation upon which confidence in water mist systems will rest. Successful control of fires by water mist should be documented for all types of water mist systems, including land-based systems and marine systems similar to automatic sprinkler systems. Failures in badly designed fire protection will potentially occur, as they do with all fire protection systems. Accurate reporting of such incidents is needed.

One avenue for improving general understanding of the capabilities of water mist systems is to invest in technology transfer opportunities, such as conferences and webinars, to provide accurate information about the successes and uses of water mist systems. This is where a strong industry organization of manufacturers could play a valuable role, in reporting incidents where water mist systems have successfully controlled fires, and at the same time providing a degree of control over the types of claims made by its members. This type of self-managed quality management among competing manufacturers would benefit all.


Maintenance of water mist systems is vital to the long-term reliability of the protection. If not properly maintained, the potential for plugging of small orifices in nozzles is higher for water mist systems than with conventional sprinkler systems. Therefore, it’s important to monitor water quality and evaluate the functionality of components.

Certain maintenance needs will be unique to a manufacturer’s particular water mist system. This may limit the competition for choice of maintenance contractors, or special training will be needed for maintenance personnel within the organization. The on-going costs of the level of maintenance needed for a water mist system must be reflected in the presentation of life-cycle costs for a water mist system.

Some organizations have a well-established culture of maintenance of fire protection systems. One large industrial end-user of water mist systems with a well-trained staff dedicated to maintenance identified the following factors that increased the costs of maintenance and the skill required to accomplish it, over what they had expected:

• Some type of water mist system releasing valves proved to be difficult to reset after annual activation tests, increasing labor costs for servicing the systems over original estimates.

• It is useful to have a borescope (video camera on a flexible probe) to inspect the interiors of water storage cylinders to evaluate the condition of liners.

• Maintaining the quality of stored water in tanks is reported to be difficult, with accumulations of “gunk” showing up on strainers and screens after annual activations.

Based on their experience, this end-user increased the frequency of maintenance inspections and specified details of the procedures. A borescope is now routinely used to inspect the interior of water storage cylinders or tanks; water tanks are drained and refilled with water of a specified quality semi-annually rather than annually, and screens and filters are inspected more frequently than was recommended in the manufacturer’s DIOM or in NFPA 750.

In discussing the possibility for installing water mist sprinkler systems in US embassies abroad, the advantages of a system requiring less water than conventional sprinklers were viewed favorably, but facilities managers were concerned about the ability of local contractors to install or maintain the systems. From their perspective, the reliability of the fire protection system should not depend on skills that are likely to be absent in the local work force.

The next edition of NFPA 750 (2013) will hopefully strengthen the message that water mist systems require quality maintenance to remain reliable over time. Inspection, testing and maintenance procedures must be frequent, thorough and sustained over the service life of the system. The true cost of maintenance should be factored into the life-cycle cost of the systems.

Jack Mawhinney is with Hughes Associates, Inc.


  1. MSC/Circ. 800 (19), “Revised Guidelines for Approval of Sprinkler Systems, Equivalent to that referred to in SOLAS Regulations II-2/12; Appendix 2. Fire Test Procedures for Equivalent Sprinkler Systems in Accommodation, Public Space and Service Areas on Passenger Ships,” International Maritime Organization, London, England: 1995.
  2. MSC/Circ. 728, “Amendments to the Test Method for Equivalent Water-Based Fire Extinguishing Systems for Machinery Spaces of Category A and Cargo Pump Rooms Contained in MSC/Cir. 668, Appendix B,” International Maritime Organization, London, England: 1996.
  3. UL 2167, Standard for Water Mist Nozzles for Fire Protection Service, Underwriters Laboratories, Northbrook, IL, 2002.
  4. Approval Standard for Water Mist Systems, Class Number 5560, FM Global Technologies, LLC, Norwood, MA, May 2005.
  5. NFPA 13, Standard for the Installation of Sprinkler Systems, National Fire Protection Association, Quincy, MA, 1991.
  6. NFPA 750, Standard on Water Mist Fire Protection Systems, National Fire Protection Association, Quincy, MA, 2010.
  7. CEN/TS 14972, Fixed Firefighting Systems – Watermist Systems – Design and Installation, European Committee for Standardization, Brussels, 2011.
  8. Mawhinney, J.R., “Principles of Water Mist Fire Suppression Systems,” Fire Protection Handbook, National Fire Protection Association, Quincy, MA, 2008.


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