Water mist fire protection systems, Part 2
By Samuel S. Dannaway, PE,
President, S.S. Dannaway Associates, Inc., Honolulu
As we noted last month, the current state of the art in water mist fire protection system design is that there is no comprehensive set of guidelines that can be used to design a water mist system. Designers are limited to providing protection for hazards that fit into the small number of approved pre-engineered system designs. This is codified in paragraph 8.1.1 of the 2010 edition of NFPA 750: “Water mist protection systems shall be designed and installed for the specific hazards and protection objectives specified in the listing.”
The explanatory information in the annex paragraph A.8.1 aptly explains the need for this approach: “Currently, no generic design method is recognized for water mist protection systems. The relationship between flux density or nozzle spacing and performance in controlling fires is not consistent between systems designed by different manufacturers. The system features, such as nozzle spacing, flow rate, drop size distribution, cone angle and other characteristics, need to be determined for each manufacturer’s system through full-scale fire testing to obtain a listing for each specific application.”
There are two established avenues for gaining approvals for pre-engineered water mist fire protection systems, one for systems on passenger ships and one for land-based hazards.
In order for water mist fire protection systems to be approved for use on passenger ships, the system must be deemed equivalent to a shipboard automatic sprinkler system based on fire testing. This is done by meeting the requirements of Appendices 1 and 2 of The International Maritime Organization publication IMO 800 (19), Revised Guidelines for Approval of Sprinkler Systems Equivalent to that Referred to in SOLAS Regulation II-2/12. Appendix 1 contains the test protocols for evaluation of water mist nozzles. Appendix 2 contains the fire test protocols for water mist systems for use in passenger accommodation spaces, public areas and service areas.
There are several approving authorities for systems on passenger ships. These include the U.S. Coast Guard, the Danish Maritime Authority and Bureau Veritas.
For water mist systems on other than passenger ships, we must rely on approvals provided by Factory Mutual Global. FM evaluates water mist components and systems by FM Approval Standard 5560, Water Mist Systems. FM approvals are provided for complete systems and for water mist nozzles for use in light hazard occupancies.
Current FM system approvals are provided in the following categories:
Local Application Protection
Combustion or Steam Turbines in Enclosures with Volumes < 9175 ft3 (260 m3)
Combustion or Steam Turbines in Enclosures with Volumes > 9175 ft3 (260 m3)
Combustion or Steam Turbines in Enclosures with Volumes < 2825 ft3 (80 m3)
Machinery in Enclosures with Volumes < 9175 ft3 (260 m3)
Machinery in Enclosures with Volumes > 9175 ft3 (260 m3)
Machinery in Enclosures with Volumes < 2825 ft3 (80 m3)
Computer Room Subfloors
Industrial Oil Cookers
Wet Benches and Similar Processing Equipment
Continuous Wood Board Presses
Light Hazard Occupancies
Fire tests of machinery enclosures and turbines include tests in which the fire is both shielded and unshielded from the water spray. Non-fire testing of combustion turbines is done to ensure that the thermal shock caused by the cooling effect of the water mist nozzles will not adversely affect turbines. Computer room subfloors, industrial oil cookers, wet benches and continuous wood board presses are fire tested in full scale mock-ups of each hazard.
The various fire tests for these hazards include at least one test in which the water spray nozzle is obstructed. Testing for light hazard occupancies in done in three compartments; a small room (130 sq ft with ceiling at 8 feet), a large room (400 sq ft room with ceiling at 8 feet) and a large open space with a minimum 860 sq ft ceiling area that is 16.4 feet high. To pass the tests the maximum ceiling temperature must not exceed 500 F. Damage to combustible fuel packages is limited to a certain percentage for each test. For the large room, a nozzle located in the doorway must not open. In the open space test, a maximum of five nozzles are permitted to open.
Both the FM and NFPA water mist standards permit a limit amounted of flexibility for purposes of adapting a specific hazard situation to a particular approved system. Section of 3.3 of FM Data Sheet 4-2 Water Mist Systems allows use of systems approved for a specific hazard to be used for protection of other hazards as long as the hazard is similar. Similarly, NFPA 750 requires the application of approved systems to situations where the hazard and compartment characteristics are “consistent with the listing of the system” (paragraph 8.1.2). The standard also cautions the designer that, “Pre-engineered water mist systems for compartment enclosures shall not be extrapolated beyond the volume, ceiling height, ventilation rate and number of nozzles tested, unless dimensions of the enclosure are such that additional nozzles are required to maintain nozzle spacing.”
To date, Underwriters’ Laboratories does not provide listings for pre-engineered systems. UL does provide listing services for water mist system components including nozzles and fire pumps.
The use of water mist fire protection systems for protection of light hazard occupancies is gaining greater acceptance. Marriott Corporation has included certain types of water mist fire protection as an alternative to sprinkler systems in the most recent edition of their Fire Protection and Life Safety Design Standards. There are currently over 30 hotels under the Marriott brand that are protected with water mist fire protection. It seems that Marriott has had particular success in using these systems to protect historic structures.
Appendix O of FM Std 5560 lists several other hazards in which FM Global is interested in providing fire protection. It would seem that FM considers that water mist fire protection systems could be of value in protecting these hazards. The hazards are:
Chemical Fume Hoods
Electronic and Telecommunication Equipment Areas
Flammable Liquid Storage in Metal Drums in Storage Rooms
Flammable Liquid Spray Operations
Occupancies Listed in Table 1 of FM Data Sheet 3-26 Fire Protection Water Demand for Nonstorage Sprinklered Properties
Table 1 of FM Data Sheet 3-26 lists several occupancies classified into one of three hazard categories, HC-1, HC-2 and HC-3. HC-1 is similar to the light hazard sprinkler hazard classification. Though Appendix O indicates FM’s interest in protection of Table 1 occupancies with water mist, paragraph 2.1.1.12 of Data Sheet 3-26 indicates that water mist should only be used to protect HC-1 occupancies. This paragraph also contains several other requirements and restraints for use of water mist fire protection systems for light hazard occupancies.
For the time being, approved use of water mist fire protection will be limited to those hazards in which full scale fire testing has been performed.
Samuel S. Dannaway, PE, is a registered fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is past president and a Fellow of the Society of Fire Protection Engineers. He is president of S. S. Dannaway Associates Inc., a 15-person fire protection engineering firm with offices in Honolulu and Guam. He can be reached via email at SDannaway@ssdafire.com.