The eruption of a fuel fire in the engine room of the cruise ship Nordic Empress (see sidebar) was controlled and extinguished expeditiously because of the swift activation of a water mist system designed to protect shipboard machinery spaces. This is the most recent success of high velocity water sprays combined with low water discharge rates.
Known as water mist systems, the technology is being installed on an increasing number of commercial vessels, especially cruise ships, replacing alternatives such as Halon-1301. The Navy is also planning to use a water mist system on its new Amphibious Transport Dock ship, the LPD-17, expected to be delivered from the Litton Avondale Industries Shipyard in New Orleans in 2003.
Water has been the traditional fire-fighting agent for centuries, but too much water combined with inappropriate fire fighting techniques has had disastrous results. One example was the capsizing of the 83,423 grt trans-Atlantic liner Normandie on February 2, 1942 at Pier 88 in New York. Sparks from a welder’s torch ignited stored life jackets and well intentioned New York City firefighters poured tons of water into the superstructure trying to extinguish the stubborn fire. However, the ship was not dewatered properly, and it rolled over on its port side, eventually putting out the fire. Unfortunately, fighting some shipboard fires, such as on the Normandie, has too often simulated the old medical joke, “The operation was a success but the patient died.”
Traditional water fire-extinguishing systems are based on water applied to a fire from straight hose streams or sprinkler systems. Typically, wet pipe sprinkler systems are activated when heat from a fire melts a fusible link in a sprinkler head allowing water to flow and spray into the area. Other systems, known as dry pipe, have open sprinkler heads and an electrically controlled main supply line valve controlling water flow to all sprinkler heads in the system. Both systems are designed to allow for a maximum volume of water to be discharged onto a fire area, effectively drowning the fire. The disadvantages of traditional water systems are that they can cause substantial damage to areas not directly affected by the flames and, for ships, jeopardize their buoyancy.
Fire fighting technologies have advanced considerably since the Normandie disaster and have been incorporated by naval architects and marine engineers into ship designs for both new-builds and overhauls. The alternate fire fighting technologies designed to reduce the use of water include improved dry chemical systems, wet foam for use against flammable liquid fires, high expansion foam and gaseous agents such as the Halons and CO2. Whichever system is used, the goal is to apply quantities of the extinguishing agent onto a smoldering heat source to prevent a full-scale eruption of flames or onto a working fire in a sufficient quantity to extinguish the flames.
Halon 1301, a now restricted alternate technology, gained fast acceptance for marine applications beginning in the early 1970s. It became a preferred extinguishing agent for local applications protecting enclosed, high value areas such as control rooms and shipboard machinery spaces, since it did not cause the secondary damage commonly associated with water systems. However, it did have the drawback of personnel evacuation before discharge of the system and the area had to be sealed to prevent dispersion of the Halon gas. A larger problem surfaced by the early 1990s because of Halon 1301’s detrimental effect on stratospheric ozone. The fire protection industry and marine engineers began searching for a suitable replacement that could be in service before production of Halon 1301 ended in 1994. One alternative was an environmentally friendlier version named FM-200.
Back to square one – water
The search for a viable replacement for the standard water sprinkler system technologies that evolved into water mist fire suppression. “Aboard ships, water mist has a number of benefits over gaseous systems,” said Randy Eberly, a fire protection engineer with the U.S. Coast Guard in Washington, D.C. “You have an unlimited supply of extinguishing agent where, with a gaseous system you have only one shot and normally no reserve backup. And water mist reduces the heat in an area so much that people can enter more quickly than with a gaseous system.”
Marioff Corporation Oy, headquartered in Vantaa, Finland, was one of the pioneers of the water mist system and introduced its technology in January 1991. According to the company, its water mist system, known as HI-FOG, “combines the extinguishing characteristics of water with the penetrative qualities of gas. Penetration and suppression of all types of fires is achieved by high momentum discharge of small water droplets in the form of a mist. Once activated, the HI-FOG system protects the space with high density mist that easily reaches shielded fire areas, absorbs and blocks heat radiation from the fire source and prevents oxygen from entering the combustion area.” A report from Factory Mutual Research Corporation’s (FMRC) Hydraulic Laboratory in West Glocester, R.I., noted that atomized water spray of fine water spray systems are made up of very small droplets of water that upon reaching the source of heat “flash instantly to steam at an expansion ratio of 17,000:1” and that effectively cuts off the oxygen necessary to sustain the combustion at the fire source.
Water mist systems differ significantly from standard sprinkler systems in that they are designed to discharge smaller volumes of water from the sprinkler heads but at higher pressures, creating a mist effect. The mist essentially performs two functions. It cools the fire area dramatically and can drop the fire zone ceiling temperature from about 1,000Â° C to under 300Â° C almost immediately. The reduction of high temperatures can help prevent dangerous flashovers and re-ignition after the fire is extinguished. The second water mist fire extinguishing function reduces the supply of oxygen to the fire when the small mist water droplets turn to steam on contact with the fire. “Oxygen reduction is a big part of the extinguishing mechanism for water mist,” Eberly said.
Recognizing the need for a quick change from Halon 1301 systems, the U.S. Navy’s Naval Sea Systems Command launched a research effort in the early 1990s to develop a suitable fire suppression system without the use of chemicals. A paper by Robert L. Darwin and Dr. Frederick W. Williams noted Navy studies as early as 1978 fighting fires aboard submarines favorably endorsed water mist principles. In Navy tests, “mist was generated by commercial impingement-type atomizing nozzles operated at discharge pressures of 17-27 bar (250-400 psi). Droplets were estimated to measure in the 80-100 micron range. Modest success was achieved in extinguishing flammable liquid pan fires in simulated submarine machinery spaces and torpedo storage rooms.” The technology was not used at that time but since then, following more research and tests, water mist fire suppression systems have been specified for the Navy’s LPD-17.
The size of the droplets of water in different water mist systems affects both fire fighting capabilities and water usage. Very fine droplets can be created using nozzles with very small openings or sprinkler heads operating under higher pressure that sprays larger droplets out of the nozzle openings against a disrupter plate breaking it into smaller droplets. To achieve sufficient water pressure creating the atomized spray necessary for effective water mist extinguishing, the systems use either high-pressure water supply pumps or compressed gas.
One of the advantages of water mist over Halon 1301 or CO2 is that people do not have to evacuate an area prior to the activation of a water mist system since the water is harmless to personnel, according to Marioff, an important benefit to vessel operators. Jim Walsh, vice president of environmental health & safety at Carnival Cruise Lines, said, “No matter what type of gas agent is used you usually have to evacuate personnel and that means you don’t want the system tripping automatically. With water mist, if it trips automatically it doesn’t bother our personnel while it’s suppressing a fire.” The very rapid cooling effect of water mist in a fire zone “can allow fire fighting personnel to approach the fire much closer when the system is operating and that is one of the perceived benefits,” Eberly said.
The characteristic of water mist to turn some water to steam in a fire area has been considered a possible risk to personnel but is felt to be mitigated by the overall cooling effect of the working water mist system. And, noted Eberly, for fire fighting personnel “walking into where the temperature is hot can be detrimental but is generally much less of a concern than the fire itself.”
The exposure of personnel in confined areas to other fire fighting agents such as CO2 or the Halon gases has always been a greater concern. At temperatures in the 9000 F (4800 C) range, Halon can break down to bromides, chlorine and fluorides that are toxic, noted Walsh. However, the U.S. Environmental Protection Agency (EPA) brought water mist systems under review as alternatives to Halon and, according to Eberly, some manufacturers’ design features have raised concerns. Water mist is acceptable for occupied spaces as long as it is just pure water, but the additives proposed by some system manufacturers to improve fire suppression capabilities may be a problem. “Water mist systems putting out droplets that are less than 50 microns reach a magic point,” Eberly said. “Larger than 50 microns, things that are inhaled normally get stopped in the nose or lungs. But smaller than 50 microns increases the chance of it being absorbed into the blood stream through the lungs. So that rules out the use of additives unless they have been EPA approved.”
Ships of Royal Caribbean International’s 3,100-passenger Voyager class, under construction at Kvaerner’s New Turku yard in Finland, have water mist fire protection in a number of areas including galleys, machinery spaces and control rooms. A growing number of vessels, including the U.S. Navy’s new LPD-17, are opting for water mist systems.
Testing to find the limits
Since the early 1990s, intensive testing has focused on finding the limits of water mist systems’ ability to control fire in shipboard machinery spaces. In March 2000, the International Maritime Organization reported that, “If properly designed and tested, water mist fire suppression systems can provide effective protection of category A machinery spaces with volumes up to 500 cubic meters. The conclusions developed during these previous investigations also suggest that water mist systems may be inappropriate for larger machinery spaces due to the need for some degree of oxygen depletion to aid in the extinguishment of obstructed fires.” The IMO report noted that, “The conservative nature of the protocol due to high ventilation rates and smaller fire sizes (i.e. 1.0 MW) will limit the use of water mist in larger machinery spaces. Based on this analysis, the IMO concluded that it is highly unlikely that any system discharging only water will ever successfully complete the protocol for volumes greater than 2000 cubic meters.” A 1.0 MW fire is equal to one cubic meter of fire. The typical test fire size used to simulate engine room fires is 5.0 MW.
Eberly said the International Water Mist Association, headquartered in Vahldorf, Germany, has questioned the testing methods being used. One of the main issues they raised is the water mist testing was conducted in an enclosure with a 4 square meter vent opening. Although the water mist was tested as a replacement for total flooding CO2 or FM-200 Halon, those fire tests were in a sealed space without any vents. “So there is a feeling that water mist is being treated unfairly because of the huge vent opening,” Eberly noted.
The Coast Guard also conducted a series of water mist tests. There were 13 different tests for fuel spray fires and various sizes of fuel pan fires. The hardest test fire to extinguish was also the smallest – a half square meter fuel pan fire under an engine mock-up. Water mist systems are most effective with larger fires due to the water mist droplets’ expansion to steam due to the fire’s heat, resulting in the greater displacement of oxygen at the fire source. Smaller fires do not create large quantities of heat, thereby reducing the ability of the water mist system to produce steam and the subsequent reduction of oxygen flow to the fire. However, Eberly noted, the shipboard operational issue comes down to one question, “If the water mist can extinguish all the fires except the very smallest ones, is it a problem or not? You do have the crew trained to put out small fires.”
Tests conducted by the Coast Guard, the IMO and FMRC have studied the overall relationship between the volume of the protected area to the water mist’s ability to extinguish fires. The fire tests were conducted based on three categories of engine spaces: 500 cubic meters, 3,000 cubic meters and over 3,000 cubic meters. Extinguishment results of the water mist systems varied based on the volume of space. “Once you get above 2,000 cubic meters there are problems with extinguishment,” Eberly said.
For Carnival Cruise Lines, which is installing water mist suppression systems on new ships, the concerns of using water mist total flooding in very large machinery spaces is a valid point, said Walsh. “But our engine rooms are not as cavernous as on some cargo ships or tankers because of the way we have them segregated.” The new-build 88,500 ton, 2,124 passenger Carnival Spirit has water mist systems protecting the six diesel engines in the segregated compartments.
Installation factors give water mist systems an advantage over other extinguishing systems. A Navy study that compared alternative fire protection systems for machinery spaces on the LPD 17 ships put the cost of water mist at $330,000 compared to $540,000 for an FM-200 gas system. The water mist system had a footprint of 450 sq. ft. compared to 1,500 sq. ft. for a FM-200 system. The weight of water mist systems is also less when compared to conventional sprinkler systems due to the smaller diameter stainless piping it employs.
Water mist fire suppression systems have also gained the acceptance of Esso, Shell and BP to protect turbines, turbine drivers and other critical equipment on North Sea oil platforms. In one test, it took only 10 seconds and less than three gallons of water to extinguish a test crude oil and propane fire.
Marioff’s HI-FOG systems were first introduced into the marine markets in 1991 with installations aboard Eurway and Silja Europa cruise ferries. Among the latest applications of Marioff systems are Fortum Oil and Gas Oy’s two new-build 106,000 dwt double-acting tankers with ice breaking capabilities that will carry crude oil from North Sea platforms to refineries in Finland. In the U.S. new-build market, American Classic Voyages’ two new 72,000 grt cruise ships, under construction at Litton Ingalls Shipbuilding in Pascagoula, Miss., will also use HI-FOG water mist systems.
Also pushing the water mist extinguishing technology for marine applications are Heien-Larssen headquartered in Spikkestad, Norway, Semco in Bronby, Denmark, and U.S.-headquartered Grinnell in Houston with its AquaMist system. Heien-Larssen, a subsidiary of Kidde PLC in Colnbrook, England, has placed water mist systems on cruise ships, including Monarch of the Seas, North Sea oil platforms and other maritime applications according to Mike Donovan, marine marketing manager with a Kidde sister company, Kidde Fenwal in Ashland, Mass. “The IMO requirement for local application fire extinguishment systems to protect and contain any fire that breaks out in high hazard areas, such as around engines, is now being phased in,” said Donovan. Heien-Larssen is supplying marine customers with their flexi-FOG total flooding water mist systems and local application water mist that are used as supplemental systems when the ship’s primary main extinguishment system is gaseous such as CO2,” according to Heien-Larssen’s Bid Manager Lars Elsrud.
The versatility of water mist systems in both machinery areas and public areas is fully utilized on Royal Caribbean International’s 142,000 ton, 3,100 passenger, Voyager class cruise ships. Installations have been made in public areas including cabins, galleys, equipment areas, machinery spaces and control rooms. Each of Royal Caribbean International’s 142,000 ton Voyager class ships built at Kvaerner-Masa’s New Turku Shipyard in Turku, Finland, have Marioff water mist fire protection systems with almost 60 kilometers (37 miles) of stainless steel piping and 11,000 sprinkler heads. Berndt Lonnberg, naval architect and project manager at the New Turku Shipyard, said the water mist sprinkler piping installed on the Voyager class ships includes both dry pipe and wet pipe zones. Tests have shown cabin fires can be extinguished with as little as two liters of water, which also virtually eliminates water damage. He said the water mist’s advantage is to provide fire protection “not with great volumes of water but through the cooling effect of the water mist and that is far more efficient and faster than normal sprinklers.”
Fast response to any fire condition aboard a ship is vital. Fire fighting systems and technologies have improved over the years and the manufacturers of all systems can claim fire stopping records. Water mist systems have been successful stopping fires in engine rooms at sea. In addition to extinguishing the fire aboard the Nordic Empress, a water mist system put out a fire in the main machinery spaces aboard Silja Line’s SuperSeaCat, a high speed ferry operating between Helsinki, Finland and Tallinn, Estonia.
On August 14, 2000 a fire condition resulted when a turbo charger failed. The engines were shut down, “the HI-FOG system was deployed immediately in accordance with company procedures and the fire was extinguished. The vessel returned to Tallinn under own power,” according to a company report. The compliment to the water mist system came from David Stocks at Sea Containers Ltd., the owners of SuperSeaCat. He said the water mist system was fitted to the vessel to provide “quick response to an engine fire” and the water mist suppression system technology met Sea Containers. Ltd. goal to “kill the fire not fight it.”