Smoke Explosion Kills Two Boston Firefighters
A sudden fire development that occurred in a Chinese restaurant fire in the West Roxbury section of the city killed two Boston firefighters on 29th August 2007. The fire, that appeared to originate within the ceiling space, was fuelled by a possible build-up of cooking grease in the ceiling area or grease duct. The fire was contained to a small area behind the fiber ceiling space as firefighters arrived. However, a sudden development of the fire, possibly caused by a smoke explosion, caused the firefighters to become immediately disoriented and the subsequent fire killed two of them and injured several others.

Source: Boston Globe

Boston Fire Chief Kevin MacCurtain stated that firefighters initially arrived at Tai Ho Mandarin and Cantonese restaurant "thinking to themselves, 'This one is going to be easy.' " But fire officials now believe that the fire had been smoldering above a false ceiling that concealed it for more than an hour while it created an explosive mix of gas and heat. The explosion and fire caused a critical collapse of the roof area that further served to trap some of the firefighters. 

A digital photo (above) taken on a bystander's cell phone showing the terrible power of a fireball that killed two Boston firefighters is helping investigators understand more about what happened at the Tai Ho Restaurant in West Roxbury. "I've never seen such a violent sudden flashover, if that is what we're looking at," said Boston Firefighters Union President Edward Kelly. The picture has been circulated throughout the entire firefighting community and scrutinized by investigators. "What I think we're seeing is possibly a backdraft condition or a violent, sudden flashover," Kelly said. Kelly said those are the two possible scenarios investigators are now looking at. FLASHOVER INFORMATION HERE

Paul Grimwood reports ... ' I have attended around 150 serious grease duct fires as I was stationed in the heart of a thriving inner city restaurant area for several years. Lots of white smoke that suddenly turns dark and black as the fire development becomes under-ventilated. I have seen these fires spread through and beyond ducting to take several floors of buildings but usually, the fire is mostly contained to the ducting itself. I have never personally heard of, or experienced, a smoke explosion in this type of fire. There was one incident in New York in 1997 where such a fire did reportedly explode in restaurant's cooking exhaust system in a high-rise building HERE. However, such events are rare. It is also worth noting that smoke explosions in fiber-based suspended ceiling spaces are not so rare. In fact there are several references to such incidents and I have been writing about this particular hazard since 1990'.

In the USA, grease ducts must be in compliance with NFPA 96 as well as the local building codes and fire codes. Cleaning takes place typically every 3 months, 6 months or annually, depending on the nature of the appliances below the hood. For instance, woks require quarterly grease duct cleaning, whereas normal stoves may necessitate the grease duct to be cleaned only every 6 months. Compliance must be proven through certificates issued by the cleaning and maintenance contractors. Purpose-designed fire suppression systems inside the hoods must also be routinely maintained. Proper cleaning must be enabled through the use of approved, fire-resistant access panels. Grease ducts should be kept as short as possible to minimize grease build-up.

Grease, of course, is highly flammable. In fact, it qualifies as a hydrocarbon due to its inherent chemistry. Regardless of what state it is in, vapor, liquid or solid, it ignites easily and burns very rapidly, necessitating special provisions to accomplish a fire-resistance rating based on an internal grease fire as well as an external fire. Special provisions also include the necessity for proof that any adjacent fire-stop must be compatible with the grease duct system. Grease duct fires can reach temperatures of 1,600°F (871°C) to 2,000°F (1,093°C) within minutes, hot enough to melt aluminum and ignite surrounding combustibles within seconds.

Kathy A. Notarianni, head of the Department of Fire Protection Engineering at Worcester Polytechnic Institute, said the fire raises questions about whether the fume hood and grease duct were properly installed. Most grease ducts contain detectors that automatically release a liquid fire retardant when they sense flames, she said. The National Fire Protection Association recommends that restaurant owners conduct quarterly inspections of their kitchen hoods and vents, Notarianni said. "One or more things seem to have broken down here," said Notarianni. "There shouldn't be that accumulation [of grease], and, even if there is and it catches fire, it should have been detected [by a detector] in the duct."

The principal fire risk in kitchen extract ventilation is created by the excessive build-up of cooking oil deposits. Different cooking styles will inevitably create different grease residues, for example, deep frying produces grease similar to translucent creosote, frozen foods containing large quantities of water create a hard shiny layer of grease. Oriental style cooking creates a very sticky, syrup-like grease that can become very adhesive to metal surfaces. Meats cooked on solid fuel ranges or charbroiled produce large quantities of grease. A first layer of grease will bond to metal surfaces, and then subsequent layers of black carbon will build-up created by ash and grease from the cooking process.

It is also important to remember that at temperatures above 200 degrees C flammable vapors are given off from cooking oils and that spontaneous ignition can occur at temperatures between 310 and 360 degrees. So it can be seen there may only be a short time from safe to potentially dangerous conditions if thermostats are not working correctly. It is also worth noting that the flashpoint of cooking oil is reduced by progressive oxidation as a result of repeated use. Deposits of some mixtures, such as chicken fat and vegetable oil are particularly easy to ignite.

Any fire will require ignition, fuel and air, and these are usually present in a commercial kitchen. All cooking equipment forms a potential source of ignition, from the flames of gas-fired equipment through deep fat frying apparatus to electric equipment such as toasters, fryers and griddles. We have seen above how fuel is available in the form of cooking oils, but other combustible materials may be sited adjacent to exhaust ducts, and of course there is the power supply to the catering equipment itself e.g. gas. Finally, plenty of air will be available to supply the extract system.

The risk assessment should be conducted in five separate stages: identifying ignition risks; the assessment of risks; evaluation of the risks; keep records; and finally review and revise the assessment after a period of time.

The fire risks in a kitchen

There are several primary risks of fire in a kitchen:

• Flames, sparks or hot gases from cooking can ignite combustible deposits inside extract ducts
• Superheated oils leading to spontaneous ignition
• Fan-motor failure or overheating caused by hardened grease when restarting in seasonal catering establishments on non-24 hour operations
• Thermostats not working correctly, and the absence of a second high level safety thermostat
• Individual equipment not switched off, especially on cessation of business
• Metal extract ducts are good conductors of heat and can ignite nearby building materials or litter
• Catalytic converters decompose grease, but operating at 1000 degreesC are a potential source of ignition
• Solid fuel cooking equipment such as barbeques
• Tandoori ovens without igniters/pilot lights lit by burning pieces of paper/absence of flame failure or safety shut off device
• Gas torches used to brown some dishes
• Cooking equipment which is left unattended during operation.

In addition to the above there are further risk factors to consider:

• Lack of a competent person on site
• Human error
• Faulty or non-tested electrical appliances
• Design aspects of the extract ventilation, such as long duct runs, horizontal ducts, type of fan, type and number of duct access doors
• Cleaning contracts may only cover hoods and easily accessible and visible areas such as those parts which are only within arm's reach
• Combustible food debris trapped in the grease filter
• Remnants of paper napkins or other combustible oddments may have been inadvertently left in cooker hoods or inside the extract ducting
• Level of competence of cleaning contractor
• Poor siting or failure of fire suppression system
• Inaccessible extract ducts
• Insufficient access doors to allow inspection and cleaning
• Unsuitable ductwork for a kitchen environment
• Lack of knowledge about the extract system.