Cooking equipment is the leading cause and is responsible for over half of fires in eating and drinking establishments (see Warren expert Chad Jones’ 2020 blog, Structure Fires in Eating and Drinking Establishments, for further reading on fire causes and NFPA 96 on duct inspection and cleaning). Fire extinguishing systems are also routinely provided over GREASE-producing cooking appliances. So why are some of these fires still so bad? GREASE is the word.
NFPA 17A, Standard for Wet Chemical Extinguishing Systems and NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations go together, like rama lama lama, ka dinga da dinga dong. Why is that? NFPA 96 covers which cooking appliances are required to be covered by a fire extinguishing system and how to build and maintain (including inspection and cleaning of) the exhaust system for them. NFPA 17A covers the fire extinguishing system itself: its installation, inspection and maintenance. Consider that, as the level of grease accumulation increases, so does the fire hazard and likelihood. At the same time, the likelihood also goes up that your hood fire extinguishing system will malfunction or be unable to put out that fire.
Let’s look at a typical wet chemical fire extinguishing system and the major components, then we’ll discuss briefly how it works. Refer to the figure below. The system will consist of:
- a storage tank for the wet chemical extinguishing agent
- distribution piping and discharge nozzles to deliver the agent to the protected areas (piping is dashed-gray, nozzles are colored yellow)
- a detection system (conduit is dashed-red; thermal links are red with black outline),
- a control/release panel or box
- a manual release, and
- a fuel shut-off (to gas or electricity; in the figure this would be gas).
If there’s a fire, a thermal link on the detection system (3) will break and release tension on the cable detection system. Components in the control panel (4) release the wet chemical extinguishing agent from the tank (1), which moves through the discharge piping and flows out from the nozzles (2), all at once. The extinguishing agent works to put out the fire by producing a synthetic foamy mass (through saponification) on the surface of hot or burning grease. This foam layer acts to smother a fire and serves to prevent re-flash until the fuel cools. Components in the control panel simultaneously release the gas valve (6) or electrical interlock, which shuts down the appliances’ energy source(s). This is critical because it starts things cooling down.
The discharge nozzles protect the appliances, plenum, and first section of the exhaust duct. The system has only one shot at a fire. Consider that the figure above does not show the entirety of the exhaust duct system. The fire protection system doesn’t cover the entirety of the exhaust duct. Consider what the “?“ in the figure above represents in the exhaust system at your place, or at places you’re familiar with. Does that duct run from the hood straight up through the roof? Does that duct elbow 90 degrees, run horizontally for a bit, then turn vertically and run to a roof 4 stories up? Or maybe it zigs and zags horizontally like a directionally-challenged snake for 100 feet before exiting a side wall? Are all portions of it, from hood to open air, safely accessible for inspection and cleaning? All portions of the exhaust system must be inspected and cleaned as needed. “Let’s keep it clean, people. Let’s keep it clean.” – Coach Calhoun
Let’s talk about how GREASE specifically (there are other factors) might keep the automatic wet chemical fire extinguishing system from working correctly.
- If GREASE is built up on the heat-detection parts, that build-up can insulate them such that they don’t operate in response to the heat of the fire.
- GREASE build-up can freeze the linkage on the detection system such that the cable can’t move and activate the system. Above there is considerable build-up around the cable and end of a thermal link’s linkage bracket. Do you think the cable was free to move even when the thermal link released?
- GREASE can enter the conduit of a cable-type detection system and plug it up.
- GREASE can plug up the distribution nozzles or glue the nozzles’ protective caps on (make sure the caps stay in place, but are free to move).
- Accumulations of GREASE can increase the fire hazard beyond what the fire extinguishing system can handle.
- Accumulations of GREASE can end up in places beyond the coverage (reach) of the fire extinguishing system. This can include behind, under, or inside cabinets of appliances. This can include those parts of the exhaust system that are not covered by the fire extinguishing system.
The one that I want – for kitchen exhaust system cleaning. Hire carefully – a guy off the street with a pressure washer is not the one that you want to clean your kitchen exhaust system. The individual performing the work needs to be CERTIFIED. Why is this important? Because they need to know the requirements. Because if they don’t clean the entire exhaust system as it should be cleaned, you could have an uncontrolled fire. The fire extinguishing system is only going to do what it’s designed to do, and if there’s more GREASE in the duct than the fire extinguishing system was designed to handle, all bets are off that it will work.
Case Study: “Let’s keep it clean, people. Let’s keep it clean.” – Coach Calhoun
Once upon a time, there was undue and unnecessary GREASE built up in the exhaust system of a restaurant. A flare-up in a broiler ignited this grease. The kitchen fire extinguishing system also failed to actuate automatically, and no one pulled the manual release either.
Case Study: Greased Lightnin’
An uncertified cleaner had just cleaned some portion of the hood and ducts at this restaurant. Upon leaving, he left the exhaust fans off. The fans were not turned on before appliances were started up the next day. Hotter-than-usual gases collected in the hood and exhaust, igniting grease deposits that remained after the cleaning. The hood fire extinguishing system did not activate, most likely due to accumulations of grease on the thermal links. The non-compliant duct system allowed the fire to spread into the combustible attic space and it took off like Greased Lightnin’. At the end of the day this restaurant was well-done.
Case Study: Fire out of reach
Pedestrian passers-by were among those to report fire shooting out the roof from this restaurant’s exhaust system termination. At least one hood had been operating for some time with broken and missing grease filters, likely permitting abnormal amounts of grease to accumulate in the ducts. Line workers were burning off a grill’s grates just prior to the fire reports. The hood fire extinguishing system did activate but was not able to affect the fire where it involved grease accumulations past its coverage area.
For further reading, also see Warren’s 2014 blog on Annual Hood Suppression System Inspection (https://www.warrenforensics.com/2014/06/19/annual-hood-suppression-system-inspection/ )
Amy Anderson, PE has a Bachelor of Science in Chemical Engineering from Clemson University. Amy has over 20 years of engineering experience including property loss prevention engineering specializing in fire protection, chemical and pharmaceutical facilities. She has partnered with clients to identify, assess, avoid, and reduce risk at their commercial and industrial properties. Additionally, she has assisted with the development of building and fire protection specifications, reviewed plans and performed site visits. She has reviewed project documents for compliance with applicable standards – construction, fire protection, process, and combustible dust hazards. Amy is a member of the National Association of Fire Investigators, the Society of Fire Protection Engineers, the National Fire Protection Association and the American Institute of Chemical Engineers.