RESPONDING TO FIRES IN INDUSTRIAL OVENS

RESPONDING TO FIRES IN INDUSTRIAL OVENS

BY BILL GUSTIN

My first encounter with an industrial oven turned out to be a dangerous, frightening fiasco. It occurred several years ago when I was a young company officer. My engine company responded to a waterflow alarm at a manufacturing facility. When we arrived, workers at the plant told us there had been an explosion and fire in one of the curing ovens but that everything was now “under control.” Since I was not familiar with the oven, the manufacturing process, or the procedures for handling such an incident, I based my decisions and course of action heavily on the information provided by the plant personnel. Employees led us to their ovens, which look similar in size and configuration to an automotive paint spray booth. The oven in question had doors, side panels, and some vent ductwork partially dislodged by the force of the explosion. Sprinklers overhead were operating in response to the heat that escaped through the damaged doors.

We learned that the plant manufactured “honeycomb” cores that are sandwiched between two sheets of aluminum to form aircraft floors and bulkheads. The core was made by dipping Nomext fabric into a polyester resin, which, after drying, became quite rigid and strong. The cores were cured and hardened in the oven by evaporating a vehicle of isopropyl alcohol from the resin. Evidently, alcohol vapors had accumulated and exploded.

Again, employees assured us that everything was under control. In fact, they said that fires in their ovens occurred frequently and that they handled them themselves. The fire department responded this time only because an automatic waterflow alarm had been transmitted.

Everything did look like it was under control. Sprinklers had extinguished fire that had escaped from the oven, and the gas supply to the burners was shut down. However, the ventilation ductwork to the oven was still hot, so I asked the plant personnel to shut off the exhaust blowers to prevent fire extension. This turned out to be a big mistake. A few minutes after I cancelled the rest of the responding companies, the first of four explosions occurred, each producing a blue fireball of alcohol vapor that blew out of the oven.

LESSONS LEARNED

This incident taught me some hard lessons:

The presence and severity of fires in industrial ovens and ductwork may not be readily apparent.

Since conditions may not be what they may seem, don`t be in a hurry to cancel other companies responding until you have thoroughly checked for fire extension.

One of the most important factors of sizing up a fire in an industrial occupancy is gauging the competency of the personnel and the reliability of the information they give.

Although shutting down the exhaust-ventilation blowers reduced the possibility of fire`s extending through ductwork, it allowed residual heat and alcohol vapors in the oven to build until they reached their explosive range and ignition temperature.

We had a lot to learn about industrial ovens.

GATHERING INFORMATION

My company and I began to familiarize ourselves with commercial ovens by surveying industrial occupancies in our response district. We found several facilities that used industrial ovens for a variety of processes: drying paint, paper and wood; curing plastics, resins, and phenolics; treating textiles; baking enamel and baked goods; cooking pharmaceuticals; and roasting coffee and cocoa. We asked personnel at each facility how they would handle a fire in their ovens and found many plants that had no procedures in place or even knowledge about fighting such a fire. In other facilities, however, we encountered highly competent engineers who taught us much about their ovens and became a valuable source of information for us concerning ovens at other facilities as well.

Each industrial facility must develop, post, and practice its own specific fire plan because ovens are designed and built for a specific industrial process; therefore, ovens and fire procedures vary greatly.

Very few general rules apply to every oven. Fire, operational, and maintenance procedures will be more accurate if they are developed with the technical expertise of oven manufacturers. Also, loss-control engineers, such as those at Factory Mutual, can be a valuable information source.

Plant managers should have realistic expectations of the fire department and not expect the firefighters to be experts in handling emergencies involving their industrial and manufacturing processes. Conversely, the fire officer will be looking to plant personnel for technical information and procedures for handling emergencies specific to their facility.

We have responded to several industrial fires in the early morning hours when no one on the night shift had a clue as to what to do. When managers went home in the evening, so did all the technical expertise. Consequently, we`ve found ourselves on the phone at 3 a.m. asking some manager who was home in bed how to handle a fire or other problem at his facility.

Fire procedures must be written, posted, and communicated to the fire department when they arrive. Without such direction, firefighters are likely to use too much water and may cause unnecessary damage and production downtime.

TYPES OF INDUSTRIAL OVENS

There are two general types of industrial ovens: batch ovens–in which contents are placed to undergo a purging-drying-heating process, and continuous ovens–which heat contents as they pass through on a conveyor or rollers. Of the two, the batch oven is inherently more dangerous because the contents are introduced into the oven all at once.

CAUSES OF INDUSTRIAL OVEN FIRES

The causes of industrial oven fires are many and varied. They include the following:

Spontaneous heating and autoignition of paint drippings, adhesives, coatings, and resins. Lacquer drippings are particularly prone to autoignition.

Slow, low-grade heating and carbonization of food crumbs, textile lint, sawdust, and other debris, which accumulate below oven racks or conveyors.

Exothermic reaction of materials such as foam rubber or composites saturated with resins. Heat-producing chemical reactions are more rapid and intense in a hot oven than at ordinary room temperatures.

Overheating due to operator inattention or error, or the failure of thermostatic controls. Food products such as coffee, cocoa, and peanuts are particularly vulnerable to overheating because they are roasted at close to their ignition temperatures.

Ignition of dusts and residue of condensed vapors in plenums and ventilation exhaust ductwork.

Explosions of flammable vapors produced by the heating and evaporation of vehicles from paints, coatings, adhesives, resins, and other low-flash-point solvents.

Factory Mutual Engineering (FM) property loss prevention data indicate that the majority of oven vapor explosions were caused by changing solvents or materials or by increasing the rate of heating in an oven without determining the evaporation rate of the new solvent or the adequacy of the ventilation system.

CHECK DUCTS FOR EXTENSION

Virtually all industrial ovens are equipped with powerful ventilation systems. Large blowers and ducts supply burners with air for combustion and dilute and exhaust flammable vapors. FM recommends that safety ventilation systems maintain an oven atmosphere of less than 25 percent of the vapor`s lower explosive limit (LEL). Further, FM recommends that ovens that evaporate flammable vapors go through an automatic preventilation cycle to purge flammable vapors before burners or conveyors can be started.

Ventilation ductwork can discharge vapors to a solvent recovery system; fume incinerator; or, where permitted, directly outdoors. Dusts are typically directed to a cyclone collector, where it is filtered by cloth filter tubes (also called “bags”; hence, dust collectors are often called “bag houses”) and deposited into a drum or hopper.

At one facility to which we responded, the oven can direct the exhaust flow to an incinerator; then to a bag house; and, finally, to the outside atmosphere. Powerful pneumatic dampers in the exhaust plenum direct the flow of exhaust to different routes of ductwork at different stages of the curing process.

An oven fire can spread through ductwork all over a building. It is critical, therefore, to determine where the ductwork is routed and to examine it at several locations before canceling any companies still responding. Residue can collect and burn at bends and low points in the duct system. Be sure to check ducts where they pass through combustible construction. Note the location of clean-outs, access covers, and dampers. Dusts and thick, gooey residue can burn hot and intensively inside ducts, especially when they are enriched and driven by airflow from powerful ventilation blowers. Fire issuing from the exhaust opening or ventilation ductwork can look quite spectacular because powerful fans give it a blowtorch effect. Also, first-arriving companies may report that the fire is “through the roof” because of oily residue burning on the roof at the location where the ductwork terminates.

Shutting down ventilation blowers to slow the spread of fire in ductwork sounds like a good idea. But, as we learned in the scenario at the beginning of this article, it can do more harm than good. Shutting down blowers reduces the flow of heat and vapors escaping from the oven; consequently, they can reach a concentration within their explosive range. Contents, such as a food product, can also overheat and burn. If installed in the ductwork, sprinklers can operate in response to elevated temperatures and cause unnecessary water damage. Also, industrial oven fires are typically smoky fires. Shutting down blowers reduces the amount of smoke that is exhausted and results in more smoke pushing from the oven and filling the building. The time to find out if and when it is appropriate to shut down ventilation fans is during prefire planning–not over the telephone at three o`clock in the morning.

Don`t spray a lot of water into exhaust ductwork; it is seldom necessary and can cause additional damage, especially when water runs into a hot oven. It can precipitate a steam explosion or warp and distort racks and conveyor belts.

We`ve had good results leaving exhaust fans running and applying short bursts of water fog or dry chemical into a clean-out or other opening. Blowers can carry water droplets or dry chemical a considerable distance in ductwork and extinguish fire remote from the point of application. Where there are no openings, you may have to make your own in ductwork to check for extension and apply extinguishing agents. An ax will effectively penetrate sheet metal ductwork, but an air chisel or piercing nozzle does a more meticulous job that is easier to repair. Get a company to the roof as soon as possible to check for and extinguish roof-covering material that has been saturated with oily residue at exhaust duct outlets.

Carefully examine the bag house to see if fire has extended and hope that it hasn`t. A fire in a dust collector can be a miserably dirty, difficult, and dangerous job. Chances are good that every filter bag will have to be removed and overhauled and that the hopper will have to be emptied of smoldering dust. Be careful; agitating finely divided dusts can cause an explosion.

Shutting down the conveyor on a continuous oven is another action that may not be advisable. We had intentions of shutting down the conveyor when we responded to a fire in a baking oven that was more than 100 feet long. Fortunately, engineers were present at the bakery to explain that stopping the conveyor would in effect be like turning the oven into a barbecue grill–directly exposing almost 100 cakes to fire in crumbs burning below the conveyor belt. Also, they explained, leaving the cakes inside the oven would eventually overheat them until they reached their ignition temperature.

Use water sparingly, if at all, inside a hot oven. Remember, this is a confined space that is well above the boiling point of water. Water liberally sprayed into an oven can precipitate a steam explosion. Also, rapid and uneven cooling of racks and conveyors can warp and distort them. Before applying any water, first determine if it is necessary. It may be better to just let burning residue consume itself. Contents, on the other hand, will probably have to be removed. This means operating the conveyor on a continuous oven or opening the doors of a batch oven–something that can be dangerous without proper direction from someone in the plant who knows how to do it safely. It`s not uncommon for overheated contents to burst into flame when they are removed from an oven. Once you`ve determined that the fire is confined to the oven and hasn`t spread through the vent system and that fuel to the burners has been cut, the problem becomes less urgent.

You may choose (hopefully in collaboration with plant personnel) to adopt a wait-and-see strategy and allow the fire to burn out. Water, if indicated, should be applied in very short bursts of fog, allowing steam to fill the oven and saturate the contents. Dry chemical can also be effective on small fires but tends to allow solid residue to smolder and reflash.

Again, prefire planning–developing and posting plans for oven fires–is extremely important. A factory manager who relies on the fire department to take the proper action when his industrial oven catches fire and doesn`t provide direction for the fire department is gambling with his business and the safety of his employees and the firefighters.

MANY VARIABLES

There are just too many variables in an oven fire to proceed without guidance from the facility`s management. Too many questions demand immediate answers:

Has the fire spread to the ventilation-exhaust system?

Have the ventilation blowers been shut off? Should they be shut off?

What is the flash point of the solvents heated in the oven?

Where does the vent ductwork lead?

Where are the clean-outs?

Where is the bag house? Has it been checked for extension?

Are there any protection systems (sprinkler, water spray, CO2) that can be used to extinguish the fire in the oven and ductwork? Note: Automatic sprinkler systems that protect ovens typically use sprinkler heads with temperature ratings far in excess of those found in the other portions of the building–if you replace the heads, use the correct replacement heads.

At what other location can the fuel be shut down if it`s too dangerous to shut it off at the oven?

Should the conveyor be shut down?

If the conveyor automatically shuts down when fuel or ventilation is stopped, how can the interlock be overridden so the contents of the oven can be removed?

Is it safe and appropriate to open batch oven doors and remove the contents? If so, when should it be done?

How should the fire be extinguished?

Should the fire be extinguished or be allowed to burn out?

FIRE PREVENTION AND PROTECTION

The real keys to preventing damage and costly production downtime in industrial ovens are proper operation, maintenance, and housekeeping and properly functioning fire detection and suppression systems. Industrial ovens must be cleaned regularly of combustible lint, food particles, dust, and drippings. Exhaust ductwork must be cleaned of dusts and oily deposits or flammable vapors.

Factory Mutual Engineering and NFPA (National Fire Protection Association) 86, Standard on Ovens and Furnaces–1995, stipulate several safeguards that can help to prevent and suppress accidental oven fires. They include the following:

Ovens, exhaust ducts, and bag houses that contain combustible vapor, dusts, or residue must be sprinklered.

Interlocks that do the following should be provided:

–prevent burner start-up before purging the oven of flammable vapors,

–shut down burners if ventilation-exhaust fans stop running, and

–shut down burners if the conveyor stops.

A temperature limit switch that automatically shuts down burners at a predetermined maximum temperature should be installed.

Infrared detectors that activate a water spray system to extinguish fire in ducts and prevent extension to dust collectors should be present.

Recommended explosion venting calls for covers and panels fastened by a weak seam or frangible shear pins in ovens and ducts; doors should be secured with latches that will release at designed pressures.

For more complete information on standards, requirements, and recommendations, consult NFPA 86 and Factory Mutual Property Loss Prevention Data Sheet #6-9.

Today`s firefighters are being pulled in many different directions. The public demands a lot from us and expects us to be proficient in many different roles: technical rescue specialist, wildland firefighter, hazardous materials technician, medic, and soon (I`m afraid) social worker.

But industry must not expect public fire departments to be the experts in handling fires and other emergencies at their facilities. Industrial ovens, for example, are too specialized and require a site-specific plan, which, hopefully, will be developed before the oven is involved in a fire or an explosion. n

I gratefully acknowledge the technical assistance provided by Factory Mutual Engineering in preparation for this article.

(1) Rolling resin-dipped blocks of Nomext fabric into batch oven. Oven dries and cures blocks by evaporating the isopropyl alcohol vehicle from the resin. Note previous fire damage from flammable alcohol vapor. (2) Oven with resin-saturated fabric blocks inside. Two cylindrical ducts at left and right are for explosion venting. Middle square duct is exhaust ventilation. At industrial oven fires, firefighters must determine the route of the vent ductwork and examine it in several places for fire extension. (3,4) Explosion vent outlets for oven in photo 2. Note the oily residue on the ducts and the roof. (5) Blocks of resin-saturated fabric underwent an exothermic reaction and autoignited as they were being cured in the oven. (6) Interior of oven coated with drippings and residue of condensed resin vapors. Industrial ovens must be cleaned at regular intervals. (Photos by Raul Torres.)

(Left) Continuous oven treats fabric as it passes through on rollers. (Right) Oily residue leaking from exhaust duct from oven in photo at left. Oily fabric lint is extremely combustible. Lack of regular cleaning is a leading cause of fires in industrial ovens and their ventilation systems. (Photos by author.)

(1) A fire extending through the ventilation exhaust ducts spread into the dust collection system and required extensive overhaul. (Photo by Justin Wasilkowski.) (2) Ventilation exhaust from industrial ovens at this factory can be directed to a fume incinerator (left, background), dust collection bag house (building in right, background), or the outside. Determining the route of ventilation ductwork can be extremely difficult for firefighters, who cannot be very familiar with every industrial occupancy to which they respond. (Photo by Raul Torres.) (3) Examining the bag house for fire extension by way of the ventilation-exhaust ductwork. (4) The clean-out in the ventilation-exhaust ductwork enabled firefighters to check for fire in the duct and to apply a fog stream. (Photos 3,4 by Justin Wasilkowski.)

(1) The powerful exhaust blower spread fire from the industrial oven to the ventilation ductwork. Whether to shut down blowers or leave them running is a critical decision. (Photo by Justin Wasilkowski.) (2) Ventilation ductwork from an industrial oven must be examined for extension in several places, especially where it passes through combustible roof construction. (Photo by Raul Torres.) (3) Fire issuing from the exhaust opening of ventilation ductwork can look quite spectacular because the powerful fans give it a “blowtorch” effect. Also, the fire may look like it`s “through the roof” because of oily residue burning on the roof. (Photo by author.) (4) Blowout panel in exhaust ductwork design relieves pressure of explosion. (Photo by Bill Gustin Jr.)

(Far left) This continuous baking oven is more than 100 feet long. (Left) Food crumbs and melted sugar accumulation below the conveyor belt is a common fire hazard. Industrial ovens must be cleaned at regular intervals. (Photos by Justin Wasilkowski.)

(Far left) Latch on blast door is designed to release to vent the pressure of an explosion in the oven. (Left) This oven, which evaporates flammable resins, is equipped with a deluge sprinkler system as required by the National Fire Protection Association and Factory Mutual. (Photos by Raul Torres.)

BILL GUSTIN is a captain with the Metro-Dade County (FL) Fire Department and lead instructor in his department`s officer training program. He began his 26-year fire service career in the Chicago area and teaches fire training programs in Florida and other states. He is a marine firefighting instructor and has taught fire tactics to ship crews and firefighters in the Caribbean countries. He also teaches forcible entry tactics to fire departments and SWAT teams of local and federal law enforcement agencies. He is an editorial advisory board member of Fire Engineering.