Fires and Explosions in Ovens and Furnaces

By: John Holecek

What are Industrial Ovens and Furnaces?

• We are not talking about home cooking and home heating systems.

• The terms oven, furnace, and kiln are used somewhat interchangeably. Often vary as much by industry than inherent differences in design.

• The range of ovens, furnaces and applied heat transfer devices used in industry is quite large.

• Lets look at some examples.

Dryers

• Dryers are used to evaporate a liquid, often water, from a product.

Dry Kiln’s are used to dry water and volatiles from green lumber.

Grain dryers are used to dry grain before storage.

Baking Oven

• Baking Ovens are used to heat products and thereby cause a physical change in the product. They will also dry products.

Paint Baking ovens cure coatings by a heat catalyzed chemical reaction.

Food Baking Ovens are used to cook foods such as chicken and bread.

Kilns

• Kilns are essentially high temperature ovens, usually operating at 1000ºF+.

Brick Kilns are used to fire bricks, typically at approximately 2500F.

Rotary lime kilns create lime from limestone by a heat catalyzed chemical reaction.

Furnaces

• Furnaces are also used to heat products, often at higher temperatures, and thereby cause a physical change in the product.

Heat treating furnaces change the properties of metal.

Furnaces come in many different sizes, from small units shown above to large units as shown on the left.

Autoclaves

• Autoclaves: Ovens that operate at substantially non-atmospheric pressures either pressurized or at vacuum. They may also have specially constituted atmospheres.

Thermal Oxidizers

• Thermal oxidizers: Used to thermally incinerate undesireable or hazardous compounds into less problematic components. May also use catalysts to assist in breaking down the compounds.

Other Heating Devices

• Many applied heat transfer machines use some of the same design elements as industrial ovens.

Thermal oil heaters. Heated parts washers share some safety design elements with ovens.

Other Heating Devices

Thermal Oxidizer

Water Heater

Biomass Fired Boiler

What types of losses occur with ovens and furnaces?

Explosions:• From fuel supply• From product being processedFires:• From product being processed• From combustible residuesWorker’s Compensation:• Injuries from fires and explosions• Electrocutions• Burns from hot surfacesLoss of Business Income Failure to Perform

What defines the correct way to design, build, and operate ovens and furnaces?

• Codes and Standards:

Codes have been written in a format suitable for adoption into law independent of other codes and standards. An example is the International Fire Code.

Standards are documents containing mandatory provisions using the word “shall” to indicate a requirement. They are written in a form such that their provisions become mandatory when referenced by another code or standard. An example is NPFA 86: Standard for Ovens and Furnaces.

• Good Engineering Practice• Design Guides• Normal Practices / State of the Art

Codes and Standards that directly reference oven design:

• International Fire Code (IFC): Has a whole chapter on industrial ovens. Incorporates compliance with NFPA 86 by reference. The IFC is legally adopted in 42 states.

• NPFA 86: Standard for Ovens and Furnaces: The “textbook” of safe oven design.

• NFPA 13: Standard for the Installation of Sprinkler Systems: Gives information on installing sprinklers in ovens.

Other codes, standards and guides that relate to safe oven design:

• NFPA 54, National Fuel Gas Code, Gives proper methods for fuel gas piping.

• NFPA 31, Standard for the Installation of Oil-Burning Equipment, Gives proper methods for fuel oil piping.

• NFPA 58, Liquefied Petroleum Gas Code, Gives proper methods for storage, handling, transportation, and use of LP-Gas

• NFPA 70, National Electrical Code®, Gives proper methods for electrical wiring of ovens.

• NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Noncombustible Particulate Solids, Gives proper methods of exhausting ovens.

• NFPA 68, Guide for Venting of Deflagrations, Gives proper methods for explosion venting.

• FM Data sheet 6-9: gives excellent guidance on the proper design of ovens.

What are the requirements for permitting ovens and furnaces?

• Requirements vary by location. Different states, counties, and cities can have different requirements.

• Where the International Fire Code is enforced, fire code officials are “authorized” to require building and operational permits for installing or operating an industrial oven.

• Most states issue specific modifications to the IFC which may define if permits for ovens are required or may give discretion to the permitting official.

Ovens are separated by class (per NFPA 86):Class A Furnace. An oven or furnace that has heat utilization

equipment operating at approximately atmospheric pressure wherein there is a potential explosion or fire hazard that could be occasioned by the presence of flammable volatiles or combustible materials processed or heated in the furnace.

(Translation) Ovens that are not under pressure or vacuum and process combustible products or dry / cure flammable coatings. Examples: Many paint baking ovens, lumber kilns.

Class B Furnace. An oven or furnace that has heat utilization equipment operating at approximately atmospheric pressure wherein there are no flammable volatiles or combustible materials being heated.

(Translation) Ovens that are not under pressure or vacuum and process noncombustible products and do not dry / cure flammable coatings. Examples: Brick kilns, lime kilns, dryers for non flammable liquids on non combustible products

More oven classifications:

Class C Furnace. An oven or furnace that has a potential hazard due to a flammable or other special atmosphere being used for treatment of material in process.

(Translation) Ovens and furnaces with potentially flammable or hazardous atmospheres. Example: Metal heat treating ovens with flammable gas atmospheres, integral quench furnaces and molten salt bath furnaces

Class D Furnace. An oven or furnace that operates at temperatures above ambient to over 5000°F (2760°C) and at pressures from vacuum to several atmospheres during heating using any type of heating system. These furnaces can include the use of special processing atmospheres.

(Translation) Ovens that operate under substantial vacuum or pressure. Example: Autoclaves for curing composite materials.

Design Requirements

We will look at the following elements of oven design:

• Housing Construction and Ductwork

• Ventilation

• Fire Protection

• Heating Systems

• Control Systems

• Operational Support

Housing Construction

• Noncombustible housing materials required– No wooden structures – No plastic foam insulation

• Provide clearance to combustible surroundings– Keep adjacent materials below

160F • Must provide clearance on all

sides and adequate maintenance access

• Class D ovens with pressurized housings above 15psi must be designed to the ASME Boiler and Pressure Vessel Code

Housing Construction

Explosion Relief: Specially designed provisions that allow for freely relieving internal explosion pressures. Prevents an exploding oven from becoming a giant hand grenade. Required on all fuel fired ovens with the following exceptions:– Indirect fired ovens with demonstrated low levels of

combustible / flammable vapors– Class D ovens (Autoclaves) or Thermal Oxidizers– Certain high temperature furnaces that are made of

minimum 3/16” thick steel, structurally reinforced, and refractory lined.

– Certain low oxygen type furnaces

Housing Construction• Explosion Relief:

– 1 ft2 of relief area is required for every 15 ft3 of oven volume

– Often explosion relief is accomplished by leaving some roof panels simply laid in place without significant restraint

– Openings, and doors with listed hardware are included in the relief area

– Basis of design should be that panels relieve explosive pressure before the oven’s safe design internal pressure limit is exceeded

– Heavy materials should not be placed on relief panels

– Extensions to the oven such as heater houses and ductwork should be included in the oven volume amount when calculating relief area

Ductwork

• Ducts must be made of noncombustible materials

• Include provisions for cleaning out ducts, especially when products generate combustible dusts or residues. Generally this requires clean out doors.

• Guard or insulate surfaces over 160F.• When vapors are likely to have heavy

concentrations of condensable gasses, the duct should be insulated. Otherwise a heavy buildup of combustible residue will form in the duct and present a fire hazard.

• Keep hot surfaces of ducts away from combustible surroundings. This is frequently improperly done at the exhaust duct penetration through the building roof.

• NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Noncombustible Particulate Solids, has additional requirements

An example of a correct penetration of an exhaust duct through a building roof

Duct

Bar Joists

Rain Skirt

Air Gap Per Code

Combustible Roof Material

Steel Support

Fire Protection

Ovens and furnaces are frequently equipped with fire protection systems. These systems include wet pipe, dry pipe, deluge systems. Some ovens are installed with special fire protection such as Clean Agent or Water Mist systems. The most common practice is to install a wet pipe system with the supply piping located outside the oven.

The International Fire Code and NFPA 86 currently differ somewhat in the requirements for fire protection systems. Let’s look at what they say.

2012 International Fire Code on Oven Fire Protection

SECTION 2106FIRE PROTECTION

2106.1 Required protection. Class A and B ovens which contain,or are utilized for the processing of, combustible materialsshall be protected by an approved automatic fire-extinguishingsystem complying with Chapter 9.

2106.2 Fixed fire-extinguishing systems. Fixed fire-extinguishing

systems shall be provided for Class C or D ovens toprotect against such hazards as overheating, spillage of moltensalts or metals, quench tanks, ignition of hydraulic oil and escapeof fuel. It shall be the user’s responsibility to consult withthe fire code official concerning the necessary requirements forsuch protection.

2012 International Fire Code

Clearly, The International Fire Code often requires that industrial ovens be equipped with fire protection systems.

Let’s see what NFPA 86 has to say about fire protection in industrial ovens.

NFPA 86 (2011)– Fire Protection System Requirements

“A study shall be conducted to determine the need for fixed or portable fire protection systems for ovens, furnaces, or related equipment. This determination of the need for fire protection systems shall be based on a review of the fire hazards associated with the equipment. Where determined to be necessary, fixed or portable fire protection systems shall be provided. “

The Explanatory material in Appendix A of the Standard does attempt to clarify things:

“Automatic sprinkler protection should be considered for ovens, furnaces, or related equipment if any of the following conditions exists: (1) The material being processed is combustible. (2) Racks, trays, spacers, or containers are combustible. (3) If there are areas where appreciable accumulations of combustible drippings or deposits are present on the inside of the oven surface or on racks, trays, and so forth.

The type of sprinklers and arrangement should be appropriate to the oven arrangement, interior ductwork, and the material passing through the oven.Note: Past versions of NFPA 86 required fire protection in ovens

processing combustible materials.

Wet Pipe System

Wet pipe sprinklers systems are the most common type of fire protection installed in industrial ovens. High temperature heads are typically rated 50F above the high temperature limit setting for the oven.

As stated in the appendix of NFPA 13, Standard for the Installation of Sprinkler Systems:

“The preferred arrangement for piping is outside of the oven; the sprinkler should be installed in the pendent position. The sprinkler temperature rating should be at least 50°F (28°C) greater than the high-temperature limit setting of the oven or applicable zone. As a minimum, the sprinkler system inside the oven or furnace should be designed to provide 15 psi (1 bar) with all sprinklers operating inside the oven/furnace. Sprinkler spacing on each branch line should not exceed 12 ft (3.7 m).”

Specialized Systems

• Deluge Systems : Open heads are positioned in the oven. Water is supplied by a valve in response to actuation of spark, flame or temperature sensors mounted in the oven or ducts.

• Steam Suppression Systems: Specialized approach that is seldom used and actually discouraged in NFPA 86. Care must be taken to contain steam in the oven. Steam can burn persons near oven openings.

• Water Mist Fire Protection Systems: Essentially, a deluge system operating at high pressure with small, strategically located nozzles. The water mist created by higher pressures control or extinguish fires by cooling of the flame and fire plume, oxygen displacement by water vapor, and radiant heat attenuation. Used when fast suppression is desired and large volumes of water may be problematic.

• Dry Chemical or CO2 Systems: Specialized systems used where the introduction of water cannot be allowed. For example, an oven used in the manufacture of products containing reactive metals like lithium or sodium.

Ventilation

• A cornerstone safety design concept for industrial ovens is that fresh air ventilation keeps the levels of flammable or combustible constituents below levels that can cause an explosion.

• In fact, NFPA 86 stipulates that four times the minimal level of ventilation be present. This should maintain a 4 to 1 safety factor against explosive atmospheres.

In many systems, solvents may not be released in a uniform manner. The way in which products are loaded may produce peak solvent levels far above the time weighted average. This must be verified, especially for batch processing type ovens.

Proper Ventilation Design

• Required fresh air supply rate =– 12,000 cubic feet of fresh air for every gallon of solvent

evaporated– Plus volume of products of combustion released into the oven

• The above is the common and simple calculation for the volume of fresh dilution air. It has been calculated using common solvents.

• The volume can be calculated in a more involved manner that uses the specific properties of the actual volatiles in use.

• It’s very important to remember that every oven has a design basis for the amount of volatiles released per unit of time. If the oven is loaded in a manner that overwhelms the safety factor, an explosive environment may result.

• Any dampers in the exhaust or fresh air ductwork must be made so that even when closed they will pass the required safe amounts of airflow.

Proper Ventilation Design• The exhaust rate for the oven is based on the fresh air supply

rate. They are equal on a pound per minute basis. However, because air expands as it heats, the volumetric flow rate of the exhaust fan must be corrected for the temperature of the oven. For example: at 600F the exhaust fan rating must be 2 times the desired fresh air supply rate.

Temp Temp Factor Temp Temp Factor Temp Temp Factor°F °C °F °C °F °C

70 21 1 300 149 1.43 950 510 2.66100 38 1.06 350 177 1.53 1000 538 2.75110 43 1.075 400 204 1.62 1050 566 2.85120 49 1.09 450 232 1.72 1100 593 2.94130 54 1.11 500 260 1.81 1150 621 3.04140 60 1.13 550 288 1.9 1200 649 3.13150 66 1.15 600 316 2 1250 677 3.23175 79 1.2 650 343 2.09 1300 704 3.32200 93 1.24 700 371 2.19 1350 732 3.42225 107 1.29 750 399 2.28 1400 760 3.51

Table 9.2.5.1 Temperature–Volume Conversion Table (at Sea Level)

Improper Ventilation Design

The designs shown lack dedicated exhaust fans and are not compliant with NFPA 86.

Proper Ventilation Design

The designs shown are compliant with NFPA 86.

High LEL Systems

NFPA 86 does allow designing up to 50% of the LEL: “Where a continuous solvent vapor indicator and controller is provided it (oven safety ventilation) shall prevent the vapor concentration from exceeding 50 percent of the LEL.”

This is typically used in conjunction with air pollution control systems. The level of solvents in the exhaust air is elevated prior to delivery to a thermal oxidizer or concentrator / absorber.

Oven Heating Systems

• Ovens may be heated using any of several energy sources:

– Fuel Gas (Typically Natural Gas or Propane)– Fuel Oil– Electricity– Hot Water– Steam – Thermal Oil– Hot Air from Recovered Heat

• Each heating method has its own requirements.

Oven Heating Systems

• All systems need to accomplish the following in a safe manner:– Allow stopping and starting the supply of

energy to the oven in a safe manner.– Keeping the energy source flow rate within

safe limits.– Have provisions that allow interlocking of the

oven’s safety controls to the operation of the energy source.

– Have appropriate maintenance provisions

Fuel Gas System

• Photograph of fuel gas piping manifold with combustion air blower.

Fuel Gas System

• NFPA Diagram of fuel gas piping manifold

Infrared Fuel Gas Fired System

Fuel gasses can also be used to fire infrared heating burners. These systems typically premix gas and air in the proper ratio. The gas / air mixture is piped to the burners.

Fuel Oil System

• Photograph of fuel oil piping manifold

Fuel Oil System

Diagram of fuel oil piping manifold

Electrically Heated System

Electric Infrared Oven with Conveyor

Electrically Heated System• Use any of several types of

resistance electrical elements to provide heating capacity.

• Convective Heating Elements: Essentially, like the heating elements in a home oven. Generally used to supply convective heat to an oven.

• Radiant Elements: Lamps in any of several shapes that operate at high element temperatures and supply a significant part of their energy as thermal radiation.

Fluid Heated Systems

• These systems use a flow of a hot fluid through a heat exchanger to supply energy to the oven. This includes hot water, thermal heat transfer fluids (thermal oil), and steam (not actually a fluid). NFPA 86 has requirements for fluid heated systems including:

• Piping per ASME B31.1, Power Piping.• Insulation requirements for thermal

oil.• Isolation valve requirements.• Locations and filtration requirements

for heat exchangers.

Steam Heated System

• Uses a flow of steam through a steam to air heat exchanger to provide heat for the oven. Typically used on lower temperature ovens. Requires a steam boiler which often supplies steam to several heated devices.

Hot Water Heated System

• Uses a flow of hot water through a water to air heat exchanger to provide heat for the oven. Typically used on lower temperature ovens. Requires a hot water boiler and a pumping circuit which often supplies water to several heated devices.

Hot Oil Heated System

• Uses a flow of hot oil (thermal heat transfer fluid) through a liquid to air heat exchanger to provide heat for the oven. Requires a thermal oil heater which normally supplies hot oil to several heated devices. Often used in plants that use waste for firing the heaters. Thermal oil heated systems often are designed with the oil temperature near or above the auto-ignition temperature of the oil in air. Extreme care must be taken to avoid leaks as a leak can easily cause a fire.

Recovered Energy Heated System

• Uses energy recovered from a higher temperature process to supply heat to the oven. This can take many forms. One example is to recover energy from an oxidizer and use it to preheat the fresh air supply to an oven. Typically uses a heat exchanger to recover the energy.

Oven Control Systems

• All ovens have electrical control systems that, when properly executed and maintained, enforce the required safe logic of operation. This logic serves to verify safe ventilation, safe fuel conditions, and to prevent excessive operational temperatures. Additionally, the control system may interface to fire protection systems and material handling systems.

Oven Control Systems: Programmable Logic Controllers

• Many newer ovens are controlled by Programmable Logic Controllers (PLC’s). NFPA 86 includes rules for using PLC’s with ovens. Generally, PLC’s can initiate actions (start and stop fans and burners) and monitor safety controls; however, they are not allowed to actually process the safety logic. Unless listed for combustion safety service.

Oven Control Systems: Data Aquisition

• Some ovens are equipped with data acquisition systems. These are often used for quality control purposes to record the oven temperature and other critical variables. Many PLC controlled systems have data acquisition systems. In the event of a fire or explosion, these systems can often provide valuable data to help in understanding the conditions at the time of the event. Securing any such data should be a top priority when investigating oven and furnace losses.

Ventilation Interlocks

• Fans are used to circulate air inside ovens, to supply fresh air to ovens and to exhaust air from ovens. The proper use of fans in all of these applications is critical to the safe operation of the oven.

Ventilation Interlocks

• The fan’s motor is typically started and stopped using a motor starter. A motor starter is simply an electrical switch that can be turned on and off by an electrical signal. The starter should include an auxiliary contact that is wired back to the control logic. Closing of this auxiliary contact upon giving a “fan start” signal proves that the motor starter switch closed and therefore the fan motor should be energized. However, that does not prove the fan is actually turning and moving air. Perhaps the V-belts between the motor and fan are broken. An additional interlock is required.

Ventilation Interlocks• Air flow or pressure switches are

used to prove that fans are actually running and moving air. Air flow switches are “sail switches” that have move and close an electrical switch due to the passage of a minimum amount of air flow. They are not used very much in industrial ovens.

• Typically, air pressure switches are used. These devices are made of a small switch attached to a flexible diaphragm. Each side of the diaphragm is connected via tubing to an appropriate point in the system ductwork so that when the fan moves air, the diaphragm flexes and actuates the switch .

Ventilation Interlocks

• Air pressure switch location and operation are often poorly designed and a misused element of safe oven design.

• The switches are typically piped to the inlet and outlet of the fan. This is acceptable as long as there are no mechanical elements such as a damper that can reduce the airflow to an level below the minimum design level. An alternate approach is to include a fixed orifice in the duct an have the switch sense the pressure drop across the orifice. This actually verifies system airflow, not just a pressure difference across a fan.

Ventilation Interlocks

– The pressures that are sensed by air flow switches are low, often less than 1/25 of a pound per square inch. Pressures this low can be difficult to sense. Sometimes the switch adjustment or location of the tubing taps in the ducts make the switch’s operation erratic. This will shut down the oven and create a real problem for maintenance personnel. Not infrequently, these switches are bypassed (jumped) to get rid of this nuisance problem. If the fan then fails (eg, broken belts), there will be no safety ventilation and the control system will not sense this fact. An explosion could be the result.

Purging

–Explosions have resulted from accumulations of flammable vapors inside ovens which accumulated when the oven was not operating. These vapors may come from leaking fuel gas valves or from some process related source. To prevent an explosion of these vapors, ovens are purged prior to attempting to light the burner(s). Purging consists of starting all the fans and running the fans for a timed period prior to allowing an attempt at burner ignition. The NFPA 86 standard requires purging four oven volumes worth of fresh supply air prior to allowing a trial for ignition. On most systems, one main gas valve must be proved closed during purging by a “proof of closure“ switch. Also, the timer must be approved for combustion system use.

Fuel System Interlocks

–Fuel pressure switches check that the fuel pressure is within safe limits. Most systems have both high and low pressure switches.

–Above 400,000 BTUH, one valve must be equipped with a proof of closure switch. This verifies that the fuel valve is closed during purging.

Temperature Controllers– Almost every oven used in industry is designed to maintain an enclosure

or a product at a specific elevated temperature. For example, an automotive electrocoat oven might operate at 350°F. Temperature Controllers are used to vary the heating system’s energy output to hold the oven temperature at a specific value. In many newer and larger installations, programmable logic controllers (PLC’s) or industrial computers are used to control the temperature. Any of these approaches use a temperature sensor such as a thermocouple or RTD to sense the temperature. NFPA 86 does not give requirements for temperature controllers or even require one. Some ovens, especially radiant heating ovens, do not have temperature controllers.

Excess Temperature Limit Controllers

– Every class A oven, and most other ovens, are required to have an excess temperature limit controller that is separate from any process temperature controller. The “high temp limit” cuts off the energy input to the oven if the oven temperature gets too high. The limit controller must be listed for its service.

Flame Supervision

–Ovens that include fuel fired burners must control the burner with a flame supervision relay and appropriate flame sensor. The flame relay is an electronic device that has onboard logic specific to safely controlling burners. Many flame relays also have logic for starting combustion air blowers or purging the oven. Flame relays use flame sensors to indicate the presence of a flame. Flame rods and UV scanners are the most popular types. A flame rod uses the electrical conductivity of a flame to sense the flame. A UV scanner is a device that senses the ultraviolet radiation emitted by the flame to sense its presence. There are also other types of flame detectors such as lead sulfide detectors.

Flame Supervision

Flame Rods are simple and reliable flame sensors. However, the metal part of the rod must contact the flame and therefore it erodes over time. Generally, flame rods are used on lower temperature processes where access to the flame is possible.

UV Scanners require only a line of sight to the flame, not actual contact. They can fail in a flame simulating manner which could cause an explosion. On ovens that run continuously, self checking scanners are required. These units continuously check for flame simulating failure.

Flame Rod

UV Scanner

Self-Checking UV Scanner

Other Interlocks– Conveyor Interlock: According to NFPA 86, “Conveyors

or sources of flammable or combustible material shall be interlocked to shut down on excess temperature or if either the exhaust or recirculation system fails.”

– Fire Protection: Some oven systems are interlocked to the fire protection system so that the oven reacts safely to a fire. An example might be to cut off the burner if a sprinkler flow switch actuates.

Operational Requirements–Operating an industrial oven or furnace is a serious matter and requires specific actions to insure it is done safely. Manufacturers are required to instruct the owner in safe operating and maintenance procedures. Only by using the oven within its design parameters can safe operation be assured. To be sure the oven is operated within safe limits, NFPA 86 requires each Class A oven have a “Safety Design Data Form” which must specify the following:

• Solvent used• Number of gallons (liters) per batch or per hour of solvent and volatiles entering the oven

• Required purge time• Oven operating temperature• Exhaust blower rating for the number of gallons (liters) of solvent per hour or batch at the maximum operating temperature

Operational Requirements

–NFPA 86 requires that oven manufacturers provide instruction in safe operating procedures including providing “clear inspection, testing, and maintenance instructions.” In fact, NFPA 86 includes several suggested oven maintenance checklists. These lists include, among many requirements :

• Verification that safety devices such as pressure switches and high temperature limits actually work.

• Checking fuel gas valves for leaks. A specific leak checking procedure is outlined.

• Cleaning debris from ovens and ducts.