Article Archive
Risk Assessment
Combustion Control Problems
Vol 21 No 2

First and foremost, if you are a first responder and have an oven or furnace in your jurisdiction, go and visit the facility to find out more information about it. If you are a manufacturer who has an oven or furnace at your facility, invite the fire department out to visit and have them become familiar with its operation and explain where the fuel shut off valves and other controls are located.

If you are a first responder and respond to an event in the vicinity of an oven, find out what the flame/fire is for. It might be there because when an oven opens, the remaining atmosphere is burned off during the process. This could be normal for the process. We had a situation where the fire department responded to a fire alarm at a local facility and attempted to extinguish a fire on a special atmosphere furnace under the impression that the furnace was on fire, when in fact, the fire was located nearby on the floor. The facility employees realized what was happening and shut off the gas to the furnace.

There are four classes of oven; Class A, Class B, Class C and Class D. Class A oven by definition is an oven, furnace or dryer that is heated and operated at approximately atmospheric pressure. The products inside the oven have a potential explosion or fire hazard because of the presence of flammable volatiles or combustible materials that are being processed, heated or are a byproduct of the process in the furnace. Examples of a Class A oven include: paint curing ovens, which are used to dry car bodies after being painted using a flammable or combustible paint mixture. Another example is a popcorn machine in the movie gallery.

A Class B oven is similar to a Class A oven, except there is a minimal chance of flammables or combustibles in the oven. An example of a Class B oven is where an unpainted car body dries after washing.

Class C ovens are ovens or furnaces that have a potential hazard due to a flammable or other special atmosphere being used for treatment of material in the process. An example of a Class C oven is a quench oven, which is used for car parts that go through a heat treat process and are immersed in a quench oil to cool off and treat the part.

A Class D Furnace operates at temperatures from above ambient temperature to over 5000?F (2760?C), and at pressures from a vacuum to several atmospheres during heating using various types of heating systems. These furnaces can include the use of special processing atmospheres. An example of a Class D furnace is a bell furnace, which is used to produce computer chips for an automobile's computer system.

Class A, Class B, Class C, and Class D ovens are covered in National Fire Protection Association's NFPA standard 86 Standard for Ovens and Furnaces. The oven can be heated by fuel-gas, oil, electricity, or hot fluids. NFPA 86 requires all types of ovens to have the proper combustion controls. These controls include a remote manual shut-off valve (located so someone can operate it easily, without the use of a ladder or special tool, in an emergency), high and low temperature controls (much like the thermostat in your home), airflow monitoring for the combustion air fan and oven ventilation, explosion venting of the combustion chamber and work chamber of a Class A oven, and a controller for the oven. These fired ovens would also be required to have additional combustion controls depending upon how they were heated. These controls include:

Fuel-Gas Fired Ovens: manually operated shutoff valve, sediment drip leg on the system side of the shut off valve, fuel strainer, pressure regulator, low or high/low combination gas pressure switch, two safety shutoff valves with leak test connections, high gas pressure switch (if the high/low combination is not provided) and a second manual shut off valve.

Oil Fired Ovens: manually operated shutoff valve, fuel strainer, pressure regulator (if needed), low or high/low combination oil pressure switch, high and low oil temperature switch if the oil is heated, two safety shutoff valves with leak test connections (if the operating pressure of the oil pump is greater that 125 psi [862 kPa] or the oil pump operates without the main burner operating or when the oil pump operates as the unit is firing in fuel-gas mode, otherwise a single valve is required) and a second manual shut off valve. Additionally, the oil is required to be atomized to a proper droplet size for the burner. This would require the atomizer, such as steam, to be monitored.

These controls along with flame supervision would reduce the chances of an explosion in the unit.

There have been cases where it was impossible to shutoff the gas supply at the ovens during a fire. The people had to locate the nearest accessible shut off valve. In some of these cases the valve was over 20 feet in the air. Since these valves did not have accessible means to shut them off, someone had to get a ladder to reach them. By the time the person got the ladder and started to shut off the valve, it took over 10 minutes. The fire lasted for over 30 minutes until the gas in these large diameter pipes bled off and stopped feeding the fire.

Another problem that often arises is the venting of gas regulators, gas pressure switches and other gas venting devices such as diaphragm-type valves, relief valves and vent valves. Unless listed for such, all line gas pressure regulators, gas appliance pressure regulators, and gas pressure switches are required to be vented to the outdoors. This is to minimize the build-up of gas near the unit that can cause an explosion. These devices require atmospheric pressure to operate properly. The atmospheric vent on these devices is to allow air to get into the device to make the device work and if the device breaks, the escaping gas to be vented to an outside, safe location. If allowed by the authority having jurisdiction, (AHJ) the vent lines for the pressure regulators can be manifolded with the vent lines for the pressure switched as long as the manifolded line is piped in such a manner that diaphragm rupture of one vent line does not backload the others.

The vent lines for the pressure relief valves, diaphragm-type valves and vent valves are vented to a safe location outdoors. Anytime these valves operate, fuel gas will be discharged outdoors. You cannot manifold these vent lines with atmospheric vents.

If you manifold lines together, the size of the vent manifold should be no less than the area of the largest vent line plus 50 percent of the additional vent line area.

For example: if the vent line for a pressure regular is ?-in. and the vent line for the pressure switch is ?-in. The vent line area for each line is 0.3038-in.? (A = ? ? r?), therefore the manifold line area needs to be 0.3038 + (0.5)?0.3038 = 0.4557-in.? The resulting pipe diameter would be 0.720-in. The vent line would have to be a ?-in. pipe.

One major problem is that each unit must have it's own vent manifold. The code does not allow the manifolding vent lines of multiple ovens or furnaces together. The reason they do not allow the manifolding is if one unit is taken out of service for maintenance, and the others are left operating, a ruptured diaphragm or normal venting of the vent valve could introduce gas into the unit that is out of service.

Another issue is the requirement for explosion venting. Vents are to be installed on all ovens and furnaces that are gas or oil fired and those containing flammable liquids, gases, or combustible dusts. The problem is the location and position of the explosion vents. Please keep in mind that what ever goes up in the air will have to come down. The vents are supposed to be near the combustion chamber (the area where the gas and fuel are ignited) for all fuel-fired furnaces and distributed along the main part of a Class A oven.

A situation that comes up often is the location of the oven in the building. Some companies located their ovens either next to a wall, to use the wall for explosion relief or adjacent to walkways. Problems occurred because the oven was constructed knowing the exterior walls were designed for explosion relief and the facility's gas meter was located outside next to the wall. If the vent panels were to operate due to an explosion, the vent panel could sheer off the gas meter causing the gas to escape into the area. Another problem has been where the oven is located adjacent to the walkway, if the vents operate, the panels could injure people walking past the unit.

If you are fighting a fire or investigating a fire in an oven, please know where the vents are so there is no likelihood of becoming a victim. If you don't know where the vents are, stand near the entrance or exit of the oven since these areas are less likely to have any vent panels.

If you are unfamiliar with the standard, NFPA 86, please obtain a copy and review the requirements. Therefore the likelihood of an explosion, fire or injury is reduced at your facility. o


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