Safety Alert: Duct Fires in Foundries.

AFS Environmental Control Div. Air Quality Committee (10-E) Principal Author: Paul Sheehan,

Foundries can prevent personnel injuries and costly damage to their facilities by knowing how and where duct fires start, and being prepared to contain them.

Recent incidents have prompted a reminder to the foundry industry that duct fires are a real possibility and a safety item to be taken seriously. While fires can occur in any area of the foundry, the duct systems collecting dust, fumes and smoke easily can be overlooked. By design, these systems take away the objectionable materials from the foundry floor, conveying and collecting them for disposal (or recycling where appropriate). The accumulation of materials within the collection and transport systems often can go unnoticed. Under the right circumstances, this accumulation can provide adequate fuel, heat or both to support or initiate a fire and, in extreme cases, an explosion.

Fires within hood, duct and collector systems can be difficult to discover due to their containment within a closed system. This initial containment can simplify the control of the fire and minimize damage if the fire is discovered early. If not discovered, the fire can damage the emissions control system, the surrounding building and support structures, and other material and equipment in the vicinity.

The nature of the materials captured in these systems and the subsequent combustion of this built-up material in these fires can lead to the generation of smoke, fumes and gases, which can present a danger to personnel fighting the fire or working in the area. The products of incomplete combustion and volatilization of the materials within the systems also can generate explosive mixtures of gases or dusts. The other more obvious danger posed is burns from contact with the hoods, ducts or other debris from the fire.

Damage from these fires may not necessarily be limited to the system within which the lire occurs. In addition to the damage to adjacent buildings and structures, damage to utility systems such as gas or oil piping and power lines can quickly turn a fire into a true disaster.

The prevention and fighting of fires in the foundry industry requires the attention of designers, operators and maintenance personnel to prevent plant damage and promote safe working conditions. This article presents the various scenarios in which duct fires can start and how foundries can prevent and fight them.

Flammable Materials

Duct fires have been known to occur in a variety of dust control and collector systems within foundries. Two of the more common types of fires that can occur are the pouring, cooling and shakeout fume collection fires in green sand foundries and thermite or dust fires in aluminum foundries. Fires also can occur in coreroom emissions control systems or in other systems where either organic materials or reactive metals are in use.

The use of seacoal in green sand formulation produces coal pyrolysis products from the mold, which can condense out of the smoke to form a carbonaceous coating on the duct walls, bags and cartridges. Depending on the type of castings being made and the relative amount and type of cores, venting patterns and sand-to-metal ratios, various amounts of condensable hydrocarbons are released from the mold. The extent and design of the fume collection system determines the amount of condensed organic material that may collect on the walls of the hood, duct system and within the fume control device. Duct and storage facility fires also can occur in the seacoal storage and feed systems.

Ducts--Some foundries have redesigned duct takeoffs to minimize the transmission of heat, direct flame contact and pickup of burning tramp materials into the duct system. The use of screens and dropout boxes or spark arrestors have been used to stop the migration of burning materials through the system and subsequent initiation of fires in downstream equipment.

The position of the gas takeoff from continuous cooling tunnels and within the shakeout enclosure in green sand foundries can greatly affect the type of buildup within the collection system.

The amount and type of buildup is a major factor in the initiation and support of fires in the hood and duct systems. Another factor observed in these systems is the flames on the gases being vented from the molds. The presence of flames tends to consume the organic materials that otherwise would condense within the hood, duct and collection system.

Some foundries that have experienced fires have installed temperature actuated dampers ahead of the collectors to prevent the spread of fires, to isolate them and to smother the flames.

Collectors--Dry-type dust collectors typically are equipped with filter media made from natural or synthetic organic materials and burn when ignited. These fabric filter media are exposed to a variety of organic compounds in liquid, solid and vapor forms. These materials can stick and build up on these media, increasing the fuel content of the collectors, making them susceptible to a fire.

Dead Spaces--Duct systems can have dead spaces either by design or through alterations. These spaces tend to accumulate materials and can be a starting spot in the system for a fire.

Sources of Ignition

Ignition of the built-up lining within duct systems has been attributed to hot work practices, flames from the molds, burning tramp material from the pouring station, heat of reaction from the buildup itself and static discharge within the duct system.

Tramp Materials--A fire within a collector system can begin in several ways. The emissions control system is designed to draw materials into the duct system and to the collector. If there is some flammable or burning material within the influence of the collector system, it most likely will be captured and conveyed to the collector. The presence of either burning or flammable tramp materials is an easy way for a fire to start. Lunch bags, newspapers, waste cardboard or sloughed-off buildup from within the hood and duct system all can catch fire and be drawn into the system.

Static Discharge--The static charge developed by air moving through duct systems is a serious consideration in the conveying of explosive and combustible materials. The design of such systems requires complete grounding of all components to prevent static discharge and its complications.

Spontaneous Combustion--In foundry duct systems, the accumulated material can be a mix of a wide variety of reactive organic materials and their breakdown products. These reactive materials can produce heat, and if sufficient buildup occurs to both sustain the reactions and insulate the reacting materials to retain the heat, the reactions will proceed more quickly with the generation of even more heat, resulting in auto-ignition.

Thermite Fires--Thermite fires in aluminum foundries can occur in fume and dust collectors and in the material handling system. Thermite fires can occur when fine aluminum chips and flakes or dust [750F (399C)] accumulate. Thermite is a general term used to describe reactions between aluminum metal and iron oxide. These reactions can occur with a mix of metal oxides in which the aluminum is oxidized and the metal oxide is reduced to metal, accompanied by the release of heat.

Thermite fires are particularly dangerous due to the extremely high temperatures produced [up to 5500F (3038C)], the rapidity with which the reaction can proceed, and the evolution of metal and metal oxide fumes, which may be hazardous. In a dust collector system, these can occur in the hoppers or duct systems that do not completely empty and may have accumulations of finely divided aluminum, alloys and metallic contaminants.

Prevention of Duct Fires

The buildup in the hood, duct and collectors on the mold lines in green sand foundries can range from a fine dust to light fluffy organic soot to heavy sticky porous or solid layers. The lighter materials are relatively easy to clean out while the heavier materials can be virtually impossible to remove. When the accumulation of resins in the duct system is a known problem, the buildup is controlled by heating the duct system to prevent the condensation of the organic materials out of the gas stream.

Optimization of the combustion of the organic materials emitted from the molding process and careful application of ventilation principles to foundry applications can help to reduce buildup. Careful attention to the contaminant concentration, retention of heat in the off gas, mixing of exhaust streams and the use of sorbent materials in the collection system and collectors can help to render the problem manageable.

Process changes offer another solution to the minimization of buildup in the duct systems. Chemical suppliers and other consultants to the industry either are developing or have installed systems to minimize the generation of the heavier constituents of the gases generated in the casting process.

Thermite fires can most readily be prevented by avoiding buildup in hoppers on the collection and duct systems. The accumulation of material in these systems may be due to inadequate carrying velocities in the duct system, by improperly designed hoppers (low-valley angles or dead spaces) or by leaky discharge valves in the collectors. Leaky valves tend to classify and retain the finer particle size materials, which also will be the most reactive.

In the finishing and machining operations of aluminum foundries, the dust and chips generated can pose a risk of either fires or dust explosions.

Preventive measures that can help avoid the conditions that can lead to a fire include:

* avoid buildup of dust in and around the duct system;

* provide inspection ports, clean out doors or other access into the duct system;

* conduct regular duct inspections to check for buildup, blockages or leaks;

* focus on housekeeping in the areas where flammable materials are used or are likely to accumulate.

Fighting Fires

Methods for fighting duct fires range from letting the fire bum itself out and watching to make sure that it doesn't spread to other areas of the plant to shutting fire dampers and fighting the fire with hoses. Some duct systems and many fabric filter systems have been fitted with either manually actuated or automatic sprinkler systems. These systems can prevent serious structural damage but amount to a containment system for the damage. Once a deluge-type system is actuated, the foundry line is shut down to clean up the water and debris, rebag the fabric filter or both.

Use of water on thermite fires or any metal fire must not be attempted. Metal fires and water create an instant explosion hazard.

Smothering the fire may be attempted by either closing the dampers to shut off the air flow through the system or by flooding the system with carbon dioxide ([CO.sub.2]). These methods are not highly effective because in the duct systems where fires tend to occur, the buildup is gummy enough that fire dampers stick in one position (open) and do not react in time to do any good.

[CO.sub.2] systems are effective at snuffing flames, but [CO.sub.2] is not effective at extinguishing a smoldering fire, such as buildup in a duct system. Attempts to douse these fires with [CO.sub.2] in the duct have not been proven successful; the flames can be suppressed but tend to rekindle once oxygen is reintroduced. While nitrogen is less expensive, it offers the same limitations as [CO.sub.2]--namely that it is ineffective at extinguishing a smoldering fire.