Handling and processing of powders at elevated temperatures is widespread in many industries. The range of temperatures associated with these operations varies depending on the nature of the material and the intent of the operation (drying, melting, agglomeration etc.). Under certain conditions, exposing powders to elevated temperatures can cause fire and explosion hazards due to self-heating, electrostatic discharges and other ignition sources.
This two-part article series will focus on how to manage fire and explosion hazards when drying powders. A fundamental understanding of the fire triangle and explosion pentagon — in addition to having the appropriate data on the physical and chemical characteristics of the powders — is essential to effectively manage the risk.
To begin, it is important to understand the requirements necessary to initiate dust fires and explosions in dryers as well as potential ignition sources that can initiate these events.
|See the second part of this two-part article series, "Strategies to Manage the Risk of Fire and Explosions Inside Drying Equipment," to learn more about conditions necessary to ignite fires and create explosion hazards in powder-drying operations.|
Elements of a Fire and Explosion
Three elements are required for a fire:
- A fuel.
- An oxidant, typically the oxygen in air.
- A sufficiently energetic ignition source.
If any one of these three elements can be removed, fire cannot be initiated. The first two of these elements — fuel and oxidant — when in an appropriate ratio, are referred to as a flammable atmosphere.
For an explosion involving a flammable gas, vapor or combustible powder, or dust to occur, two additional requirements are necessary:
- Mixing of the gas, vapor, suspension of the combustible powder, or dust in air above the lower flammable or explosive limit.
In the case of combustible powders or dusts, without mixing dust and air, “only” a fire will follow an ignition. But, when a flammable powder or dust cloud is ignited, a flash fire will ensue, and the developing fireball can cause serious injury to people and damage to plant. If the burning dust/air mixture is confined inside a vessel such as a dryer, the expansion of the hot combustion products will be impeded, and the pressure will increase, leading to an explosion. Due to the pressures generated, damage to the dryer — and possibly the building — is likely. This article will focus primarily on combustible powders but will include the treatment of hybrid mixtures, where gas or vapor may mix with a combustible powder or dust cloud inside of drying equipment.
Assessment of dust-fire and explosion hazards inside of drying equipment requires a fundamental understanding of both:
- The ignitibility and explosibility characteristics of the powders.
- The actual process conditions, including equipment operation.
Absent these two important requirements, conducting a fundamental process hazard assessment (PHA) is not likely to provide meaningful results.
A fire or explosion cannot be initiated in the absence of an effective ignition source. Inside of drying equipment, there can be many potential sources of ignition, dependent upon the specific type of equipment involved. The most common types of ignition sources associated with drying operations are self-heating, friction and electrostatic discharges.
Self-Heating of Dry Solid Particulate During Industrial Thermal Processing
When dry solid particulates such as powders or granules are heated, there may be a tendency for these materials to auto-oxidize or self-heat. This tendency will depend on the chemical and physical properties of the particulate, including the nature of the molecule, particle size, thermal conductivity, density and heat capacity. Inside of drying equipment, while the moisture is being removed, the evaporated water serves as a cooling mechanism that prevents the overheating of the powder.
Once dried, however, if the heat is not soon removed from the powder, a self-heating process may be initiated. Typically, when heated powder builds up inside of the equipment, the interior of the heap may be shielded from the air movement inside the dryer, and this can exacerbate the powder’s heat-retention rate and lead to self-heating. Over a period of time, if the rate of heat generation exceeds the rate of heat dissipation, the powder temperature will increase, possibly resulting in autoignition of the powder.
In situations where humid atmospheres are present, condensation on the powder will generate heat and, in some cases, this heat can initiate an exothermic reaction. The heat generated from this process can serve as an ignition source to initiate fires or explosions. Understanding the phenomena associated with thermal instability is essential in order to effectively manage the risk. Once understood, measures for ensuring safety can be taken, based on the type of dryer being used. (These measures will be discussed in the second part of this article series.)
The onset temperature — the temperature above which the powder temperature will increase uncontrollably — depends on a number of factors:
- The time the powder is exposed to the high temperature (induction time).
- The scale of the powder deposit.
- The availability of air for the self-heating reaction.
- The presence of contaminants.
Therefore, there is not a single onset temperature, and it is not possible to characterize all self-heating processes with a single test.
Several laboratory tests can be used to predict the self-heating behaviors of solid particulates, including powders. These tests can simulate process conditions and, in some cases, the results can be extrapolated to predict the behavior of these powders in larger process equipment such as industrial dryers. Figure 1 shows a number of laboratory tests that can be used to predict self-heating behaviors.
In some cases, more than one test may be necessary. For instance, once the range of self-heating is known, it may be necessary to conduct additional tests under isothermal conditions. During them, the oven temperature is held constant, and the behavior of the powder is observed at that temperature over a period of time to see if an exothermic reaction take place.
Sources of Industrial Dryer Fires
Self-Heating Trigger: Friction
Heat or mechanical sparks generated from friction may have sufficient energy to ignite combustible powder/dust clouds or layers, depending upon the characteristics of the particular material. These types of ignition sources are of concern in drying equipment that has moving parts such as rotary, conveyor or trough dryers.
Sources of friction inside of a dryer include:
- Misaligned parts or overheated bearings.
- Tramp metal or other foreign objects entering the dryer.
- The material itself.
In the case of the material itself, if the feed rate causes buildup inside the dryer, or if sticking or plugging occurs, friction can occur inside of equipment that feeds product using drag plates or screws.
Electrostatic discharges associated with powder handling inside of drying equipment often have been a source of ignition for fires and explosions. In order to determine if charging of powders or dusts presents an ignition hazard, three elements must be determined:
- If the charge can accumulate.
- If the accumulated charge can discharge.
- If the discharge energy can exceed the minimum ignition energy of the powder or dust cloud under the conditions (in particular, the heating/temperature conditions) that exist in the dryer.
Charge-accumulation mechanisms include insulating powders, insulating powder-handling equipment, electrically isolated conductors and the human body.
Insulating Powders. The ability of powder materials to retain static charge even if the powder is in contact with an electrically grounded conductive surface can be evaluated by determining its volume resistivity. Generally, powders are divided into three groups:
- Powders with volume resistivity up to about 105 Ω•m are considered conductive.
- Powders with volume resistivity in the range 105 Ω•m to 108 Ω•m, are medium resistivity or semi-conductive.
- Powders with resistivity above 108 Ω•m are high resistivity or insulating powders.
The higher the resistivity, the higher will be the probability of the powder retaining electrostatic charge.
Insulating Powder-Handling Equipment. Electrostatic charge also can accumulate on insulating powder-handling equipment such as plastic containers, liners, pipes and hoses. The propensity of a solid material to accumulate static charge is reflected in its volume and surface resistivity. Materials having a volume resistivity of 108 Ω•m or greater or a surface resistivity of 109 Ωper square or greater are generally considered insulating (high resistivity) in electrostatic terms and are prone to charge accumulation. It must be noted that the volume resistivity of powders at elevated temperatures that exist in a dryer or some downstream equipment may be different that those obtained under ambient temperature conditions.
Electrically Isolated Conductors. Static charge can accumulate on conductors such as metal piping, flanges, vessels, tools, decking and other metal items that are isolated from electrical ground with a resistance-to-ground exceeding 1x106 Ω. It is recommended that for all-metal items of plant and components, the resistance to ground is maintained at 106 Ωor less. While a resistance to ground of less than 106 Ωwill prevent accumulation of static-electric charge in most cases, if periodic testing reveals a significant increase in the as-installed resistance, that increase could be the result of corrosion or other damage, which could lead to sudden loss of continuity.
Human Body. The human body is an electrical conductor, and if left electrically isolated from ground, it can accumulate any electrostatic charge that might be placed on it. Charge accumulated on personnel can give rise to discharges upon approaching a grounded object. Such discharges can be sufficiently energetic to ignite flammable atmospheres with a minimum ignition energy (MIJ) less than about 30 mJ.
Regardless of the charge-accumulation mechanisms, four types of electrostatic discharges typically may be encountered during materials handling and processing that may pose an ignition hazard. They are:
- Spark discharges.
- Brush discharges.
- Cone or bulk discharges.
- Propagating-brush discharges (PBDs).
Spark Discharges. This type of discharge arises between two conductors at different electrical potentials. For example, conductive components inside of drying equipment can become electrostatically charged when isolated from electrical ground. When such charged conductors are exposed to another conductor at a lower electrical potential — typically ground (zero potential) — a spark discharge may occur. While dependent upon the ability of the conductor to store charge and the extent to which it becomes charged, the effective energy of spark discharges can be high (greater than 1 J) and is typically sufficient to ignite many flammable atmospheres, including flammable gases, vapors and dust clouds.
Brush Discharges. This type of electrostatic discharge arises from electrostatically charged insulating (nonconductive) surfaces to a conductor at a lower electrical potential — typically ground. For example, a plastic drum liner can become electrostatically charged when the drum contents are poured out. These liners may give rise to brush discharges upon exposure to a conductor at a lower electrical potential such as an operator or vessel manway. Brush discharges also can occur when opening plastic bags or separating plastic sheets.
Brush discharges are known empirically to have an effective energy of as much as 4 mJ and are capable of igniting flammable gases, vapors and, theoretically, some flammable dust clouds. However, the ignition of an explosible dust cloud by a brush discharge has not been documented except in cases where a flammable vapor also was present (hybrid mixture).
Cone Discharges. Electrostatic discharges that may occur on the surface of highly charged, high resistivity (insulating) bulk solids and powders as they accumulate (bulk) in vessels or containers are known as cone discharges. As high resistivity bulk solids or powders accumulate in a vessel, the electric field on the surface of the material intensifies — a phenomenon known as charge compaction. The strong electric field causes ionization of the air just above the surface of the material, giving rise to cone discharges from the powder pile to the vessel walls.
The effective energy of cone discharges — also referred to as bulking brush or bulk-surface discharges — is dependent upon the volume resistivity, particle size and mass-charge density of the bulking solid and the diameter of the vessel in which it is accumulating. Also, cone discharges are known empirically to exhibit effective energies on the order of 10 to 20 mJ and, thus, are only capable of igniting flammable atmospheres, including dust clouds, having relatively low minimum ignition energy values, evolved during vessel filling.
Propagating-Brush Discharges (PBDs). PBDs are highly energetic electrostatic discharges capable of igniting many flammable atmospheres, including gases, vapors and dust clouds. These discharges — exhibiting effective energies of as much as several joules — derive their energy from the formation of a double-layer charge on both sides of a surface such as a fabric, film or coating.
In one mechanism, a double-layer charge forms when the charge on one side of an insulating surface is sufficiently strong so as to induce an equal and opposite charge on the other side by atmospheric ionization. PBDs also may arise from the double-layer charge that forms when a highly charged insulating film or coating is adjacent to a grounded plane. The latter mechanism may arise when an electrically insulating coating is applied to a grounded metal vessel or container.
Only materials possessing a certain dielectric strength are capable of supporting the double-layer charge needed to produce PBDs. The measure of dielectric strength relevant to PBDs is breakdown voltage. Breakdown voltage refers to the point at which the insulating property of a material breaks down upon application of high voltage.
Breakdown voltage can be used to evaluate the propensity for a material to produce PBDs. It is known empirically, that only materials having a breakdown voltage of 6 kV or greater are capable of giving rise to PBDs. A potential would exist for PBDs when dumping from plastic-lined fiberboard boxes or fiber drums under certain conditions. An explosion or flash fire then could occur if a suspension of fine combustible dust or powder existed in proximity to the discharge. If combustible powders/dusts, in high concentrations, are vacuumed or pneumatically conveyed using plastic nonconductive hoses, PBDs are possible, particularly if the hose is manufactured with a metal wire helix that could provide a breakdown path to ground.
Testing to Determine Minimum Ignition Energy of Powders
Testing on powders or dusts can be performed to determine the smallest electric spark that can ignite a dust-cloud atmosphere at its most easily ignitable concentration (in air). This is called the minimum ignition energy (MIE) test, and the results are typically reported in millijoules.
As it is widely recognized, the sensitivity of the dust cloud to ignition is influenced by particle size and moisture content. A reduction in particle size and/or moisture content would result in a dust cloud that is more sensitive to ignition. Another influencing factor — one that is quite relevant to powder-drying operations — is the dust cloud temperature. The MIE of a dust cloud is reduced with an increase in the temperature (figure 2). For some powders, the reduction in MIE can be significant; therefore, for situations where a dust cloud may be present — under elevated temperatures during drying or in the downstream handling or storage equipment — the MIE test must be conducted under conditions that reflect the particle size, moisture content and the (elevated) temperature conditions.
In conclusion, an important component of any dust-fire and explosion hazard assessment is having a proper understanding of the ignition sensitivity, explosion severity and electrostatic properties of the particulate solid materials under process conditions. This article has addressed potential ignition sources inside of drying equipment, including friction, self-heating and electrostatic sparks. In the second part of this two-part article series, I will discuss how typical powder dryers operate, what hazards they present and strategies that can be used to manage the risk of fire and explosion inside of this equipment.