46 Powder and Bulk Engineering, March 1993 0 0 73 ra 5 2. d 0 cn 0 ow to - minimize electrostatic discharge zards in your pneumatic conveying S m C. James Dahn Safety Consulting Engineers, Inc, Sparks can initiate catastrophic dust explosions in pneu- matic conveying systems that operate with air. To safe- guard your conveying system from such electrostatic dischargehazards, you need to understand and identify the hazards and find ways to minimize them. This article first reviews some pneumatic conveying system basics, then explains how to analyze electrostatic discharge mechanisms and how to minimize electrostatic dis- charge hazards in your conveyingsystem. powder’s electrostatic charging capability can create high potential for a dust explosion in an air pneumatic A conveying system. Under the right conditions, the high electrostatic voltage fields produced by the flowing powder can cause electrical breakdown in the powder-air mixture, in the ac- cumulated powder in the receiver or the silo, or in the convey- ing system’s isolated conductive parts (system materials and components). Such a breakdown results in an electrostatic dis- charge that can initiate a dust explosion. Before examining electrostatic discharge mechanisms and ex- ploring ways to minimize these hazards, let’s review some pneumatic conveying system basics. A pneumatic conveying system typically consists of a feed powder silo, a blower (or fan), conveying line (tubing or piping), a powder-air separator, and a receiver or a silo at the powder destination (or destina- tions). The blower causes gas (usually air) to flow through the conveying line as powder feeds into the system. The powder is suspended in the airflow and transported along the conveying line. When the powder arrives at its destination, the powder-air separator (a cyclone, a cartridge filter, or a baghouse) separates the powder from the air and routes the powder to the receiver or the silo. A pneumatic conveying system typically conveys the powder by either pulling it by vacuum or pushing it with air pressure. A vacuum-driven system (typically a dilute-phase system) has a low powder-to-air ratio. A pressure-driven system (often a dense-phase system) has a low to high powder-to-air ratio and usually transports the powder at a lower rate than a vacuum-dri- ven system. Figure 1 shows some typical pneumatic conveying system configurations. [Editor’s note: For more information on pneumatic conveying systems, refer to articles listed in the pneumatic conveying section of Powder and Bulk Engineer- ing’s “Index to articles, Volumes 1-6 (1987-1992)” in the De- cember 1992 issue.] In an air pneumatic conveying system, the powder’s flow through the conveying line and other system components can create a high voltage field. You can prevent the creation of a high voltage field in several ways, thus minimizing the poten- tial for an electrostatic discharge to initiate a dust explosion. Analyzing various electrostatic discharge mechanisms can help you find the most effective ways to prevent a high voltage field.
Transcript
46 Powder and Bulk Engineering, March 1993 0 0 73
ra 5
2. d
0 cn 0
ow to - minimize electrostatic discharge zards in your pneumatic conveying
S m C. James Dahn Safety Consulting Engineers, Inc,
Sparks can initiate catastrophic dust explosions in pneu- matic conveying systems that operate with air. To safe- guard your conveying system from such electrostatic discharge hazards, you need to understand and identify the hazards and find ways to minimize them. This article first reviews some pneumatic conveying system basics, then explains how to analyze electrostatic discharge mechanisms and how to minimize electrostatic dis- charge hazards in your conveying system.
powder’s electrostatic charging capability can create high potential for a dust explosion in an air pneumatic A conveying system. Under the right conditions, the high
electrostatic voltage fields produced by the flowing powder can cause electrical breakdown in the powder-air mixture, in the ac- cumulated powder in the receiver or the silo, or in the convey- ing system’s isolated conductive parts (system materials and components). Such a breakdown results in an electrostatic dis- charge that can initiate a dust explosion.
Before examining electrostatic discharge mechanisms and ex- ploring ways to minimize these hazards, let’s review some pneumatic conveying system basics. A pneumatic conveying system typically consists of a feed powder silo, a blower (or
fan), conveying line (tubing or piping), a powder-air separator, and a receiver or a silo at the powder destination (or destina- tions). The blower causes gas (usually air) to flow through the conveying line as powder feeds into the system. The powder is suspended in the airflow and transported along the conveying line. When the powder arrives at its destination, the powder-air separator (a cyclone, a cartridge filter, or a baghouse) separates the powder from the air and routes the powder to the receiver or the silo.
A pneumatic conveying system typically conveys the powder by either pulling it by vacuum or pushing it with air pressure. A vacuum-driven system (typically a dilute-phase system) has a low powder-to-air ratio. A pressure-driven system (often a dense-phase system) has a low to high powder-to-air ratio and usually transports the powder at a lower rate than a vacuum-dri- ven system. Figure 1 shows some typical pneumatic conveying system configurations. [Editor’s note: For more information on pneumatic conveying systems, refer to articles listed in the pneumatic conveying section of Powder and Bulk Engineer- ing’s “Index to articles, Volumes 1-6 (1987-1992)” in the De- cember 1992 issue.]
In an air pneumatic conveying system, the powder’s flow through the conveying line and other system components can create a high voltage field. You can prevent the creation of a high voltage field in several ways, thus minimizing the poten- tial for an electrostatic discharge to initiate a dust explosion. Analyzing various electrostatic discharge mechanisms can help you find the most effective ways to prevent a high voltage field.
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How to analyze electrostatic discharge mechanisms Analyzing electrostatic discharge mechanisms requires evalu- ating the powder’s charging tendencies and the likelihood that the charges will build up during conveying. Follow these steps to analyze electrostatic discharge mechanisms:
Identify the powder’s physical and chemical properties.
Determine the powder’s electrostatic charging properties rela- tive to the conveying system parts the powder contacts.
Use testing or analysis to identify the electrostatic charge drainoff properties of the powder and the conveying system parts.
Identify where and how electrostatic discharge could initiate a dust explosion in the conveying system.
Identify the powder’s physical and chemicalproperties. Iden- tifying your powder’s physical and chemical properties is im- portant because the properties have a significant effect on both the powder’s potential for dust explosion and how easily the powder can be ignited by electrostatic discharge.’ For instance, if your powder has a high concentration of fine particles (<200 mesh), the powder-air mixture can be ignited as easily as a pow- der-air mixture containing only fine particles. If your powder contains 0.1 percent to 1 percent flammable volatiles (such as acetone or hexane), the powder-air mixture can also be ignited easily, similar to a vapor-air mixture. Some other properties and factors that can contribute to a powder’s potential for dust ex- plosions are particle shape, porosity, powder coatings, and con- taminants.
Determine the powder’s electrostatic charging properties rel- ative to the conveying system parts the powder contacts. Much r e ~ e a r c h ~ . ~ has related electrostatic powder charging in pneu- matic conveying systems to theories of particle contact charg- ing -that is, charging caused by contact both between particles and between particles and conveying system parts. However, the research has found very little correlation between particle contact charging theory and actual measurements. Par- ticle contact charging is difficult to measure, probably because very few electrons (which create charging) are needed to create saturation (maximum) charging in the powder and because variations in the surface conditions of the particles and the con- veying system parts can easily change charging values. Lab re- search in this area has also been difficult, especially for low-resistivity powders, because charge quickly drains off the powders. And lab instruments haven’t been sensitive enough or fast enough to measure electrostatic charging on small particles.
But in the last 10 years, lab-scale electrostatic charging tests‘ have been developed that correlate to full-scale field condi- tions. The tests measure the electrostatic charge in 0.1 to 5 kilo- grams of powder accumulated in a Faraday cage after flowing down an inclined chute. Table I shows some typical test results (electrostatic charge per accumulated powder surface area and electrostatic charge per accumulated powder mass) for four powders. Conducting such tests on your powder can help you analyze the electrostatic charge in your full-scale conveying system.
Use testing or analysis to identifj electrostatic charge drain- offproperties of the powder and the conveying system parts. Charge drainoff from the powder and the conveying system parts through grounded conductive components prevents high voltage fields, thus minimizing electrostatic discharge hazards. But if your powder is highly resistive and dusty, dust layers can accumulate on the conveying line and the receiver or the silo walls and prevent charge drainoff through grounded parts. The highest voltage fields tend to occur at the receiver or the silo, where the powder accumulates. The greater the powder accu- mulation, the greater the voltage field. When a sufficiently high voltage field is created, the dust layer can sustain electrical breakdown and produce an electrostatic discharge. Under the right conditions, this can ignite the conveying system’s pow- der-air mixture and initiate a dust explosion.
You can calculate the voltage field in your system’s conveying line and receiver or silo using the following equations, which are based on researchers’ experience. The equation for calcu- lating the voltage field on the accumulated powder surface in a conveying line and a vessel (the receiver or the silo) after trans- port is:
E = (QIA) I eo
where QIA is coulombs per square meter of powder surface and eo is 8.85 X lo-’? coulombs per voltmeter. The equation for cal- culating the voltage field in the vessel’s vapor space after trans- port is:
E = (Qkg) X (kglm’) X R I (3 X eo>
where Qikg is coulombs per kilogram of powder, kg/m3is kilo- grams of powder per airflow rate, and R is the vessel radius (meters).
s for four powder
Char (micro
Powder per squ
I I
Using these calculations to determine voltage fields for the powders listed in Table I produces the results shown in Table 11. Data for three powders tested by other researchers5-’is also in- cluded in Table 11. Voltage fields for five powders were mea- sured in full-scale conveying systems to evaluate the calculations’ accuracy. The table shows very good agreement between predicted and actual voltage fields for these powders. Notice that for the plastic resin, the actual voltage field ex- ceeded the air breakdown level (2,000 kV/m); this initiated a dust pop (a localized reaction), but didn’t propagate a full-scale dust explosion through the conveying system.
Identi& where and how electrostatic discharge could initiate a dust explosion in the conveyingsystem. An electrostatic dis- charge can occur where a sufficiently high voltage field causes breakdown across the conveying system’s electrically isolated conductive parts (those parts isolated from a grounded conduc-
Powder and Bulk Engineering, March 1993 49
tive part). You can calculate the voltage field if you know the configuration of parts that yields capacitance (the ability to store electrostatic charge) and the size of the gap between the isolated conductive parts in this configuration.
For instance, a pneumatic conveying system that transports plastic resin powder (which has an ignition threshold greater than 10 millijoules) was evaluated to determine the electrostatic energy stored in the system’s potentially isolated conductive parts at various gap thicknesses. The results, which show elec- trostatic energy stored for discharge, are shown in Table 111. Ac- cording to the results, a baghouse door or an isolated receiver in this system stores enough electrostatic energy at any gap thick- ness to initiate an explosion of the powder-air mixture. (The re- ceiver coupling and the receiver’s access door also store enough energy to initiate a dust explosion, but only at the greater gap thicknesses.)
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Table 111 Stored electrostatic energy in isolated conducting
parts of the pneumatic conveying system’ e.
Gap Sizeor Width Stored thickness length or energ
Part (inch) (feet) diameter (milliiourer)
isolated conveying line 0.10 20.0
0.20 20.0 4 inches 2.0 4 inches 4.0
Conveying line coupling 0.20 20.0 4 inches 0.2
access door [ I-inch-wide gasket 0.02 2.0
0.06 2.0 0.10 2.0 0.20 2.0
Baghouse door 0.06 1.5 0.10 1.5 0.20 1.5
Receiver isolated from conveying line and ground 0.03 5.0
Note: a Based on voltage field [E) = 2,200 kVlm in the receiver and 311 kV/m in the
conveying line.
Apropagating brush discharge can also occur in silos when an insulated part, backed by a conductive grounded material, is contacted by a conductive grounded part. This luminous dis- charge starts from the conductive part when the part’s electro- static potential exceeds a certain value but remains too low to form an actual spark. For this reason, you should never lower a conductive or metal sampling device into a silo during convey- ing system operation. If extremely high electrostatic charging occurs within the silo and thin layers of insulating powder coat the silo wall, lowering such a device into the silo can cause high- energy discharges (from 5 to 1,000 millijoules) from within the powder heap to the silo wall. This effect has been tested by in- creasing voltage fields (up to 15,000 volts on one apparatus) in powder resistivity tests’and has produced high-energy dis- charges.
Apowder bulking discharge can also occur as powder compacts in a heap in the receiver or the silo.”Criteria based on electrosta-
tic charge per area and the receiver or silo radius have been de- veloped (shown in reference 9’s Figure 4) to determine whether an electrostatic discharge will occur in a powder heap. The val- ues for these criteria verify that the calculated voltage field is near the air breakdown level (which varies for each powder).
Dust concentrations throughout a large receiver can vary widely depending on turbulence within the vessel. In a horizon- tal conveying line, dust is highly concentrated at the line’s bot- tom and less concentrated higher in the line, where the concentration falls below the minimum concentration at which a dust explosion can occur, called the minimum explosible con- centration (MEC). A powder-air mixture is usually easier to ig- nite after the dust reaches its lowest concentration (or reaches a point of lowest energy,) and then rapidly reconcentrates. Even after the dust rapidly reconcentrates, an electrostatic discharge can initiate a dust explosion only under certain conditions (dust concentration, spark condition and duration, and explosion propagation conditions).
How to minimize electrostatic discharge hazards in your conveying system
High electrostatic charging will occur in your pneumatic con- veying system, but the hazard’s seriousness depends on the powder’s ability to drain off the charges and the conveying sys- tem’s ability to store the charges. To prevent an electrostatic- discharge-initiated dust explosion in your system, you need to carefully test and analyze these charge drainoff and storage ca- pabilities.
The lab-scale tests we’ve discussed to predict voltage fields can help you evaluate the electrostatic discharge hazards in your full-scale system. By using this information and calculating ca- pacitances for isolated conductive parts, you can estimate the system’s electrostatic discharge energies. The potential for ig- niting a dust explosion during conveying is highest if the energy required to ignite your powder-air mixture is low (<30 milli- joules) and your powder contains fine particles.
Once you’ve identified the electrostatic discharge hazards, you can minimize them in several ways. To reduce electrostatic charging, convey round particles (which have a low surface-to- volume ratio), large (>60 mesh) particles, or a powder with a different particle size distribution (a greater proportion of large particles). You can also reduce the powder’s flammable volatile content, increase the powder’s moisture content, or reduce the conveying velocity.
You can increase the powder’s charge drainoff capability by adding moisture, conductive particles, or an antistatic (water getter) material to the powder. You can reduce the energy in the conveying system’s voltage fields by reducing the powder-to- airflow-rate ratio, reducing electrostatic charging, or using a smaller receiver or silo.
You can reduce the electrostatic discharge potential in your conveying system in several ways. To minimize sparking, ground all conductive parts in the system, reduce the voltage fields’ energy, and convey large or round particles. To mini-
mize propagating brush discharges, use a smaller receiver or silo, convey smaller particles, and reduce the voltage fields’ energy.
You can reduce the potential for a dust explosion by reducing or removing fines (<60 mesh), preventing dust accumulations, and reducing the potential for electrostatic discharge. PBE
References 1 , C. Dahn, “Method to evaluate electrostatic hazards in bulk powder handling
operations,” Tenth Symposium on Explosives and Pyrotechnics, February 1979.
2. A. Bailey, “Electrostatic hazards in powder silos,” Institufe of Physics Conference Series 85: Electrostatics, 1987.
3. J. Hughes and A. Bright, “Electrostatic hazards associated with powder handling in silo installations,” IEEE Pansactions on Industrial Applications, Volume IA-15, Number 1, JanuaryFebruary 1979.
4. C.J. Dahn, “FIBC potential electrostatic hazards,” Proceedings of the 1991 Powder & Bulk SolidsTM Conference, Reed Exhibition Companies, Rose- mont, Ill.
5. R. Kuczynski, W. Kordylewski, and W. Kucinski, “Electrification of flour during filling of small silos,” Journal of’Electrosratics, Volume 14, 1983, pages 135- 147.
Powder and Bulk Engineering, March 1993 5 1
6. M. Glor, G. Luttgens, B. Maurer, and L. Post, “Discharges from bulked polymer granules during filling of silos - characterization by measurements and influencing factors,” Jonrnal of Electrostntics, Volume 23, 1989, page.; 35-43.
7. P. Cartwright, S. Singhs, A. Bailey, and L. Rose, “Electrostatic charging characteristics on polyethylene powder during pneumatic conveying,” lEEE Trrrnsaction.~ on Industritrl Applications, Volume IA-2 I, Number 2, MarcWApril 1985.
8. Tests conducted at Safety Consulting Engineers, Inc., Schaumburg, Ill
9. M. Glor, “Conditions for appearance of discharges during the gravitational compaction of powders,” Journal of Electrostutics. Volume 15, 1984, pages 223-235.
C. James Dahn is president of Safety Consulting Engineers, Inc., 2131 Hammond Drive, Schaumburg, IL 601 73; 708/925- 81 00. He holds a BS in aeronautical engineering from the Uni- versity of Minnesota, Minneapolis, and has also done graduate work in mechanical engineering at the University of Min- nesota. This article is adapted from a paper the author pre- sented at the 1992 Powder & Bulk SolidsTM Conference in Rosemont, Ill.
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