Ashok Ghose Dastidar
Safety Consulting Engineers, Inc.
Hazards of Powder Metal Dusts & Hybrid Compositions
Ashok works for Safety Consulting Engineers, Inc. a company specializing an mitigating the hazards of explosion from dusts. They use many standard and specialized tests to evaluate dust and powder hazards. Ashok's initial focus was on coal dust explosions, but now many other systems are considered including corn starch, polymers, paper products and metals.
A dust explosion is a "gas dynamic phenomenon" caused by rapid oxidation. For a dust explosion to occur there must be five components present (this is similar to the triangle required for fire, but for "explosion" there must be two additional).
1. Fuel
2. Oxidant
3. Ignition Source
4. Dispersion (meaning fuel is not confined to a surface)
5. Confinement
To eliminate explosions, one simply has to remove one of these five.
Explosion potential is evaluated as a risk. This "risk" is the product of:
1) The probability of occurence and
2) The Severity of the Consequences
(These consequences include the destructiveness, violence, and losses of property, labor, and capital)
In the end, the risk is usually given a monetary value.
When tested using an electrical spark, most powder materials exhibit at lower value of concentration, and an upper value of concentration between which explosions can occur. When you are below the lower level or above the upper level there is no likelihood of explosion.
Another way of rating materials is looking at energy release during test explosions. When plotting energy, pressure and time, materials show explosive severity in several ways. A high "overpressure" (the peak of presure increase) indicates particulary damaging explosive potential. Aluminum powder has a high peak, Polyethylene a medium peak, and Iron disulfide a rather low one. Another measure is the slope of rising pressure plotted against time. Again Aluminum powder has a very steep slope, Polyethylene is also fairly steep, and Iron disulfide powder is quite low. Materials are compared in this way.
Looking a historical explosion data is somewhat revealing. In order the top five (by number of reported incidents) are:
Silos and bunches
Dust Collectors
Mills and crushing plants
Conveyor systems
Dryers
The top five ignition sources for explosions are:
Mechanical Sparks
Smoldering Nests
Mechanical Friction
Electrostatic Discharges (this is the top cause for explosions of plastic particulate)
Fire
The three types of measurable output from and explosion are: Thermal, Pressure, and speed.
For metal dust explosions a book by Eckhoff is an authoritative reference and should be consulted. 22 micros Aluminum powder has a very high rate of pressurization (1100 bar/sec). 170 micron Aluminum powder however will not explode under ordinary ignition conditions. From this we see that physical properties of a dust are very important. The second most violent explosions are 28 micron Magnesium powder at 508 bar/sec.
There is a "rule-of-thumb" which works very well for most plastic and polumer dusts. The max overpressure of an explosion of these material will be about 8 time the starting pressure. This rule absolutely DOES NOT WORK for metal powders however. For metals overpressure varies all over the map.
The significant variables which effect the three output properties are:
Dust properties
Nature of oxidative atmosphere
Dispersion Mechanisms
Type, magnitude, and location of ignition
Nature of the confinement structure
There are several testing methods to characterize the material that you have.
Standard tests exist for Max. Pressure and dP/dt rates.
Min Ignition Energy (MIE) can be evaluated using a capacitive discharge test.
Min Explodable Concentration (MEC) is measurable.
Other include:
AIT Test
Dust layer test
Friuction test
Impact tests
Electrostatic charging tests
and resistivity
Other approaches to study the problem include:
FMEA (Failure Mode and Effects Analysis)
HAZOP (Hazard Inoperability Study)
FTA (Fault Tree Analysis)
Mitigation is always based on breaking one or more of the five components required for explosion.
It is most often necessary to study each process and powder combination individually to really assess the risk. However, many follow certain code which at least reduce the risks under specific conditions. Such codes include:
NFPA (sections 651, 68 and 69)
BOCA (sections 417.0 and 418.0)
and OSHA
Despite 200 years of research, each material and process can be unique.
A dust explosion is a "gas dynamic phenomenon" caused by rapid oxidation. For a dust explosion to occur there must be five components present (this is similar to the triangle required for fire, but for "explosion" there must be two additional).
1. Fuel
2. Oxidant
3. Ignition Source
4. Dispersion (meaning fuel is not confined to a surface)
5. Confinement
To eliminate explosions, one simply has to remove one of these five.
Explosion potential is evaluated as a risk. This "risk" is the product of:
1) The probability of occurence and
2) The Severity of the Consequences
(These consequences include the destructiveness, violence, and losses of property, labor, and capital)
In the end, the risk is usually given a monetary value.
When tested using an electrical spark, most powder materials exhibit at lower value of concentration, and an upper value of concentration between which explosions can occur. When you are below the lower level or above the upper level there is no likelihood of explosion.
Another way of rating materials is looking at energy release during test explosions. When plotting energy, pressure and time, materials show explosive severity in several ways. A high "overpressure" (the peak of presure increase) indicates particulary damaging explosive potential. Aluminum powder has a high peak, Polyethylene a medium peak, and Iron disulfide a rather low one. Another measure is the slope of rising pressure plotted against time. Again Aluminum powder has a very steep slope, Polyethylene is also fairly steep, and Iron disulfide powder is quite low. Materials are compared in this way.
Looking a historical explosion data is somewhat revealing. In order the top five (by number of reported incidents) are:
Silos and bunches
Dust Collectors
Mills and crushing plants
Conveyor systems
Dryers
The top five ignition sources for explosions are:
Mechanical Sparks
Smoldering Nests
Mechanical Friction
Electrostatic Discharges (this is the top cause for explosions of plastic particulate)
Fire
The three types of measurable output from and explosion are: Thermal, Pressure, and speed.
For metal dust explosions a book by Eckhoff is an authoritative reference and should be consulted. 22 micros Aluminum powder has a very high rate of pressurization (1100 bar/sec). 170 micron Aluminum powder however will not explode under ordinary ignition conditions. From this we see that physical properties of a dust are very important. The second most violent explosions are 28 micron Magnesium powder at 508 bar/sec.
There is a "rule-of-thumb" which works very well for most plastic and polumer dusts. The max overpressure of an explosion of these material will be about 8 time the starting pressure. This rule absolutely DOES NOT WORK for metal powders however. For metals overpressure varies all over the map.
The significant variables which effect the three output properties are:
Dust properties
Nature of oxidative atmosphere
Dispersion Mechanisms
Type, magnitude, and location of ignition
Nature of the confinement structure
There are several testing methods to characterize the material that you have.
Standard tests exist for Max. Pressure and dP/dt rates.
Min Ignition Energy (MIE) can be evaluated using a capacitive discharge test.
Min Explodable Concentration (MEC) is measurable.
Other include:
AIT Test
Dust layer test
Friuction test
Impact tests
Electrostatic charging tests
and resistivity
Other approaches to study the problem include:
FMEA (Failure Mode and Effects Analysis)
HAZOP (Hazard Inoperability Study)
FTA (Fault Tree Analysis)
Mitigation is always based on breaking one or more of the five components required for explosion.
It is most often necessary to study each process and powder combination individually to really assess the risk. However, many follow certain code which at least reduce the risks under specific conditions. Such codes include:
NFPA (sections 651, 68 and 69)
BOCA (sections 417.0 and 418.0)
and OSHA
Despite 200 years of research, each material and process can be unique.