Extension of accreditation to new ASTM procedures
In 2023, the testing laboratory of VVUÚ, a.s., No. 1025, expanded its accreditation with new procedures in accordance with the standards of the American Society for Testing and Materials (ASTM).
The following test procedures have been added:
- Determination of the ability to generate an explosive dust atmosphere – screening test (Go/No-Go test) (EN ISO/IEC 80079-20-2 / ASTM E1226)
- Determination of lower explosion limit of dust clouds (LEL, MEC) (EN 14034-3+A1 / ASTM E1515)
- Determination of explosion characteristics of dust clouds (pmax, (dp/dt) max, Kst) (EN 14034-1+A1, EN 14034-2+A1 / ASTM E1226)
- Determination of the limiting oxygen concentration (LOC) (EN 14034-4+A1 / ASTM 2931)
- Determination of minimum ignition energy of dust clouds (MIE) (EN 13821, EN ISO/IEC 80079-20-2 / ASTM 2019)
- Determination of minimum ignition temperature of dust clouds (MIT, MITc) (EN ISO/IEC 80079-20-2 / ASTM E1491)
- Determination of the minimum ignition temperature of dust layer (LIT, MITd) (EN ISO/IEC 80079-20-2 / ASTM E2021)
Requirements for the Test Sample
EN ISO/IEC 80079-20-2
- Finely divided solid particles with a nominal size of up to 500 μm (if finer fractions are encountered during use, a fraction smaller than 63 μm is recommended).
- Determination of minimum, mean, and maximum particle sizes, as well as the particle size distribution.
- Determination of moisture content.
EN 13821
- Finely divided solid particles with a nominal size of up to 500 μm (if finer fractions are present during use, a fraction smaller than 63 μm is recommended).
- Determination of grain size distribution.
- Determination of moisture content.
EN 14034-1+A1, EN 14034-2+A1, EN 14034-3+A1, EN 14034-4+A1
- Finely divided solid particles with a nominal size of up to 500 μm.
- Determination of grain size distribution.
- Determination of moisture content.
ASTM E1226, E1515, 2931, 2019, E1491, E2021
- Tests can be performed on the sample as received. It is recommended that 95% of the particles in the test sample be smaller than 200 mesh (75 μm).
- Moisture content should not exceed 5%. For ASTM E1491 only, the moisture content may be up to 10%.
Individual Test Procedures and Comparison of Standards
Determination of the ability to generate an explosive dust atmosphere - screening test (Go/No-Go Test) (EN ISO/IEC 80079-20-2 / ASTM E1226)
A screening test is the initial step in a dust hazard analysis, aimed at determining the potential for an explosive dust cloud within a plant or facility. The result of this test is qualitative, providing information on whether the tested dust sample is classified as a combustible material capable of creating an explosive atmosphere or, conversely, is non-combustible. If the test sample is found to be combustible, further testing is recommended to determine specific explosion parameters such as minimum ignition temperature (MIT), layer ignition temperature (LIT), minimum ignition energy (MIE), explosion severity (KSt/pmax), dust resistivity, and other relevant characteristics.
Figure 1 Modified Hartmann tube
| EN ISO/IEC 80079-20-2 | ASTM E1226 | |
|---|---|---|
| Hartmann tube | ✓ | X |
| 20 litre spheres | ✓ | ✓ |
| Ignition energy in 20 litre spheres | 2000 J | 5000 J |
The screening test consists of two parts as defined by the EN ISO/IEC 80079-20-2 standard:
- The first part is conducted using a modified Hartman tube with two ignition sources:
- A continuous induction spark with an ignition energy exceeding 1 J.
- A glow wire with an energy exceeding 10 J.
- The second part is performed in a 20-litre sphere, where two chemical ignitors, each with an energy of 1000 J (total energy 2000 J), serve as the ignition source.
In contrast, the ASTM standard only describes the part conducted in the 20-litre sphere and does not include tests using the modified Hartman tube. The ignition source specified by the ASTM standard is a chemical ignitor with an energy of 5000 J or higher. In other respects, the procedure is identical for both standards.
Determination of lower explosion limit of dust clouds (EN 14034-3+A1/ ASTM E1515)
The lower explosive limit (LEL/MEC) of combustible dust represents the lowest concentration of a mixture of combustible dust and air at which the mixture remains explosive. This value is crucial for defining hazardous zones and establishing conditions to prevent explosive atmospheres as part of a risk assessment.
Figure 2 20 litre spheres
| EN 14034-3+A1 | ASTM E1515 | |
|---|---|---|
| Ignition energy | 2000 J (2x 1000 J) | 2500 J nebo 5000 J |
| Verification of negative concentration | 3x | 2x |
In EN 14034-3+A1, the use of two chemical ignitors, each with an energy of 1000 J (total energy 2000 J), is prescribed, whereas ASTM E1515 requires a single ignitor with an energy of either 2500 J or 5000 J. Another difference between the standards is the number of verifications required: EN 14034-3+A1 mandates that the explosion-capable concentration be verified three times, while ASTM E1515 requires only two verifications.
Example of test results
Figure 3 Dependence of explosion pressure on dust concentration
Figure 4 Dependence of pressure rise rate on dust concentration
The lower explosive limit is 110 g/m3.
Determination of explosion characteristics of dust clouds (pmax, (dp/dt)max, KSt) (EN 14034-1+A1, ČSN EN 14034-2+A1/ ASTM E1226)
- Maximum Explosion Pressure (pmax): The highest overpressure generated during the explosion of an explosive atmosphere of a given substance in a closed container, under specified test conditions and standard atmospheric conditions.
- Maximum Rate of Pressure Rise (dp/dt)max: The highest rate of pressure increase per unit of time observed during the explosion of a mixture in a closed container.
- Explosion Constant (KSt): A dust-specific constant used, for instance, in the design of explosion protection systems.
- Explosion Class (St): Indicates the explosive properties of a substance and classifies powders into three categories (St1–St3) based on the value of KSt.
| KSt Value (bar × m × s-1) | Explosion Class St |
|---|---|
| 1–200 | St 1 |
| 200–300 | St 2 |
| 300 + | St 3 |
Testing takes place in a 20-litre explosion-proof sphere. Both standards use the same equipment, identical ignition energy, to determine explosion parameters. The standards are virtually identical, and the determination procedure and test equipment are the same for both standards. The equipment and work procedure are verified regularly in the international laboratory comparison Calibration-Round-Robin CaRo.
Example of test results:
Figure 5 Dependence of explosion pressure on dust concentration
Figure 6 Dependence of explosion pressure on dust concentration
- Maximum explosion pressure (pmax): 7,4 bar
- Rate of pressure rise after ignition (dp/dt)max: 648 bar/s
- Explosion constant (KSt): 176 m*bar/s
- Explosion class St: St1
Determination of the limiting oxygen concentration (EN 14034-4+A1/ASTM 2931)
The limiting oxygen content (LOC) is defined as the highest concentration of oxygen in a test mixture of air and nitrogen at which combustion or explosion does not occur. Understanding this parameter is crucial for protecting equipment and technologies from explosion hazards by using inert gases. The determination is conducted in a 20-liter sphere. The primary distinction between the standards lies in the initiation energy used during the test. EN 14034-4+A1 employs two chemical ignitors, each with an energy of 1000 J, whereas ASTM 2931 utilizes a single ignitor with an energy of 2500 J.
Determination of minimum ignition energy of dust clouds (MIE) (EN 13821, EN ISO/IEC 80079-20-2 / ASTM 2019)
The minimum ignition energy (MIE) is the lowest energy level of a spark capable of igniting airborne stirred-up dust when discharged. Understanding this parameter is essential for evaluating the ignition risk posed by combustible and explosive dust mixtures.
The test can be conducted using two different methods: with or without inductance. Testing with inductance assesses the hazard posed by electronic or electrical discharges, while testing without inductance evaluates the risk of electrostatic discharges.
Figure 7 Device for determining the minimum ignition energy
All standards utilize the same test equipment to determine the minimum ignition energy (MIE). The procedure outlined in EN ISO/IEC 80079-20-2 specifies exact dust concentrations (750, 1200, 2000, and 3000 mg) and ignition delay times (60, 120, and 180 ms). In contrast, EN 13821 and ASTM 2019 only indicate that dust concentration and ignition delay times should be varied, without providing specific values.
Aside from these differences, the determination procedure is identical across all three standards. We regularly verify the equipment and procedures through the international laboratory comparison program, Calibration-Round-Robin (CaRo).
Example of test results:
Figure 8 Dependence of ignition energy on dust concentration
The minimum ignition energy (MIE) falls within the range of 100 mJ < MIE < 300 mJ, with a specific value of Es = 220 mJ.
- A circle: represents a negative test result.
- A square: represents a positive test result.
- Tv set: refers to the set ignition delay time, which is the interval between dust sample agitation and spark discharge.
Determination of the Minimum Ignition Temperature of Stirred-Up Dust (MIT, MITc) (EN ISO/IEC 80079-20-2 / ASTM E1491)
The minimum ignition temperature (MIT) of stirred-up dust is defined as the lowest temperature of a furnace's hot surface at which the most easily ignitable dust-air mixture inside the furnace will ignite. This parameter is critical for assessing the likelihood of ignition in dust-air mixtures that come into contact with hot surfaces, such as in ascending ducts. Additionally, it plays a essential role in the design and application of protective systems for explosive atmospheres.
Figure 9 Device for determining the minimum ignition temperature of agitated dust
EN ISO/IEC 80079-20-2
- Initial conditions: Start with a temperature of 500 °C, a recommended powder charge of 0.3 grams, and an ignition pressure of 30 kPa.
- Incremental increase: If no ignition occurs, the temperature is increased by 50 °C.
- Optimization: Once ignition occurs, the optimum combination of powder charge and ignition pressure is determined.
- Temperature reduction: After identifying the optimum combination, the temperature is reduced by 20 °C. (For temperatures below 300 °C, the reduction step is 10 °C.)
- Verification: The negative temperature (the lowest temperature at which ignition does not occur) is verified 10 times using the same powder charge and ignition pressure.
- Final confirmation: The negative temperature is further verified by adjusting the powder charge and ignition pressure.
- Result: The final verified temperature is recorded as the negative temperature.
ASTM E1491
- The initial temperature is 500 °C, the recommended powder weight is 0.1-0.3 grams, and the boiling pressure is not specified
- When no ignition occurs, the temperature is increased by 50-100 °C.
- When ignition occurs, the temperature is reduced by 25 °C or less.
- At the non-ignition temperature, we change the dust concentration.
- The negative temperature must be verified by at least 5 different dust concentrations.
- Positive temperature must be verified in at least 3 different dust concentrations.
- The resulting temperature is the average of the positive and negative temperatures rounded to the nearest ten.
Determination of the Minimum Ignition Temperature of Dust Layer (EN ISO/IEC 80079-20-2 / ASTM E2021)
The minimum ignition temperature (MIT) of a dust layer is defined as the lowest temperature of a hot surface at which a dust layer of a specified thickness ignites when placed on the surface. This value is essential for evaluating the risk of ignition of deposited dust layers in contact with hot surfaces.
Figure 10 Device for determining the minimum sensing temperature of settled dust
EN ISO/IEC 80079-20-2
- Maximum surface temperature: 400 °C
- Positive result: Defined as ignition or glowing, a temperature rise of at least 250 °C in the dust layer above the surface temperature, or when the dust layer temperature reaches 450 °C.
- Dust layer thickness: Typically, 5 mm.
- Verification: The negative temperature must be verified three times. Positive temperature verification is not required.
ASTM E2021
- Maximum surface temperature: 450 °C
- Positive result: Defined as ignition or glowing, along with a temperature rise of at least 50 °C in the dust layer.
- Dust layer thickness: 12.7 mm.
- Verification: The negative temperature must be verified twice. Positive temperature must also be verified.
Example of test results:
Figure 11 The graph shows the dependence of the temperature in the settled dust layer on time
Conclusion
The comparison above demonstrates that European and American standards differ in certain aspects. The most significant differences lie in the requirements for the test sample, the number of verification tests, and the magnitudes of ignition energies.
When comparing test sample requirements across all standards, ASTM standards are more stringent. They specify that the test sample must be 95 % particulate with sizes smaller than 200 mesh (75 μm) and establish maximum moisture content values. In contrast, European standards recommend that the test sample contain particles smaller than 500 μm (for EN ISO/IEC 80079-20-2 and EN 13821, the recommendation is that fractions smaller than 63 μm be used if finer fractions are present in service). Additionally, European standards only require determination of the moisture content, without setting a maximum limit.
The ignition energy used in 20litre sphere tests is higher in ASTM standards than in European standards, affecting screening (Go/No-Go) tests, as well as Lower Explosion Limit (LEL) and Limiting Oxygen Concentration (LOC) determinations. Due to the higher ignition energy, ASTM standards can yield lower LEL and LOC values. In the screening (Go/No-Go) test, a test sample might be classified as flammable under ASTM standards but not under European standards, owing to the stronger ignition energy used in the ASTM method.
Regarding verification of negative temperatures or concentrations, European standards are more stringent. In contrast, ASTM standards require fewer tests to verify a given parameter.
To meet customer requirements, the VVUU Testing Laboratory offers dust explosion performance testing according to either European or American standards, depending on the customer's preference. The determinations are carried out in compliance with the selected standard.