EN 14034 3 2006 64 e stf BRITISH STANDARD BS EN 14034 3 2006 Determination of explosion characteristics of dust clouds — Part 3 Determination of the lower explosion limit LEL of dust clouds ICS 13 230[.]
General
The standard apparatus for assessing the lower explosion limit of dust clouds is a 1 m³ explosion pressure-resistant vessel This equipment is utilized to determine the maximum explosion pressure, the maximum rate of explosion pressure rise, and the limiting oxygen concentration in dust/air/inert gas mixtures.
The main components of the test apparatus are
NOTE The 20 l sphere apparatus is an alternative explosion vessel for these determinations (see Annex C).
Explosion vessel
The standard explosion vessel, designed to withstand explosion pressure, is a spherical or cylindrical container with a volume of 1 m³, as specified by EN 14460 For cylindrical vessels, the aspect ratio must be maintained at 1:1 ± 10%.
NOTE It is recommended that the explosion vessel be designed to withstand an overpressure of at least 20 bar
The equipment must be equipped with electrical and/or mechanical cut-offs to ensure that all openings in the vessel, such as the main door, instrument ports, and inlet or outlet, are securely closed before initiating any test procedures.
The apparatus shall also be equipped as far as possible to ensure that any residual pressure inside the vessel is vented before the vessel can be opened
2 chemical igniters 6 fast acting valve
3 inlet for purge air 7 connecting tube
4 dust disperser 8 outlet for exhaust gas
Dust dispersion system (dust container, fast acting valve, connecting tube, dust disperser)
The dust is stored in a 5.4 dm³ container with a 3:1 aspect ratio, engineered to endure an overpressure of at least 20 bar.
The dust container features a base outlet sealed by a fast-acting valve, which is triggered by a blasting cap This valve, equipped with a mushroom-shaped seal, is held in place by a small ring that is destroyed upon firing the blasting cap, allowing the valve to open due to the internal pressure of the dust container in under 10 ms The fast-acting valve is connected to the explosion vessel, with a connecting tube to the dust disperser limited to a maximum length of 350 mm For alternative valve options, refer to Annex A.
2 mushroom shaped seal 6 protective hood
Figure 2 — Dust container with blasting cap activated valve as commonly used for explosion suppression (schematic; it is commercially available)
A perforated semicircular spray pipe, known as a dust disperser, is installed inside the explosion vessel, aligned concentrically with its wall This spray pipe features an internal diameter of 21.7 mm and is equipped with 13 holes, each measuring 6 mm in diameter, including one hole in each end cap, as illustrated in Figure 3 and referenced in Figure 1.
For dust samples that are coarse, voluminous, fibrous, or poorly flowing, standard dust dispersers may not effectively discharge the material, as illustrated in Figures 3 and B.1 In these instances, it is essential to utilize specialized dust dispersers, examples of which are shown in Figures B.2 and B.3 The specific dust disperser employed must be documented in the test report.
Using dust dispersing systems not outlined in this standard may allow for the propagation of explosions from the explosion vessel to the dust container Therefore, it is essential to implement additional safety measures, such as ensuring the dust container has higher pressure resistance.
2 end cap with 6 mm hole
Figure 3 — Location of the 6 mm holes in the dust disperser
Ignition source
The ignition system consists of two electrically activated pyrotechnical igniters, each delivering a nominal calorimetric energy of 5 kJ based on the pyrotechnic powder mass Upon activation, these igniters produce a dense cloud of extremely hot particles with minimal gas emission They are triggered by electrical fuse heads, and the power supply circuit is designed to activate the fuse heads in under 10 ms Positioned at the center of the explosion vessel, the igniters fire in opposite directions.
Control unit
The control unit sequences the start of the dust injection, the activation of the ignition source and the start of the recording system.
Pressure measuring system
The pressure measuring system consists of a minimum of two pressure sensors and recording equipment, with the sensors installed in the test vessel and their heads aligned with the internal wall It is essential to implement measures to mitigate temperature effects on the pressure sensors.
The pressure measuring system shall have an accuracy of ± 0,1 bar or better and a time resolution of 1 ms or better
The lower explosion limit of dust clouds is inversely related to particle size, meaning that smaller particles lead to a lower explosion threshold Consequently, it is essential to determine the particle size distribution of the tested sample, which should be included in the test report.
The moisture content of dust clouds significantly influences their lower explosion limit, which decreases as moisture content diminishes Consequently, it is essential to measure the moisture content of the tested sample and include this information in the test report.
The dispersion process can reduce the size of dust particles To assess the significance of this effect, it is essential to evaluate the particle size distribution again after dispersion, without ignition.
NOTE 2 A rough classification of the shape of the dust particles may be also required („spherical“, „flat“ or „fibrous“)
NOTE 3 A volatile content may affect the explosion characteristics of the dust In this circumstance it may be necessary to measure the volatile content
Explosion tests with defined dust/air mixtures shall be carried out according to the following procedure
The required amount of the dust is placed in the dust container The container is then pressurised to an overpressure of 20 bar
Before starting the test procedure the temperature inside the vessel shall be measured and recorded
At the start of dust dispersion, the pressure within the 1 m³ vessel must be at atmospheric levels It is essential to measure and document the actual pressure in the vessel at the time of ignition, referred to as the initial pressure (\$p_i\$).
The dust container's bulk volume must not exceed ắ to ensure proper pressurization If this limit cannot be met, two parallel dispersion systems with 5.4 dm³ dust containers should be utilized.
The ignition delay time (\$t_v\$) for dust dispersion is set at (0.6 ± 0.01) seconds Pressure is monitored over time, and the explosion pressure (\$p_{ex}\$) is calculated by averaging the maximum values recorded by pressure sensors, as illustrated in Figures 4 and 5.
If the difference in the pressures measured by the pressure sensors is more than 10 %, the accuracy of the sensors shall be checked and the measurements repeated
An ignition of the dust (dust explosion) shall be considered to have taken place, when the measured overpressure relative to the initial pressure p i is ≥ 0,3 bar [p ex ≥ (p i + 0,3 bar)]
After each test, the explosion vessel shall be cleaned
Begin the procedure with a dust concentration of 500 g/m³ or another level that triggers an explosion, then systematically reduce the dust concentration by 50% of the previous level, as outlined below.
For dust concentrations exceeding 500 g/m³, the step width should be set at 250 g/m³ This process should be repeated until reaching the concentration level at which no explosion takes place, which will be identified as the lower explosion limit.
Y1 overpressure in the dust container
The Y2 explosion overpressure occurs in a 1 m³ vessel, with the fast-acting valve initiating at time \( t_a \) Dust dispersion begins at time \( t_0 \), while the reaction time of the valve is denoted as \( t_r \) The release of dust from the container into the vessel happens at time \( t_{nd} \), and pressure equalization between the dust container and the vessel is achieved at time \( t_{rd} \) The ignition source is activated at time \( t_i \), with an ignition delay of \( (0.6 \pm 0.01) \) seconds.
Figure 4 — Dust dispersion and pressure-time curve
Y2 explosion pressure p ex, in bar
Figure 5 — Determination of the lower explosion limit LEL
Calibration
Only calibrated systems for measuring temperature, time and pressure shall be used.
Verification
The test apparatus and procedures must be verified annually or after any major maintenance or repair This verification process, outlined below, focuses on the dust dispersing system and utilizes the explosion pressure, denoted as \$p_{ex}\$.
Verification shall be carried out using the test procedure given in Clause 6 by one of the following two ways:
Internal verification requires at least one reference dust with a known maximum explosion pressure (\$p_{ex}\$) for the relevant concentrations The results of \$p_{ex}\$ must not differ by more than 5% from previously obtained results with the reference dust.
External verification of explosion pressure (\$p_{ex}\$) for relevant concentrations must be conducted through comparative measurements with at least one other laboratory using the same dust type The results of \$p_{ex}\$ should not differ by more than 10% from those previously obtained by the other laboratory.
For the purpose of internal verification reference dusts shall be chosen on the basis of evidence that their p ex does not change significantly over the period between verifications
The instructions for use shall include at least the following warnings:
Precautions shall be taken to prevent accidental ignition by electrostatics, friction, impact or other means during the handling of the dust samples, blasting caps and chemical igniters;
Precautions shall be taken to ensure any openings in the explosion vessel, e.g doors and ports, are properly closed before a test;
To ensure personnel safety during explosion vessel tests, it is crucial to implement precautions that protect against flying fragments This can be achieved through the use of shielding or by maintaining a safe distance from the test area.
Before opening the explosion vessel any build up of internal pressure in the vessel shall be released;
After a test, glowing material may remain on the walls of the explosion vessel It is essential to take precautions to protect personnel from potential risks if this material ignites when the vessel is opened.
Toxic samples and reaction products shall be handled and disposed of in a way that will not cause harm to personnel or the environment
The lower explosion limit of dust clouds can be assessed using alternative testing equipment and procedures, provided that the method demonstrates consistent results across at least ten different dust types This validation must include a minimum of two metal powders, two natural organic powders, two synthetic organic powders, and two coal dusts, ensuring that the results align with those obtained from standard methods.
Details of an alternative method using the 20 l sphere, for which conformity has been proven, are given in Annex C
The modified Hartmann tube can be used to determine the lower explosion limit of dust recognized as explosible However, a "No reaction" result in this apparatus does not indicate that the dust is non-explosible or lacks a lower explosion limit.
The test report shall include at least the following information:
Name and address of the testing laboratory;
Unique identification of the test report;
Name, description and identification of the tested dust (characteristics);
Preparation of the dust sample for the tests;
Particle size distribution of the tested dust (incl method);
Moisture content of the tested dust (incl method);
If relevant a volatile content of the tested dust (incl method);
Type of the test equipment and the test procedure used;
Any changes to the test equipment or test procedures specified in this standard, the reasons for the changes and any other information relevant to specific tests;
A table or graph showing the measured values of p ex versus the dust concentration (corresponding to Figure 5);
Lower explosion limit LEL in g ã m -3 ;
A statement to the effect that the test results relate only to the samples tested;
A statement that the result may deviate up to 10 %
An alternative type of fast acting valve for which conformity has been proven, is the:
A ball valve equipped with an electro-pneumatic drive can serve as an alternative to the fast-acting valve mentioned in section 4.3 It must be engineered to endure an internal overpressure of at least 20 bar, with an opening time of less than 100 ms Commercially available valves meet these specifications.
Figure A.1 — Electro Pneumatic Valve (schematic)
One indication of conformity is a dispersion characteristic lying between the ranges given in Figure A.2 (without dust)
Figure A.2 — Discharge characteristic of dust dispersers (without dust)
To ensure compliance, the delay between the start of dust dispersion, triggered by the electro-pneumatic valve, and the ignition source activation must be within the range of (0.6 ± 0.1) seconds The ignition delay time, represented as \( t_v = (x ± 0.01) \) seconds, will be established through external verification as outlined in section 7.2.
Dust disperser with 5 mm holes
An alternative type of semicircular dust disperser for which conformity has been proven, is the:
Dust disperser with 5 mm holes
The dust disperser, with an internal diameter of 21,7 mm (see 4.3) is fitted with 20 holes of a diameter of
5 mm (incl one hole in each end cap) which are located according to Figure B.1
The dust disperser nozzles, developed over many years in Europe, exhibit variations in size and hole count due to historical factors However, the two configurations outlined in the standard and this annex have demonstrated nearly identical performance results.
For dust samples that are coarse, voluminous, fibrous, or poorly flowing, standard dust dispersers may not effectively discharge the material, as illustrated in Figures 3 and B.1 In these instances, it may be essential to utilize specialized dust dispersers, examples of which are shown in Figures B.2 and B.3 The specific dust disperser employed should be documented in the test report.
1 end cap with 5 mm hole
Figure B.1 — Location of the 5 mm holes in the dust disperser
General
An alternative type of test equipment, for which conformity has been proven, is the:
For dusts that are coarse, voluminous, fibrous, or poorly flowing, effectively discharging them through the specified dispersing system may be challenging In such instances, utilizing a 1 m³ vessel is recommended whenever feasible.
Test apparatus
The explosion vessel, designed in accordance with EN 14460, is a stainless steel hollow sphere with a volume of 20 dm³, featuring a water jacket to dissipate heat from explosions During testing, dust is introduced into the sphere from a pressurized container through a fast-acting valve and rebound nozzle, with the valve operated pneumatically by an auxiliary piston and electrically activated compressed air valves The ignition source is centrally located within the sphere, and the pressure measuring system consists of at least two pressure sensors along with recording and control equipment.
Before the dust is dispersed, the sphere must be partially evacuated to a pressure of 0.4 bar, ensuring that the initial pressure (\$p_i\$) inside the sphere reaches 1013 mbar after the dust injection.
4 dust container (0,6 dm 3 ) 9 water inlet
5 air inlet 10 outlet (air, reaction products)
Figure C.1 — Test equipment 20 l sphere (schematic)
Test conditions
Initial pressure p i = 1013 mbar (pre-evacuation of the explosion vessel down to 0,4 bar);
The ignition source consists of two chemical igniters, each with an energy output of 1 kJ For the verification process outlined in section 7.2, it is necessary to utilize two chemical igniters, each providing an energy of 5 kJ to accurately determine the pressure.
Test procedure
In general the test procedures described for the 1 m³ vessel (see Clause 6) shall be applied for the 20 l sphere
A dust explosion is defined as the occurrence of ignition when the measured overpressure, including the effects of chemical igniters, reaches or exceeds 0.5 bar relative to the initial pressure (\$p_{ex} \geq (p_i + 0.5 \text{ bar})\$).
The highest concentration of a combustible dust at which no ignition occurs in three consecutive tests shall be taken as the lower explosion limit (LEL)
! Relationship between this European Standard and the Essential
Requirements of EU Directive 94/9/EC
This European Standard was developed under a mandate from the European Commission and the European Free Trade Association to ensure compliance with Essential Requirements.
Once cited in the Official Journal of the European Union and implemented as a national standard in at least one Member State, compliance with this standard's normative clauses provides a presumption of conformity with the relevant Essential Requirements of the Directive, specifically Annex II, Clause 1.0.1, and related EFTA regulations.
WARNING — Other requirements and other EU Directives may be applicable to the product(s) falling within the scope of this standard."
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