Minimum Ignition Energy Test Apparatus for Dust Cloud

Transcription

1 Minimum Ignition Energy Test Apparatus for Dust Cloud Industrial Explosion Protection Institute Northeastern University, P. R. China Jan. 3, 2010

2 1 Introduction Minimum ignition energy is the lowest energy of spark (as measured in standard procedure) that is capable of igniting the most sensitive dust/air mixture with sustained combustion. The criterion of ignition is if the flame propagation length from the center of the spark is more than 60mm. The apparatus consists of a Hartmann explosion test apparatus (MIE ETD-1.2L) and a spark generator (MIE SPG-10J), and the parameters of the apparatus are shown in Table 1. Table 1 Parameters of MIE test apparatus for dust cloud Range of energy 0.2mJ~10J Humidity limit <80% Range of temperature 15~25 Rating voltage of power supply AC 220V 50Hz Rating current of power supply 2A Compressed air supply 0.7 MPa Volume of gas reservoir for dust dispersion 50mL High voltage relay Design voltage 20kV, Rating voltage 15kV Charging voltage 12kV 1 Discharge load, selected by human interface or computer None; Inductance (1.5mH) Resistance (100K) Trigging method Moving electrode; Switch; Dust cloud Method of capacity selection Automatic Note: 1 For safety consideration, the charging voltage is limited to 12kV. The advantage of the MIE test apparatus are as follows: (1) The distributed capacity (capacity of the circuit without connection of capacities) is around 10pF. (2) Capacities are connected into the circuit automatically. (3) Human interface is used to specify operation mode and parameters, such as discharge load specification (with or without inductance), triggering method, ignition delay et al.

3 2 Mechanism 2.1 General mechanism The Hartmann explosion test apparatus includes Hartmann tube, electrodes, gas driven piston, micrometer, sample chamber (the bottom of the Hartmann tube), dust dispersion nozzle, inlet valve, dispersion valve, gas reservoir and the supporting case (Fig. 1). The left electrode is moveable with a gas driven piston. The piston stand and micrometer are put on a track, and both of them are moveable to adjust the electrode distance. Figure 1 Schematic diagram of the minimum ignition energy test system Dust is put in the dust container on the bottom of Hartmann tube and can be dispersed to form dust cloud in the Hartmann tube. The capacity in the spark generator is charged before ignition, and a spark is triggered in given ignition delay either by moving electrode or by a switch. The simplified schematic diagram of spark generator is shown as Fig. 2. Switch K1, K2 is used to charge capacity C when they are on their normal open positions. K2 is used to trigger ignition when Switch triggering mode is selected. K3 is used to shortcut the electrodes for earthing the two electrodes or guarantee equal potential when charging. K3 is also used to release electricity of the capacity C when an ignition is finished. K4 is used to discharge the capacity C1 slowly.

4 -HV GND High voltage power R1 Current limit resistance K1 Charge C R2 904 Explosion vessel K3 Protection 908 Charge K2 Ignition K4 Slow discharge GND Figure 2 Simplified schematic diagram of the spark generator 2.2 Mechanism of the spark generator Spark generator can be used together with Hartmann tube (MIE-1.2L), gas explosion test apparatus (MIE-G0.2L) and 20L spherical explosion test vessel (ET-20L). The discharge load may be none, resistance and inductance. The voltage and current during discharge can be measured dynamically using a recording oscilloscope in case of necessary. R1 Current limiting resistance 10M/10W JS1 Charge -HV GND HV Power JK5-1 Capacity measurement Resistance/inductance JK2 R2 L1 1mH JK1 Discharge load Explosion test vessel JK3 Protection JK6-1 Voltage measurement JS2 Charge/Ignition Current sampling R4 Capacity group HD HS Capacity meter LD LS 900 R3 测流电阻 Voltage divider 0.3Ω 1/10000 JK4 Voltage sampling JK6-2 Current measurement JK5-2 Capacity measurement 900 Figure 3 Schematic diagram of the spark generator (Voltage divider is not equipped by default)

5 2.3 Triggering modes of spark generator Triggered by moving electrode Before a single run of test, firstly the moving electrode is moved to left side, and the electrode distance is more than 10mm, which is more than break distance for the defined voltage. Then the capacity C is charged. In case of ignition, K1 is disconnected, and terminal of K3 is connected to its terminal, at the same time the terminal of K2 is connected to earth. Because the distance of two electrodes is too big, the voltage of the capacity is not enough to break the electrode gap. Finally, the moving electrode is quickly moved to the setting electrode distance. Discharge occurred when the electrode distance is small enough. This mode is only valid for Hartmann tube Triggered by a switch When the apparatus works in this mode, the left electrode is always on its right position, and the electrode distance if fixed. Discharge is triggered by switch K2. Since the switch itself consumes part of the energy stored in the capacity, the minimum ignition energy is bigger than the MIE determined by other triggering mode. This mode is used with the gas explosion test apparatus MIE-G0.3L, and the 20L spherical explosion test apparatus (ET-20L, gas and dust cloud) Triggered by dust cloud When the apparatus works in this mode, the left electrode is always on its right position, and the electrode distance if fixed. During charging, K2 is off (Terminals of and are connected). Increase the voltage of the high voltage power, until sparks occur between the electrodes, and then decrease the voltage a little so that breakdown just not occurs. When dust is dispersed, the electrical field is disturbed by the dust cloud and spark ignition is induced between the electrodes. This mode is used for Hartmann tube (MIE-1.2L) and the 20L spherical explosion test apparatus (ET-20L, explosion test for dust cloud) Triggered by increasing voltage The current limiting resistance is big enough, so for defined capacity, the charging time is more than several tens seconds. During charging, K2 is off (Terminals of and are connected). Increase the voltage of the high voltage power step by step, until sparks occur between the electrodes. This mode is used for the gas explosion test apparatus MIE-G0.3L, and the 20L spherical explosion test apparatus (ET-20L, gas explosion test)

6 3 Operation guide of MIE 1.2L The test procedure is as follows: (1) Adjust the electrode distance using the micrometer. (2) Weight defined mass of the sample dust, and distribute the dust in the dust container on the bottom of the Hartmann tube. (3) Switch on the high voltage power, and press Charge button, charge the capacity for 20s. Press Prepare air button to charge compressed air in the air reservoir. Press Ignition button, programmable logic controller will disperse the dust, and dust cloud will form in the Hartmann tube. After a pre-defined ignition delay, moving electrode will trigger an electrostatic discharge. (4) If the dust cloud is ignited, clean the electrode and carry out the next run of test. If the dust is not ignited, samples can be reserved in the Hartmann tube and try ignition again. If ten tries fail, then ignition fails for the defined test condition. The various test conditions include: Electrode distance, sample mass, dispersion pressure, ignition delay, charged voltage and capacity. Before systematical tests, preliminary tests shall be carried out to determine sensitive conditions, include sensitive electrode distance, sample mass, dispersion pressure. In case of systematical tests, first define energy, then change voltage (capacity will be determined automatically). If the sample dust is ignited at an energy value, decrease the discharge energy, until no ignition occurs for more than 20 tries. 4 Packing List Table 2 Packing list Items Unit Quantity Hartmann explosion test apparatus, MIE ETD-1.2L Suit 1 Control box, MIE SPG-10J (Discharge voltage and current Suit 1 measurement terminals are not equipped) Pipe for compressed air, Pipe-MIE 1.2L Suit 1 Control cable, Cable-MIE 1.2L Suit 1 High voltage cable, Cable-HP Pair 1 Explosion test software - MIE 2010 CD 1 Hartmann tube (Glass tube), as backup, MIE Tube G02 1 Electrodes, as consumable goods Pair 10

7 5 Requirement for installation Table 3 Requirement for installation Item Dimension of field Grounding terminal Fume hood Power supply Compressed air supply, 0.7 MPa Movable dust collector Computer Description 3m 4m With grounding resistance less than 4 Ω For removing combustion product of MIE ETD-1.2L. 220V, 50 Hz, 5A One 40L compressed air steel bottle, maximum pressure 15 MPa. A pressure reducing valves (adapter), 15 MPa to 1 MPa, operated at 0.7 MPa. For cleaning the Hartmann explosion test apparatus. Optional, for experimental process control and database maintain. 6 Related pictures The photos of Hartmann tube and spark generator are shown as Fig. 4 and Fig. 5.

8 Fig. 4 Minimum ignition energy test apparatus MIE 1.2L

9 Fig. 5 Minimum ignition energy test apparatus control unit (spark generator ETC SPG-10J) 7 Contact us Dr. Zhong Shengjun Box 339, Northeastern University Industrial Explosion Protection Institute, Northeastern University Tel: , Fax: zhongsj@smm.neu.edu.cn