Risk assessment of explosion of epoxy powder coatings

Abstract:
used 20L ball explosion test device, Hartmann tube explosion test device and Godbert-Greenwald furnace to study the different particle size epoxy powder coatings in different mass concentrations and powder pressure of the explosion sensitivity and strength parameters of the law.
results show that the minimum explosion mass concentration of dust cloud is 30 to 40g/m3, and it shows non-monotonous change with the increase of particle size; The maximum explosion pressure had no significant effect, and the maximum pressure increase rate decreased significantly with the increase of particle size, the minimum ignition energy and the minimum fire temperature of dust cloud increased with particle size and increased monotonically, and the minimum fire temperature of dust cloud decreased gradually with the increase of powder spray pressure.with the development of modern industry, the domestic spraying industry production scale continues to expand, spraying, painting and electrophoresis and other coating processes have been widely used
China
. Static powdering technology in the powdering process has many advantages, such as solvent-free loss, small pollution, energy saving, uniform coating, etc., is widely used in metal surface coating, but static powder spraying process there is an explosive dust environment, once there is an accidental fire source, dust explosion accident will occur.
dust explosion has the characteristics of great destructive power, strong suddenness and great personal injury. There have been many explosions of powder coating in our country. On May 8, 2018, a paint company exploded during production, injuring several people.For more than
years, some scholars have studied the explosive hazard in the process of powder spraying from different angles, and the Guangzhou Institute of Electrical Science in Guangdong Province has focused on the process equipment and process construction of electrostitration spraying of epoxy powder coating. Li and other tests the sensitivity parameters of the spray powder, and proposes the explosion protection measures based on explosion insulation and explosion leakage.
Liu Daochun analyzed the process of possible explosion accident in electrostational spraying operation, Shen Li summarized the current safety status of the spraying process from the process of powder spraying, and put forward a series of suggestions for improvement.
and abroad for the spraying industry explosion risk research mainly concentrated in the production process, for spraying raw materials explosive research is very few. In this paper, the explosive risk parameters of epoxy powder coatings are systematically studied, and the semi-quantitative assessment of the risk of epoxy powder coatings is carried out according to the results, which provides the basis for the safety protection of the late spraying industry.
1, experiment
1.1 sample treatment
sample for a spray plant epoxy powder coating, composed of epoxy resin, pigments, fillers and additives. The particle size distribution tested by the granularity meter (Mastersizer 2000 Laser Particle Meter, Shanghai Quyuan Scientific Instruments Co., Ltd.) is 81 m and belongs to the micron grade powder in the ultra-fine powder. Large differences in particle size were observed under the microscope (see Figure 1), indicating that epoxy powder particles had a reunion effect and were unevenly dispersed.different particle sizes of powder coatings were selected as experimental samples by vibration screening method (particle size results can be found in Table 1). The sample was hot-dryed to ensure that its moisture quality score was less than 5% during the experiment.1.2 method
experiments were conducted in 20L spherical explosive containers (see Figure 2), Hartman tubes (see Figure 3) and G-G furnaces (see Figure 4).Dust explosion parameters are divided into explosive ferocity parameters and explosion sensitivity parameters, the explosion ferocity is mainly dust cloud maximum explosion pressure (p), dust cloud maximum explosion rate (dp/dt) and other parameters, sensitivity parameters are mainly by dust cloud minimum explosion mass concentration (ρLEL), dust cloud minimum ignition energy (EMIE) and dust cloud minimum ignition temperature (TMIT).
epoxy powder weighed in a certain mass, placed in a dust tank, sealed. Pump the exploding ball into a vacuum (-0.06MPa) and start the computer application. Automatically turn on the dust solenoid infringing powder to ensure that the exploding ball is at atmospheric pressure when ignited. The ignition head is ignited after a lag of 60ms, and the pressure-time curve generated after the explosion is analyzed to arrive at the explosive pressure and pressure rise rate of the dust explosion.
the epoxy powder explosion curve with a particle size of 63 to 98 m, as shown in Figure 5. Where t1 is the combustion duration (ms), t2 is the induction time (ms), td is the exit valve delay (ms), tv is the ignition delay (ms) ;pd is the tank expansion pressure (MPa) ;dp/dt is the pressure rise rate (MPa/s) ;p ex is the maximum explosive pressure (MPa).A certain mass of epoxy powder is placed in the powder storage chamber at the bottom of the Hartmann tube, and a certain concentration of dust clouds are formed in the glass tube after the air at the set pressure is blown away, releasing the set-value energy pre-stored by the capacitor through a certain distance electrode. Through 10 experiments, it was observed whether the dust cloud in the tube spread combustion or explosion to determine the minimum ignition energy of the dust cloud.
, the change of dust mass concentration at the determined ignition energy continues to be tested until no combustion or explosion occurs 10 times in a row at each mass concentration. The minimum ignition energy is between the maximum energy (E1,mJ) for 10 ignition failures and the minimum energy (E2, mJ) for 10 consecutive ignitions.
in order to more clearly compare the differences between the ignition energy of different samples, the statistical value Es(mJ) value is used in this paper to indicate the size of ignition energy.: the number of successful ignition experiments for ignition energy is E2, and the total number of E2 experiments for the ignition energy is E2.
the amount of epoxy powder weighed in the experiment is added to the dust storage tank, the furnace is heated to a pre-set temperature according to the furnace wall temperature control, and then the air pressure in the air reservoir is adjusted to the experimental pressure. When the solenoid valve is open, compressed air in the tank quickly sprays the epoxy powder from the tank into the furnace, creating a dust cloud. Change the pressure and dust quality in the tank until a fire occurs. Keep the experimental conditions and lower the temperature to continue the test until the flame is not observed in 10 experiments, which is the lowest fire temperature of the dust cloud.
2, Results and Discussion
2.1 Explosion Ferocity
2.1.1 Particle Size, Concentration effect on explosion pressure
According to GB/T 16426-1996 experimental method, the explosion pressure of different particle size ranges and concentrations of epoxy powder was studied separately to explore the effects of mass concentration and particle size on it. Figure 6 shows the mass concentration and explosion pressure diagram of dust of different particle sizes. by the figure, the epoxy powders of different particle size ranges show similar variations. At mass concentration of 125 to 500g/m3, the explosion pressure increases rapidly with the increase of mass concentration.
with the increase of dust concentration, the explosion pressure of dust cloud increased slightly, and the explosion pressure of epoxy powder of different particle size ranges reached a peak of 0.689, 0.704 and 0.706MPa at 1000g/m3. After that, the dust mass concentration continued to increase, the explosion pressure stabilized and decreased, showing the overall "U" pattern of change, indicating that particle size and mass concentration have a certain impact on the explosion pressure.
In the range of low mass concentration, oxygen quantity is sufficient, the reaction is sufficient, the explosion pressure increases, in the case of high epoxy powder concentration, a certain powder pressure, too many powder particles will affect the dispersion of the powder, coupled with the limit of oxygen, the explosion pressure is getting smaller and smaller. In general, with the decrease of particle size, the effective ratio of epoxy powder to surface area will be greatly increased, the greater the explosion pressure.
2.1.2 Concentration, particle size on the rate of pressure increase
according to GB/T 16426-1996 on different particle size, different mass concentration of epoxy powder coating explosion pressure was carried out experimental research, to explore the mass concentration and particle size of its effect. Figure 7 shows a graph of the mass concentration and pressure rise rate of dust of different particle sizes. the figure, for epoxy powders in different particle size intervals, dp/dt increases significantly with the increase of mass concentration at mass concentrations of 125 to 500g/m3. Subsequently, there were varying degrees of fluctuation, with epoxy powders in the 0 to 32 m particle size range reaching a peak of 44.0MPa/s at 2000g/m3 and powder particles in the other particle size ranges peaking at 1500g/m3. Fluctuations in the increase near the maximum value are mainly due to the thermososticity of epoxy powder, which softens the flow easily throughout the combustion process of the explosion and causes incomplete reaction by sticking to the wall of the reaction vessel.
time, powder particle size and mass concentration have a certain effect on dp/dt, particle size has a greater effect on dp/dt, with the gradual increase of particle size dp/dt will decrease. Because at low dust concentrations, epoxy powders with smaller particle sizes are susceptible to heat, releasing more volatile substances under the same conditions, resulting in a higher combustion rate and a higher pressure rise rate, reducing the epoxy powder particle size will accelerate the combustion rate of epoxy powder and significantly increase the maximum explosive pressure rise rate.
2.2 explosion sensitivity 2.2.1 Dust Cloud Minimum Explosion Concentration
In the 20L ball explosion test device, the lowest explosion concentration was used to find out whether the explosion pressure exceeded 0.15MPa as the explosive standard, as a result of Table 2. the effect of small-size epoxy particles is reduced compared to the surface area, and the reaction is incomplete. Large-particle-size epoxy powder at the same concentration of the effective number of epoxy particles is less, the total than the surface area is reduced, the reaction is not complete, so from the lowest explosion concentration, the particle size of 32 to 63 m epoxy particles are more prone to explosion.
In addition, the experimental study of the lowest explosive concentration is using 10kJ ignition head, the minimum explosion concentration of different particle size range is not very different, it may be that the ignition head's energy is too large, too strong ignition behavior will cover the explosion behavior of low concentration of dust itself, there may be "overload" phenomenon.
2.2.2 Dust Cloud Minimum Ignition Energy
The effects of particle size on MIE are explored by reference to EN13821's experimental method of testing the minimum ignition energy, and 300, respectively, under a certain ignition energy. 600, 900, 1200, 1500, 1800, 2400, 3000g/m3 a total of 8 mass concentration points of the experiment, 10 experiments per point.
the ignition energy of different particle size dusts is shown in Figure 8. As can be known from the figure, with the increase of particle size, the overall trend of the minimum ignition energy of dust cloud is gradually increased. When the particle size of dust is 75 to 98 m, the required ignition can be increased significantly.
This is because the change in particle size affects the surface area of dust particles, dust particle size increases, decreases than surface area, the contact area with oxygen is also relatively small, the dust particles involved in the reaction can not be fully burned, so the dust particles participate in the reaction required to increase the energy. 2.2.3 Minimum fire temperature of dust clouds
based on GB/T 16425-1996 experimental method, at mass concentrations of 0.1, 0.2, 0.3, 0.4, 0.5, 0 Experiments were conducted under .6, 0.7, 0.8, 0.9, 1.0g for the lowest fire temperature of dust clouds with powder pressures of 0.08, 0.09, and 0.10MPa for different particle size range epoxy powder coatings.
3 is the result of the lowest fire temperature of epoxy dust clouds with different particle sizes and different powder pressures. The reunion effect of epoxy powder coating makes the powder pressure less than 0.08MPa not enough to blow the powder out to form a dust cloud state. When the powder pressure is 0.08, 0.09, 0.10MPa, the three groups of powder coatings with different particle size ranges change the same law, that is, as the powder pressure increases, the minimum fire temperature of the dust cloud decreases.
mainly due to the interaction between epoxy particles, in a certain range, the greater the powder pressure is conducive to the formation of epoxy dust clouds, so that the more dispersed epoxy particles, the more full contact with the air, epoxy particle combustion provides less heat.
and powder pressure is low, epoxy particles are not fully dispersed, and the heat required for combustion needs to be passed on to each other, requiring a higher fire temperature. Under low injection pressure conditions, epoxy powder particles are not fully dispersed, the heat required for combustion is transmitted to each other, and the ignition temperature is higher.
the same powder pressure, the minimum fire temperature of the dust cloud increases with the increase of particle size. The fire temperature of the dust cloud with particle size range of 0 to 32 m is 480 to 490 degrees C, which is lower than the fire temperature of dust cloud with particle size range of >63 to 98 m, mainly because the burning of dust mainly occurs on the surface of epoxy particles, the smaller the particle size, the larger the surface area, more full contact with air and more complete reaction. In addition, the smaller the epoxy particles, the increase in the number of dust particles in the same volume, the more heat produced by the dust particles that are effectively involved in the reaction, the more intense the reaction, so that the minimum fire temperature is reduced, the dust is more easily ignited, and the potential risk of dust explosion increases.
3, semi-quantitative risk assessment
In this paper, epoxy powder coatings are studied experimentally in terms of explosion sensitivity and ferocity, corresponding to the probability and severity of dust explosion when the risk assessment is made. The risk assessment and evaluation of dust explosion can be carried out from qualitative and quantitative aspects, and no unified and effective risk assessment method has been formed at home and abroad.
German Explosion Index Grading Methodology (VDI 2263-1: Dust fires and dust explosions hazards-assessment-protective measures) is related to the volume of the explosive container and the maximum rate of increase in explosive pressure, and the explosion index Kst is (dp/dt) maxV1/3 (0.1MPa/m/s), as measured in Table 4. Based on the above experimental results, the most dangerous explosion parameters are selected for generational calculation, Kst=1190.1MPa.m/s is a weak explosion type, but even if the risk level of dust explosion is "weak", it is not