| As a typical method to prevent the explosion hazards,dust explosion venting technology is widely used in the equipment with explosion risk.With the development of the industry,small chemical engineering units such as high-pressure bunker and hydrogenation reactor have rapidly developed.Processing technology under high temperature and pressure have been widely used as well.Therefore the accurate explosion venting technology at elevated activation pressure becomes an urgent problem to be solved.Previous studies have mostly focused on the venting with the activation pressure lower than 0.1 MPa.They mainly focused on the analysis and discussion of the venting phenomenon like reduced pressure and vented flame.However,there is a lack of the quantitative studies on venting behaviors,especially for the engineering calculation research related to dust explosion venting at elevated activation pressure.In this paper,dust explosion venting experiments with high activation pressure(70k Pa～280 k Pa)were carried out with 30μm aluminum dust to study the effect of activation pressure,venting area and dust concentration on venting behaviors.In addition,the precise prediction models of reduced pressure and vented flame are established respectively.A design method of venting area for equilibrium venting at elevated activation pressure is proposed as well.The main conclusions from this research are as follows:(1)The vented flame propagation process at elevated activation pressure often divide into two stages.In the first stage,the vented with high pressure near the venting disc ejects and then expands.In the second stage,the burning dust cloud is pushed by the subsequent vented dust cloud and continues to propagate forward.When the dust concentration is 400g/m~3,600 g/m~3,800 g/m~3and 1200 g/m~3,the maximum vented flame length is 0.98 m,1.44 m,1.40 m and 1.84 m,respectively.The length and width of the vented flame are both promoted by the increase of the activation pressure,dust concentration and venting area.Both the maximum reduced pressure and the pressure rise rate in the vessel increase with the rise of activation pressure.And the reduced pressure continues to rise when the concentration is higher than the optimal concentration of confined explosion.(2)By the quantitative calculation of the physical change during the venting process,the prediction model of dust explosion venting pressure is established.The model can predict the process of internal reduced pressure during venting at high static activation pressure with a great precision.By the predicted results from models,it is found that the thermal energy loss accounts for more than 80%of the total energy loss,which is the main energy loss form during the venting.The proportion of thermal energy loss to the total energy loss has a positive correlation with the change of the activation pressure,and a negative correlation with the change of the venting area.During the venting process,the change transition of state parameters lags behind the change of energy loss rate,which are not synchronized.Based on the analysis above,a design of venting area which is suitable for the equilibrium venting at elevated activation pressure is proposed,and the designed results are verified by the experimental results(3)Based on the analysis from the jet dynamic,the prediction model of the maximum length and maximum width of vented flame is established.The new model takes the influence of the experimental parameters into consideration.And the predicted values are verified by experimental results.For the external pressure,the predicted results from the standard EN14491 has a positive correlation with the rise of venting area,which is consistent with the experiments,while the standard NFPA 68 shows an opposite result.Based on the fitting analysis of experimental results,it is found that the the external pressure attenuation is inversely proportional to the distance.So it is suggested to modify the external pressure prediction in EN14491 to have a better performance. |
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