Characteristics Of Dust Explosion Venting At Elevated Static Activation Overpressures

Dust explosion venting technology is widely used in process industry to prevent vessels and equipment from being damaged by dust explosion overpressures. However, venting technology is still not completely clear due to the numerous influencing factors and the complex mechanisms. The existing research is mainly focused on the characteristics at low static activation overpressures, usually lower than0.1MPa. Research on dust explosion venting characteristics at elevated static activation overpressures is less. Even those studies at low static activation overpressures, the synchronous changes of flame and overpressure during dust explosion venting process and the secondary explosion during duct-vented dust explosions are still not fully understood. In this paper, both the simply vented dust exploions and duct-vented dust explosions were performed at elevated static activation overpressures ranging from0.1MPa to0.6MPa. The characteristics of explosion overpressure and vented flame in dust explosion venting process at elevated static activation overpressures were studied. Experimental results and predictive results calculated according to the design standards existed were compared. The influence mechanism and laws of the existence of vent duct on the maximum reduced overpressure were systemically studied. The main work and conclusions are listed below:(1) The spherical experimental apparatus both for dust explosion and explosion venting was established. The standard20L spherical chamber in accordance with international standards was used as the explosion vessel. The venting structures used for the elevated static activation overpressures, the vent duct with variable duct diameters and lengths, the control and multi-channel synchronous data acquisition system were all accomplished. The confined dust explosion experiments, the simply vented dust explosion experiments, and the duct-vented dust explosion experiments can be performed with this apparatus. Electric igniters with0.5kJ ignition energy and the lycopodium dust with stable explosion characteristics were selected in experiments. The static activation overpressures of venting structures at different venting membrane layers and different vent diameters were tested, by using compressed air to fail the vent membrane. The calculating formulae at various vent diameters and membrane layers were fitted.(2) The explosion characteristic parameters of lycopodium dust with half-content diameter of66μm were tested in the confined20L chamber. The results show that the dust explosion process can be divided into three stages:explosion initiation, explosion acceleration, and explosion deceleration. The maximum overpressure, the maximum rate of pressure rise increase rapidly then decrease slowly with the increase of dust concentration. The maximum overpressure reaches maximum at the concentration of750g·m-3, indicating that the maximum explosion overpressure is0.65MPa. The maximum rate of pressure rise reaches maximum at the concentration of1000g·m-3, indicating that the maximum rate of explosion overpressure is28.3MPa·s-1. The dust explosion index is7.68MPa·m·s-1.(3) The simply vented dust explosion experiments at elevated static activation overpressures of0.1MP～0.6MPa were performed. The results indicate that the maximum reduced overpressure increases with the increase of the static activation overpressure. When the static activation overpressure reaches at about0.42MPa-0.45MPa, the increasing tendency gets slow and the venting type is close to the equilibrate venting. The experimental results and the calculated results according to NFPA68and EN14491standards at elevated static activation overpressure were compared. It is shown that the predictive results of NFPA68standard may or may not conservative but have high precision. However, the predictive results of EN14491standard are always conservative, and have poor accuracy.Three types of venting features for exlosion overpressure and flame at different static activation overpressures and vent diameters were founded according to experimental results: no secondary flame, no secondary explosion type; secondary flame type; secondary explosion type. Three physical models were established to explain the cause of the venting types. Secondary explosion type occurs more easily at larger vent diameters and lower static activation overpressures.(4) The suppression technology to the secondary explosion occurred out of the vent during simply vented dust explosions was studied. Results indicate that both the porous wire mesh and the bubble iron-nicked metal can suppress secondary explosions though the maxium reduced overpressure increases. The vented flame out of the vent is totally extinct when the layers of the wire meshes are enough, which is called flameless venting. The design idea and the design process of the flameless venting were raised.(5) The duct-vented dust explosion experiments at elevated static activation overpressures of0.1MPa～0.6MPa were performed. The results show that the increase of the maximum reduced overpressure results from the occurrence of secondary explosion near the vent in the duct.The secondary explosion in the duct becomes weak with the increase of static activation overpressure of venting device at a fixed duct diameter. Hence the influence of secondary explosion on the maximum reduced overpressure is also weak. The severity of secondary explosion is larger at duct with smaller duct diameter. However, the influence on the maximum reduced overpressure is weaker. The influence of duct length on the secondary explosion can be negelected.When conducting duct-vented design at high static activation overpressure from0.1MPa to0.6MPa, the predictive results of NFPA68have certain accuracy. The predictive results of EN14491are too conservative and have low accuracy.