Dust explosions suppression systems

Table 7-31 lists the explosibility index that is a relative measure of the potential damage from a dust explosion. A rating of 2 to 4 requires large vent areas. Above 4, for most cases, the explosion cannot be controlled by venting design and therefore requires the use of protection such as inert gas or explosive suppression systems, some of which are commercially available. 

Steam snuffing in solids dryers can also provide effective fire protection. In some cases, the major concern for a fired product dryer may be explosion involving product dust. Where there is a possibility of an internal dust explosion in a product dryer, a fast response explosion suppression system should be considered in addition to fire protection. 

The explosion suppression system used in the South Africa HS mine is an active explosion suppression system developed exclusively by Hans in South Africa. It could change traditional passive explosion suppression system to active explosion suppression system and timely block reaction chain of gas explosion effectively. This technology of explosion suppression has been at the international advanced level (Ma Li et al. 2010). In the previous work (Wang Jun-feng et al. 2010), the four basic parameters of this system, i.e., lag time of spraying, time for fogging, duration of effective fog and rate for spraying were systematically measured and calculated. These results shows that the system can effectively restrain the combustion and explosion of gas and dust in coal mine. In this paper, based on the theory of the explosion suppression... 

The sodium bicarbonate suppressant attains almost equivalence in performance with the monoammonium phosphate powder. Water has proven to be a very effec tive suppressant of dust, especially grain and Fodder dusts. A suppressant is regarded as being veiy effec tive when an increase of the artivatiou pressure of the explosion system leads... 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

Optical detectors shall be used in more open configurations where ressure buildup due to the incipient explosion is limited. Optical etectors shall not be used where high dust concentrations limit the reliability of the suppression system. Both uv and ir detectors are available for optical detection. The use of daylight-sensitive sensors shall be avoided to avoid spurious activation. The sensor shall be mounted such that the angle of vision allows it to cover all the protected hazard area. The performance of an optical detector will also be affected by any obstacles within its vision, and this shall be overcome by the introduction of more detectors. Optical detectors shall be fitted with air shields to keep the optical lens clean. 

The concentration of particles inside the pipeline is extremely high and beyond the upper explosible limit (UEL). This forms a natural buffer zone or barrier for any explosion that may occur in a vessel at either end of the pipeline (i.e. flame propagation is unlikely). This could avoid the need for explosion suppression and/or isolation, which would be required normally for a dilute-phase or dust extraction system. 

Open systems use atmospheric air for drying. In cases where products pose a potential for dust explosion, plants are provided with pressure relief or suppression systems. For recycle systems, the drying system medium is recycled, and the evaporated solvent recovered as condensate. There are two alternative designs. In the self-inertizing mode, oxygen content is held below 5 percent by combustion control at the heater. This is recommended for products with serious dust... 

Automatic suppression systems are of particular use when the dust is toxic and safe discharge of the explosion products from a vent is not permitted. It is also important to bear in mind that the suppressant is not effective against exothermic decomposition reactions resulting in large volumes of gas. 

Typically applied where dust explosion potentials exist in enclosures from potential overpressures generated from dust or vapor. Spark detectors (i.e., infrared detectors) or pressure detectors are typically used that instantaneously initiate a suppression system to inhibit or extinguish the spark and prevent an explosion. See also Explosion Isolation. 

The pressure rise accompanying a dust explosion is rapid but it can be detected in time to initiate some action to suppress the explosion. Suppression involves discharging a quantity of inert gas or powder into the vessel in which the explosion has commenced. Modem suppression systems triggered by the pressure rise accompanying the start of the explosion have response times of the order of a few milliseconds and are able to effectively extinguish the explosion. The fast acting... 

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