BS EN 14491:2012 pdf download – Dust explosion venting protective systems
NOTE 1 There is no direct guidance for estimating an endangered area for toxic or other harmful emissions, but the safe discharge area for external flames calculated according to the formulae in 6.2 gives some indication of the area required in direct line from the vent. Harmful emissions will be dispersed by air movements, however, and an extensive area in lateral directions can be required. This European Standard shall be used together with EN 14797 and EN 14460. Venting neither prevents or extinguishes an explosion; it only limits the explosion overpressure. Flame and pressure effects outside the enclosure and flying debris are to be expected and suitable precautions shall be taken.
Fires inside the enclosure can also occur. NOTE 2 If burning continues inside the vented vessel after the explosion, it can cause damage to the vessel, even though it has been protected from damage caused by overpressure. The increase of the length-to-diameter ratio of an enclosure results in an increase of the rate of flame propagation. This is taken into account in the formula for vent sizing (see Clause 5). Enclosures in this European Standard are limited to L/D ≤ 20. In a system consisting of connected enclosures, a dust explosion ignited in one enclosure can propagate through the connection, generating increased turbulence, perhaps causing some pre-compression and then act as a large ignition source in a connected enclosure. This combination of effects can enhance the violence of the secondary explosion. The venting requirements of the system thus need to be increased, or the enclosures isolated (see 5.4). Internal dust explosions can endanger buildings or parts of buildings and explosion venting can be applied to protect the integrity of the building.
A separate method for calculating the venting requirements for buildings is given in Annex D. The effects of internal or external obstructions on venting effectiveness shall be taken into account. Recoil forces shall be taken into account when considering the location and distribution of the vent area. Explosion venting devices shall be positioned so that the effectiveness of the venting process is not impeded. Positioning shall be such that personnel and the nearby plant will not be at risk from the venting action.
If the enclosure is small and relatively symmetrical, one large vent can be as effective as several small vents of equal combined area. For large enclosures, the location of multiple vents to achieve uniform coverage of the enclosure surface to the greatest extent practicable is recommended. NOTE 3 In the formulae presented in this standard, it is important to use the correct units, which are not always SI- units. The units are indicated for every parameter used in the limits of application. Where log is used in the formulae, log 10 is meant.
5 Sizing of vent areas
5.1 General Accurate sizing of vents is the most important aspect of vent design. The size of the vent depends on the explosion characteristics of the dust, the state of the dust cloud (concentration, turbulence and distribution), the geometry of the enclosure and the design of the venting device. Two explosion characteristics of the dust are the maximum overpressure p max and the dust explosion constant K St . For cubical enclosures, p max and K St are essentially independent of enclosure volume. The volume of the enclosure and the length-to-diameter ratio L/D relevant to the shape of the enclosure and the position of the explosion vent are required for sizing vents. The explosion resistance of the enclosure p red,max is also required for vent sizing. All parts of the enclosure, e.g. valves, sight-glasses, man-holes and ducts, that are exposed to the explosion pressure shall be taken into account and the explosion resistance of the weakest part shall be taken as the explosion resistance for the enclosure.