Cement Manufacturers Association (CMA)

K St The VM percentage, the particle size and particle size distribution, the dust-in-air concentration, the particles’ humidity, the ignition energy, the turbulence and the geometry of the enclosure in which the explosive combustion takes place all play a role in the development of explosion pressure, but the K St and P max values, if evaluated using a representative sample, will cover all this. In order to calculate the necessary explosion venting capability (dimensioning the European (EN) standard 14491 and the USAmerican standard NFPA 68. The equation needs the following input and is to be applied assuming the normal 21 % O content of the explosive atmosphere: - volume and geometry information (re the vessel to protect) - K St and P max - EPSR expressed as pred, in which pred is withstand the maximum mitigated explosion pressure that will be the result of the explosion venting In many cases, the EPSR of the DC is not known. It is not easy to calculate the strength that the typical DC geometries with rectangular sections in them have. Only in very rare cases the manufacturers of bag houses had their bag house designs’ strength tested, normally by using modular designs and testing module combinations. This is what often goes wrong: - A standard DC design (as used for dust-air separation with non-combustible dusts) is used and just a couple of explosion vents are installed on it. - In practise, in many cases, all kinds of designs are realised without their real EPSR ( P red ) being known. - in the way of the explosion venting are not accounted for. EN 14491 and NFPA a design fault. - The more complicated geometry of DCs with multiple funnel hoppers is not accounted for, simply due to the fact that the responsible designers are completely ignorant of the fact that they have to look into this not-so-easy-to-deal-with condition. EN 14491 and NFPA 68 don’t mention - The explosion vents don’t qualify as such (have not been type-tested) and are non-self-reclosing. Many phantasy-designs exist, some of them are very dangerous, will not work - The explosion vents have been positioned incorrectly and will vent the venting blast into a dangerous direction. - The recoil force has not been taken into account in the design of the DC’s support. - The penthouse covers are not EPSR or after maintenance work on the DC have not been put back into place correctly. - The extraction screw conveyor doesn’t have the necessary EPSR, e.g. in its coaction could open like a zipper, when exposed to explosion pressure. - The screw conveyor connection of the DC with (a) downstream pump pre-hopper(s) or, if connected directly, for gravity in-feed, with (a) PF silo(s), doesn’t have the necessary explosion isolation system(s) installed in it and lack(s) EPSR, or both. It is important that the penthouse is kept clean. Damaged bags will cause deposits of coal dust to form in the penthouse. Depending on the thickness of the coal dust layers that have formed and the characteristics of the coal, auto-ignition of an explosion in the bag section of the DC, coal dust deposits in the penthouse may become part of a scenario for which the protection of the DC was not designed. Page 41