About the mass of combustible dust deposited in the room
Authors: Земский Геннадий Тимофеевич , Зуйков Владимир Александрович , Кондратюк Наталья Валентиновна , Зуйков Александр Владимирович 
Published: June 2021

Темой данной статьи является рассмотрение расчетного метода оценки пыленакопления в помещении с оборудованием, содержащим дисперсные материалы, а задача заключается в его совершенствовании на основании современных представлений. Витающие в воздухе мелкие пылинки постепенно осаждаются на горизонтальных поверхностях, создавая угрозу внезапного перехода в состояние аэровзвеси и взрывообразного сгорания с образованием волны давления. Для определения категории помещения по взрывопожарной и пожарной опасности и разработки профилактических противопожарных мероприятий необходимо иметь математический инструмент, позволяющий оценивать количество пыли, которое может участвовать во взрыве. В статье предлагается определять критический диаметр частиц пыли, исходя из равенства скорости осаждения частиц и скорости распространения пламени. При этом скорость осаждения частиц следует определять по известному соотношению Стокса, а скорость распространения пламени принимать или по данным эксперимента, или среднестатистическую, равную 3 м/с.

The dust that settles on the horizontal surfaces of equipment and building structures is inevitably contained in the premises in which technological process is carried out with the presence of combustible dispersed materials. The amount of dust deposited in a room can be determined in several ways. 1. Weighing the dust collected during cleaning. 2. Experimentally by placing dust collectors on different surfaces for a certain period of time. It should be considered the incompleteness of the previous dust collection, which depends on the dust collection method when determining the amount of dust deposited in the room. The amount of the remaining uncollected dust is estimated as 10 % of the detected dust if the cleaning was carried out by a mechanized method (dust extraction coefficient K = 0.9, and insufficient decontamination coefficient n = 0.1). The amount of remaining uncollected dust is estimated as 40% of the detected dust if cleaning was carried out manually using a dry method (dust extraction coefficient K = 0.6 and insufficient decontamination coefficient n = 0.4). The amount of remaining uncollected dust is estimated as 30% of the detected dust if the cleaning is carried out manually with a wet method (K coefficient = 0.7, and insufficient decontamination coefficient n = 0.3). Dust factor is the ratio of amount of dust suspended in the airspace to the total amount of dust in the room. It should be taken equal to 0.5, if the diameter of dust particles is more critical, and equal to 1, if the diameter of dust particles is less than the critical diameter. The diameter of the dust particles is critical, above which the dust ceases to spread the flame. The critical diameter can be determined based on the following considerations. Flame propagation through the air suspension will not occur if the particle deposition rate exceeds the flame propagation rate from bottom to top. The deposition rate is easily determined by the Stokes coefficient. The flame propagation rate can be determined experimentally or estimated as slightly higher than the average for most industrial dusts. The authors obtain a formula for calculating the critical diameter of dust particles equating the terminal velocity to flame propagation rate (3 m/s) and solving the Stokes equations of the particle diameter. The critical particle diameter of air suspension of combustible materials in most cases does not exceed 300 microns at flame propagation speed of 3 m/s. The obtained calculated values of the critical diameters are in satisfactory agreement with the literature data, and also confirm the correctness of the choice of dust formation coefficient K = 1 for dust with dispersion of less than 350 mkm. The air suspension ready for a secondary explosion is formed when combustible particulate material leaves the equipment as a result of an internal explosion. The aerosol with a high concentration of solid phase is formed in case of free precipitation from the device or from damaged packaging located above the floor level of the room. In this case, the amount of fuel capable for explosive combustion is determined by the amount of oxygen in the cloud, since the diffusion supply of oxygen from the outside does not have time to ensure complete combustion of the fuel. When the product spills unhindered, the resulting cloud has the shape of a volumetric cone with a base diameter equal to the height of the cone. It is possible to determine the amount of burned dust knowing the stoichiometric concentration of dust (%) required for its combustion per unit of air volume, as well as the volume of the dust cloud (V). The Pc value can be determined by the combustion reaction if the chemical composition of the dust is known. There are proposed refined equations for determining the critical diameter of combustible dust, the stoichiometric concentration during its combustion in a unit of air volume as well as the mass of dust deposited in the room. It is recommended to estimate the value of the stoichiometric concentration according to the combustion heat of dust for dusts of indeterminate chemical composition.