Niacin, lycopodium and polyethylene powder explosibility in 20-L and 1-m3 test chambers

Abstract

An experimental program has been undertaken to investigate the explosibility of selected organic dusts. The work is part of a larger research project aimed at examination of a category of combustible dusts known as marginally explosible. These are materials that appear to explode in laboratory-scale test chambers, but which may not produce appreciable overpressures and rates of pressure rise in intermediate-scale chambers. Recent work by other researchers has also demonstrated that for some materials, the reverse occurs – i.e., values of explosion parameters are higher in a 1-m3 chamber than one with a volume of 20 L. Uncertainties can therefore arise in the design of dust explosion risk reduction measures.The following materials were tested in the current work: niacin, lycopodium and polyethylene, all of which are well-known to be combustible and which cover a relatively wide range of explosion consequence severity. The concept of marginal explosibility was incorporated by testing both fine and coarse fractions of polyethylene. Experiments were conducted at Dalhousie University using the following equipment: (i) Siwek 20-L explosion chamber for determination of maximum explosion pressure (Pmax), volume-normalized maximum rate of pressure rise (KSt), and minimum explosible concentration (MEC), (ii) MIKE 3 apparatus for determination of minimum ignition energy (MIE), and (iii) BAM oven for determination of minimum ignition temperature (MIT). Testing was also conducted at Fauske & Associates, LLC using a 1-m3 explosion chamber for determination of Pmax, KSt and MEC. All equipment were calibrated against reference dusts, and relevant ASTM methodologies were followed in all tests.The explosion data followed known trends in accordance with relevant physical and chemical phenomena. For example, Pmax and KSt values for the fine sample of polyethylene were higher than those for the coarse sample because of the decrease in particle size. MEC values for all samples were comparable in both the 20-L and 1-m3 chambers. Pmax and KSt values compared favorably in the different size vessels except for the coarse polyethylene sample. In this case, KSt determined in a volume of 1 m3 was significantly higher than the value from 20-L testing. The fact that the 20-L KSt was low (23 bar m/s) does not indicate marginal explosibility of the coarse polyethylene. This sample is clearly explosible as evidenced by the measured values of MEC, MIE, MIT, and 1-m3 KSt (at both 550 and 600 ms ignition delay times).