Journal of Loss Prevention in The Process Industries, Vol.61, 147-159, 2019
Measurement of minimum ignition energies (MIEs) of dust clouds - History, present, future
Nearly 130 years ago Holtzwart and von Meyer (1891) demonstrated by experiments that explosible dust clouds could be ignited by inductive electric sparks. Then more than half a century passed before the publication of the important quantitative research of Boyle and Llewellyn (1950) and Line et al. (1959). They worked with capacitive electric sparks and found that the minimum capacitor energies 1/2CU(2) required for ignition of various dust clouds in air decreased substantially when a large series resistance, in the range 10(4)-10(7) Omega, was introduced in the discharge circuit. When considering that the net energies of the sparks themselves were only of the order of 10% of the 1/2CU(2) discharged, the minimum net spark energies required for ignition with a large series resistance were only a few per cent of the net energies required without such a resistance. Line et al. observed that the essential effect of increasing the series resistance, and hence increasing the discharge time of the sparks, was to reduce the disturbance of the dust cloud by the blast wave from the spark. This phenomenon was explored further by Eckhoff (1970, 2017), and subsequently by some simple experiments by Eckhoff and Enstad (1976). Franke (1974, 1977) and Laar (1980) confirmed the additional finding of Line et al. (1959) that the minimum 1/2CU(2) for ignition is also substantially reduced by including a series inductance in the discharge circuit, rather than a series resistance. The basic reason is the same as with a large series resistance, viz. increased spark discharge time and hence decreased disturbance of the dust cloud by blast wave from the spark. For this reason inclusion of an appreciable series inductance in the spark discharge circuit is an essential element in current standard MIE test methods. In experiments with spark ignition of transient dust clouds produced by a blast of air in a closed vessel, it is necessary to synchronize the occurrence of the spark with the formation of the dust cloud. The precision required from this type of synchronization is typically of the order of 10 ms, which can be obtained even by mechanical arrangements, such as rapid change of spark gap length, or of the distance between two capacitor plates. The present paper reviews some methods that have been/are being used for achieving adequate synchronization of dust cloud appearance and spark discharge. Some current standard experimental methods for determining MIEs of dust clouds experimentally have also been reviewed. The same applies to some theories of electric-spark ignition of dust clouds. At the end of paper some suggestions for possible future modifications of current standard methods for measuring MIEs of explosible dust clouds are presented. With regard to justifying significant modifications of existing standard methods, the "bottom line" is, as quite often in many connections, that any modifications should be based on realistic cost/benefit evaluations.