Ignition of sensitive materials by low energy electrical discharges

Jeremy Smallwood PhD Thesis 1992

Abstract

Electrical discharges pose an ignition threat to sensitive materials in manufacturing industry, medical technology, minimg and the pyrotechnics and explosives industies. A Minimum Ignition Energy (MIE) is used as a guide to the sensitivity of the material. Observed values of MIE are dependent on the reactive mixture, ambient conditions, the nature of the discharge and experimental conditions. Researchers to date have used a wide variety of test methods yielding a range of MIE results. It cannot be confirmed that any of these corresponds to an absolute MIE and yet the term implies considerable uathority. Careful standardisation of the ignition test is recommended here using carefully chosen discharge characteristics, material presentation and other conditions. Simple capacitive discharge techniques are probably appropriate in routine ignition ensitivity testing once tandard test conditions have been identified. Stored energy for ignition as a function of peak discharge current, breakdown voltage and series resistance are important parameters which have easy applications in the field.

In this study high voltage pulse and capacitive discharge apparatus and techniques are described for the generation and measurement of low energy gaseous discharges. Capacitive and pulse discharges could be unidirectional arc-like, unidirectional glow-like or pulse train forms as a function of discharge current. A capacitive discharge could also take oscilliatory form. As the discharge current was reduced through the 10^-1A range transition from arc-like to glow-like characteristics was observed, corresponding with an observed descrese in stored energy and optimum time constant for capacitive discharge ignition. A decrease in observed sensitivity corresponded to low discharge currents (10^-3 A or less) where pulse train discharges could form.

Table of contents

• Abstract

• Acknowledgements

• List of symbols

• Part 1 Ignition by electrical discharges

  • 1.1 Introduction

  • 1.2 Combustion and ignition processes

  • 1.3 Electrical discharge phenomena in ignition experiments

  • 1.4 Electrical discharge ignition sensitivity

• Part 2 The experiemntal apparatus

  • 2.1 Discharge current measurement apparatus and techniques

  • 2.2 Capacitive discharge ignition apparatus

  • 2.3 Pulse discharge igniton apparatus

  • 2.4 Test cell design for sensitive materials

• Part 3 The experimental program

  • 3.1 Measurement of discharges from small capacitances

  • 3.2 Electrical sicharges from the human body

  • 3.3 Capacitive discharge measurements

  • 3.4 pulse discharge measurements

• Part 4 Ignition by electrical discharges

  • 4.1 Ignition by electrical discharges

  • 4.2 A comparison of pulse and capacitive discharge ignition systems

  • 4.3 The assessment of electrical discharge ignition hazard of sensitive dust layers

• 4.4 Conclusions

• References