When an electrostatic spark occurs in a flammable atmosphere, the critical question is whether or not the spark has energy sufficient to cause an ignition. The details of electrostatic ignition are quite complicated and, despite many years of study, questions still remain about the controlling parameters. Despite the absence of a completely accepted theory, engineers and safety personnel must do their job effectively. The generally accepted approach to electrostatic hazard assessment for the case of capacitive discharges is to compare the available capacitive energy
to the minimum ignition energy (MIE) of the flammable mixture known or expected to be present. If
then it is assumed that ignition is likely and that appropriate abatement measures must be taken. For most realistic production operations, estimation of Ue is not a simple matter, but once an estimate has been obtained, then the safety hazard assessment can be performed.
shown below provides an easy graphical means to assess capacitive electrostatic hazards. Various
forms of this nomogram have appeared in the literature over the years [Bodurtha, 1980; Jones and
King, 1991]. The version below adds the charge Q to the voltage and capacitance scales.
Note the placement of markers for the measured MIE values of some representative ignitable gases,
vapors, and suspended powders along the Ue scale.
The red line intersecting all four scales on the nomograph above represents an estimate for the level of charging of the electrophorus on dry days when big sparks can be observed:
For these parameters, we may calculate Q = 1 mCoulomb and Ue = 30 milliJoules. There are several interesting observations to be made about these values. First, the value of 1 mCoulomb defines the generally accepted threshold for human perception of electrostatic discharges [Carstensen, 1987]. This result is certainly consistent with our observation that the electrophorus delivers a noticeable shock on a dry day. Second, the energy of 30 milliJoules exceeds the MIE of typical hydrocarbon vapors by an order of magnitude. Therefore, we should expect to be able to ignite common HC solvents such as acetone or isopropyl alcohol quite readily in the chamber. From experience, we find that ignition is reliably achieved if the humidity is not too high. Detailed information about capacitive discharge nomograms and a unique ON-LINE INTERACTIVE NOMOGRAM are available. Also, please CLICK HERE to test an interactive JAVA-based tool for creating custom nomograms on-line.
The critical issue of the flammability of vapor/air mixtures is distinct from the electrostatic ignition condition. When difficulty is experienced in achieving an ignition, the reason is very often that the vapor and air are not adequately mixed, or that the mixture is either too rich or too lean.
F.T. Bodurtha, Industrial explosion prevention and protection, (McGraw-Hill, New York) 1980.
E.L. Carstensen, Biological effects of transmission lines, (Elsevier Science Publishers, Amsterdam) 1987.
T.B. Jones and J.L. King, Powder handling and electrostatics, (Lewis Publishers - CRC Press, Boca Raton, FL) 1987.