The nomograms provided in this page may be used to assess the ignition risk of flammable vapor/air mixtures or suspensions of flammable dust by a capacitive electrostatic discharge. They are based on the well-known relationship between the voltage V (in volts), capacitance C (in Farads), charge Q (in Coulombs), and electrostatic energy Ue (in Joules). In SI units, these relationships are:
The quantity Ue should be thought of as the maximum possible energy released (and converted to heat) if the capacitor is fully discharged in a spark. If the minimum ignition energy or MIE (in Joules) of a flammable atmosphere of vapor, gas, or dust is known, then one important requirement for an ignition is:
Despite a number of factors complicating the physical circumstances of ignition, this inequality is accepted nevertheless as the criterion for assessment of ignition risks associated with capacitive discharges. Measurements of MIE for suspended dusts are not straightforward, and there is evidence that capacitive discharges slowed by series resistance actually yield lower MIE values than earlier published values obtained by standard ignitability tests.
A very important restriction on the use of these relations is that the capacitance C must be reasonably well-identified by the geometry. For example, the capacitance between two conductors is well-defined and measurable, and distinct, capacitive spark discharges will occur between them for the right set of conditions on voltage and charge. Use of the nomograms in this bulletin is restricted to the assessment of ignition risks for such discharges between conducting bodies.1 When a charged, insulating surface is involved, other types of electrostatic discharges occur requiring other, more qualitative methods of evaluation.
Nomogram #1, which is based on that of Bodurtha2 and also Jones and King3, allows quick graphical determination of any one of the parameters C, V, or Ue, as long as the other two are already known. The two known points are marked on the appropriate log scales and a line is drawn through them. The unknown quantity is given by the intersection of this line with the third log scale. For convention's sake, the nomogram uses the following units:
Commonly accepted values for the capacitance of implements and plant components are marked on the capacitance log scale. On the energy log scale, some commonly accepted values for the MIE of flammable mixtures and airborne dusts are superimposed. Likewise, the voltage scale has some important values indicated, the most important of which are the minimum sparking potential of air at STP (~350 V) and the recommended safe upper limit of 100 V for the potential drop between adjacent ungrounded conductors.
In hazard assessment, there are several ways to use the nomogram. For example, consider the human body which has a typical capacitance of 200 pF. A person wearing shoes with insulating soles can readily become charged to ~104 volts walking across a carpet or insulating mat on a dry winter day. Using the nomogram, the electrostatic energy storage Ue is found to be 10 millijoules. This value is sufficient to ignite hydrocarbon (HC) vapors mixed with air; therefore, operators in a plant where flammable vapors or liquids may be present must wear safety shoes with conductive soles or ankle straps. The value of 10 millijoules is considered marginal for ignition of the airborne dusts of polymer compounds, thus conductive soled shoes might be recommended but not required where the only flammable materials is dust.
An alternative way to employ the nomogram is to use the capacitance C and minimum ignition energy (MIE) values to determine the maximum safe voltage. As an example, consider a rubber-wheeled tanker truck with capacitance of 1000 pF and figure out the voltage that the truck would have to acquire in order to create an ignition risk for a flammable mist. Connecting the points C = 1000 pF and Ue = 1 millijoules, the extended line intersects the voltage scale at V ~ 1300 V. The risk assessment issue then reduces to an estimate of the maximum voltage that could be developed between the truck and ground potential. In fact, V = 1300 V is rather easy to achieve due to rolling friction if conditions are dry. We can thus conclude that provision for grounding for a tanker truck during loading or unloading operations is vital.
Note that if the line defined by the given values of C and Ue intersects the voltage scale below V = 350 V, no spark should occur. The recommended value of 100 volts provides an appropriate margin of safety for ESD risk assessment, given the inevitable uncertainty about actual capacitance values.
Another way to assess the ignition risk of a capacitive electrostatic discharge is to determine the maximum amount of charge transferred by the spark. This maximum transferred charge can not exceed the charge Q (in Coulombs) stored by the capacitance.
It is found that spark discharges involving the transfer of less than 0.1 microCoulomb (10-7 Coulombs) will not ignite flammable gases or vapors. Therefore, Nomogram #1 may be modified by the addition of a log scale for the stored electrostatic charge Q. See below.
This additional scale provided on Nomogram #2 is marked off in units of microCoulombs (10-6 Coulombs). If the line defined by any two of the three parameters (C, V, Ue) intersects this scale below Q = 0.1 microCoulomb, then the risk of ESD ignition is minimal. For example, a metal scoop with capacitance C = 20 pF charged to V = 2000 V is unlikely to ignite flammable mixtures, because the line defined by these two values intersects the Q scale below 0.1 microCoulomb.
A usable definition of MIE depends on clear identification of both the charged object's capacitance and the path of the electrical discharge. If there happens to be significant effective resistance in the circuit, then the apparent minimum ignition energy is altered. One example where discharge path resistance becomes important is the human body, which does become charged readily, but discharges more slowly across a gap than a metallic conductor due to the finite conductivity of the body. One study, performed with acetone vapor, reported that the effective MIE is increased by a factor of approximately 4 for the case of an electrically charged person4.
Please click here to test an on-line nomogram. A blank copy of the nomogram in pdf format may be downloaded. Also available for testing is an interactive, tool for creating custom nomograms on-line.
1 The capacitance between a metallic conductor and a charged, insulating surface is ill-defined and difficult to estimate. Furthermore, discharges between such surfaces will be of either the brush or propagating brush type. Such discharges represent recognized ESD ignition hazards, but quantitative assessment of the inherent risks can not be based on a capacitive discharge model.
2 F.T. Bodurtha, Industrial explosion prevention and protection, (McGraw-Hill, New York) 1980.
3 T.B. Jones and J.L. King, Powder handling and electrostatics, (Lewis Publishers (CRC Press), Boca Raton, FL) 1987.
4 R.W. Johnson, "Ignition of flammable vapors by human discharges," Loss Prevention (AIChE), vol. 14, 1981, pp. 29-34.