“Take off your sweater in the darkness and static flares as a tiny lightning storm.” — John Geddes
The dry air of winter makes it much more likely that we will be shocked by static sparks. So instead of visions of sugar-plums dancing in their heads, the thoughts of many of our clients turns to the inadvertent ignition of flammable vapors and combustible dusts.
Minimum Ignition Energy for Flammable Vapors
For a static spark to ignite a flammable vapor cloud or a combustible dust cloud, it must exceed the minimum ignition energy (MIE) of the vapor or dust.
Vapors, being dispersed at a molecular level, tend to be reported consistently, although it depends some on the test method. The vapors with the lowest MIE are carbon disulfide (0.009 mJ), hydrogen (0.016 mJ), trichlorosilane (0.017 mJ), and acetylene (0.019 mJ).
Most flammable gases and liquids, however, have MIEs over an order of magnitude higher. Here are some reported values for commonly encountered flammable materials:
- Methane 28 mJ
- Ethane 25 mJ
- Propane 25 mJ
- Butane 25 mJ
- Pentane 28 mJ
- Acetone 55 mJ
- Methyethylketone 27 mJ
- Methanol 20 mJ
- Ethanol 28 mJ
- Benzene 20 mJ
- Toluene 24 mJ
Not surprisingly, chlorinated solvents, which are widely regarded as non-flammable, have very high MIEs: dichloromethane is 9300 mJ and trichloroethane is 4800 mJ.
Minimum Ignition Energy for Combustible Dusts
As with flammable vapors, a static spark can only ignite a combustible dust cloud by exceeding the minimum ignition energy (MIE) of the dust. The MIE of a dust, however, depends on more than the chemical composition of the dust. It also depends on the size, shape, and moisture content of the dust particles. So, instead of exact values, the MIE of dusts is typically reported as a range of values. Here are some examples:
- Flour, wheat 30 – 300 mJ
- Sugar, powdered 10 – 100 mJ
- Wood, dust 10 – 100 mJ
- Aluminum 10 – 100 mJ
- Sulfur 0.3 – 3 mJ
- Polyethylene 30 – 100 mJ
- Polypropylene 10 – 300 mJ
- Polystyrene 10 – 300 mJ
- Polyvinylchloride 10 – 100 mJ
What About that Static Shock?
The energy of a spark that is just strong enough to be visible in the dark is only 0.04 mJ. It’s strong enough to ignite carbon disulfide, hydrogen, trichlorosilane, or acetylene gasses, but not much else.
The energy from a static spark in wintertime is often reported as around 0.5 to 1.5 mJ, although it has been reported to be as much as 60 mJ. That’s a huge difference—the difference between being just enough to ignite flammable vapors (and sulfur) and being able to ignite many combustible dusts. So, what is it?
The energy from a static spark can be calculated using the equation, E = 0.5 C V2, where C is capacitance in farads, V is voltage in volts, and E is energy in joules.
The company that published the 60 mJ value sells static electricity control systems, so it’s possible they have an interest in exaggerating the hazard. They reported that the capacitance of the body is 100 – 300 picofarads, and that the voltage carried by the body is as much as 20,000 volts. Using 300 picofarads and 20,000 volts, they calculated the energy of a static spark:
0.5 x 300 E-12 Farads x (20,000 V)2 = 0.06 J = 60 mJ
However, they reported that “the maximum potential commonly achieved on the human body ranges between 1 and 10kV.” (So 20,000 V is a bit of an exaggeration.) While they used 300 pF as the capacitance of the human body, it is generally accepted that the capacitance of the human body is around 100 pF. (300 pF is also a bit of an exaggeration.) So, if the capacitance is 100 pF and the voltage is 1 kV, the energy of a static spark from a person is
0.5 x 100 E-12 Farads x (1,000 V)2 = 0.00005 J = 0.05 mJ.
If the voltage is bumped up to the top of the range, 10 kV, the energy of a static spark from a person is
0.5 x 100 E-12 Farads x (10,000 V)2 = 0.005 J = 5 mJ.
This range, 0.05 mJ to 5 mJ, is consistent with the values generally reported, 0.5 to 1.5 mJ, as the energy of a spark of static electricity from a person.
The Need to Control Static Sparks
Even though we are more likely to generate static sparks in the dry air of winter than in the humid air of summer, static electricity can be a concern all year round. We can take some comfort that the static spark that we feel is not likely to be energetic enough to ignite a combustible dust cloud, which allows us to focus our attention on other ignition sources for combustible dusts.
On the other hand, static sparks from people are a concern when we are working with flammable solvents. A static spark strong enough to be felt is strong enough to ignite just about any flammable vapor cloud.
Controlling Static Shocks
One of the easiest things to do to control static is to wear all cotton clothing, which does not accumulate either a positive or a negative charge. Silk, wool, hair, glass, fur, and skin all tend to accumulate a positive charge, while synthetic polymers tend to accumulate a negative charge.
Another way to control static is to stay grounded. Charges, positive or negative, cannot accumulate if we are grounded. In an industrial environment, that may include wearing conductive or static dissipative footwear or wearing personnel-grounding devices. If those are not practical, consider carrying a small metal object such as a coin and while holding it in your bare hands, touching it to a surface that normally be a source of a shock. The static charge will drain away without the painful or destructive spark.
Happy holidays. May the sparks this season be the sparkle in the eyes of your loved ones, and not the shock at their fingertips.