“Electricity can be dangerous. My nephew tried to stick a penny into a plug. Whoever said a penny doesn’t go far didn’t see him shoot across that floor. I told him he was grounded.” — Tim Allen
When I was fresh out of grad school, I had a job in process development. One project involved recrystallizing a reactant from hot tetrahydrofuran, which has a flashpoint of -14 C (7°F). It was a cold, dry January day and my hair would not stay put. Apparently, a spark of static electricity jumped from my hair to the THF and it burst into flames in my face. Fortunately, I was wearing goggles, a face shield, and a lab coat, and suffered no injury. I quickly closed the lid and the fire went out. We didn’t even lose the batch.
But it sure got my attention. I described the event to my wife when I got home that evening, and she suggested that I should start using conditioner after I shampooed my hair. Honestly, I wasn’t sure that would help, but I figured it wouldn’t hurt. What I really wanted to know, and what I assume everyone wants to know is, what are the protections against static and when do we need to use them.
How Much Energy Does It Take for Ignition?
The minimum ignition energy (MIE) is measured in millijoules (mJ). MIE depends greatly on the type of material. The really flammable gases have the lowest MIEs:
- Carbon disulfide 009 mJ
- Hydrogen 016 mJ
- Acetylene 017 mJ
The unsaturated hydrocarbons have MIEs about ten times higher:
- Methane 21 mJ
- Propane 24 mJ
- n-Pentane 24 mJ
Ordinary solvent vapors have about the same MIE has saturated hydrocarbon gases:
- Acetone 19 mJ
- Ethanol 23 mJ
- Toluene 24 mJ
Even my nemesis, THF, has an MIE of 0.19 mJ, the same as acetone.
Combustible dusts, the MIE of which depends not only on composition, but also particle size and morphology, tend have MIEs in the neighborhood of 1 to 50 mJ. Or higher.
How Much Energy is In Static Electricity?
The amount of energy in static electricity depends on the size of the spark. An average bolt of lightning is about one trillion mJ. A lightning bolt will ignite anything that is flammable or combustible.
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, or acetylene gasses, but not much else.
The spark of static that you feel when you touch a doorknob in the wintertime is about 1.2 mJ. This is strong enough to ignite just about any flammable gas or vapor, but not most combustible dusts.
The energy from a really nasty spark is about 4 mJ, and Tasers, er, electroshock weapons, is over 10 mJ. These can be enough to ignite some combustible dusts when they are well dispersed in a cloud above the minimum ignitable concentration. The minimum ignitable concentration is generally taken to be thick enough that you cannot discern a 25 watt incandescent bulb 2 meters away.
What Are the Protections Against Static Electricity?
An excellent reference regarding the hazards of static electricity is NFPA 77, Recommended Practice on Static Electricity. The most current edition, 2019, is 80 pages of great advice. None of it includes using conditioner after shampooing. While there is no way a few words here can capture all the information there, there are some key points to be aware of.
- Assure that all equipment is bonded and grounded, and periodically verify that it remains bonded and grounded.
- Avoid non-conductive materials of construction.
- Keep flow velocities of low-conductivity liquids below 1 m/s initially or in static-prone situations and below 7 m/s in any case.
- Avoid allowing potentially charged materials from free falling into a vessel, which would allow a spark to jump from the material to the vessel wall.
- Protect personnel from becoming a source of static charge by installing conductive or static dissipative flooring and wearing conductive or static dissipative footwear, wearing personnel-grounding devices, and wearing antistatic or conductive clothing and cloth.
Static Electricity Just Is
Dissimilar materials moving past each other, as with any powder, liquid, or gas flowing in a pipe, or with two-phase flow, will build a charge. A difference in charges will seek to equilibrate, and if there is no way to do it without a spark, then the difference in charge will equilibrate via a spark. That spark can be enough to ignite flammable or combustible materials.
So, it is important to be aware that flowing material will always be a potential source for static electricity with sufficient energy to touch off a fire or explosion. Whether or not you want static, static is something you must consider and address.
The best time to address the hazard of static electricity as an ignition source is during the design of a process. If it wasn’t addressed then, the process may require retrofit. Whether included in the original design or in a retrofit, though, the countermeasures for static electricity require constant vigilance. Controls for static electricity are not something that can be installed and forgotten. Make sure that your measures for controlling static electricity are well-grounded.