Electrostatic Ignitions Toluene – An Underestimated Fire Hazard
Research into the cause of fires involving toluene has always unearthed one certain fact, which is that devastating results are inevitable if there is an electrostatic charge. But why does the chemical ignite so quickly?
In the huge blaze at a refinery in Köln-Godorf on January 9th, 2014, a tank filled with toluene burnt out after an explosion. No details were given but electrostatics was blamed as an ignition source.
In the last years we investigated several toluene fires and so the question arose, why toluene in confirmed electrostatic ignitions of flammable liquids was affected in an above-average degree.
Toluene is assigned to explosion group IIA and is about as sensitive to ignitions as, e.g., benzene, methanol, and acetone. But in which way does toluene differ from these liquids?
How Electrostatic Charges Can Occur
First of all there is the electrical conductivity which is the lower the higher the tendency for electrostatic charging is. In the range of liquids, toluene shows the lowest value with its conductivity of 0.1 – 10 Picosiemens/m. It follows, when toluene is flowing through supply lines, it will be electrostatically charged very quickly, e.g., negatively, when flowing through a metal pipe.
Charges created in this way only show ignition energies of about 0.5 Millijoule. This is sufficient for igniting many flammable liquids whose minimum ignition energy (MIE) lies in between 0.2 und 0.5 Millijoule but not for igniting combustible dusts.
The "Explosion Range" – In the Danger Zone
It is well known that gases and vapours are only ignitable within their explosion range, that means, between the lower explosion limit (flashpoint) and the upper one. Below the mixtures are »too lean«, above they are »too rich«. But within the explosion range the need of ignition energy is in no way constant, it reaches a little above the stoichiometric mixture, well known as λ=1 -Lambda sensor in car engines- its lowest value, the MIE.
Towards the borders of the explosion range, the need of ignition energy rises for more than an order of magnitude. It follows, that common electrostatic discharge appearances (brush discharges from charged isolating materials, spark discharges from charged persons) can only ignite stoichiometric fuel/gas- mixtures.
Toluene and Propyl Acetate Prone to Electrostatic Ignition
The mixing ratio created above a flammable liquid is determined largely by its flashpoint (FP). For example the mixtures of benzene are too rich (FP < -40 °C), on the other hand those of xylene FP = 28°C are too lean.
Particularly significant are the properties of toluene and propyl acetate. The flashpoint of both are at 5 °C, leading to a mixture ratio of aboutλ = 1 at room temperature. Thereby both liquids may be optimally ignited by electrostatic charges. However, propyl acetate shows a much better conductivity than toluene and, accordingly, is electrostatically charged much less.
Assuming that most refilling procedures take place at room temperature it becomes obvious that toluene is more often ignited electrostatically than other flammable liquids.
In the four following case histories, refilling procedures with toluene will be investigated at which fires occurred. All of them took place at room temperature. In view of these case histories it should be noted that the cause of an ignition can be proven only with more or less probability.
1. How to Transfer Toluene Into a Plastic Drum
Toluene from a distilling apparatus is being collected in an intermediate container and emptied into metal drums. Because of prevailing explosion danger in the area appropriate safety measures against electrostatic ignition dangers were taken:
- Filling hose and fill nozzle conductive and earthed
- Steel drums earthed
- Flooring dissipative
- Safety shoes dissipative
As all of a sudden no metal drum was available any more the worker took an empty plastic drum standing nearby, so that emptying of the intermediate container did not stagnate.
Shortly before the plastic drum was full, ignition of the toluene vapours occurred which led to a fire. As in the course of time the plastic drum burnt down, the entire toluene was set free causing a conflagration. A devastating widespread fire was the result but the worker could get himself to safety in time.
Cause of Fire
The electric charge carried with the toluene accumulated in the plastic drum as it could not be drained off to earth, a possibility that had existed with the earthed metal drum. When the thus highly charged liquid level approached the filling nozzle a brush discharge occurred and its energy was just sufficient to ignite a stoichiometric vapour/air-mixture. As the accident took place at room temperature, optimal ignition conditions were given.
Recommendation:In explosion endangered areas no plastic packaging materials should be present.
2. Flowing of Toluene Through an Isolating Hose
Contaminated toluene had to be filled into an earthed metal container at a printing ink manufacturer. In general, this took place in aspects of electrostatic safety measures as explained in case history 1.
During the action the worker noticed a leaking point at the hose coupling, liquid dripped out. For such a case a supplementary hose was planned but it could not be found. However, in a similar plant – but for water – a like hose was found, but made of isolating PVC. Quickly it was replaced against the leaking conductive hose and now the action could continue.
But when the filling nozzle approached the rim of the metal container weak sparks were to be noticed and immediately a tongue of flame came up. Luckily the container withstood the flames and the fire soon could be suffocated by extinguishing foam.
Cause of Fire
The filling nozzle was electrostatically charged by the flowing toluene. This charge was capable to drain off to earth via the conductive hose but not via the isolating PVC hose. Unfortunately the worker was wearing isolating safety gloves which prevented the charge to be drained off via the handheld filling nozzle and the person.
Providing Conductive Hoses is Highly Recommended
It is not always known whether a liquid is isolating and thus electrostatically chargeable. Therefore it is recommended always to provide conductive hoses when handling flammable liquids. When working in explosion endangered areas only conductive safety gloves should be worn. So it will be ensured that electrostatic charges from handheld parts can be drained off to earth via the person.
3. Filling Toluene Into Small Metal Drums
Toluene was delivered in a composite-IBC (1000 litres volume) and had to be filled via the drain cock into 60 l metal drums. For this task the IBC was placed onto a concrete pedestal of such height that the drum fit below the drain cock. As the drums were transported on a drum trolley the drum did not reach the drain cock at its side. Therefore a metal extension of 500 mm length was taken but it did not show an appropriate thread to the drain cock. A transition piece was made that fit and now action could start.
First the worker attached earthing clamps to the drum and the housing grating of the IBC and afterwards opened the drain cock. When the drum was almost filled, suddenly the toluene inside was on fire. The worker could not reach the drain cock to close it unchallenged but he could escape to safety. So the entire content of the IBC could outflow into the fire and caused a devastating conflagration.
Cause of Fire
As every part was properly earthed, an electrostatically caused ignition first was not in focus. When inspecting the location of fire the metal extension was found as well. It was remarkable that at the end which had been connected to the IBC a charred remnant was found which could not be assigned to the plastic drain cock. Queries brought up that it most likely was the rudiment of the transition piece between the extension and the drain cock. It was made of PVC. At material residues of this part found in the workshop an insulation resistance of R > 1PetaOhm was measured.
Thus the metal extension had been isolated against the earthed IBC by means of the transition piece made of plastic. The extension had been electrostatically charged by the toluene flowing through and so a discharging spark towards the rim of the small metal drum could occur igniting the prevailing optimally ignitable toluene/air-mixture.
Recommendation: Take Extra Care of Earthing
All conductive (metal) parts which can be charged by the process (flowing through pipe) have to be reliably earthed. In general, mixed structures (conductive – isolating) should be avoided in explosion endangered areas.
It would be unlawful to blame the worker for the fact that he did not try hard enough to close the drain cock already in flames to avoid a conflagration, before running away. He had acted absolutely correct to get out of danger because fighting a fire is the task of the fire brigade.
4. How About Earthing?
When toluene is on fire strong soot generation will occur; thus exact determination of causes becomes more or less difficult. This was the way with distilled toluene in a partly filled composite-IBC which had burnt down entirely. In the housing grating entirely blackened by soot, the vague image of the molten down plastic inner container could be seen.
As an ignition source it was considered to be likely that the charge of the flowing toluene had been accumulated in the IBC. This hypothesis proved to be true, when it became known that the pallet of the IBC was made of plastic. And then the »gleaming polished« earthing clamp fixed at the soot blackened housing grating was detected. The reason of the fire was clear to everyone, and to override this embarrassing situation no report was compiled.
In theory and on the basis of case histories it could be depicted why toluene in confirmed electrostatic ignitions of flammable liquids was affected in an above-average degree.