The vacuum truck was deployed to a below-grade sump that contained mostly of “off-spec” toluene. As the vacuum truck operation was nearing completion of the removal of the toluene from the sump an ignition of the vapors occurred resulting in a fire. In the ensuing investigation of the incident, it was determined that the vacuum truck had not been grounded by the operator. Although other ignition sources would have been considered, the fact that the truck was not grounded and the material being transferred was toluene, it was highly plausible that a static spark was the cause of the fire.
In the case of this incident, a neoprene hose with an embedded metal wire helix was used. As the metal helix would have been connected to the truck via the metal couplings of the hose, the full length of the hose would have been at the same voltage as the truck. To put some “hard numbers” on the case for static being the source of ignition we need to look at some of the physical characteristics of this operation.
A streaming current for a resistive liquid flowing through a pipe (including hoses) can be estimated from the equation:
Streaming current, IS = (2.5×10-5)(v2)(d2)
v = velocity of liquid (metres per second)
d = internal diameter of hose (metres)
It is known that the suction flow rate was 500 gallons per minute which is equivalent to 3.9 meters per second. The internal diameter of the hose was 4 inches which are equivalent to 0.102 meters. Hence the streaming current would have been somewhere in the region of 3 micro-amps (3×10-6 A). It was estimated that the resistance between the truck and ground was at least 1×1010 ohms. Most of the resistance would have been provided by the truck’s wheels and the asphalt on which the truck was positioned.
The voltage of the truck could be assumed to have reached a level of at least:
V = R x I
Where V = the voltage of the truck-hose system
I = the streaming current provided by the charged toluene
R = the resistance to ground of the truck-hose system
V = (1×1010).(3×10-6)
Minimum truck voltage = 30,000 volts
As the ignition of the vapors occurred in the sump, the discharge must have been from the hose to the metal rim of the sump. However, in order to prove this theory, we must investigate the hose itself. As stated earlier the hose was made from neoprene with an embedded metal wire helix. This means that the metal spiral was not in direct physical contact with any external objects. In addition, neoprene is a high resistivity material with a dielectric strength that is several times greater than air with a typical value of 10,000 volts required per mm of spark gap.
The neoprene layer was 2 mm (0.08 in.) in thickness. This would mean that for every millimeter of the distance between the metal helix and the metal sump there must have been a high enough voltage to discharge a spark with enough energy to exceed the MIE of the toluene. In order to breakdown the neoprene, at least 20,000 volts must have been present to achieve this. Given that the minimum voltage on the truck-hose system is estimated to be at least 30,000 volts it means an extra 10,000 volts would have been present to discharge from the neoprene surface to the sump. So, at some point in the operation, a gap between the surface of the hose and the sump must have been created resulting in a complete spark being discharged across a gap that had toluene vapors present.
The final part of the jigsaw is the potential energy of the spark itself. We can estimate the amount of energy available for discharge via the spark from the equation:
Potential energy of spark (Joules) = ½ x capacitance of charged object x square of the object’s voltage.
E = ½ CV2
If we estimate a low value of 1 nano-farad for the truck’s -9 capacitance value (1×10 F) combined with a voltage of 30,000 volts on the truck-hose system at the time of ignition, we can assume a total potential energy of 450 milli-joules, which is far in excess of the MIE of toluene which is 0.24 milli-joules. A certain percentage of the energy would be given off in the form of sound and radial heat, but a significant percentage would have been released in the spark gap, igniting the toluene in the process.
For the full analysis and further explanation about the risks of static electricity, click here.For the full analysis and further explanation about the risks of static electricity, click here.Click here for related articles and news
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