Although static electricity can be regarded as a difficult subject to grasp, we hope that our series of case studies give you an insight into the reasons why static electricity provides an ignition source for serious fires and explosions that occur during everyday operations involving the handling and processing of flammable products.
This case study investigates the factors resulting in an electrostatic ignition incident involving toluene, a prolific charge generator filling a metal bucket via gravity fed 0.75” metal pipping.
In this scenario, an operator opened a valve to draw toluene into a metal bucket with toluene from an overhead tank by gravity flow at approximately 5 gallons per minute. The operator hung a metal bucket with a wire bail and plastic handle over a globe valve. The plastic handle on the bail isolated the metal bucket from ground.
On opening the valve, the operator backed away from the bucket allowing the toluene to flow as he had previously done several times. Within a few moments the toluene had ignited causing the operator to immediately leave the scene returning with a small fire extinguisher, which proved inadequate to put the fire out. The operator then left the scene returning with a larger fire extinguisher, however by the time he had returned the fire was out of control and he was unable to close the valve to prevent the flow of toluene to the bucket which was already over flowing.
The investigation into the incident outlined that the operator had opened the valve and backed away from the metal bucket. The operator stated “I was just standing there looking at it when it caught fire”. As a result, discharge from the operator could be ruled out as a cause of the incident and the scenario of a streaming current was considered.
(I) Is = 2.5×10-5 ∙ v2 ∙ d2
(II) Is = 2.5×10-5 ∙ 1.10692 ∙ 0.019052
(III) Is = 0.01μA
Is = Streaming Current (A)
v = Velocity (m/s)
d = Pipe diameter (m)
The streaming current was found to be in the order of 0.01 µA were it not for the presence of the in-line filter. The residence time of the toluene between the in-line filter and the exit of the pipe was less than a second, much shorter than the recommended 30 seconds; therefore, a reasonable estimate of the streaming current at the exit of the pipe can be calculated around 0.1 µA. In any case, an estimate for the streaming current can be assumed to be between 0.1 µA and 0.01 µA.
Assuming that the toluene flow had continued for 30 seconds, there would have been a charge of 3 µC on the bucket provided that the bucket was completely isolated from earth.
The potential energy on the bucket can be found using the equation:
(I) Q = Charge on the bucket
(II) C = Capacitance of the bucket
Therefore the potential energy on the bucket:
And the voltage on the bucket can be found using the Equation:
With the breakdown strength of air at 3 x 106 V/m a spark from the bucket could easily jump across a gap of 50 mm (1.96″) meaning it was probable discharge from the wire of the bail to the body of the globe valve could occur.