Static electricity is often perceived as an invisible risk. This case study explains why static electricity provides an ignition source for serious fires and explosions that could occur during everyday operations involving the handling and processing of flammable products.
A company supplying aluminium powder had an order cancelled when the bulk truck transporter carrying the powder was en route to a railcar hopper loading station. The truck driver was instructed to return the aluminium to the plant from where it was manufactured. As this scenario had never occurred before there was no standard operating procedure in place to offload the aluminium from the truck back into the production facility. Shortly after the operators had worked out how to overcome some practical challenges for moving the powder back into the plant, an explosion occurred which propagated throughout the plant.
On return to the plant it was noted that there was no direct loading point for the finished powder to be injected back into the production stream directly from the truck. A decision was taken to convey the powder into the pneumatic transport system entry point of the plant using the 3” hoses on the truck. Unfortunately the hoses could not reach the entry to the plant’s pneumatic system so an additional length of hose from the plant was added to the line of 3” hoses running from the truck. Both hose types were constructed of rubber tubing that contained internal metal helixes that ensured the hose flanges were electrically bonded together.
The truck was grounded so the 3” hoses (assuming they were in good condition) were also grounded, hence the risk of an accumulation of electrostatic charge on the truck and hoses was minimal. One issue that was encountered, however, was that the plant hose that was used to complete the distance between the truck’s hoses and the entry to the plant’s pneumatic system was wider in diameter to the truck’s hoses. This meant that a sealed connection between the hoses could not be made. The operators overcame this issue by stuffing rags into the gap between the hose flanges. This had the effect of electrically isolating the plant hose from the truck’s hose, potentially impeding the transfer of electrostatic charges from the plant hose to ground via the grounded truck. The other end of the hose was assumed to have been resting on the concrete floor inside the plant. The other issue was that the density of the air-powder phase coming from the truck was above the minimum explosive concentration of the aluminium powder.