Grounding, Bonding, and Static Electricity Control
Static electricity is an unavoidable byproduct of flammable liquid flowing through pipes and hoses. As petroleum products move at high velocity through loading hoses, friction generates static charge that accumulates on the liquid, the hose, and the vehicle's metal surfaces. If this charge finds a path to ground through a spark gap — such as a disconnected hose fitting — the resulting spark can ignite flammable vapors and cause a fire or explosion. Grounding and bonding procedures prevent this by ensuring all conductive surfaces are at the same electrical potential before any product flows.
Bonding connects two conductive objects to each other so electrons flow freely between them, eliminating any charge difference. Grounding connects the vehicle or equipment to the earth, dissipating accumulated charge harmlessly. At a petroleum loading terminal, the driver must connect the grounding cable (typically a spring-loaded alligator clip or magnetic clamp) from the loading rack to the tanker's designated grounding point — usually a clean unpainted metal surface near the dome cover — before opening any covers or connecting loading hoses. Most modern loading racks have an interlock system that prevents product flow until the ground connection is verified electronically.
After grounding, the driver connects the loading arm or hose to the tank's fill opening. Top loading (through dome covers) is becoming less common due to vapor exposure risks and fall hazards; bottom loading (through valves at the bottom of each compartment) is now the industry standard. Bottom loading is faster, produces less vapor, and eliminates the need for a worker to stand on top of the tanker. The loading arm is connected to the bottom of the compartment through a dry-break coupling, which seals when disconnected to prevent drips.
During loading, the driver must monitor the flow of each compartment and not leave the vehicle unattended. Overfill prevention probes (OPP) installed in each compartment automatically signal the terminal's rack controller to stop product flow when the compartment reaches its maximum fill level. However, these sensors can fail, and the driver must watch the compartment meters and be prepared to manually stop loading if the automatic system does not activate.