On cold, dry afternoons, walking across a carpet to open the front door can create a spark that jumps between the hand and the door knob. That release of electricity comes from a buildup of static electricity in the human body that can reach an 10-15 kV (kilovolts). The discharge of that voltage can measure just 20-30 mJ (milliJoule), well above the threshold for igniting propane or even natural gasoline vapors.
When trucks or trains load and unload liquids, the friction builds up in the transfer of those liquids creating static electricity. The charge level is higher for poorly conductive solvents flowing through plastic tubes. In addition, fast flow rate or large amounts of air bubbles flowing through the tube can amplify the static electricity. Tank trucks loading propane, gas or flammable liquids can accumulate enough static electricity to emit 2,250 milliJoules. It only takes 26 milliJoules to ignite propane and 24 for gasoline vapor.
When liquid passes through a pipe at a high flowrate, as it typically does in loading and unloading liquids from Truck and trains, the electrostatic charge of the flowing liquid generates static electricity. Large amounts of air bubbles and an increased flow rate can amplify the static electricity.
A: Charge that moves along with liquid flow
B: Charge that is fixed to a solid surface and cannot move
It’s called Grounding because the safest way to discharge the built-up voltage that comes from flowing fuel, dragging hoses, or railcars down the line is to send it to the Earth. The voltage to be discharged is connected to an electrode stuck in the ground. The connections are critical. The lines are usually copper or steel, but even those have some resistance. Lines that are too long can have enough resistance to prevent the static electricity from totally dissipating. Both the NFPA and API recommend that the resistance of these lines measure no more than 10 ohms end to end.
Railcars and tank cars have many parts, metal and otherwise. To discharge the static electricity by grounding, all the parts have to be connected—bonded—in a way that electricity can flow horn each to the ground. That prevents one part from discharging to another even though they do not touch, just like a charged hand on a doorknob receives a startling result. That spark happened in nanoseconds before contact between the two as the charge jumped across air.
Tank Truck Grounding
Loading procedure during transfers involves the driver connecting the truck to the Earth (grounding) before any other operations. The truck grounding system should have circuits that prevent transfer of the fuel if the ground connection has not been made. Typically, the loading rack has a grounding system that connects to the truck.
The tracks that railcars run on have their own grounding system. The wheel assemblies have metal-to-metal contacts so they are always grounded. But many railcars have wheel bearings that are not conductive, which makes the rest of the carriage assembly isolated electrically. Similar to truck, when railcars are loaded or unloaded, operators will use railcar grounding systems to discharge static electricity,
The wear pads on the carriage cast also isolate the assembly from the tank and fitting, so the procedure calls for grounding the tank to the loading/unloading rack during the transfers. Again, the system should have failsafe circuitry that shuts down transfers of fuel if the ground connection is lost.
SafeRack’s Grounding, Overfill & Monitoring Solutions
When working with petrochemical or other combustible liquids, safety cannot be overemphasized. It’s important to take every precaution to avoid accidents, and SafeRack makes that easy. Our industry experts and our technologically advanced overfill protection and ground verification systems help boost both safety and productivity. And we can help you select and install electronic rack equipment for accurate and reliable monitoring regardless of your railcar or tank-car operation’s needs.
More on Tanker Truck Safety
Around 25% of all freight hauled in the US is transported in tanker trucks,with nearly half of that being flammable liquids. With so much hazardous cargo on the nation’s roads, and the occurrence of cars driving under road tankers not uncommon, why are there so few disasters?
Read about all the tanker truck safety devices.