The transportation of flammable products by rail, whether that be as a result of the shale driven boom in crude by rail or the transportation of chemicals from petrochemical manufacturing centres to end user markets, is still one of the most flexible and cost effective methods by which to move flammable products across the continent of North America in bulk quantities. And while this industry has a host of safety and environmental regulations to contend with, one area of safety that is often misinterpreted or misunderstood, is the ignition hazard associated with static electricity and the measures that can be put into practice to control this risk.
Static electricity: what’s the big deal?
Let’s start with static electricity itself. The clue is in the term. Static electricity is, essentially, electricity that is static. It is electricity that is temporarily “stuck” in the same position. It’s made of the same “stuff” that powers your refrigerator or lighting, but its characteristics are different to the line power delivered to your home or place of work.
In the hazardous process industries, more commonly referred to as the HAZLOC industries, static electricity is generated virtually all of the time. Various grades of crude oil, refined petroleum products like LPG, and a host of chemicals fall into a category of materials that are often referred to as “static accumulators”. What this term means is that materials in this category are known to be powerful attractors of electrons from other materials and resist “letting go” of electrons they come into contact with. They “accumulate” static charge.
In a typical Lease Automatic Custody Transfer (LACT) unit or rack loading operation, the static accumulating product is transferred from, say, a truck, via the LACT unit or from a storage tank via a rack loading system into a receiving tank car. We can refer to the equipment involved in the transfer of product collectively as the product “transfer system”. As the product makes its way through the transfer system to the customer side of the transfer, the molecules in the product become electrostatically charged.
If the tank car is not grounded contact with the charged product will cause it to become electrified. If this situation is allowed to exist and persist throughout the transfer operation, it will present a potentially serious source of ignition in the presence of flammable atmospheres.
As the tank car builds up electrostatic charges on its surface, the voltage present on the tank car rises dramatically in a very short space of time. Because the tank car is at a high voltage, it is seeking to find ways of discharging this excess potential energy and the most efficient way of doing this is to discharge the excess electrons to objects at a lower potential in the form of a spark. The best object to discharge to is the Earth or an object with a direct connection to it, i.e. something that is grounded. This is because the Earth can absorb an infinite amount of charge due its size and mass. This is what happens during an electrical storm, where lightning strikes result from huge potential differences that are present between the Earth and the layer of storm clouds above its surface. Static electricity is no different; it’s the same stuff, the only difference being the amount of electrons being discharged via a static spark or via a lightning strike.
Energy discharged in static sparks.
Grounded objects that are in close proximity to charged objects are good targets for electrostatic sparks and permitting the uncontrolled accumulation of static electricity in a HAZLOC atmosphere is no different to having an engine’s spark plug exposed to a potentially flammable atmosphere.
The magnitude of the energy present during the discharge of a static spark is a product of the capacitance of the tank car and the voltage present on the tank car at the time the spark is discharged. The electrostatic voltage that is present on the tank car is a combination of the charging current generated by the flow of the liquid, the capacitance of the tank car and the tank car’s isolation from ground.
Increased flow rates and turbulence can increase the size of the charging current, but even when safe recommended flow rates are taken into consideration, if the transfer system is not grounded, the electrostatic voltage of the tank car can build up to hazardous levels in less than 20 seconds. Table 1 illustrates how much energy can be discharged by a spark from a tank car charged to 20,000 volts.