In the hazardous process industries, more commonly referred to as the “EX” 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”. 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. In other words they “accumulate” static charge.
In a typical rail tanker loading operation, the static accumulating product is transferred from a storage tank via a gantry loading system into a receiving rail tanker. 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 rail tanker, the molecules in the product become electrostatically charged.
If the rail tanker does not have a direct connection to earth it will accumulate electrostatic charges on its surface, which will result in the voltage of the rail tanker rising dramatically in a very short space of time. Because the rail tanker is at a high voltage, it will seek to find ways of discharging this excess potential energy and the most efficient way of doing this is to discharge the excess electrons in the form of a spark.
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 an EX atmosphere is no different to having an engine’s spark plug exposed to a flammable atmosphere.
If the rail tanker is not grounded, its electrostatic voltage 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 rail tanker charged to 20,000 volts.
Table 1. Potential energy of sparks from various objects.
When the energy of sparks discharged by static electricity is compared with the minimum ignition energies of a wide range of petroleum products and flammable chemicals, it’s easy to see why the rail tanker and any equipment connected to it, like flexible hoses and piping, should be bonded and grounded.
As can be seen in Table 1, electrostatically charged rail tankers can discharge sparks with a huge amount of energy. At these energy levels the prevention of electrostatic shocks to workers is also an important safety consideration. Involuntary physiological reactions caused by electrostatic shocks could lead to trips and falls and could be particularly hazardous when personnel are working above ground level.
Of the several factors that contribute to static charging, the one variable that must definitely be controlled is the grounding of the rail tanker. Grounding the rail tanker ensures that the rail tanker’s resistance to the general mass of the earth is maintained at a level that does not impede the safe transfer of static charges from the rail tanker to ground.
In North America the grounding of rail tankers with dedicated static grounding systems is common practice. In Europe, the practice of rail tanker grounding is mixed. Some sites do it, some don’t. For sites that do not actively ground rail tankers there is an assumption made that the tank of the rail tanker is well bonded to the chassis and that static charges generated by the product transfer operation can pass from the chassis through the rail tanker’s wheels to earth or back to the loading gantry via bonding connections.