The loading and unloading of tank trucks with flammable and combustible products, presents one of the most serious fire and explosion risks for site operations within the hazardous process industries. A study conducted by the American Petroleum Institute (API) in 1967 identified static discharges as being responsible for over 60 incidents in tank truck loading operations, demonstrating just how long this potential threat has been acknowledged. The natural presence of static electricity in product transfer operations, combined with its associated ignition hazards, ensures that regulators take static control precautions for tank trucks very seriously.
Static electricity and tank truck product transfer operations.
Powders and liquids with low electrical conductivities are the prime sources of static charge generation because their electrical properties do not easily permit the transfer of excess charges. Instead, non-conductive and semi-conductive liquids and powders retain and accumulate charges after they make contact with conductive objects. The most common interface for charging of non-conductive and semi-conductive product is contact with metal plant equipment including pipes, filters, pumps, valves, barrels, totes, mixers and agitators. When the electrostatically charged liquid (or powder) is deposited into a container like a barrel, tote, or tank truck charging of the container will occur if there is nowhere else for the charges to go. In this situation the charges are “static”, accumulate on the surface of the container and set up a potential difference with respect to ground.
Fig.1 Levels of voltage generated on a tank truck by electrostatically charged liquid at approved flow rates.
Over a short time period (less than 20 seconds) potentials in excess of 50,000 volts can be induced on a tank truck’s container when it is being filled at normal flow rates with a product that is electrostatically charged. The magnitude of the voltage induced is directly proportional to the quantity of charges making contact with the container. This voltage represents the ignition source and the potential energy available for discharge via a static spark at voltage levels of 50 kV can, for a typical tank truck, be in excess of 1250 mJ. The vast majority of flammable vapours and combustible dusts can be ignited at these energy levels.
For sparking to occur in tank truck product transfer operations, other conductive objects must come into close proximity with the charged container of the tank truck. Examples of conductive “objects” include the fill pipe entering the opening on the top of the container, fall prevention systems like folding stairs, and drivers or operators working around the tank truck.
The charges on the tank truck’s container attract opposite charges to the surface of the object and rapidly create an electric field between their respective surfaces. It is the strength of this electric field that causes the “breakdown” of the air between the container and the object. When the air is “broken down” a conductive path for the excess charges to rapidly discharge themselves is created, leading to a static spark discharge. If a combustible atmosphere is present in this space, ignition of the atmosphere is very probable. Under ambient conditions an average field strength of 30 kilo-volts is capable of causing the electrical breakdown of air over a spark gap of 0.8 inches.
Fig.2 Potential minimum ignition energies present on tank trucks based on the time period of tank truck filling operations.
In addition loose conductive items located inside the container could become charged by contact with the liquid and discharge to the container if they are capable of floating on top of the liquid. It is important to carry out regular visual inspections of the container to ensure loose debris is not present inside the tank truck container.
Standards and recommended practice governing the static control of tank truck product transfers.
As outlined earlier, regulators are extremely cautious about the ignition hazards presented by static electricity in tank truck product transfer operations. Three standards, in particular, provide clear guidance on what precautions should be taken. NFPA 77, API RP 2003 and IEC 60079-32 state that grounding of the tank truck should be the first procedure carried out in the transfer process. Grounding effectively creates an electrical circuit that connects the tank truck to the Earth and it is this connection to earth which prevents static charges accumulating on the tank truck’s container. The reason the charges can transfer from the tank truck to earth is because the Earth has an infinite capacity to absorb and redistribute static charges, with the positive effect of removing the ignition source from a potentially combustible atmosphere.
The electrical resistance of this circuit from the tank truck to the “ground source” (or “grounding point”) which is in contact with the earth, is a key performance indicator of the entire grounding circuit’s capacity to provide a secure and safe product transfer operation. NFPA 77 and API RP 2003 state the resistance in a healthy metal circuit should never exceed 10 ohms, therefore the entire circuit between the truck and grounding point should be measured and be equal to, or less than, 10 ohms. If a resistance above 10 ohms is measured this will indicate problems with parts of the grounding circuit including the tank truck connection, the ground point connection or the condition of the conductor cable.