Tank truck static grounding protection
As the product (liquid or powder) moves through the transfer system and interacts with pumps, valve, filters meshes and pipe walls, the product will be building up the amount of electrostatic charge is carries. In electrical terms this is commonly described as static charge accumulation. When the product is transferred into the tank truck, the tank truck, will in turn, become electrified and be subjected to a rising voltage.
For example, a typical tank truck when it is being filled with a liquid at recommended flow rates, but is without static grounding protection, could have its voltage raised to between 10,000 volts and 30,000 volts within 15 to 50 seconds. This voltage range is very capable of discharging a high energy electrostatic spark towards objects at a lower voltage potential, especially anything at ground potential. Examples of objects at ground potential could be operators working in the vicinity of the truck, or the filling pipe situated in the hatch on top of the tank truck.
1. Electrostatically charged tank truck (electrified tank truck).
It is possible to estimate the energy of such sparks by combining the capacitance of the tank truck with the voltage carried by the tank truck. The capacitance is a measure of how much charge can accumulate on the outer surface of the tank truck. Because tank trucks have a very large surface area, they can accumulate very large amounts of charge, which in turn, creates the presence of very high voltages on the surface of the tank truck.
2. Voltage build-up on charged tanker truck.
For example, a truck with a capacitance of 1000 pico-farads that is electrified to 30,000 volts has approximately 450 milli-joules of potential spark energy. Given that most hydrocarbon vapors and gases have MIEs of less than 1 milli-joule and most combustible dusts have MIEs of less than 200 milli-joules, it’s easy to see why tank trucks that do not have static grounding protection in place can be a major ignition source in a hazardous area.
3. Example of potential spark gap during
the tanker truck loading operation.
To counteract this risk, it is important to ensure that the tank truck does not have the capacity to accumulate static electricity. The most practical and comprehensive way of achieving this is to make sure that the tank truck is at ground potential, especially before the transfer process starts. When we describe “ground potential” we mean that the tank truck is connected to the general mass of the Earth, which is commonly referred to, in electrical terms, as “True Earth”. This is because the general mass of the Earth has an infinite capacity to pull static charges from the tank truck, which in turn eliminates the generation and presence of voltages on the tank truck.
The Earth-Rite RTR performs three critical functions which ensure the fire and explosion risk of an ignition caused by static electricity is eliminated. The first function the RTR performs is in determining if the driver or operator has made a secure connection to the body of the tank truck. This minimises the risk of the driver obtaining a permissive condition for the static grounding system by connecting to objects like the loading rack, or objects on the tank truck that could be isolated from the main body of the tank truck, as this would defeat the objective of passing electrostatic charges from the tank truck to ground. The RTR then verifies if it has a low resistance connection to True Earth via the structure to which it is connected, e.g. the loading rack.
As any static charges generated by tank truck loading (unloading) process will travel to ground via the RTR, it is important to ensure the RTR itself has a low resistance connection to True earth. When both of these conditions are positive, i.e.:
1. The RTR knows it is connected to a tank truck.
2. The RTR knows it is connected to a verified earth ground.
The RTR will then establish if the connection resistance between the tank truck and the verified earth ground is 10 ohms or less. 10 ohms is the benchmark requirement repeated in several international standards, the most prominent of which is the American NFPA 77 “Recommended Practice on Static Electricity” and Europe’s IEC 60079-32 standard. If the resistance is not more than 10 ohms the RTR will indicate that the tank truck is connected to ground and indicate this via its ground status indicators, a cluster of green LEDs that pulse continuously.
4. How the Earth-Rite RTR ensures static electricity cannot build-up on the tank truck.
The reason the LEDs pulse is to indicate that the RTR is continuously monitoring the static grounding circuit between the tank truck and the verified earth grounding point (e.g. loading rack) for the duration of the loading (unloading) process. If the resistance of the tank truck’s connection to the verified earth ground ever rises above 10 ohms, the RTR will go non-permissive.
Both of the standards listed above recommend that interlocks controlling the flow of product to or from the tank truck are provided by the static grounding system. To comply with this requirement, the RTR has two volt free contacts that can interface with control circuits for pumps, valves and PLCs.
If the RTR determines that the tank truck has lost its connection to ground, the volt free contacts can be used to halt the transfer process. The benefit of halting the transfer process removes the charging mechanism that would otherwise charge up the tank truck while it has no active static grounding protection in place.