Safe Grounding of Tank Trucks
- What causes electrostatic charging while loading tank trucks?
- How does grounding the tank truck help combat this danger?
- Which requirements does the truck’s ground connection need to fulfill?
- What does best practice look like for grounding tank trucks?
Due to the high filling speeds, there is always a potential danger of electrostatic charging of the material, the delivery lines or even the tank truck itself when the tank truck is loaded. This is why a means to safely ground the tank truck is necessary to prevent an ignitable spark discharge in potentially explosive atmospheres..
This article provides you with an explanation on what you should keep in mind when grounding the truck, which standards and requirements must be observed and what the best practice entails.
What causes electrostatic charging while loading tank trucks?
During the filling and emptying of trucks, electrostatic contact charging is a frequent occurrence. Due to the high delivery speeds of the liquid or solid material, brief contact with the ambient pipelines and tanks does take place. In the process, electrons are exchanged between these components.
However, due to the high delivery speed, the electrons do not have time to return to their starting point. Gradually, the number of electrons on the material with higher electron binding force increases, while the number on the other material decreases. As a result, what we can detect is a difference in electric potential – an electrostatic charge occurs.
If the electrostatic potential difference is too great, the accumulated electrical energy can discharge in a high-energy spark within fractions of a second. The energy released in the spark is often sufficient to ignite explosive atmospheres, causing serious damage to the equipment and the surrounding area.
How does grounding the tank truck help combat this danger?
Electrical energy can always accumulate when the charge ratio exceeds the discharge capability of the material. Specifically, this means:
If conductive material is insulated from the ground potential and contact charging occurs there, a dangerous electrostatic potential difference inevitably builds up, because the energy cannot be discharged fast enough in sufficient quantity. Examples of potentially at-risk areas are the couplings of filling hoses, system pipes, or even the truck’s tank.
This is the point at which the significance of truck grounding becomes so evident. If all conductive areas are equipped with a good grounding connection, the speed of the discharge exceeds the speed of charging. This means that energy does not accumulate, thus effectively preventing a hazardous potential difference and possible spark generation.
Which requirements does the truck’s ground connection need to fulfill?
Having to review all of these variables with respect to the hazard potential takes a lot of time and is very costly. Thankfully, standards and guidelines make things easier for the operators of tank truck filling systems, allowing them to take the correct measures for ensuring safe processes. They describe workflows, provide guidance on best practice measures, and define maximum leakage resistances. In particular, what must be observed is:
Establishing a safe grounding connection before performing any activities, such as opening the cap or connecting pipes and hoses, is recommended.
Generally, electrostatic grounding can be executed unmonitored, for example, with the use of a simple grounding cable with a clamp. However, the standard explicitly specifies an automatic monitoring device for the ground connection to ensure optimum protection for employees and the equipment.
In addition, a maximum leakage resistance to ground potential of 1 MOhm (10^6 ohms) is specified. This resistance between the truck and the grounding point must be maintained or be lower at all times during the process to prevent potentially dangerous charges.
What does best practice look like for grounding tank trucks?
The type of grounding that meets all requirements and which as such is viewed as best practice depends on the type of tank truck. Here, we differentiate between the following types:
Bottom-loading tank trucks
With this modern model, filling is performed through hose couplings in the lower, side area of the truck. The liquids are filled into the tank chambers at a high speed, typically at approx. 2,500L/min.
To prevent overfilling and to ensure grounding at the same time, a combination unit, such as the TIMM EUS-2 should be used. These overfill protection devices are connected to the tank truck using standard plugs in order for grounding to be monitored, while simultaneously reading the fill level sensors of the chambers.
These devices are integrated into the system controller and can thus stop the work process in the event of danger to prevent bursting of the tank chambers and electrostatic charging.
If the tank truck is filled through caps on the top of the tank, ground monitoring is used without additional overfill protection. However, this also requires an interlocking function in the event of an insufficient ground connection to meet best practice requirements.
With a simple ground connection using cable and a clamp, there is always a risk that an undetected cable breakage or other impairment of the discharge capability may have occurred. This potential hazard is effectively ruled out as a result.
In addition to automatic shutdown, best practice grounding devices, such as the Grounding Control Device EKX-4 that uses object detection rule out any capability of manipulating the device or the ground circuit. These devices measure the electrical characteristics of the connected object and verify whether the values are plausible for a truck.
This makes it impossible for the user to inadvertently connect the clamp to a conductive point of the system and in doing so bypass the system or set it to permanent release. A filling release is not issued until the tank truck has been properly grounded.