Problem:
- ignition risk as a result of electrostatic discharge during road and railroad takers unloading at the power unit unloading stations
Solution:
- furnishing 9 unloading stations with certified earthing control systems dedicated particularly for road and railroad tankers
- this solution enables blocking of the unloading process in case proper earthing is missing and informing the operator about the system status by virtue of light signals
The project of building a new power unit comprises a broad spectrum of works. The common denominator for all of the elements of such a huge project is to design optimal technical solutions (often subject to legal regulations) which will provide the proper level of safety at the further stage of the unit operation.
The need to furnish unloading stations with earthing control systems
Within the construction of a new power unit, we have furnished nine stations for road and railroad tankers unloading with earthing systems including earthing system status control. This resulted from the risk of electrostatic discharge during the unloading process which is going to be made at the power plant, whereas unloading may be a potential source of ignition in explosive atmosphere.
Important was also to find a solution to prevent or block the unloading process in the absence of proper earthing. Worth noting is the fact that in such situations the simple earthing clamps without monitoring system will not be sufficient. The offered systems fulfil the design recommendations. Apart from Ex certification, the solution fulfils SIL2 conditions (Safety Integrity Level), which means that the risk of their failure is between 1,000 and 10,000 times lower than in the case of standard solutions.
Monitoring of the earthing of road and railroad tankers
As regards this project, road and railroad tankers are unloaded at different stations. Therefore, there was no necessity to apply systems capable of serving both types of vehicles. Instead, earthing control systems in two different versions were applied.
The systems intended for road tankers earthing control are provided with a function, which first of all recognises a road tanker and an attempt to cheat the system by attaching the earthing clamp to another vehicle ends up in a failure (the system does not allow for starting the tanker unloading). As soon as the first step is properly verified, the system checks whether the earth electrode to which it is connected (usually hoop iron) provides for fast and safe disposal of electric charge to the earth. If both conditions are fulfilled, the unloading starts, whereas the system is constantly monitoring the earthing status. If the connection between the tanker and the earth is broken, for example, due to the clamp loosening or damage of cables, the process will be blocked. The information is conveyed to the operator by way of a diode, which will light red. Depending on the needs, it is also possible to apply additional sound signals.
As regards railroad takers, earthing control system has been planned, which after the clamp connection to the tanker structure measure the resistance of the earthing connection. If it is lower than 10 ohms, the system starts up the devices responsible for the medium transfer (e.g. pumps, valves, etc.). If the resistance exceeds the limit value (again as a result of cable damage or clamp loosening), the system will stop the process and inform the operator by light signals.
For one of the unloading stations the assembly of the monitoring system was not possible in the direct vicinity of the tanker. Therefore, instead of the spiral cable, the system was provided with 15 m long self-winding straight cable.
Earthing systems fit for heavy-duty working conditions
Earthing systems are often installed in places where they are exposed to the operation of external factors and corrosive atmospheres. Owing to the fact that their operation affects the level of safety and cannot be interrupted, particular attention must be paid to the proper finishing and installation. A standard earthing control system consists of a monitoring unit, a clamp, a hanging-up bolt, and an earthing cable. The system monitoring units supplied by us within this solution possess Ex d fire‑proof casing, which means they may be used in explosion hazard zones. Additionally, the are provided with LEDs that serve informing the operator about the earthing status.
The earthing clamps are made of stainless steel and sharp tungsten carbide teeth, which enables the clamp to penetrate the layers of rust, paint or other dirt which may be present on the tanker elements at the moment of clamping. Further, the earthing cables are provided with Hytrel sheath to guarantee resistance to chemical and mechanical conditions. Properly engineered systems guarantee reliable operation even in heavy-duty conditions.
Hazard awareness as a basis for ensuring safety
The awareness of the need to apply proper earthing systems as well as earthing monitoring systems significantly contributes to ensuring proper safety level at a plant. Thanks to the dedicated solutions our customer may limit the risk of ignition due to electrostatic discharge during tanker unloading at the power unit, although such unloading may seemingly be perceived as a trivial activity which does not require additional protective measures.
