A tank farm is a facility where petroleum products are stored prior to being disbursed to end consumers or retail facilities. The facilities at a tank farm are usually very basic. Tanks can be above or below ground, with plumbing so that tanks can be connected to tankers and piping for the purpose of dispensing fuel and refilling the tanks. Many tank farms are located near refineries, ports, rail yards and major trucking terminals (for convenience). These locations make it easy to move fuel in and out of the farm. A tank farm can also be located along a pipeline used to transport petroleum products. Cathodic protection systems, as well as grounding systems, may be applied in tank farms for corrosion control and safety considerations respectively. (For a primer on cathodic protection, read The Basics of Cathodic Protection.)
Cathodic Protection vs. Earthing Systems
Although both cathodic protection systems and earthing systems are applied through earth, each one of them has its own unique function. Cathodic protection is a corrosion control technique used to protect underground metallic structures from corrosion and increase their lifespan. Adequate cathodic protection is achieved by shifting the potential of the buried structure to a certain criteria, -850 mv with respect to a Cu/CuSO4 reference electrode.
An earthing system, on the other hand, is related to the safety and effective operation of electrical circuit protection devices, such as circuit breakers. Earthing systems are also used for lightning protection and the drainage of static charges that are accumulated in metallic structures due to the inflow and outflow of fluids.
Cathodic Protection of Earthed Tank Farms
In order to overcome corrosion and safety problems in tank farms, both cathodic protection and grounding systems need to be applied. The question, however, is how these systems can be made to be compatible with each other in order to ensure effective operation. Here we will talk about the different cathodic protection system arrangements that are used for corrosion control of earthed tank farms, some of the problems that arise, and the most suitable solutions for these problems. (Also see: Troubleshooting Cathodic Protection Systems and Function Systems.)
Remote Cathodic Protection System
This system is a remote one, where the groundbed, or anode, is placed at a remote place, which may be at any corner of the tank farm, and is used to cathodically protect all buried metallic structures in the tank farm including the earthing grid.
For this system, all metallic structures must be bonded, either mechanically or electrically, to each other so that the cathodic protection current can be distributed without interference problems.
- It's common for earthing grids to be made of copper rods. A copper earthing grid, connected to the structure under cathodic protection, may drain more than 90% of the protection current. In fact, the polarization of copper is less than that of steel, and requires 10 to 20 times more current. According to the configuration, it may even be impossible to correctly polarize the steel structure and, as a result, there is a risk of corrosion of the structure due to the galvanic coupling between the copper and the steel, to the detriment of the steel.
- It's very difficult to determine any discontinuity in the whole protected structure. Therefore, corrosion risk due to interference problems is possible.
- For complex structures equipped with cathodic protection, it's recommended to make the earthing out of galvanized steel instead of copper. The current requirement is less than with copper; the current losses are therefore diminished and the galvanic coupling remains favorable for the steel. Of course, mixing earthed copper and galvanized steel in the same geographic area and on the same structure should be avoided. So, this solution is only possible for a new installation, or when the existing copper earthing system is disconnected. If the copper earthing system can't be disconnected, the cathodic protection system should be redesigned considering the earthing copper system. However, the current required to ensure adequate cathodic protection will be very large in this case.
- Any electrical bonding should be made through junction boxes, so that it can be examined for any discontinuity.
Localized Cathodic Protection System
In this system, each buried metallic structure—piping, vessels or tanks—has its own cathodic protection system, each of which is isolated from the others using an insulating kit, insulated flange or isolating joint. Localized cathodic protection is achieved by placing anodes close to the protected structure.
In order to achieve adequate cathodic protection, a separate earthing grid is required for each metallic structure and should be isolated from the main earthing grid of the electrical installation in the tank farm.
- Although the earthing grid of aboveground tanks used for lightning protection and static charge discharge can be separated from the main earthing grid, all electrical equipment installed on the tank is connected to the main earthing grid for safety and effective operation of electrical circuit protection devices as mentioned above. As a result, the cathodic protection current can be drained, creating interference problems.
- Any deterioration in the insulated kit, insulated flange or isolating joint, which isolates the metallic structures from each other, may lead to inadequate cathodic protection of the protected structures.
- In order to isolate the DC circuit of the cathodic protection system applied on the metallic structures from the main earth grid, decoupling devices such as polarization cell replacement (PCR) can be installed at certain locations. This should be done according to field surveys to ensure the continuity of the grounding system and block the DC current.
- Periodic inspection of the insulating kit is required in order to determine its status and do the required maintenance or replacement.
In order to avoid the malfunction of the cathodic protection systems of earthed tank farms, the design of the earthing system must be agreed upon with the suppliers and designers of cathodic protection systems.