Earthing of Floating Roof Tanks

The fire hazard potential in fuel storage tanks is very high to the extent that if lightning strikes these tanks or static electricity accumulates, unfortunate incidents may occur. Considering this issue, the necessity of having a proper path for discharging electrical charges is clear and obvious.
To prevent such unfortunate events, the earthing system installed in the ground must be properly connected to the floating roof of the tanks. There are two general solutions in this regard. The first method is the shunt method, where metal strips establish multiple connections between the tank body and the roof to discharge the electrical charges accumulated on the tank roof. However, in many cases, a proper electrical connection does not exist, which reduces the reliability of the system.

Another proposed solution that addresses the problem of improper permanent connection is the permanent electrical bonding of the tank roof to the earthing system using a strap. This is done in two ways. The first method involves leaving the strap freely laid on the upper surface of the roof. Considering the long length of the strap, when the tank roof is at its highest position, the entire length of the strap lies in the current conduction path; therefore, without providing any advantage, the impedance from the roof to the ground becomes very high.

In the second method, the connection is made using a wire and a reel. In this case, considering the floating nature of the tank roof, the strap connecting the roof to the earthing system has a variable length, allowing for length adjustment based on the position of the floating roof. The advantage of the second method over the first is that the impedance of the path from the tank roof to the ground is reduced to one-sixth of that in the first method. The system that provides this feature (the second method) is called the RGA (Grounding Retractable Assembly) system.

The RGA must have certain specific features, among which the following can be mentioned:

1- Since these tanks usually contain corrosive materials, the connecting wire must be resistant to corrosion. Therefore, aluminum cables are used in this device, which are highly resistant to most gases such as hydrogen sulfide (H₂S).
2- The connection must be strong, permanent, and have low impedance.
3- The installation of the system should be as easy as possible.
4- The RGA should be installed at the highest point so that its cable can be fully extended.
5- The cable and RGA should be securely bolted and sealed at the most appropriate points on the floating roof and the tank.

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