Thank you for your question regarding tank farm grounding, it is our pleasure to be of assistance.
There are a number of issues that can occur if your ground systems are not bonded together, step potentials being just one of them. The biggest issue is that unbounded and independent grounds systems such as you are describing are illegal under the National Electrical Code. NEC 250.54 specifically states that the earth may not be used as the sole equipment grounding conductor. The primary reason for this is that Over-Current Protection Devices (OCPD) such as circuit-breakers and fuses may not function properly should a fault or short-circuit occur at the tank. This is your biggest human safety concern.
Others issues from corrosion, to Ground Potential Differences (GPD), static, pipeline stress-voltages, current distribution, lightning strikes, longitudinal current flow, leakage current rates, magnetic and electric fields, harmonics, and more may all be negatively impacted by having independent ground systems. If you are handling flammable liquids, you will have even more adverse issues. Let alone the signaling problems you will have with data lines and control wires.
Adding the additional grounding can be quite expensive and sometimes hard to justify to the powers-that-be. A good grounding design using computer modeling to prove the engineering points, can be very useful in justifying the expense. Please let us know if we can help you with this.
Also, you may want to consider conducting some point-to-point direct-current (DC) resistance checks around your tank farm, back to some central control point. The data may be very revealing to just how good/bad your situation is. If you see DC resistances of greater than about 0.5-ohms, you may have a problem. A well grounded tank farm will have point-to-point resistances of less than 0.1-ohms (some increase may be expected for very large systems).
Feel free to call us so that you can speak immediately and directly to an engineer who will gladly answer your questions, free of charge.
The Engineering Team at E&S Grounding Solutions
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