PetroStreet

Q: If a storage tank is subjected to CP & earthing simultaneously, how do the two systems work separately without interfering each other despite they have somewhat opposite science?

At our site, in 40 aboveground tanks, CP has not been done. Only earthing and lightening arrangement have been installed. I seriously doubt the correct operation of CP and earthing simultaneously. Earthing always tend to keep the potential to zero whereas in CP the potential is always raised to avoid galvanic corrosion. May be there are systems existing for operating both these systems in parallel but some design considerations / modifications must be needed.
Also for above ground piping, an insulation flange is used where underground piping comes aboveground in order to separate both installments that is CP and earthing.

As pe my information there are devices known as "Surge Diverters" which are used to keep the two systems operational simultaneously. One lead of earthing wire & one of CP wire is connected to this device. Its function is not that much known to me but simply When a surge diverter is inserted in series in the grounding circuit the grounding system is isolated such that DC current of CP system will not flow through it but the AC high voltages when encountered are grounded because the surge diverter does not block high voltage ac current.