Preventing Galvanic Corrosion when repiping a hydronic expansion tank

I'm an operating engineer in a high-rise commercial building which was built in 1972. Since beginning work there six months ago I've been familiarizing myself with the systems and I noted that our heating hot water loop needs to be periodically topped off manually. It does have an expansion tank of about 500 gallons located in the penthouse which is showing a level in the sight glass, but the level never changes and when I isolated the tank from the system and opened a strainer drain valve in the piping nothing came out. My first guess is that either the galvanized 3/4" piping which connects the tank to the loop has corroded shut or else the Bell & Gossett "Airtrol" fitting which connects the tank to the system has failed shut.

I want to have this system repiped so that the automatic loop fill can work again, but my question regards galvanic corrosion with the repiping material. This large steel tank is in a 36th floor penthouse and if it ever goes bad the only way it's coming out of there is with a crane lift or helicopter. Frankly, I'd rather sacrifice the piping to preserve the tank. The present piping, as I said, is 3/4" galvanized and Bell & Gossett fittings (Airtrol, pressure reducing valve, relief valve) are used from the fill meter to the tank and some corrosion on the piping is noticeable now. What suggestions does the readership have for how I should spec out the repiping job with an eye to keeping this system operating long-term? And, assuming I use all-new fittings, what is present-day preferred practice for piping hydronic expansion tanks?

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