When taking a shell-and-tube heat exchanger with a removable tube bundle such as a bayonet-tube type heavy fuel oil service heater out of service, to avoid thermal shock what statement represents the correct operating procedure?

Correct Answer: Option D — Gradually secure the steam first, then secure the heavy fuel oil to avoid thermal shock.

Gradually securing steam first allows the heat exchanger to cool down slowly, preventing thermal shock to the tube bundle and tube sheets. Maintaining fuel flow during this process ensures the unit does not experience localized overheating or rapid temperature fluctuations that could compromise the integrity of the seals.