When running a large, dual fuel, main propulsion diesel engine on heavy fuel, which of the following precautions should be observed when switching from heavy fuel oil to diesel oil?

Correct Answer: Option A — The temperature of the fuel from the preheater should be gradually reduced after switching over the three-way valve.

Thermal shock can fracture fuel injection equipment and cause vapor binding when switching fuels. Fuel preheater temperatures must be reduced gradually after shifting to diesel fuel to prevent rapid temperature differentials and associated heavy fuel flashing or pump seizure.