🔍 Key Concepts

• Study the flow paths from the DO settling tank and DO day tank through valves 4 and 5. • Think about how the system changes when switching between DO (distillate oil) and HFO (heavy fuel oil) operation. • Consider why valves are marked NC (normally closed) and how that relates to preventing back‑flow/cross‑contamination.

💭 Think About

• For each operating condition in the choices, trace the fuel path from the settling tanks, through the purifiers, to the mixing tank and main engine. In which condition do you want the DO line isolated from the common header? • Look at valve 4 on the DO day tank line and valve 5 on the DO settling tank/common header line. When would you NOT want HFO to be able to flow into those DO tanks, or DO to flow into the HFO system? • Which mode requires only the HFO system to supply the common header and keeps the DO system separated except for its own purifier loop?

✅ Before You Answer

• Verify which tanks and purifiers are feeding the mixing tank/common supply header in each operating mode. • Check the legend: valves 4 and 5 are marked NC (normally closed). Ask: in which operating condition would they remain in their normal state instead of being opened? • Confirm in which mode the main engine is expected to burn only one type of fuel and why the other fuel’s day/settling tanks should be kept isolated by having valves 4 and 5 closed.