According to the illustrated diesel engine fuel treatment and fuel service systems schematic, what statement is true concerning the valves labeled "4" and "5"? Illustration MO-0077

Correct Answer: Option A — Valves 4 and 5 are closed during normal operation, but when opened will allow one of the HFO purifiers to be used as a backup DO purifier, should the DO purifier become disabled.

Valves 4 and 5 act as cross-connection isolation valves. Opening them redirects flow to permit a heavy fuel oil purifier to function as a diesel oil separator during maintenance or emergency conditions, maintaining continuous dual-fuel plant readiness.