Question 25 OSE02 - Assistant Engineer - OSV

The Correct Answer is A **Explanation of why Option A is correct:** Duplex fuel filters (like those described as duplex secondary spin-on filters) are designed to ensure continuous engine operation even when a filter element becomes clogged and needs replacement or cleaning. The system consists of two parallel filter bodies (labeled '1' and '2') and a selector valve. * **Normal Operation:** The engine is typically run using only one filter element at a time (Selector handle in position '1' or '2'). The selected single filter is designed to have sufficient throughput capacity to meet the full fuel requirements of the engine, even at maximum load and RPM. Running on one element maximizes the life of the elements because the flow is concentrated through one side, allowing the other side to remain clean and ready for immediate switchover. * **Switchover (Transfer):** The "BOTH" position is a temporary, intermediate setting. When the operator notices that the running filter (e.g., filter '1') is becoming restricted (indicated by a vacuum gauge or pressure drop), they must switch to the clean filter (filter '2'). To avoid starving the engine of fuel momentarily during the transfer, the selector handle is briefly passed through the "BOTH" position. This allows fuel to flow through both elements simultaneously for a moment, ensuring continuous supply while the valve transitions from closing the restricted side to fully opening the clean side. **Explanation of why the other options are incorrect:** * **Option B is incorrect:** This option incorrectly links the selector position to engine load. Duplex filter elements are sized to handle maximum fuel flow even individually. There is no operational requirement to place the filter in "BOTH" at low load or to run only one side specifically at high load. Furthermore, the selection of filter 1 or 2 is based on cleanliness, not engine speed. * **Option C is incorrect:** This is the opposite of option B but is equally wrong. The "BOTH" position is not used during high load/high RPM to increase throughput. If one filter element cannot handle the maximum required flow, it indicates a restriction or an improperly sized filter system, not a situation where the operator must select "BOTH." The system is designed to run continuously and efficiently on a single, clean element at any load. * **Option D is incorrect:** While running in the "BOTH" position might double the theoretical handling capacity, it is not standard operating procedure. Running continuously in "BOTH" accelerates the clogging of both elements simultaneously, removing the primary advantage of the duplex system (i.e., having a clean standby filter). The selection of the filter is a maintenance decision based on element restriction, not a function related to convenience or engine speed changes.