Question 53 OSV01 - Master/Chief Mate - Offshore Supply Vessels

The Correct Answer is D. **Why option D ("Quick operation") is correct:** A butterfly valve is a quarter-turn valve, meaning it only requires a 90-degree rotation of the stem (or handle) to move from fully closed to fully open. This mechanical characteristic allows for extremely rapid opening and closing compared to a gate valve. A gate valve is a multi-turn valve; it uses a rising or non-rising stem mechanism that requires numerous rotations to lift the gate entirely out of the flow path, making its operation significantly slower. The quick operation of a butterfly valve is one of its primary advantages. **Why each of the other options is incorrect:** * **A) Less maintenance required:** While modern butterfly valves are generally reliable, the soft seats they often employ are susceptible to wear, erosion, and chemical attack, requiring eventual replacement. A gate valve (especially a solid-wedge, metal-seated type) often requires less frequent specialized maintenance, although the packing gland may need adjustment or replacement. This statement is debatable and not a definitive advantage for the butterfly valve. * **B) No resistance to cargo flow when open:** This is characteristic of a gate valve. When a gate valve is fully open, the gate is entirely withdrawn, leaving a full bore (pipeline diameter) passage, resulting in very low-pressure drop (minimal resistance). A butterfly valve, even when fully open, has the disc situated permanently in the middle of the flow path, which generates a noticeable pressure drop and turbulence (resistance to flow). * **C) Precise control over cargo flow:** Neither a standard butterfly valve nor a standard gate valve is ideal for precise throttling or control applications. Gate valves are strictly designed for on/off service. While a butterfly valve can be used for throttling, its flow characteristic is generally poor and non-linear at low openings, making dedicated control valves (like globe valves) far superior for precise flow control. Therefore, this is not a major advantage over a gate valve, and in fact, globe valves are better suited for control than butterfly valves.