When a valve plug tapers from wide to narrow from top to bottom, how must the plug travel to close?

The correct answer highlights the operational mechanics of a tapered valve plug. When the valve plug tapers from a wider top to a narrower bottom, it is designed so that as the plug moves downwards, it effectively covers the opening of the valve seat, thereby sealing it off. This downward movement increases the surface contact area against the seat, ensuring a tight closure.

Conversely, to open the valve, the plug needs to move upward, retracting from the valve seat. This upward motion allows fluid to flow through the now-open valve passage.

Understanding this relationship between the direction of the plug's movement and the valve's operation is crucial for effective control in instrumentation applications. The design of the valve plug plays a significant role in both its sealing capability and its responsiveness in the system.