Question 9

The Correct Answer is A. ### Why Option A ("close regulation of flow") is correct: A needle valve is a type of globe valve designed for extremely fine flow adjustment. Its name comes from the sharp, tapered, needle-like point on the disc, which fits precisely into a corresponding seat. This design creates a small orifice that changes gradually as the valve stem is turned. Because the effective flow area changes very slowly relative to the stem movement, the needle valve allows for precise, repeatable, and **close regulation of flow**, making it ideal for metering applications, maintaining specific flow rates in small pipes, and throttling. ### Why the other options are incorrect: * **B) no pressure drops:** All valves, by their nature of creating resistance (throttling), introduce pressure drops into the system. Needle valves, specifically designed to resist flow for fine control, inherently cause significant pressure drops. * **C) no back flow:** While needle valves are designed to control forward flow, they are not specialized check valves. Preventing backflow (reverse flow) requires a dedicated check valve, which operates automatically based on pressure differential. A needle valve, like most manual valves, simply blocks flow when closed, but it is not the recommended device specifically for preventing backflow. * **D) high-pressure drops:** While needle valves **do** cause high-pressure drops (as explained in B), this is a consequence of their primary function, not the reason they are *recommended*. They are recommended for their ability to achieve **close regulation** (A). If the goal were simply a high-pressure drop without requiring fine control, a cheaper, more robust valve (like a standard globe valve or an orifice plate) might be used.