Control valves — application limits:\nNeedle valves are precise throttling devices. In which service are they generally not used?

Introduction / Context:
Control valves regulate flow, pressure, or level. Needle valves feature a slender, tapered plug and fine-pitch stem threads, delivering precise metering of small flow rates. Their geometry dictates where they excel and where they do not.

Given Data / Assumptions:

• Needle valves provide small flow coefficients (Cv) and excellent resolution.
• They are typically constructed in small sizes (fractional to about 1–2 inches nominal).
• System may require high turndown and leak-tight shutoff at low flows.

Concept / Approach:
The practicality of a needle design diminishes as pipe size increases because the long, tapered stem would need to accommodate very large flow passages, becoming mechanically unwieldy and causing excessive pressure drop or poor controllability. For large lines, globe, ball, butterfly, or segmental ball valves are preferred depending on service.

Step-by-Step Reasoning:

Verification / Alternative check:
Standards and catalogs list needle valves primarily up to small sizes; large sizes are rare and impractical.

Why Other Options Are Wrong:

• High-pressure precise throttling, accurate low-flow control, small-bore lines, and sampling: typical needle valve applications.

Common Pitfalls:
Confusing needle with globe valves; assuming the same trim scale applies to all line sizes.

Final Answer:
Very large diameter pipes