Plant utilities design: for compressed air service in industrial pipelines, what is a typical maximum permissible flow velocity (order-of-magnitude value) used to limit pressure drop and noise while maintaining reasonable line sizes?

Introduction:
Compressed air distribution must balance capital cost (pipe size), operating cost (pressure drop and compressor power), and practical concerns such as noise and erosion at fittings. A rule-of-thumb maximum velocity helps designers choose economical line sizes without detailed iteration at early stages.

Given Data / Assumptions:

• General industrial compressed air networks (not high-pressure gas transmission).
• Acceptable pressure drops over reasonable distances.
• Target is a widely cited maximum permissible velocity.

Concept / Approach:
As velocity increases, frictional pressure drop and acoustic noise rise sharply. Many design guides suggest keeping main line velocities around 6–10 m/s for efficiency, with branches and short runs up to about 20 m/s. The question asks for a typical maximum permissible value—an upper bound used where line lengths are limited or when cost pressure favors smaller pipes.

Step-by-Step Solution:
Relate velocity to pressure drop using Darcy–Weisbach: ΔP ∝ f * (L/D) * (ρ * v^2 / 2).Recognize noise and efficiency concerns above ~20 m/s in plant air lines.Select 20 m/s as a representative maximum often tolerated in practice.

Verification / Alternative check:
Compressed-air handbooks frequently present tables with recommended velocities around 6–10 m/s for mains and up to ~20 m/s for branches; beyond this, pressure drops and noise become problematic.

Why Other Options Are Wrong:

• 5 or 10 m/s: Conservative and common for mains, but the question seeks a maximum permissible order-of-magnitude value.
• 40 or 60 m/s: Excessive for plant air; would cause large pressure losses and noise.

Common Pitfalls:
Applying a single velocity criterion without checking length, roughness, and allowable pressure drop; detailed design should verify with friction calculations.

Final Answer:
20