Measuring discharge in very large pipes (diameter > 3 m) Which instrument is generally preferred for measuring flow in very large-diameter water mains (greater than about 3 m)?

Introduction / Context:
Flow measurement in very large pipelines presents practical challenges: installing full-bore primary elements (like venturimeters or orifice plates) becomes costly and intrusive, and differential heads can be large. Velocity-probe methods often become the pragmatic choice for large conduits and tunnels.

Given Data / Assumptions:

• Pipe diameter exceeds roughly 3 m.
• In-situ, pressurized flow of water.
• Desire for a practical, minimally invasive measurement method.

Concept / Approach:

A Pitot tube measures stagnation and static pressure to infer local velocity via Bernoulli. By traversing the cross-section (or using multiport averaging Pitot tubes), the average velocity can be determined and discharge computed as Q = V̄ * A. This approach is widely used in large mains because it avoids full-bore meter installations and large permanent head losses.

Step-by-Step Solution:

Verification / Alternative check:

Where access allows, acoustic (ultrasonic) transit-time systems are also used; however, among the listed classical devices, the Pitot tube best suits very large diameters for practicality and low permanent pressure loss.

Why Other Options Are Wrong:

(b) Venturimeters of that size are expensive and bulky; (c) orifice plates impose significant head loss and plate handling issues at very large sizes; (d) rotameters are for small pipe/line sizes, not large mains.

Common Pitfalls:

Failing to perform adequate section traverses; neglecting Reynolds-number effects on Pitot coefficient; installing near elbows causing distorted velocity profiles.

Final Answer:

pitot tube