Water-supply engineering: Where should an air inlet (air-release/air-vacuum) valve be located on a pressurised water main to prevent air locking during filling and to admit air during draining or negative pressures?

Introduction / Context:
Air valves in water-distribution mains are critical appurtenances. They release entrapped air during filling and operation, admit air during draining or column separation, and thus prevent air locking, surges, and inaccurate flow capacity.

Given Data / Assumptions:

• Pressurised (closed-conduit) water main.
• Normal operational needs: filling, draining, and transient protection.
• Typical appurtenance layout with sluice (isolation) valves.

Concept / Approach:

Air accumulates at local high points where velocity and pressure patterns allow gas pockets to collect. During draining or sudden flow reversal, air must enter to avoid vacuum and collapse. Locating air valves at summits and just downstream of isolating valves provides proper release/admission during switching operations.

Step-by-Step Solution:

Verification / Alternative check:

Standard waterworks guidelines and manufacturer siting diagrams show combination air valves at high points and immediately downstream of isolating devices, and sometimes near pump discharges.

Why Other Options Are Wrong:

• (a) alone omits operational needs near valves.
• (b) upstream of a sluice is less effective for draining a downstream isolated reach.
• (d) valleys collect less air; vacuum risk is different.

Common Pitfalls:

• Placing valves only at summits but not near isolations.
• Underestimating vacuum risks during rapid shut-off.

Final Answer:

Both (a) and (c).