Siphon with air vessel at the summit — purpose of the vessel In a siphon line, an air vessel is provided at the summit primarily to:

Introduction / Context:
Siphons can experience very low pressure at the summit, risking air separation or cavitation that breaks the continuous liquid column. Fitting an air vessel (or vacuum breaker/air chamber) near the summit helps stabilize operation and prevents flow interruption.

Given Data / Assumptions:

• Siphon conveys liquid over a summit higher than the free surfaces.
• Risk: pressure at the summit may drop below vapor pressure, causing separation.
• Device: air vessel installed at or near the summit.

Concept / Approach:
The air vessel acts as a cushion and helps maintain continuity by smoothing pressure fluctuations and preventing the absolute pressure from falling too low. It also allows entrained air to collect and be managed so that the siphon does not lose its prime.

Step-by-Step Solution:
Recognize the failure mode: separation at low summit pressure.Introduce air vessel to stabilize pressure and avoid loss of prime.Primary function: avoid interruption and maintain continuous flow.Hence choose “avoid interruption in the flow.”

Verification / Alternative check:
Practical siphon designs specify maximum allowable elevation of the summit; adding a vessel reduces pressure swings and sustains operation, which confirms the purpose.

Why Other Options Are Wrong:

• Increase discharge/velocity: discharge is governed by head difference and losses; the vessel is not a pump.
• Maintain fixed pressure difference: while it moderates fluctuations, it does not fix a specific Δp like a regulator.

Common Pitfalls:

• Confusing air vessels in reciprocating pumps (which reduce pulsations) with siphon summit vessels; both smooth flow but serve different systems.

Final Answer:
avoid interruption in the flow