Siphon hydraulics: What is the maximum practical vacuum (height of water column) that can be created at the summit of a siphon under standard atmospheric conditions?

Introduction / Context:

A siphon lifts liquid over a summit by creating sub-atmospheric pressure at the highest point, sustained by atmospheric pressure at the source reservoir. The summit height is limited by cavitation and vapor pressure; exceeding this limit causes the liquid to vaporize and the siphon to break.

Given Data / Assumptions:

• Standard atmospheric pressure approximately equivalent to a 10.3 m water column at sea level.
• Negligible friction and minor losses for the limiting argument.
• Water at typical temperatures; vapor pressure non-negligible near the limit.

Concept / Approach:

The theoretical limit is when absolute pressure at the summit tends to the vapor pressure of water. Subtracting vapor pressure head and allowing safety margin yields a practical maximum around 7–8 m of water column. Textbook design values commonly cite ~7.6 m; examination options often round to 7.4 m.

Step-by-Step Solution:

Verification / Alternative check:

Field practice rarely exceeds ~7 m to ensure reliability, aligning with the selected option.

Why Other Options Are Wrong:

• 2.7 m and 5.5 m are too conservative for the physical limit.
• 10.3 m is a theoretical atmospheric head ignoring vapor pressure and losses; siphons cannot sustain this in practice.
• “None” is incorrect because a well-known practical limit exists.

Common Pitfalls:

• Confusing theoretical atmospheric head with practical siphon summit height.

Final Answer:

7.4 m of water