Siphons – pressure requirement to avoid cavitation For a siphon to operate satisfactorily, the minimum pressure anywhere in the pipe should be __________ the vapour pressure of the liquid.

Introduction / Context:
A siphon conveys liquid over a summit higher than the source free surface by maintaining continuity of liquid and pressure below atmospheric at the crest. Cavitation risk constrains the allowable minimum pressure within the siphon.

Given Data / Assumptions:

• Incompressible liquid with known vapour pressure p_vap.
• Barometric pressure sufficiently high to sustain the siphon column.
• Negligible dissolved gas release for this conceptual check.

Concept / Approach:
To avoid cavitation (formation of vapour cavities), the local absolute pressure must not fall to the vapour pressure. Therefore, the minimum absolute pressure in the siphon must be greater than p_vap. At the summit, pressure is lowest due to elevation head and head losses.

Step-by-Step Solution:

Verification / Alternative check:
Practical design limits the summit elevation so the computed p_abs remains above p_vap even after accounting for head losses and temperature-dependent vapour pressure.

Why Other Options Are Wrong:

• Equal to/less than vapour pressure invites cavitation and siphon failure.
• “Independent of” is untrue; vapour pressure sets the limiting condition.
• “Twice” is arbitrary and not a governing criterion.

Common Pitfalls:
Ignoring temperature effects on vapour pressure or neglecting head losses, leading to underestimation of cavitation risk at the crest.

Final Answer:
more than