Question

The allowable maximum negative head on an average is equal to __________

(a) 2.5 m or so

(b) 5 m or so

(c) 7.5 m or so

(d) 10 m or so

I got this question in a job interview.

I want to ask this question from Shaft Spillway in division Spillways, Energy Dissipators and Spillway Gates of Irrigation Engineering

Answer 1

The correct answer is (c) 7.5 m or so

The explanation: The vertical distance from the crown of the siphon down to the discharging point or HGL should not exceed a value of 7.5 m under average conditions. This is necessary to avoid cavitation.

Limiting vaccum pressure = Atmospheric pressure – Vapour pressure = 10 – 2.5 = 7.5 m.