Question

The head loss in the flow of water through a penstock pipe of given length ____________

(a) increases with the increase in flow velocity

(b) decreases with the increase in flow velocity

(c) decreases with the increase in the roughness of the pipe surface

(d) decreases with time

I got this question during an interview.

I'm obligated to ask this question of Pressure Conduites in chapter Pressure Conduits, Outlet Works through Dams and Rivers of Irrigation Engineering

Answer 1

The correct choice is (a) increases with the increase in flow velocity

To explain I would say: The head loss (HL) by pipe friction is given by Darcy-Weisbach equation –

HL = f’. V^2. L / 2gd where L is the length of pipe in meters, d is the diameter of the pipe, V is the velocity of the pipe and f’ is the friction factor which depends on Reynold’s number and the relative roughness of the pipe.

It is clear from the given equation that the head loss is directly proportional to the length of the pipe hence the head loss increases with the increase in the length of the pipe.