If a coarse screening/bar rack screen is 30% clogged, what is the effect on velocity and head?

When a coarse screening or bar rack screen is 30% clogged, the flow area through the screen is reduced due to the obstruction created by the clogging. This reduction in flow area has a direct impact on the behavior of fluid passing through the screen.

As the clogging increases, the amount of water that can flow through the screen decreases, which causes the velocity of the water entering the screen to increase as it constricts to pass through the available openings. This increased velocity occurs because of the principle of conservation of mass, where the same flow rate must be maintained. Therefore, when the cross-sectional area decreases due to clogging, the velocity must increase to allow water to flow through the smaller opening.

On the aspect of head loss, as the velocity increases due to the clogging, the friction losses also increase because of the interaction between the high-velocity flow and the obstructions. Consequently, this results in a higher energy loss, causing a decrease in the total head available downstream of the screen.

Thus, the correct answer reflects that with increased velocity due to a reduced flow area, there is a corresponding decrease in the available head, highlighting the relationship between velocity, flow area, and head loss in hydraulic systems.