In the hydro world, an intake is the structure used to convey water from a reservoir or head pond to the water conveyance system.
It may also be called the “water intake” or “power intake“.
Intakes must be set deep enough to stop surface vortices from entraining air into the water conveyance. If air enters the water conveyance system, it can accumulate in conduits with a shallow slope, causing a partial flow obstruction and creating head loss. If air passes through the water conveyance system into the turbine, it might create an unbalanced force and damage the turbine.
Intakes are typically fitted with trash racks to stop large debris from entering the water conveyance. When there is a vulnerable population of small fish in the stream or reservoir, a fine wedge-wire screen may be used to exclude fish; this greatly increases the trash rack head loss.
Intakes are usually fitted with two gates:
- a control gate; and
- a guard gate or bulkhead gate, upstream of the control gate, installed under zero flow, to isolate the control gate during inspection or maintenance.
The control gate:
The control gate is designed as an emergency closure gate, i.e. it is designed to close under normal flow rates, and even the very large flow rates that might occur due to a breach in the water conveyance system. The control gate is equipped with a hoist. This is usually a wire rope hoist or a hydraulic hoist. Control gates may be upstream-sealing or downstream sealing. Upstream sealing gates do not suffer from downpull, which can cause gates to oscillate when closing under flow.
An air vent is placed downstream of the control gate, because when the gate closes during flow, the water column can separate downstream of the gate, causing a large air demand. If this air demand is not satisfied, negative pressure develops in the water conveyance. Numerous penstocks on older stations have failed under this negative pressure.
NOTE: The term “control gate” could be misleading – it is NOT used to regulate the flow. Flow regulation is taken care of by the turbines.
The bulkhead gate:
The bulkhead gate might be a single gate or a system of stop logs. The gate is normally provided with a lifting beam that can be set to grapple the top of the stop log or to release it once it lands on top of another stop log in the stop log slot.
Other intake features:
The intake is usually equipped with pressure transducers to measure the reservoir level and the pressure downstream of the trash rack or screen. When the differential pressure across the rack rises, the station control system raises an alarm to indicate that the screen needs to be cleaned.
A by-pass pipe and valve are provided to bypass the control gate for filling the water conveyance. Sometimes a filling valve is installed in the control gate instead. A transducer is also installed downstream of the intake gate to indicate when the water conveyance is full of water.