Hydro Terms

Flow

Refers to the quantity of water supplied from a water source or exiting a nozzle per unit of time. Commonly measured in gallons per minute (gpm).

Francis Turbine

A type of reaction hydro-turbine used in low to medium heads. It consists of fixed vanes on a shaft. Water flows down through the vanes, driving the shaft.

Friction Loss

Lost energy due to pipe friction. In hydro systems, pipe sized too small can lead to serious friction losses.

Head

The difference in elevation between a source of water and the location at which the water from that source may be used (synonym: vertical drop). Expressed in vertical distance or pressure.

Headrace

A flume or channel that feeds water into a hydro turbine.

Hydroelectricity

Any electricity that is generated by the flow of water.

Impulse Turbine

Turbines with runners that operate in air, driven by one or more high-velocity jets of water from nozzles. Typically used with moderate- to high-head systems. Examples include Pelton and turgo.

Intake

The structure that receives the water and feeds it into the penstock (pipeline). Usually incorporates screening or filtering to keep debris and aquatic life out of the system.

Pelton Wheel

A common impulse turbine runner (named after inventor Lester Pelton) made with a series of cups or “buckets” attached to a hub.

Penstock

The pipe in a hydro system that carries the water from the intake to the turbine.

Pipe Loss (Frictional Head Loss)

The amount of energy or pressure lost due to friction between a flowing liquid and the inside surface of a pipe.

Pressure

The “push” behind liquid or gas in a tank, reservoir, or pipe. Water pressure is directly related to “head”—the height of the top of the water over the bottom. Every 2.31 feet of vertical head gives 1 psi (pound per square inch) of water pressure.

Reaction Turbine

Turbines with the runner fully immersed in water, typically used in low- to moderate-head systems with high flow. Examples include Francis, propeller, and Kaplan.

Runner

The wheel that receives the water, changing the pressure and flow of the water to circular motion to drive an alternator, generator, or machine.

Tailrace

The pipe, flume, or channel in a hydroelectric system that carries the water from the turbine runner back to the stream or river.

Trash Rack

A strainer at the input to a hydro system. Used to remove debris from the water before it enters the pipe.

Turgo

A type of impulse hydro runner optimized for lower heads and higher volumes than a Pelton runner.