Hydraulic turbines require a minimum head of water to operate and have several advantages including simple construction, easy controllability, and efficiency. They are classified based on the action of water on moving blades, direction of water flow, position of the shaft, head of water, speed, and specific speed. Impulse turbines have wheel passages not completely filled with water and water acts on buckets at atmospheric pressure, while reaction turbines have fully filled water passages and water acts under greater than atmospheric pressure all around the wheel periphery.