Draft tube – A draft tube is one important part of a turbine, which is used to transform water into energy. It's found within the piping system in jets, dams, or anywhere else where turbines help do difficult mechanical work. Turbines need to have a minimum amount of water to propel them in order to produce enough energy. Without these tubes, the pressure could drop because of lack of water, and in turn, the entire turbine could fail to work and power could be lost.
The primary purpose of the draft tube is to monitor the influx of water. The turbine has something called a "tail race," which is the lowest portion of a dam or construction project where water pools. The tube connects the turbine to this tail race, permitting the turbine to be outside of the water but still have access to the water.
Source:
http://en.wikipedia.org/wiki/Draft_tube
