Draft Tube is used in Reaction Turbine to

1. Pass water downstream without eddies formation
2. Convert the kinetic energy into pressure energy by the gradual expansion of the flow cross-section
3. Provide safety to the turbine
4. Prevent water splitting

Draft tube is used in reaction turbine to Convert the kinetic energy into pressure energy by the gradual expansion of the flow cross-section

Draft Tube

The draught tube connects the runner exit to the tailrace. It is a diverging tube. The fluid exits a turbine runner with noticeable kinetic energy; in order to recover some of this kinetic energy that would otherwise be lost, the fluid enters a draught tube, an expanding area diffuser. It only works with reaction turbines.

The drafting tube serves the following two objectives:

1. It enables the establishment of a negative or suction head at the runner exit, allowing the turbine to be installed above the tailrace level without experiencing a loss head.
2. It is utilised to transform kinetic energy into pressure energy at the turbine’s outlet.

To get the lowest possible output velocity, a draught tube is made divergent. As a result, a draught tube is essentially a diffuser and must be appropriately designed with the angle between the tube walls being limited to roughly 8 degrees in order to prevent the flow from separating from the wall and to minimise the energy loss in the tube.