Draft tube purpose in reaction turbines Explain the primary function of the draft tube fitted at the outlet of a reaction turbine runner operating with a tail race below the runner elevation. Choose the correct statement.

Given / Concept

• A draft tube is a gradually enlarging passage fitted at the runner exit of a reaction turbine.
• The tail race level is below the runner outlet level.

Approach / Why draft tube

Key relationsEffective head gained ≈ elevation difference (runner outlet to tail race) + portion of velocity head recovered in the diffuser-like draft tube.

Common pitfallsThinking it merely 'transports' water. Its main duty is head recovery and permitting the runner to run full under sub-atmospheric pressure at outlet.

Final AnswerIt increases the effective head by an amount about equal to the runner outlet elevation above the tail race (plus some recovered velocity head).