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.
Mechanical Engineering
Hydraulic Machines
Difficulty: Easy
Choose an option
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ATo run the turbine casing completely full of water
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BTo prevent air from entering the turbine casing
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CTo increase the effective head by recovering the elevation difference between runner outlet and tail race (and part of exit kinetic energy)
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DTo convey water to the downstream channel only, without affecting head
Answer
Correct Answer: To increase the effective head by recovering the elevation difference between runner outlet and tail race (and part of exit kinetic energy)
Explanation
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
The draft tube converts a part of the exit kinetic energy into static pressure (pressure recovery) and allows the runner to be set above tail race while still utilising that elevation difference as useful head.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).