Consider the following statements regarding draft tube of Francis turbine : 1. It permits a suction head to be established at the runner exit. 2. It makes possible to install the turbine above the tail race level without loss of head. 3. It converts a large proportion of velocity energy rejected from the runner into useful pressure energy. Which of the above statements is/are correct ?
- A1 and 2 only
- B2 and 3 only
- C3 only
- D1, 2 and 3Correct
Solution
Concept
Functions of a draft tube
Step-by-step solution
1. The diverging tube lowers the pressure at the runner exit below atmospheric (suction head) (true). 2. It lets the turbine be set above tail water without losing the head (true). 3. It recovers kinetic energy as pressure energy (true).
Explanation
The draft tube is a gradually expanding tube that does all three jobs.
Why the other options are wrong
Each of the other options leaves out a true statement.
Shortcut
Draft tube = suction + height + KE recovery.
Common mistake
Leaving one out.
Correct answer: D) 1, 2 and 3