--- title: "Draft tube objectives and governors" subject: "Hydro Power" chapter: 7 key: "semester_6/hydro_power:draft-tube-objectives-governors" source: https://www.bctengineer.com/q/semester_6/hydro_power/draft-tube-objectives-governors site: BCT Engineering --- # Draft tube objectives and governors Hydro Power · Chapter 7: Hydro-Electric Machines - Asked in: 2070 Magh - Video: [Draft tube,function of draft tube and types of draft tube](https://www.youtube.com/watch?v=YIEdO63BvgQ) — BEST MECHANICAL ENGINEERING - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/draft-tube-objectives-governors - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-7 ## Question What are the objectives of the provision of draft tube for the reaction turbine? Write notes on the working principle of governors. ## Short answer A draft tube recovers kinetic energy at runner exit, creates suction head to avoid head loss, and allows setting turbine above tailwater for easier maintenance. A governor maintains constant speed by varying flow: a fly-ball sensor detects speed change, moving a control valve that sends pressure oil to a servomotor to adjust guide vanes or spear, with feedback to prevent hunting. ## Answer #### Objectives of a draft tube for a reaction turbine A **draft tube** is a diverging pipe from the runner exit of a reaction turbine to the tailrace, with its outlet submerged. It is provided: 1. **To recover kinetic energy** at the runner exit by reducing velocity, converting it into pressure energy. 2. **To create a suction head** (pressure below atmospheric) at the runner exit, so that the head between the runner and tailwater is not lost. 3. **To allow the turbine to be set above tailwater** for easy access, inspection and repair without loss of head. 4. **To increase the net working head, efficiency and output** of the turbine. 5. **To discharge water to the tailrace** with low velocity, preventing scour. Types: conical (straight divergent), elbow with varying section, simple elbow, Moody spreading tube. Its depth is limited by cavitation (Thoma's criterion). #### Working principle of governors A **governor** automatically keeps the turbine speed constant by matching the water supply to the load. A conventional **oil-pressure (hydraulic) governor** has: - **Speed sensor:** centrifugal (fly-ball) governor or electronic speed transducer driven from the turbine shaft. - **Control (relay/distributor) valve:** directs pressure oil to either side of a servomotor piston. - **Servomotor:** a cylinder and piston that moves the guide vanes (Francis/Kaplan) or the spear and deflector (Pelton). - **Oil sump, pump and pressure accumulator:** supply oil under pressure. - **Feedback (restoring) mechanism / dashpot:** prevents over-correction and hunting. ```text turbine shaft -> fly-ball -> lever -> control valve | oil v guide vanes / spear <---- servomotor piston ^ | |____ feedback linkage ___| ``` **Sequence (load decreases):** 1. The turbine speed rises; fly-balls move outward and the sleeve rises. 2. The lever moves the control valve so that pressure oil enters one side of the servomotor. 3. The servomotor piston moves and **closes the guide vanes** (or pushes the spear forward), reducing discharge. 4. The speed returns to normal; the feedback linkage brings the control valve back to the neutral position. When the load increases, the reverse happens and the gates open. Modern plants use **electro-hydraulic or digital (PID) governors**, which sense frequency electronically but still use oil servomotors to move the gates. For Pelton turbines a **jet deflector** acts first on sudden load rejection to avoid water hammer, while the spear closes slowly. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is an objective of the draft tube in a reaction turbine? A. To create suction head at runner exit so the runner-to-tailwater head is not lost B. To increase the exit velocity C. To control the speed D. To admit air to the runner Answer: A. Suction head at the exit allows setting above tailwater without losing head. (Key point: Draft tube objectives) 2. What is the role of the servomotor in a hydraulic governor? A. Sense the speed B. A piston that moves the guide vanes or spear using pressure oil C. Store oil D. Prevent hunting Answer: B. The control valve directs oil to the servomotor, whose piston moves the gates. (Key point: Governor components) 3. When the load decreases, what does the governor do? A. Opens the guide vanes B. Closes the guide vanes (or advances the spear) to reduce discharge C. Increases speed setting D. Stops oil flow permanently Answer: B. Speed rises, the sensor moves the valve and the servomotor closes the gates until speed returns to normal. (Key point: Load decrease sequence) 4. What is the purpose of the feedback (restoring) linkage or dashpot? A. To raise the speed B. To bring the control valve back to neutral, preventing over-correction and hunting C. To pump oil D. To measure head Answer: B. Without feedback the gates would overshoot and the speed would oscillate (hunting). (Key point: Feedback mechanism) ## Related in Hydro Power - [Why draft tube is used in Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-francis) — Hydro Power, chapter 7 - [Draft tube function and turbine selection criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-turbine-selection) — Hydro Power, chapter 7 - [Draft tube functions and inlet pressure](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adraft-tube-pressure-inlet) — Hydro Power, chapter 7, asked 2× - [Conical draft tube inlet pressure and efficiency](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aconical-draft-tube-numerical) — Hydro Power, chapter 7 - [Impulse and reaction turbine performance characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aimpulse-reaction-performance) — Hydro Power, chapter 7 - [Working principle of centrifugal and reciprocating pumps](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Acentrifugal-reciprocating-working) — Hydro Power, chapter 7 ## Related in other subjects - [Draft tube: purpose, setting and efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adraft-tube-purpose-setting-equation) — Hydropower Engineering, chapter 7 - [Francis overall efficiency with draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-efficiency-32mw) — Hydropower Engineering, chapter 7 - [Max height of straight draft tube (Francis)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afrancis-draft-tube-max-height-11760kw) — Hydropower Engineering, chapter 7 - [Functions of draft tube](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adraft-tube-functions) — Hydropower Engineering, chapter 7