--- title: "Layout of a medium head Francis station" subject: "Power Plant Design" chapter: 3 key: "semester_8/power_plant_design:francis-medium-head-layout" source: https://www.bctengineer.com/q/semester_8/power_plant_design/francis-medium-head-layout site: BCT Engineering --- # Layout of a medium head Francis station Power Plant Design · Chapter 3: Hydro Power Plant Design - Asked in: 2079 Jestha, 10 marks; 2072 Asoj, 8 marks (asked 2 times) - Video: [Francis Turbine: Working Principles, Components, Advantages, and Disadvantages](https://www.youtube.com/watch?v=8X-FwurAhOw) — E-TECH - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/francis-medium-head-layout - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3 ## Question Sketch a typical layout of a medium head hydroelectric power station fitted with Francis turbine. Briefly describe the main elements. ## Short answer Medium-head plants use a Francis turbine with a layout: reservoir, intake, headrace tunnel, surge tank to absorb water hammer, penstock, spiral casing, guide vanes, Francis runner, draft tube to recover kinetic energy, generator, tailrace. Surge tank confines pressure surges to the penstock. ## Answer A **medium head** station (roughly 30–300 m head) normally uses a **Francis turbine**, a mixed-flow reaction turbine. Water reaches the turbine under pressure through a long waterway and penstock, and leaves through a draft tube to the tailrace. #### Typical layout ```text Reservoir / pond ~~~~~~~~~~~~~~~~~~~~~ | Dam | Intake + trash rack | |====================\ Headrace tunnel |_______| \ [Surge tank] | | Penstock | Valve --> +-------v------+ | Spiral casing| | Guide vanes | | Francis | | runner |---Generator +------+-------+ | Draft tube v Tailrace --> River ``` #### Main elements 1. **Reservoir or pondage and dam:** stores water and creates part of the head. A spillway passes floods. 2. **Intake with trash rack and gates:** admits water and keeps out debris; gates allow the waterway to be dewatered. 3. **Headrace tunnel / pressure conduit:** carries water to the surge tank with a gentle slope; it is often lined to reduce friction loss. 4. **Surge tank:** an open vertical shaft at the end of the tunnel. When the turbine gates close suddenly, the rising water level in the tank absorbs the water hammer; when load increases, it supplies water until the tunnel flow speeds up. It keeps water hammer confined to the short penstock. 5. **Penstock:** steel pipe from the surge tank to the powerhouse; designed for static head plus water hammer. Anchor blocks and expansion joints are provided. 6. **Main inlet valve:** usually a butterfly valve at medium head; closes in emergency. 7. **Spiral (scroll) casing:** distributes water evenly around the runner; its area decreases along the flow so the velocity stays uniform. 8. **Stay vanes and guide vanes (wicket gates):** stay vanes support the casing; guide vanes, moved by the governor, control the flow and direct it onto the runner at the correct angle. 9. **Francis runner:** water enters radially and leaves axially; both pressure and kinetic energy are converted into torque. Medium specific speed (about 60–300, metric, P in kW). 10. **Draft tube:** a diverging tube from runner exit to tailwater. It recovers kinetic energy and creates suction head so the runner can be set above tailwater; its setting is limited by cavitation (Thoma's coefficient). 11. **Generator:** vertical-shaft synchronous generator directly coupled to the runner; its speed is fixed by $N = 120f/p$. 12. **Governor and excitation system:** control speed (frequency) and voltage. 13. **Powerhouse:** surface or underground; contains units, EOT crane, control room and auxiliaries. 14. **Tailrace:** channel that returns water to the river. 15. **Transformer and switchyard:** step up the voltage and connect to the transmission line. #### Example Kaligandaki "A" (144 MW, about 115 m net head) and Middle Marsyangdi (70 MW) in Nepal use vertical Francis units with this type of layout. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When turbine gates close suddenly, the surge tank: A. Lets the water level rise to absorb water hammer, keeping it confined to the short penstock B. Drains the tunnel C. Opens the draft tube D. Raises the generator voltage Answer: A. The rising level absorbs the pressure surge; on load increase it supplies water. (Key point: Surge tank action) 2. The spiral casing has a decreasing area along the flow so that: A. Water velocity stays uniform around the runner B. Pressure rises to infinity C. The runner speeds up D. Sediment settles Answer: A. Area reduces as flow is delivered, keeping uniform velocity. (Key point: Spiral casing) 3. Guide vanes (wicket gates) are moved by the governor to: A. Support the casing B. Control flow and direct it onto the runner at the right angle C. Recover kinetic energy D. Measure head Answer: B. Stay vanes support the casing; guide vanes control flow. (Key point: Guide vanes) 4. In a Francis runner water enters and leaves: A. Axially and radially B. Radially and axially C. Tangentially and radially D. Axially and axially Answer: B. It is a mixed-flow turbine: radial inlet, axial outlet. (Key point: Francis runner flow) 5. The draft tube setting is limited by: A. Cavitation (Thoma's coefficient) B. Generator poles C. Transformer rating D. Trash rack size Answer: A. Too high a setting lowers pressure and causes cavitation. (Key point: Draft tube limit) ## Related in Power Plant Design - [Components of a medium head high discharge plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amedium-head-high-discharge-components) — Power Plant Design, chapter 3, asked 2× - [Sketch of a medium head PROR plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Apror-medium-head-sketch) — Power Plant Design, chapter 3, asked 2× - [Head, specific speed and part load in turbine choice](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aturbine-head-speed-part-load) — Power Plant Design, chapter 3 - [Design: 245 m head plant, standby generator](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2073-245m-standby) — Power Plant Design, chapter 4, asked 14× - [Design: 250 m head plant at Q25%](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2072-250m-q25) — Power Plant Design, chapter 4, asked 14× - [Design: 200 m head plant, bus MVA levels](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2070-200m-bus-mva) — Power Plant Design, chapter 4, asked 14× ## Related in other subjects - [Powerhouse layout with Francis turbine](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apowerhouse-layout-francis) — Hydro Power, chapter 5 - [Powerhouse layout with vertical axis Francis turbine](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apowerhouse-layout-vertical-axis-francis) — Hydropower Engineering, chapter 8, asked 3× - [Francis turbine design (55 m³/s, 25 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afrancis-design-55) — Hydro Power, chapter 7 - [Francis turbine design (45 m³/s, 150 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afrancis-design-45) — Hydro Power, chapter 7