--- title: "Head, specific speed and part load in turbine choice" subject: "Power Plant Design" chapter: 3 key: "semester_8/power_plant_design:turbine-head-speed-part-load" source: https://www.bctengineer.com/q/semester_8/power_plant_design/turbine-head-speed-part-load site: BCT Engineering --- # Head, specific speed and part load in turbine choice Power Plant Design · Chapter 3: Hydro Power Plant Design - Asked in: 2072 Asoj, 8 marks - Video: [GOVERNING OF TURBINES | Selection of Turbines | Hydraulic Turbines| Centrifugal Governors | PPE](https://www.youtube.com/watch?v=GfCYTl-BU7A) — Amizhdhagam - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/turbine-head-speed-part-load - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3 ## Question Discuss the effect of the following factors in selecting of a turbine for a hydroelectric plant? (i) head (ii) speed and specific speed (iii) part load operation. ## Short answer Head sets turbine type: Pelton high (above ~300 m), Francis medium (30–300 m), Kaplan low (2–40 m). Specific speed combines N, P, H and fixes runner shape; choose highest synchronous speed satisfying cavitation and runaway limits. Part-load: Pelton/Kaplan have flat efficiency; Francis peaks at 80–90% and is poor below 40–50%, so use multiple units. ## Answer Turbine selection depends mainly on head, speed/specific speed and the expected part-load operation. #### (i) Head Head is the first and most important factor because it decides the velocity of water. | Head range | Suitable turbine | |---|---| | High (above ~300 m, up to 1300 m) | Pelton (impulse) | | Medium (30–300 m) | Francis (reaction) | | Low (2–40 m) | Kaplan / propeller | | Small plants, 3–250 m | Crossflow, Turgo | - High head gives high jet velocity; an impulse turbine with small flow suits it. - Low head needs large flow to give the same power, so a large flow-area axial runner (Kaplan) is used. - Head variation also matters: Kaplan copes with varying head better than propeller. #### (ii) Speed and specific speed $$ N_s = \frac{N\sqrt{P}}{H^{5/4}} $$ - Specific speed joins head, power and speed into one value that fixes the runner shape: Pelton 10–35, Francis 60–300, Kaplan 300–1000. - A **higher running speed** gives a smaller, cheaper generator (fewer poles, $N = 120f/p$), so designers prefer the highest speed that is safe. - But higher $N_s$ means a higher Thoma cavitation coefficient, so the runner must be set lower (more excavation), and the runaway speed is higher. - Therefore the speed is chosen as the highest synchronous speed for which cavitation and runaway limits are satisfied. #### (iii) Part-load operation ROR plants often run at part load in the dry season, so the shape of the efficiency curve matters. ```text eff | ____________ Pelton/Kaplan (flat) | / ___ | / / \__ Francis (peaked) |/ / \__ Propeller (sharp) +--------------------> % load 0 25 50 75 100 ``` - **Pelton:** efficiency stays high from about 20% to 100% load (flow controlled by needle; multiple jets can be shut). - **Kaplan:** adjustable runner blades and guide vanes keep high efficiency over a wide range. - **Francis:** best efficiency near 80–90% of rated load; falls quickly below about 40–50%, with vibration and cavitation. - **Propeller (fixed blade):** very poor at part load. If part-load running is expected, choose Pelton/Kaplan, or use several smaller Francis units so each runs near full load. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which turbine suits high head above about 300 m? A. Kaplan B. Pelton C. Propeller D. Francis Answer: B. High head gives high jet velocity and small flow, suiting an impulse turbine. (Key point: Head ranges) 2. What is the benefit of a higher running speed? A. Lower cavitation risk B. Lower runaway speed C. Smaller, cheaper generator with fewer poles D. Greater head Answer: C. N = 120f/p, so higher speed needs fewer poles. (Key point: Higher speed) 3. What is the drawback of higher specific speed? A. Larger generator B. Lower jet velocity C. Lower efficiency at full load only D. Higher Thoma coefficient, so deeper setting and higher runaway speed Answer: D. Cavitation and runaway limits cap the speed chosen. (Key point: Cavitation limit) 4. How does Francis behave at part load? A. Efficiency falls quickly below about 40-50% load, with vibration B. It stays flat down to 20% load C. It improves at low load D. It is best at 20% load Answer: A. Francis is best near 80-90% of rated load. (Key point: Francis part load) ## Related in Power Plant Design - [Specific speed of a turbine](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aspecific-speed-purpose) — Power Plant Design, chapter 3, asked 2× - [Design: 300 m head plant, GCB rating](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2081-300m-gcb) — Power Plant Design, chapter 4, asked 11× - [Design: 230 m head plant, 55 km line](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2074-230m-55km) — Power Plant Design, chapter 4, asked 12× - [Design: 310 m head plant, HV breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2080-310m-hvcb) — Power Plant Design, chapter 4, asked 3× - [Design: 330 m head plant, 132 kV](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2082-330m-q45) — Power Plant Design, chapter 4, asked 12× - [Design: 120 m head plant, 33 kV](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2079-120m-33kv) — Power Plant Design, chapter 4, asked 3× ## Related in other subjects - [Model testing and specific speed of turbine](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aturbine-model-similarity-numerical) — Fluid Machines, chapter 3 - [Specific speed of a turbine](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Aspecific-speed-turbine-derivation) — Fluid Machines, chapter 3 - [Types of hydropower plants by head, turbine, load](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Ahydropower-plant-types-classification) — Hydropower Engineering, chapter 2 - [Choice of specific loadings in DC machine](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Adc-specific-loadings-selection) — Electric Machine Design, chapter 5, asked 2×