--- title: "Discharge exceedance and turbine criterion" subject: "Power Plant Design" chapter: 3 key: "semester_8/power_plant_design:exceedance-turbine-criterion" source: https://www.bctengineer.com/q/semester_8/power_plant_design/exceedance-turbine-criterion site: BCT Engineering --- # Discharge exceedance and turbine criterion Power Plant Design · Chapter 3: Hydro Power Plant Design - Asked in: 2072 Magh, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/exceedance-turbine-criterion - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3 ## Question What do you mean by discharge exceedance? Describe the criterion for selecting turbine in hydro-electric power plant. ## Short answer Discharge exceedance is percentage of time a flow is equalled or exceeded, read from flow duration curve; ROR plants often designed at Q40-Q45. Turbine selection criteria: net head, specific speed, discharge variation, part-load efficiency, cavitation, sediment, cost. ## Answer #### Discharge exceedance **Discharge exceedance** is the percentage of time a given river discharge is **equalled or exceeded**. It is read from the **flow duration curve (FDC)**. For example, $Q_{45} = 10$ m³/s means the river flow is 10 m³/s or more for 45% of the time (about 164 days a year). - Low exceedance (e.g. $Q_{25}$) → large flow, available for a short time. - High exceedance (e.g. $Q_{95}$) → small but dependable (firm) flow. - In Nepal, ROR projects are usually designed at about $Q_{40}$–$Q_{45}$; firm power is judged from $Q_{90}$–$Q_{95}$. - Exceedance is found by ranking the recorded flows in descending order and computing $P = \dfrac{m}{n+1}\times 100\%$ for rank $m$ out of $n$ values. - The choice of design exceedance is a trade-off: a lower exceedance gives more installed capacity and energy but a lower plant factor and higher cost per kW. ```text Q |\ | \ | \___ Q45 | \_____ Q65 | \______ Q95 +---------------------> % exceedance ``` #### Criteria for selecting turbine 1. **Net head:** Pelton for high head (50–1300 m), Francis for medium head (10–350 m), Kaplan/propeller for low head (2–40 m). Crossflow (Michell–Banki) and Turgo are used in small plants. 2. **Specific speed:** $N_s = N\sqrt{P}/H^{5/4}$; Pelton 10–35, Francis 60–300, Kaplan 300–1000 (metric, kW). 3. **Discharge and its variation:** if flow varies greatly, choose a turbine with good part-load efficiency (Pelton, Kaplan) or use several units. 4. **Part-load operation:** Francis efficiency falls below about 40–50% load; Pelton remains efficient down to 20%. 5. **Rotational speed:** higher speed gives smaller generator; limited by cavitation and runaway speed. 6. **Cavitation:** Thoma's coefficient fixes the setting below tailwater for reaction turbines. 7. **Sediment and abrasion:** important for Himalayan rivers. 8. **Cost, efficiency, maintenance and local experience.** #### Example For a site with $H_{net} = 250$ m and $Q_{45} = 6$ m³/s split into two units of 3 m³/s, unit power $\approx 9.81 \times 3 \times 250 \times 0.9 \approx 6.6$ MW. At 750 rpm, $N_s = 750\sqrt{6622}/250^{5/4} \approx 61$, at the boundary of multi-jet Pelton and slow Francis; the Pelton is preferred if the river is silty or long part-load running is expected. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does Q45 = 10 m3/s mean? A. Flow is exactly 10 m3/s for 45 days B. Flow is 10 m3/s or more for 45% of the time C. Flow is below 10 m3/s for 45% of the time D. The average annual flow is 45 m3/s Answer: B. Exceedance is the percentage of time a flow is equalled or exceeded. (Key point: Definition) 2. Which exceedance is used to judge firm power? A. Q10-Q20 B. Q40-Q45 C. Q90-Q95 D. Q1 Answer: C. High exceedance means small but dependable flow. (Key point: Firm flow) 3. How is exceedance computed from ranked flows? A. P = n/m x 100% B. P = (n+1)/m C. P = m x n D. P = m/(n+1) x 100% Answer: D. Flows are ranked descending and rank m out of n gives the exceedance. (Key point: Exceedance formula) 4. If river flow varies greatly, which turbines are preferred? A. Pelton or Kaplan, for good part-load efficiency B. Fixed-blade propeller C. Francis only, at a single unit D. Any turbine, since part-load does not matter Answer: A. Pelton and Kaplan keep efficiency high over a wide load range. 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