---
title: "Flow duration curve and unit sizing"
subject: "Power Plant Design"
chapter: 3
key: "semester_8/power_plant_design:fdc-unit-sizing"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/fdc-unit-sizing
site: BCT Engineering
---

# Flow duration curve and unit sizing

Power Plant Design · Chapter 3: Hydro Power Plant Design

- Asked in: 2078 Chaitra, 4 marks
- Video: [HYDROGRAPH,MASSCURVE AND FLOW DURATION CURVE](https://www.youtube.com/watch?v=Xjls-BiLd10) — Asw Education
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/fdc-unit-sizing
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3

## Question

How would flow duration curve affect while designing unit sizes of power plant?

## Short answer

The FDC fixes the design flow and hence plant capacity. Minimum stable flow of one unit must be met at dry-season flow: Francis about 40%, Kaplan 20–30%, Pelton 10–20%. A steep FDC demands more smaller units; a flat FDC allows fewer larger units, with at least two for maintenance.

## Answer

The **flow duration curve (FDC)** shows how long each flow is available, so it decides how many units are needed and how big each should be, so that turbines run near their best efficiency for most of the year.

#### How the FDC affects unit sizing
1. **Total design flow:** the plant's design discharge is taken at a chosen exceedance (e.g. $Q_{45}$). This fixes total capacity $P = \eta \rho g Q H$.
2. **Minimum turbine flow:** every turbine has a minimum stable flow (Francis about 40% of rated, Pelton about 10–20%, Kaplan about 20–30%). The dry-season flow from the FDC (e.g. $Q_{85}$–$Q_{95}$) must be at least the minimum flow of **one** unit, or the plant must shut down in the dry season.
3. **Steep FDC → more units:** if the flow drops far below the design flow for long periods, divide the capacity into two or more units. In the dry season one unit runs near full load instead of all units at poor part load.
4. **Flat FDC → fewer, larger units:** steady flow allows one or two large units, which are cheaper per kW.
5. **Equal-sized units** are preferred for interchangeable spares and simple operation; unequal units are used only when the FDC is very steep.
6. **Reliability and maintenance:** with at least two units, maintenance can be done in the low-flow season while the other unit uses all the available water.

#### Example
If $Q_{45} = 10$ m³/s and $Q_{95} = 2.2$ m³/s, a single Francis unit would need at least $0.4 \times 10 = 4$ m³/s and would stop in the driest months. With two units of 5 m³/s each, one unit needs only 2 m³/s, so it keeps running even at $Q_{95}$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What condition on dry-season flow must hold for unit sizing?
   A. It must exceed Q45
   B. It must be zero
   C. It must equal total design flow
   D. It must be at least the minimum flow of one unit
   Answer: D. Otherwise the plant shuts down in the dry season. (Key point: Minimum flow)

2. What does a steep FDC suggest?
   A. More, smaller units
   B. One large unit
   C. No units
   D. Equal flow all year
   Answer: A. One unit can keep running near full load in the dry season. (Key point: Steep FDC)

3. With Q45 = 10 and Francis minimum 40%, why use two 5 m3/s units?
   A. It needs 4 m3/s so it still stops
   B. One needs only 2 m3/s so it runs even at Q95 = 2.2
   C. Two units remove the minimum
   D. It doubles the head
   Answer: B. The single unit needs 4 m3/s and would stop; a 5 m3/s unit needs 2. (Key point: Example)

## Related in Power Plant Design

- [Flow duration curve and its applicability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aflow-duration-curve) — Power Plant Design, chapter 3, asked 3×
- [Hydrograph and flow duration curve](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydrograph-flow-duration) — Power Plant Design, chapter 3, asked 3×
- [Parameters for planning and sizing a plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aplant-sizing-parameters) — Power Plant Design, chapter 2
- [Tidal versus wind power plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Atidal-vs-wind) — Power Plant Design, chapter 1
- [Design: 95 m head plant at Q65%](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2079-95m-q65) — Power Plant Design, chapter 4, asked 12×
- [Design: 220 m head plant, optimal voltage](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2081-220m-optimal-voltage) — Power Plant Design, chapter 4

## Related in other subjects

- [Load duration curve; diesel plant; ROR flow duration](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aload-duration-diesel-import-fdc) — Hydropower Engineering, chapter 3
- [Flow duration curve and power at mean flow](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-capacity-fdc-flow-table-79) — Hydropower Engineering, chapter 3
- [Power duration curve and primary/secondary energy](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Apower-duration-curve-energy) — Hydro Power, chapter 3
- [Flow duration curve and planning parameters](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afdc-planning-parameters) — Hydro Power, chapter 3