---
title: "Desander and its design criteria"
subject: "Hydro Power"
chapter: 5
key: "semester_6/hydro_power:desander-design-criteria"
source: https://www.bctengineer.com/q/semester_6/hydro_power/desander-design-criteria
site: BCT Engineering
---

# Desander and its design criteria

Hydro Power · Chapter 5: Components of Hydropower System

- Asked in: 2079 Chaitra, 1+5 marks; 2082 Kartik, 5 marks; 2075 Baisakh, 5 marks; 2071 Magh, 5 marks (asked 4 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/desander-design-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5

## Question

What is desander? Discuss its design criteria.

## Short answer

A desander is a settling basin to remove suspended sand by slowing flow. It removes particles 0.1-0.5 mm with ≥90% trap efficiency at velocity 0.2-0.4 m/s, using Camp's Vc=a√d. Surface area A=Q/w, L/B 4-10, two chambers, flushing slope 1:20-1:50, spillway and gradual transitions.

## Answer

#### Desander
A **desander (settling basin)** is a structure placed after the intake (and gravel trap) of a hydropower scheme, in which the flow is slowed down so that suspended sand particles larger than a design size settle and are flushed out, protecting the headrace and turbines from abrasion.

```text
         +---------------------------+
 from -->| inlet  | settling zone  | outlet |--> headrace
 intake  | trans. |  V = 0.2-0.4   | weir   |
         +--------+----------------+--------+
                    \____ sediment ____/ -> flushing
```

#### Design criteria
1. **Design particle size:** 0.2 mm for medium heads; 0.1–0.15 mm for high-head Pelton plants; up to 0.5 mm for low heads. Smaller size for hard quartz sediment.
2. **Trap efficiency:** 90% or more of the design particle size should be removed.
3. **Flow velocity:** horizontal velocity below critical (scouring) velocity, by Camp's formula $V_c = a\sqrt{d}$ (cm/s, d in mm; a = 44 for 0.1 < d < 1 mm). Typical 0.2–0.4 m/s.
4. **Fall velocity:** from Stokes' law or standard curves (e.g. about 0.021 m/s for 0.2 mm quartz at 20 °C).
5. **Dimensions:**
   - Surface area $A = BL = Q/w$ for ideal settling.
   - Length $L = \dfrac{hV}{w}$, corrected for turbulence (Camp or Vetter's equation $\eta = 1 - e^{-wA/Q}$).
   - Depth 3–5 m (plus sediment storage depth); $L/B$ about 4–10.
6. **Transitions:** gradual inlet expansion (1:5 to 1:8) and outlet contraction to give uniform flow with no dead zones; guide walls/baffles if needed.
7. **Number of chambers:** at least two parallel chambers so the plant runs during flushing or repair.
8. **Flushing arrangement:** continuous or intermittent hydraulic flushing with gates/pipes; bed slope about 1:20–1:50; enough head to return sediment to the river.
9. **Spillway/overflow:** to release excess water.
10. **Location:** stable ground, close to the intake, with space for flushing return.

## Check yourself

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

1. A desander is best described as:
   A. A structure after the intake where flow is slowed so sand above a design size settles and is flushed
   B. A tank that stores water for peak load
   C. A device that removes floating logs only
   D. A pipe that increases flow speed
   Answer: A. It protects headrace and turbines by settling out sand larger than the design size. (Key point: What a desander is)

2. Camp's formula for the limiting horizontal velocity is:
   A. Vc = a/d
   B. Vc = sqrt(2gh)
   C. Vc = a sqrt(d), with a = 44 for 0.1-1 mm
   D. Vc = a d^2
   Answer: C. V = a sqrt(d) cm/s with d in mm, a = 44 for 0.1 < d < 1 mm. (Key point: Camp's critical velocity)

3. Why are at least two parallel chambers provided?
   A. So the plant keeps running while one chamber is flushed or repaired
   B. To reduce L/B below 1
   C. To avoid needing a spillway
   D. To double the velocity
   Answer: A. One chamber works while the other is flushed or maintained. (Key point: Number of chambers)

4. Which design particle size is typical for high-head Pelton plants?
   A. 1-2 mm
   B. 5 mm
   C. 0.1-0.15 mm
   D. Any size above 0.5 mm
   Answer: C. High-head Pelton plants need finer sediment removed (0.1-0.15 mm); low heads allow up to 0.5 mm. (Key point: Design particle size)

## Related in Hydro Power

- [Surge tank functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-functions-design) — Hydro Power, chapter 5, asked 2×
- [Settling basin design principle and components](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-principle-components) — Hydro Power, chapter 5

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