Desander and its design criteria

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.

         +---------------------------+
 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.