Pumps, Turbines & Dam Spillways
Centrifugal pump curves and cavitation, turbine selection by head, and the spillway/sedimentation design issues of dam and hydropower interviews.
Key formulas
Ns = N·√Q / H^(3/4) (Q in m³/s, H in m, N in rpm)
Pump specific speed: Pelton-range Ns ≈ 10–30 (in turbine form); pumps: radial < ~30, mixed 30–80, axial > 80 (metric).
P = η · γ · Q · H (γ = 9810 N/m³)
Power from water: 1 m³/s at 100 m head with η = 0.9 gives ≈ 883 kW — the quick hydropower estimate interviewers expect.
Cheat sheet
- ▸Centrifugal pump: energy given as head H ∝ N²; BHP = γQH/η. The system curve (static lift + friction) intersects the pump curve at the operating point.
- ▸Specific speed Ns = N√Q / H^(3/4) picks the type: low Ns → radial centrifugal (high head), mixed flow, high Ns → axial (very low head, high Q).
- ▸Pumps in series add heads (high-head systems); pumps in parallel add flows (large Q, modest head). Series risks operation left of the 'unstable' zone.
- ▸Cavitation: local pressure falls below vapour pressure → bubbles collapse violently, pitting impellers. Guard with NPSH_available > NPSH_required; raise suction level/shorten suction line.
- ▸Turbine selection: Pelton (impulse, very high head >250 m, jet on bucket wheel), Francis (reaction, medium head 30–300 m, mixed radial-axial), Kaplan (reaction, low head <60 m, high discharge, adjustable blades).
- ▸Draft tube (reaction turbines) recovers kinetic energy at runner exit by diffusing velocity into pressure, letting the turbine sit above tailwater.
- ▸Dam spillways: ogee (smooth S-shaped crest matching the nappe), siphon, chute/ski-jump; energy dissipated via hydraulic jump basin, flip bucket, or roller bucket. Gates: radial, vertical lift, breast walls.
- ▸Siltation and sluice design: settling velocity decides sediment traps; flush sluices near the base keep the dead storage alive.
Turbine selection by site head
| Aspect | Pelton | Francis | Kaplan |
|---|---|---|---|
| Type | Impulse | Reaction (mixed flow) | Reaction (axial flow) |
| Head range | Very high (>250 m) | Medium (30–300 m) | Low (<60 m) |
| Discharge | Low–moderate | Moderate | Very high |
| Flow control | Needle valve + deflector | Wicket gates | Wicket gates + adjustable blades |
| Cavitation risk | Almost none (atmospheric jet) | Moderate | High — needs deep setting |
Top 4 campus interview questions
MCQ mastery quiz
1. A very high head (500 m) site with low discharge is best served by a…
2. Pumps connected in series are used when the system needs…
3. Cavitation in a pump occurs when local pressure drops below…
4. The draft tube of a reaction turbine mainly…
5. An ogee spillway crest is shaped so that…