Head, specific speed and part load in turbine choice

Turbine selection depends mainly on head, speed/specific speed and the expected part-load operation.

(i) Head

Head is the first and most important factor because it decides the velocity of water.

• High head gives high jet velocity; an impulse turbine with small flow suits it.
• Low head needs large flow to give the same power, so a large flow-area axial runner (Kaplan) is used.
• Head variation also matters: Kaplan copes with varying head better than propeller.

(ii) Speed and specific speed

$$N_s = \frac{N\sqrt{P}}{H^{5/4}}$$

• Specific speed joins head, power and speed into one value that fixes the runner shape: Pelton 10–35, Francis 60–300, Kaplan 300–1000.
• A higher running speed gives a smaller, cheaper generator (fewer poles, $N = 120f/p$), so designers prefer the highest speed that is safe.
• But higher $N_s$ means a higher Thoma cavitation coefficient, so the runner must be set lower (more excavation), and the runaway speed is higher.
• Therefore the speed is chosen as the highest synchronous speed for which cavitation and runaway limits are satisfied.

(iii) Part-load operation

ROR plants often run at part load in the dry season, so the shape of the efficiency curve matters.

 eff
  |   ____________ Pelton/Kaplan (flat)
  |  /    ___
  | /    /   \__ Francis (peaked)
  |/    /       \__ Propeller (sharp)
  +--------------------> % load
  0   25  50  75  100

• Pelton: efficiency stays high from about 20% to 100% load (flow controlled by needle; multiple jets can be shut).
• Kaplan: adjustable runner blades and guide vanes keep high efficiency over a wide range.
• Francis: best efficiency near 80–90% of rated load; falls quickly below about 40–50%, with vibration and cavitation.
• Propeller (fixed blade): very poor at part load.

If part-load running is expected, choose Pelton/Kaplan, or use several smaller Francis units so each runs near full load.