Hydraulic Machines MCQ with answer and solution

Concept

Specific speed classification hierarchy of hydraulic turbines

Formula

N_s = (N × √(P)) / H^5/4

Step-by-step solution

1. Turbine specific speed is given by N_s = (N × √(P)) / H^5/4, where H is operating head. 2. Pelton wheels operate under very high head (H > 250 m), giving the lowest specific speed (10 to 50). 3. Francis turbines operate under medium head (60 m to 250 m), giving intermediate specific speed (60 to 300). 4. Kaplan turbines operate under low head (H < 60 m), giving the highest specific speed (300 to 1000). 5. Therefore, the correct descending order is Kaplan > Francis > Pelton. Answer: A

Explanation

Specific speed (Ns = N × √(P) / H^5/4) represents turbine geometry independent of size. Pelton wheels operate under high heads and low flows, having the lowest specific speed (10-50). Francis turbines operate under medium heads (60-300). Kaplan turbines operate under low heads and high flow rates, possessing the highest specific speed (300-1000). Thus, the correct descending order is Kaplan > Francis > Pelton.

Why the other options are wrong

B) Francis has a higher specific speed than Pelton, so Pelton cannot precede Francis. C) Francis has a lower specific speed than Kaplan, so Francis cannot precede Kaplan. D) Completely inverts the relationship; Pelton has the lowest specific speed.

Shortcut

Specific speed ranking: Kaplan (300-1000) > Francis (60-300) > Pelton (10-50).

Common mistake

Inverting the order by ranking Pelton highest due to confusing high operating head with high specific speed.