Full load is supplied by the turbine shaft when the diameter of jet issuing from the nozzle is 150 mm. If the load suddenly drops to 36% of the full load, the jet diameter to regulate the speed will be
- A75 mm
- B80 mm
- C85 mm
- D90 mmCorrect
Solution
Concept
Power of a Pelton wheel versus jet diameter
Formula
P ∝ Q ∝ d² (constant head and speed)
Step-by-step solution
1. At constant head the jet velocity is constant, so the power is proportional to the discharge, i.e. to d². 2. d₂² / d₁² = 0.36 ⇒ d₂ = 0.6 × 150 = 90 mm.
Explanation
A spear valve governs a Pelton wheel by changing the jet diameter. Since power varies with the area of the jet, 36% of the load needs 60% of the jet diameter.
Why the other options are wrong
75, 80 and 85 mm would give 25%, 28% and 32% of full load.
Shortcut
Load ratio = (d₂/d₁)².
Common mistake
Taking d proportional to the load (54 mm).
Correct answer: D) 90 mm