The 'cavitation' damage to hydraulic turbines and pump impellers appears as:
Choose the correct answer
Uniform smooth wear over entire blade
Pitting (small craters) from implosion shockwaves on blade surfaces — progressive material loss changes blade profile
Corrosion from dissolved salts
Thermal expansion cracks
Correct Answer
B. Pitting (small craters) from implosion shockwaves on blade surfaces — progressive material loss changes blade profile
Why is this the answer?
Cavitation damage: implosion of vapour bubbles on solid surface releases high-pressure shockwaves + micro-jets → pitting (small craters) on metal surface. Progressive pitting → loss of material → change in blade profile → reduced efficiency → vibration. Cavitation index σ = (Hatm − Hvapour − Hs)/H used to assess risk. Prevented by: limit suction head, stainless steel or cavitation-resistant material, coatings.
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