Question 39 1AE01 - First Assistant Engineer

The Correct Answer is D **Why Option D is Correct:** Excessive pitting on the internal components (like the gears) of a hydraulic pump, particularly after a short period (two months), is a classic symptom of **cavitation erosion**. Cavitation occurs when the absolute pressure of the fluid drops below its vapor pressure. If a "vacuum condition" (meaning very low or negative gauge pressure, approaching a vacuum) develops in the reservoir or, critically, at the pump's inlet line (suction side), the fluid cannot properly fill the spaces between the pump gears. This pressure drop causes dissolved air or oil vapor to flash out of the liquid (forming bubbles). As these bubbles are carried into the high-pressure zone of the pump, they rapidly collapse (implode). This implosion releases tremendous localized energy, physically hammering and eroding the metal surfaces of the gears, leading directly to the characteristic "pitting" damage. **Why Other Options are Incorrect:** * **A) Abnormal pressurization is occurring in the reservoir:** If the reservoir is abnormally pressurized (above atmospheric pressure), this would *increase* the pressure at the pump inlet (suction side). This higher inlet pressure would suppress bubble formation and effectively *prevent* cavitation, thus reducing pitting, not causing it. * **B) Operating oil temperature is determined to be below normal:** Low oil temperature generally increases viscosity. While very high viscosity can sometimes restrict flow slightly and cause noise, it is not the primary mechanism for excessive pitting damage (cavitation requires a pressure drop below vapor pressure). If the oil were excessively hot, it would lower the vapor pressure and increase the likelihood of cavitation, but cold oil does not typically cause this specific pitting damage. * **C) A partial restriction in the return line has developed:** A restriction in the return line (the line returning oil from the system back to the reservoir) would increase the overall system pressure and potentially cause heat, leakage, or component stress. However, it does not affect the *suction pressure* at the pump inlet, which is the mechanism that causes cavitation and pitting.