Hydraulic Machines MCQ with answer and solution

Concept

Cavitation is avoided by maintaining the pressure at the suction eye above the vapor pressure of the liquid, which is mathematically expressed as NPSH_available > NPSH_required.

Formula

NPSH_available = (P_atm / ρg) - (P_v / ρg) - h_s - h_fs

Step-by-step solution

1. Identify the condition for cavitation: Cavitation occurs when the local pressure drops below the vapor pressure (P_v). 2. Define NPSH: Net Positive Suction Head is the difference between the absolute suction head and the vapor pressure head. 3. Analyze the requirement: For safe operation, NPSH_available must be greater than NPSH_required. 4. Evaluate options: Increasing NPSH_available (by increasing suction head or reducing losses) directly prevents the pressure from dropping to the vapor pressure limit.

Explanation

Cavitation occurs when the pressure at the pump inlet falls below the vapor pressure of the liquid, causing bubbles to form and collapse. To prevent this, the Net Positive Suction Head (NPSH) available from the system must be greater than the NPSH required by the pump. Increasing the NPSH available ensures that the liquid remains in the liquid phase at the impeller eye.

Why the other options are wrong

A: Increasing speed increases the required NPSH and decreases local pressure, making cavitation more likely. B: Delivery head affects the pump's discharge point but does not directly prevent cavitation at the suction side. D: Decreasing suction pipe length actually increases NPSH available (by reducing friction loss), so while this is technically a way to increase NPSH, 'Increasing NPSH available' is the more direct and comprehensive engineering answer covering all methods.