BME Hydraulic Machines 2 — Questions and Answers
Question 1: What is cavitation in a centrifugal pump?
- Formation and collapse of vapor bubbles due to local pressure drop below vapor pressure (Correct answer)
- Blockage of impeller by solid particles
- Reverse flow through the pump at low speeds
- Air entrainment through the suction flange
Correct answer: Formation and collapse of vapor bubbles due to local pressure drop below vapor pressure
Cavitation occurs when local pressure falls below the liquid's vapor pressure, causing vapor bubbles that implode violently and damage pump components.
Question 2: The Net Positive Suction Head Available (NPSHA) must be:
- Less than NPSHR to avoid cavitation
- Equal to NPSHR for optimum efficiency
- Greater than NPSHR to prevent cavitation (Correct answer)
- Independent of NPSHR
Correct answer: Greater than NPSHR to prevent cavitation
NPSHA must exceed NPSHR (required) by a sufficient margin to ensure local pressures never drop to vapor pressure and cavitation does not occur.
Question 3: In a Pelton wheel turbine, the jet of water strikes:
- Curved reaction blades submerged in water
- Cup-shaped buckets tangentially (Correct answer)
- A spiral casing that increases pressure
- Diffuser vanes at the turbine exit
Correct answer: Cup-shaped buckets tangentially
A Pelton wheel is an impulse turbine where high-velocity jets strike cup-shaped (double-hemispherical) buckets tangentially, converting kinetic energy to mechanical work.
Question 4: The specific speed of a turbine is used to:
- Calculate the mass flow rate at rated conditions
- Classify turbine type and predict performance similarity (Correct answer)
- Determine the shaft material strength requirements
- Estimate bearing lubrication intervals
Correct answer: Classify turbine type and predict performance similarity
Specific speed is a dimensionless (or dimensional) similarity parameter that classifies turbine types and allows performance prediction across geometrically similar machines.
Question 5: Which hydraulic turbine is best suited for high head and low flow rate conditions?
- Kaplan turbine
- Francis turbine
- Pelton wheel (Correct answer)
- Propeller turbine
Correct answer: Pelton wheel
The Pelton wheel operates efficiently under very high heads (hundreds to thousands of meters) with relatively low volumetric flow rates.
Question 6: The draft tube in a reaction turbine primarily serves to:
- Increase the flow velocity entering the runner
- Recover kinetic energy at runner exit and allow turbine placement above tailrace (Correct answer)
- Filter sediment from the water before it enters the turbine
- Reduce the rotational speed of the runner
Correct answer: Recover kinetic energy at runner exit and allow turbine placement above tailrace
The draft tube decelerates the flow leaving the runner, converting kinetic energy back to pressure energy, and enables the turbine to be installed above the tailwater level.
Question 7: In a double-acting reciprocating pump, fluid is discharged during:
- Only the forward stroke
- Only the return stroke
- Both the forward and return strokes (Correct answer)
- Neither stroke — discharge is continuous via accumulators only
Correct answer: Both the forward and return strokes
A double-acting reciprocating pump has cylinders on both sides of the piston, so discharge occurs during both strokes, giving more uniform flow than a single-acting design.
BME Hydraulic Machines 2
What is cavitation in a centrifugal pump?