BME Hydraulic Machines 3 — Questions and Answers
Question 1: The hydraulic efficiency of a turbine is defined as:
- Ratio of shaft power output to water power available at inlet
- Ratio of runner power to water power at inlet (Correct answer)
- Ratio of electrical output to mechanical input
- Ratio of actual head to net head across the machine
Correct answer: Ratio of runner power to water power at inlet
Hydraulic efficiency is the ratio of the power transferred to the runner to the water power supplied, accounting for hydraulic losses in guide vanes, runner passages, and draft tube.
Question 2: Affinity laws for centrifugal pumps state that at constant impeller diameter, flow rate varies as:
- Square of speed ratio
- Speed ratio to the first power (Correct answer)
- Cube of speed ratio
- Square root of speed ratio
Correct answer: Speed ratio to the first power
The first affinity law states Q ∝ N, so doubling the rotational speed doubles the flow rate when diameter is held constant.
Question 3: A hydraulic accumulator stores energy in the form of:
- Kinetic energy of a rotating flywheel
- Potential energy of pressurized fluid or elevated weight (Correct answer)
- Chemical energy in a pressurized gas reaction
- Thermal energy in a heated fluid reservoir
Correct answer: Potential energy of pressurized fluid or elevated weight
Hydraulic accumulators store energy as pressure potential energy — either via a compressed gas bladder, a weighted piston, or an elevated column of fluid.
Question 4: The volute casing of a centrifugal pump is designed to:
- Increase fluid velocity before it reaches the impeller
- Convert kinetic energy of the impeller discharge into pressure energy (Correct answer)
- Filter debris from the entering fluid stream
- Cool the motor by circulating fluid around the casing
Correct answer: Convert kinetic energy of the impeller discharge into pressure energy
The volute's gradually increasing cross-sectional area decelerates the high-velocity fluid leaving the impeller, converting kinetic energy to pressure (diffusion).
Question 5: Which parameter does NOT change when two geometrically similar pumps operate at homologous conditions?
- Flow rate
- Head developed
- Specific speed (Correct answer)
- Power consumed
Correct answer: Specific speed
Specific speed is a similarity parameter that remains constant for geometrically similar pumps operating at homologous (dynamically similar) points on their performance curves.
Question 6: In a Francis turbine, the guide vanes (wicket gates) are adjustable primarily to:
- Change the direction of the tailrace flow
- Control the flow rate and thus regulate power output (Correct answer)
- Prevent cavitation at the runner inlet
- Align the draft tube with the runner exit
Correct answer: Control the flow rate and thus regulate power output
Adjustable wicket gates vary the angle and quantity of water entering the runner, allowing the turbine output to be regulated to match load demand.
Question 7: The term 'water hammer' in hydraulic systems refers to:
- Mechanical impact of floating debris on pipe walls
- Pressure surge caused by rapid change in flow velocity (Correct answer)
- Erosion of pipe interiors by high-velocity sediment
- Resonant vibration of pump impellers at critical speed
Correct answer: Pressure surge caused by rapid change in flow velocity
Water hammer is a pressure transient (shock wave) produced when flow is suddenly stopped or redirected, with pressure spikes potentially many times the normal operating pressure.
BME Hydraulic Machines 3
The hydraulic efficiency of a turbine is defined as: