BME Hydraulic Machines 4 — Questions and Answers
Question 1: The slip in a reciprocating pump is defined as:
- Difference between theoretical and actual discharge expressed as a fraction of theoretical discharge (Correct answer)
- Speed difference between motor and pump shaft
- Pressure difference across suction and delivery valves
- Leakage flow past the piston rings per stroke
Correct answer: Difference between theoretical and actual discharge expressed as a fraction of theoretical discharge
Slip = (Q_theoretical − Q_actual) / Q_theoretical, representing losses due to leakage and valve timing; negative slip is possible if inertia effects increase actual discharge.
Question 2: A Kaplan turbine differs from a propeller turbine in that:
- Its runner blades are fixed, while propeller turbine blades are adjustable
- Its runner blades are adjustable, allowing efficient part-load operation (Correct answer)
- It operates under higher heads than propeller turbines
- It uses impulse action rather than reaction
Correct answer: Its runner blades are adjustable, allowing efficient part-load operation
Kaplan turbines have adjustable (controllable-pitch) runner blades that, combined with adjustable guide vanes, maintain high efficiency over a wide range of flows and heads.
Question 3: Manometric efficiency of a centrifugal pump is the ratio of:
- Actual head developed to Euler's (theoretical) head (Correct answer)
- Shaft power to fluid power
- Flow rate delivered to theoretical displacement volume
- Brake power to indicated power
Correct answer: Actual head developed to Euler's (theoretical) head
Manometric efficiency = H_m / H_Euler, reflecting hydraulic losses (friction, turbulence) between the ideal energy added by the impeller and the actual manometric head.
Question 4: The purpose of a surge tank in a hydropower system is to:
- Increase the head available to the turbine during peak load
- Protect the penstock from water hammer during load changes (Correct answer)
- Filter sediment before water enters the turbine
- Store excess electrical energy as potential energy
Correct answer: Protect the penstock from water hammer during load changes
A surge tank provides a water surface close to the turbine that absorbs pressure transients when turbine load changes, protecting the penstock from water hammer.
Question 5: In the Euler turbomachinery equation, the torque on the runner depends on:
- Change in fluid pressure between inlet and outlet
- Change in the whirl (tangential) component of absolute velocity (Correct answer)
- Change in the axial component of relative velocity
- Total kinetic energy of flow at the runner inlet alone
Correct answer: Change in the whirl (tangential) component of absolute velocity
Euler's equation states T = ρQ(r₁V_w1 − r₂V_w2); only the whirl (tangential) velocity components contribute to angular momentum change and thus torque.
Question 6: Which pump characteristic curve feature indicates a stall or unstable operating region?
- A continuously falling head-flow (H-Q) curve from zero to maximum flow
- A rising portion of the H-Q curve at low flow rates (drooping characteristic) (Correct answer)
- A constant-power curve at all flow rates
- A perfectly flat efficiency curve across the operating range
Correct answer: A rising portion of the H-Q curve at low flow rates (drooping characteristic)
A rising (or humped) H-Q curve at low flows indicates potential instability because two different flow rates can produce the same head, leading to flow oscillation in parallel pump systems.
Question 7: The volumetric efficiency of a pump accounts for:
- Friction losses in the impeller passages
- Internal recirculation and leakage reducing useful discharge (Correct answer)
- Bearing and seal mechanical losses
- Hydraulic losses in the suction pipe
Correct answer: Internal recirculation and leakage reducing useful discharge
Volumetric efficiency = Q_actual / Q_theoretical, capturing losses due to internal leakage past wearing rings and back through clearances rather than hydraulic or mechanical losses.
BME Hydraulic Machines 4
The slip in a reciprocating pump is defined as: