BME Hydraulic Machines 5 — Questions and Answers
Question 1: Priming a centrifugal pump is required because:
- Air in the pump casing creates enough pressure head to start flow
- The pump cannot develop sufficient pressure to expel air and start flow unaided (Correct answer)
- The impeller must be lubricated before starting to prevent wear
- The motor requires a reduced-load startup to avoid overheating
Correct answer: The pump cannot develop sufficient pressure to expel air and start flow unaided
Centrifugal pumps cannot self-prime because air is much less dense than liquid; the pressure developed by spinning air is insufficient to overcome the static head and initiate liquid flow.
Question 2: The runaway speed of a hydraulic turbine is the speed at:
- Which mechanical efficiency reaches its maximum value
- Which the turbine operates if the load is suddenly disconnected with full flow maintained (Correct answer)
- Which cavitation first appears in the draft tube
- Which the governor closes the wicket gates completely
Correct answer: Which the turbine operates if the load is suddenly disconnected with full flow maintained
Runaway speed is the maximum overspeed reached when the external load is lost but flow continues; turbine design must ensure structural integrity at this speed.
Question 3: In series pump operation, compared to a single pump:
- The total flow rate is doubled at the same head
- The total head is increased while flow rate remains approximately the same (Correct answer)
- Both head and flow rate double proportionally
- Efficiency improves because losses are shared between pumps
Correct answer: The total head is increased while flow rate remains approximately the same
Pumps in series add their heads at the same flow rate, making this configuration suitable for high-head, moderate-flow applications.
Question 4: The pressure coefficient (ψ) in pump similarity analysis is defined as:
- ψ = gH / (N²D²) (Correct answer)
- ψ = Q / (ND³)
- ψ = P / (ρN³D⁵)
- ψ = ρND² / μ
Correct answer: ψ = gH / (N²D²)
The head (pressure) coefficient ψ = gH/(N²D²) is a dimensionless group relating the developed head to the square of tip speed, used in pump similarity and scaling.
Question 5: A centrifugal pump operating far to the right of its Best Efficiency Point (BEP) will likely experience:
- Recirculation at the impeller inlet and cavitation (Correct answer)
- Increased efficiency and reduced NPSHR
- Higher developed head with stable, quiet operation
- Reduced bearing loads due to lower pressure forces
Correct answer: Recirculation at the impeller inlet and cavitation
At flows much higher than BEP, NPSHA may become insufficient relative to the elevated NPSHR, causing cavitation; inlet recirculation also develops at very high flows.
Question 6: The speed ratio (φ) of a Pelton wheel at maximum efficiency is approximately:
- 0.26 (bucket tip speed ≈ 26% of jet velocity)
- 0.46 (bucket tip speed ≈ 46% of jet velocity) (Correct answer)
- 0.70 (bucket tip speed ≈ 70% of jet velocity)
- 1.00 (bucket tip speed equals jet velocity)
Correct answer: 0.46 (bucket tip speed ≈ 46% of jet velocity)
Theoretical maximum efficiency for a Pelton wheel occurs when bucket speed is 0.5 × jet velocity, but friction reduces the optimum speed ratio to approximately 0.46 in practice.
Question 7: Which dimensionless number is most relevant for determining the onset of cavitation in a hydraulic machine?
- Reynolds number
- Froude number
- Thoma's cavitation number (σ) (Correct answer)
- Weber number
Correct answer: Thoma's cavitation number (σ)
Thoma's cavitation coefficient σ = NPSHA / H relates available suction head to the machine head; when σ falls below the critical value σ_c, cavitation begins.
BME Hydraulic Machines 5
Priming a centrifugal pump is required because: