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Hydraulic Machines Flashcards

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  1. Priming a centrifugal pump is required because:

    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.

  2. The runaway speed of a hydraulic turbine is the speed at:

    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.

  3. In series pump operation, compared to a single pump:

    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.

  4. The pressure coefficient (ψ) in pump similarity analysis is defined as:

    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.

  5. A centrifugal pump operating far to the right of its Best Efficiency Point (BEP) will likely experience:

    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.

  6. The speed ratio (φ) of a Pelton wheel at maximum efficiency is approximately:

    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.

  7. Which dimensionless number is most relevant for determining the onset of cavitation in a hydraulic machine?

    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.