Francis Turbine Specific Speed & Cavitation Setting Calculator

Determine kinematic specific speed (nq), Thoma cavitation parameter (σ_c), and draft tube setting height to prevent runner blade pitting.

Francis Operating Parameters

Design Net Head H (m):

Turbine Rated Flow Q (m³/s):

Generator Speed N (RPM):

Site Tailrace Elevation (m a.s.l.):

Water Temperature (°C):

Cavitation Safety Margin (SF): USBR recommended 1.10 - 1.25

Specific Speed & Runner Elevation

Metric Specific Speed (n_q):

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Power Specific Speed (N_s, kW):

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Thoma Cavitation Factor (σ_plant):

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Atmospheric Pressure Head (H_atm):

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Max Allowable Suction Head (H_s):

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Runner Centerline Setting:

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Francis Turbine Cavitation & Runner Setting

In reaction turbines, local pressure at the suction side of runner blade exit edges can drop below the saturated vapor pressure of water, precipitating cavitation vapor bubbles. When these bubbles wash into higher-pressure zones downstream, they violently collapse against runner surfaces, causing severe metal erosion and vibration.

Draft Tube Setting Height (Hs)

The runner centerline elevation relative to downstream tailwater is governed by Thoma's cavitation equation: $$H_s \le \frac{P_{atm} - P_{vap}}{\rho g} - \sigma_{crit} H$$ For higher specific speed runners, $H_s$ is often negative, requiring deep excavation to place the powerhouse runner several meters below minimum tailwater elevation.

Francis Turbine Specific Speed & Cavitation Setting Calculator

Francis Operating Parameters

Specific Speed & Runner Elevation

Francis Turbine Cavitation & Runner Setting

Draft Tube Setting Height (Hs)

Frequently Asked Questions