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Draft Tube Efficiency Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Based on IEC 60193 hydraulic turbine acceptance testing. Last verified 14 June 2026.

The draft tube converts the kinetic energy of the water leaving a hydraulic turbine runner into pressure energy, effectively extending the available head and increasing plant efficiency. This calculator computes draft tube hydraulic efficiency from the inlet and outlet velocity heads and the hydraulic loss head within the tube. It also computes the available kinetic energy at the runner exit and the energy recovered.

Runner exit velocity = draft tube inlet velocity. Typical range 5 to 12 m/s.
Tailwater exit velocity. Should be much less than V1 for good efficiency. Typically 1 to 3 m/s.
Total head loss (friction + separation + exit loss) in the draft tube. Estimated from CFD or model testing.
-- m
-- m
-- m
-- %

Draft tube efficiency formula (IEC 60193)

hv1 = V12 / (2g) (inlet velocity head)
hv2 = V22 / (2g) (outlet velocity head)
hr = hv1 - hv2 - hf (recovered head)
etaDT = hr / hv1 = 1 - (V2/V1)2 - hf/hv1

Where: V1 = draft tube inlet velocity (m/s), V2 = draft tube outlet velocity (m/s), hf = total head loss in draft tube (m), g = 9.81 m/s^2. The recovered head hr is added to the plant net head, increasing turbine output.

Design targets for draft tubes

  • Well-designed elbow draft tubes (most common for Francis turbines) achieve efficiency of 80 to 90%.
  • Straight conical draft tubes (Kaplan turbines in low head sites) achieve 85 to 95% efficiency due to gentler diffusion angle.
  • Recommended half-angle of expansion: 3 to 5 degrees for straight conical draft tubes to avoid flow separation.
  • IEC 60193:2019 defines the model test procedures used to determine draft tube performance coefficients.

Draft tube efficiency calculator: frequently asked questions

What is a draft tube in a hydraulic turbine?

A draft tube is a diverging conduit connecting the turbine runner exit to the tailwater. Its purpose is to recover kinetic energy from the water leaving the runner by decelerating it and converting velocity head to pressure head. A well-designed draft tube can recover 70 to 90% of the kinetic energy at the runner exit, significantly improving overall plant efficiency.

How is draft tube efficiency defined?

Draft tube efficiency (eta_DT) = (V1^2/2g - V2^2/2g - hf) / (V1^2/2g), where V1 is the inlet velocity, V2 is the outlet velocity, hf is the hydraulic loss head in the draft tube, and g = 9.81 m/s^2. It measures what fraction of the available kinetic energy at the runner exit is successfully recovered. Typical values are 0.75 to 0.90 for modern draft tubes.

What is the pressure recovery coefficient Cp?

The pressure recovery coefficient Cp = (P2 - P1) / (0.5 * rho * V1^2), where P1 and P2 are the static pressures at inlet and outlet (Pa), rho is water density, and V1 is the inlet velocity. A perfect draft tube with no losses would have Cp = 1 - (V2/V1)^2 (Borda-Carnot ideal). The ratio of actual to ideal Cp is related to draft tube efficiency.

What are typical inlet velocities for draft tube design?

Draft tube inlet velocities (= runner exit velocities) typically range from 5 to 12 m/s for Francis turbines. The Thoma cavitation coefficient sigma and the available net positive suction head determine the allowable draft head (submergence). IEC 60193 provides the methods for model testing and field acceptance testing of hydraulic turbines.

How does draft tube design affect cavitation?

The draft tube creates a low-pressure region at the runner exit. If this pressure falls below the vapour pressure of water, cavitation occurs. The plant sigma (sigma_plant = Ha - Hs / Hn, where Ha is atmospheric head, Hs is draft head, Hn is net head) must exceed the critical sigma from model tests. IEC 60193 and the manufacturer's model test results define the cavitation limits.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.