Draft tube: function and types

A draft tube is a gradually expanding tube fitted at the exit of the runner of a reaction turbine; its lower end is submerged in the tailwater.

Functions

1. The runner can be placed above the tailwater level for easy maintenance without losing the head between the runner and the tailrace.
2. It converts the large kinetic energy at the runner exit into pressure energy (by the diverging area), so the water is released to the tailrace at low velocity.
3. It creates a pressure below atmospheric at the runner exit. This increases the pressure drop across the runner and hence the effective head.

Head gain (theory)

Let $H_s$ be the height of the runner exit (section 1) above the tailwater surface, $V_1$ the velocity at the runner exit, $V_2$ the velocity at the draft tube outlet (section 2), and $h_f$ the loss in the tube. The outlet is submerged, so the pressure there is atmospheric plus the depth of submergence; this depth cancels against the extra elevation difference. Applying Bernoulli's equation between sections 1 and 2:

$$\frac{p_1}{\rho g}+\frac{V_1^2}{2g}+H_s=\frac{p_{a}}{\rho g}+\frac{V_2^2}{2g}+h_f$$ $$\frac{p_1}{\rho g}=\frac{p_a}{\rho g}-H_s-\left(\frac{V_1^2-V_2^2}{2g}-h_f\right)$$

So the pressure at runner exit is below atmospheric by $H_s$ plus the recovered kinetic energy $(V_1^2-V_2^2)/2g - h_f$. The efficiency of the draft tube is

$$\eta_d=\frac{(V_1^2-V_2^2)/2g-h_f}{V_1^2/2g}$$

Also $H_s$ must be limited (about 5 to 6 m) so that the pressure at the runner exit does not fall to the vapour pressure, which would cause cavitation.

Types

 Conical      Simple elbow    Moody spreading   Elbow with
 (straight)                   (bell mouth)      varying section
   \  /          |  |           \      /          |   \
    \/           |  |_____       \    /           |    \_____
  (vertical)     \________       \__/ (hollow      \________
                 horizontal      center, flared)