Draft tube functions and inlet pressure

A draft tube is a gradually expanding pipe or passage that connects the runner exit of a reaction turbine to the tailrace, with its outlet always submerged below the tailwater level.

Functions

1. Recovers kinetic energy: the increasing area reduces the high exit velocity, converting kinetic energy into pressure energy instead of wasting it.
2. Creates a negative head (suction) at the runner exit, so the turbine can be installed above the tailwater level without losing head, and the full head $H$ is used.
3. Allows turbine to be set above tailwater for easy inspection and maintenance.
4. Increases turbine output and efficiency (net head on the runner increases).
5. Discharges water safely to the tailrace with low exit velocity.

Pressure at inlet of the draft tube is below atmospheric

     runner
   ===[  ]===   (1) inlet, p1, V1
       \  /        |
        \/         | Hs (above TWL)
 ~~~~~~~~|~~~~~~~~~|~~~~ TWL
        |  |       | y
        |__| (2) outlet, V2

Let $H_s$ = height of draft tube inlet above the tailwater, $y$ = depth of outlet below tailwater, $V_1, V_2$ = velocities at inlet and outlet, $h_f$ = loss in the draft tube, $p_a$ = atmospheric pressure. Take the datum at section 2. Bernoulli between 1 and 2:

$$\frac{p_1}{\rho g} + \frac{V_1^2}{2g} + (H_s + y) = \frac{p_2}{\rho g} + \frac{V_2^2}{2g} + 0 + h_f$$

At the outlet, $\dfrac{p_2}{\rho g} = \dfrac{p_a}{\rho g} + y$. Substituting:

$$\begin{aligned} \frac{p_1}{\rho g} + \frac{V_1^2}{2g} + H_s + y &= \frac{p_a}{\rho g} + y + \frac{V_2^2}{2g} + h_f \\ \frac{p_1}{\rho g} &= \frac{p_a}{\rho g} - H_s - \left(\frac{V_1^2 - V_2^2}{2g} - h_f\right) \end{aligned}$$

Since $H_s > 0$ and $V_1 > V_2$ with $\dfrac{V_1^2 - V_2^2}{2g} > h_f$ (the draft tube recovers more energy than it loses), the bracketed term is positive. Hence

$$\frac{p_1}{\rho g} < \frac{p_a}{\rho g}$$

i.e. the pressure at the draft tube inlet (runner exit) is less than atmospheric (a vacuum). This suction is the gain in head given by the draft tube. It must not fall below the vapour pressure, otherwise cavitation occurs; this limits $H_s$ (Thoma's criterion).