Draft tube is a pipe of gradually increasing cross-section that connects the outlet of turbine runner to the tailrace. It is used for discharging the water from exit of a reaction turbine to the tail pool and is provided only for reaction turbines eg. Francis turbine. Its cross section gradually expands and also changes its shape along its length from circular at inlet to the rectangular at the end. A draft tube plays an important role in optimizing the performance and efficiency of turbine.
Working Principle of Draft Tube
Applying Bernoulli's equation between runner exit (1-1) and draft tube outlet (2-2):
$$z_1+\frac{p_1}{\gamma}+\frac{v_1^2}{2g}=z_2+\frac{p_2}{\gamma}+\frac{v_2^2}{2g}+h_f$$
$$or, (H_s+h)+\frac{p_1}{\gamma}+\frac{v_1^2}{2g}=(\frac{p_{atm}}{\gamma}+h)+\frac{v_2^2}{2g}+h_f$$
$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-H_s-\frac{v_1^2-v_2^2}{2g}+h_f$$
$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-(H_s+\frac{v_1^2-v_2^2}{2g})+h_f \tag{1}$$
where,
$H_s$ = Static suction head
$\frac{v_1^2-v_2^2}{2g}$ = Dynamic suction head
$h_f = k\frac{v_1^2-v_2^2}{2g}$
$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-(H_s+\frac{v_1^2-v_2^2}{2g}-h_f )$$
$$or, \frac{p_1}{\gamma}=\frac{p_{atm}}{\gamma}-\left[H_s+(1-k)\frac{v_1^2-v_2^2}{2g}\right]\tag{2}$$
Now, draft tube efficiency can be written as:
$${\eta}_d=\frac{\text{Actual regain of pressure head}}{\text{Velocity head at entrance of draft tube}}$$
$$ =\frac{v_1^2-v_2^2}{2g}-h_f$$
$$=(1-k)\frac{v_1^2-v_2^2}{2g}$$
$$\therefore {\eta}_d=\frac{\frac{v_1^2-v_2^2}{2g}-h_f}{\frac{v_1^2}{2g}}$$
$$\therefore {\eta}_d=\frac{(1-k)\frac{v_1^2-v_2^2}{2g}}{\frac{v_1^2}{2g}}$$
From equation (2), it is clearly known that there exists a negative pressure at runner exit which is equal to $H_s+(1-k)\frac{v_1^2-v_2^2}{2g}$. From this, following two conclusions can be drawn:
- Due to the use of draft tube, the turbine will not lose head $H_s$ becasue of equal reduction in pressure head at runner exit.
- Due to use of draft tube of increasing cross-section, the pressure value at runner exit further reduced by $(1-k)\frac{v_1^2-v_2^2}{2g}$.
Purpose / Function of Draft Tube
- It helps to achieve the recovery of velocity head at runner outlet which otherwise would have gone to waste as an exit loss.
- It allows the turbine to be set at higher elevation without losing advantage of elevation difference.
- It serves as a passage for water from runner exit to tail pool.