The Complete Guide to Draft Tubes: Design, Function, and Applications

Draft tubes are essential components in most types of turbines such as Kaplan, Francis, and reaction turbines. The component is like a pipe designed with areas increasing gradually that connect the outlet of the runner to a tailrace. With its two ends, one is connected to the runner outlet and the other end is submerged below the level of water in the tailrace. In this component, kinetic energy is converted into static pressure.

Today you’ll get to know the definition, applications, function, diagram, types, and working of a draft tube. You’ll also get to know the advantages and disadvantages of this draft tube in its various application.

What is a draft tube?

A draft tube is a connecting pipe fitted generally at the outlet or exit of a turbine in order to convert the kinetic energy of water at the outlet into static pressure. with this component, wastage of the kinetic energy of water is avoided. In a draft tube, the diameter is smaller near the inlet and large near its outlet. This outlet is always submerged in water. Cast steel and cement concrete is the material used in making a draft tube.

Just as earlier mentioned, draft tubes are generally used in power turbines like reaction, Kaplan, and Francis turbines. The system is located just under the runner and allows to decelerate the flow velocity exit from the runner.

Draft tubes are used in turbines to avoid the problem of backflow which is why it located between the turbine exit and tailrace. Turbines like Pelton or impulse do not need a draft tube because the head available to the turbine is very large. This results in an above atmospheric pressure at the turbine exit. Because of the above atmospheric pressure of water at the turbine exit, backflow of water will never occur.

Applications of draft tubes

Applications of draft tubes in turbines have already been mentioned above. They are used to increase the pressure from low turbine exit pressure to the pressure of the surrounding to which the fluid is rejected. The primary function of a draft tube is to convert water kinetic energy into pressure energy, decrease the velocity of water, and raise the pressure of the water before joining the tailrace. This pipe is used to steadily increase the cross-sectional area.

Diagram of a draft tube:

Draft tube in Kaplan turbine.

Types of draft tubes

Below are the various types of draft tubes:

Conical draft tube:

Conical types of draft tubes allow their flow direction in straight and divergent. They usually made with mild steel plates, having a tapered shape design and greater outer diameter than the inlet. The tapered angle of the draft tube is not too wide to induce a divergence of the flow from the wall of the draft tube. Also, the angle should not be too short, because a longer draft tube is required to offer a substantial loss of kinetic energy.

Simple elbow draft tube:

These types of draft tubes carry the shape of an elbow. It is often used in a Kaplan turbine. The cross-section area remains the same for the entire length of the draft tube, while its inlet and outlet are circular. Elbow draft tubes are used at low head positions and the turbine is mounted next to the tailrace. This helps to minimize the expense of drilling and the exit diameter is as wide as a position to recover kinetic energy at the runner outlet.

Moody draft tube:

In moody draft tube types, the outlet is split into two sections and has one inlet. This tube is similar to a conical draft tube. Moody draft tube types help to reduce the swirling motion of water.

Elbow draft tube with varying cross-section:

These draft tube types with varying cross-sections are improvements of a simple elbow draft. Its inlet is circular while the outlet is rectangular. Generally, the horizontal section of the draft tube is inclined up to prevent air from reaching the exit area. The draft tube varies in cross-section from inlet to outlet. This outlet is beneath the tailrace.

Working of draft tubes

The working of draft tubes in a turbine is less complex and can be easily understood. In the case of the Kaplan and Francis turbine, the head available at the inlet is low, making the turbine to be placed much closer to the tailrace. This helps to get a maximum head. Most of the pressure of water is converted into mechanical energy of the turbine. The pressure head at the outlet of the turbine is below the atmospheric pressure.

Backflow can occur since the exit of a turbine is located near the tailrace and the pressure of water at the exit of a turbine is less than the atmospheric pressure. This occurs because the water flows from high pressure to low pressure and pressure at the exit of the turbine is less than atmospheric and at tailrace, there is atmospheric pressure.

This backflow can cause serious damage to the turbine and its parts, causing total breakdown. To avoid this backflow problem, a draft tube is used between the outlet of the turbine and tailrace. A draft tube will increase the pressure of water to atmospheric pressure.

Watch the video below to learn more on the working of draft tube:

Applying Bernoulli’s Principle at section 1-1 and 2-2

[Pressure Head + Velocity Head + Elevation Head]_1-1 = [Pressure Head + Velocity Head + Elevation Head]_2-2

Let
P₁ = pressure of the fluid at section 1-1 (an inlet of a draft tube)
V₁ = velocity of the fluid at section 1-1 (an inlet of a draft tube)
Similarly,
P₂ = pressure of fluid at section 2-2 (outlet of draft tube)
V₂ = velocity of fluid at section 2-2 (outlet of draft tube)
ρ = density of flowing fluid
g = gravitational force
h_f = loss of head (energy) in draft tube
H_s = vertical height of draft tube above the tailrace
y = distance of the bottom of draft tube from tailrace.
P_a = atmospheric pressure of a fluid.

( P₁ / ρg ) + ( V₁² / 2g ) + ( H_s + y ) = ( P₂ / ρg ) + ( V₂² / 2g ) + ( 0 + h_f )
( P₁ / ρg ) = ( P₂ / ρg ) – ( H_s + y ) + ( V₂² / 2g ) – ( V₁² / 2g ) + h_f

The pressure head at section 2 – 2 is equal to atmospheric pressure head and distance y.
( P₂ / ρg ) = ( P_a / ρg ) + y
( P₁ / ρg ) = ( P_a / ρg ) + y – H_s – y + ( V₂² / 2g ) – ( V₁² / 2g ) + h_f( P₁ / ρg ) = ( P_a / ρg ) – H_s + ( V₂² / 2g ) – ( V₁² / 2g ) + h_f

Converting the equation for our requirement (i.e., in the middle of R.H.S taking “-” common)
( P₁ / ρg ) = ( P_a / ρg ) – H_s – [ ( V₁² / 2g ) – ( V₂² / 2g ) – h_f ]

In the above equation [ ( V₁² / 2g ) – ( V₂² / 2g ) – h_f ] is called kinetic head.
Here [ ( V₁² / 2g ) – ( V₂² / 2g ) ] is the dynamic head.
From the above equation, we can write
( P₁ / ρg ) < ( P_a / ρg )
So        P1 < Pa

The pressure head at the inlet of the draft tube or outlet of a turbine is less than the atmospheric pressure. So, the net head on the turbine with the draft tube increases.

Advantages and disadvantages of draft tubes

Advantages:

Below are the benefits of draft tubes in their various applications:

• Draft tube prevents splashing of water from the runner and leads the water straight to the tailrace.
• The amount of kinetic energy required at the tailrace is vastly decreased.
• Performance of the system is increased.
• The turbine head is raised as the height is increased between the turbine exit and the tailrace.

Disadvantages:

The only disadvantages of draft tubes are that extra weight is added to the system and the initial of the draft tube is high.

Conclusion

The draft tube is an essential component in most types of turbines such as Kaplan, Francis, and reaction turbines. The component is like a pipe designed with areas increasing gradually that connect the outlet of the runner to the tailrace. Its purpose is to convert the kinetic energy of water at the outlet into static pressure. That is all for this post where the definition, applications, function, types, and working of draft tubes were discussed.

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