Test: Draft Tube - Civil Engineering (CE) MCQ

Concept:
Draft Tube:

• The draft tube is a divergent tube that connects the runner exit to the tailrace.
• At the exit of the turbine runner, the fluid has appreciable kinetic energy, and to recover some of this kinetic energy that would otherwise be wasted, the flow enters the expanding area diffuser called a Draft tube.
• It is used with reaction turbines only.

Purposes of Draft tube:

• It permits a negative or suction head to be established at the runner exit, thus making it possible to install the turbine above the tailrace level without loss of head.
• It is used to convert kinetic energy at the exit of the turbine to pressure energy.

The efficiency of the draft tube is given by:

Head gained by draft tube is given by:

Continuity equation, Q = A × V
Calculation:
Given:
Q = 9.42 m³/s, D₁ = 2 m, D₂ = 4 m, Draft tube efficiency, η_d = 90 %

Head gained by draft tube is given by
= 
 = 0.38 m

Draft Tube: The draft tube is a conduit that connects the runner exit to the tailrace where the water is being finally discharged from the turbine. It is used with reaction turbines only.
The draft tube has two purposes as follows if
i) It permits a negative or suction head to be established at the runner exit, thus making it possible to install the turbine above the tailrace level without loss of head, thus increasing the net head
ii) It converts a large proportion of velocity energy rejected from the runner into useful pressure energy.
A draft tube is made divergent so as to reduce the velocity at the outlet to a minimum. Therefore, a draft tube is basically a diffuser and should be designed properly with the angle between the walls of the tube to be limited to about 8 degrees so as to prevent the flow separation from the wall and to reduce accordingly the loss of energy in the tube.

From the equation of the continuity,
A₁V₁ = A₂V₂
V ∝ 1/A
The area at the exit is higher, therefore the velocity of discharged water will be lower.

Draft Tube: The draft tube is a conduit that connects the runner exit to the tailrace where the water is being finally discharged from the turbine. It is used with reaction turbines only.
The draft tube has two purposes as follows if
i) It permits a negative or suction head to be established at the runner exit, thus making it possible to install the turbine above the tailrace level without losing the head.
ii) It converts a large proportion of velocity energy rejected from the runner into useful pressure energy.
A draft tube is made divergent to reduce the velocity at the outlet to a minimum. Therefore, a draft tube is basically a diffuser and should be designed properly with the angle between the walls of the tube to be limited to about 8 degrees to prevent the flow separation from the wall and reduce accordingly the loss of energy in the tube.

Compared to a cylindrical draft tube, a tapered draft tube converts more of the kinetic head into a pressure head.

Concept:

• In Impulse Turbine, the available hydraulic energy is first converted into kinetic energy by means of an efficient nozzle
• The high-velocity jet issuing from the nozzle then strikes a series of suitably shaped buckets fixed around the rim of a wheel
• The buckets change the direction of the jet without changing its pressure
• The resulting change in momentum sets buckets and wheels into rotary motion and thus mechanical energy is made available at the turbine shaft
• The fluid jet leaves the runner with a reduced energy
• The impulse turbine doesn't use a draft tube. Ex. Pelton wheel
• The draft tube is used only in Reaction turbines. Ex. Francis turbine, Kaplan turbine, ​Propeller turbine

Draft Tube:

• The draft tube is a conduit that connects the runner exit to the tailrace where the water is being finally discharged from the turbine. It is used with reaction turbines only.
• The draft tube has two purposes as follows if
  1. It permits a negative or suction head to be established at the runner exit, thus making it possible to install the turbine above the tailrace level without loss head.
  2. It converts a large proportion of velocity energy rejected from the runner into useful pressure energy.
• A draft tube is made divergent so as to reduce the velocity at the outlet to a minimum. Therefore, a draft tube is basically a diffuser and should be designed properly with the angle between the walls of the tube to be limited to about 8 degrees so as to prevent the flow separation from the wall and to reduce accordingly the loss of energy in the tube.

From the equation of the continuity,
A₁V₁ = A₂V₂
V∝1/A
The area at the exit is higher, therefore the velocity of discharged water will be lower.

Draft tube is one of the most commonly used in the Kaplan turbine. It works as an outlet at the Kaplan turbine. Draft tube is also called straight divergent tube.

Most common application of the draft tube is different types of pumps. It plays an important role in the putlet of the pump and the turbine.

Draft tube in a turbine and pumps helps to prevent flow separation in order to increase the turbine efficiency and increase its performance.

The simple elbow draft tube is placed close to the tail race. It consists of an extended elbow type tube. It is mainly used in the Kaplan turbine. It is placed close to the tail race of the turbine.

A turbine that consists of moving nozzles which are alternating with the fixed nozzles is called as a reaction turbine. When the steam hits the nozzle, the pressure is decreased and the temperature is increased. It is also called as Parson’s turbine.

Temperature does not affect the performance of the draft tube. With increase and decrease of temperature of fluid in a draft tube, the draft tube remains the same.