# Hydraulic Machines Questions and Answers – Theory on Draft Tube

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This set of Hydraulic Machines Multiple Choice Questions & Answers (MCQs) focuses on “Theory on Draft Tube”.

1. The efficiency of the draft tube is ratio of ________
a) Pressure energy by kinetic energy
b) Kinetic energy by Pressure energy
c) Kinetic energy into mechanical energy
d) Pressure into mechanical

Explanation: The efficiency of the draft tube is defined as the ratio of actual conversion of kinetic energy into the pressure energy. The pressure energy and kinetic energy is present at the inlet of the draft tube.

2. Draft tubes are not used in which of the following turbines?
a) Francis
b) Reaction
c) Kaplan
d) Pelton

Explanation: Turbine extracts energy and converts it into useful work. Turbine is a vortex related device. Draft tubes are not used in Pelton wheels. Draft tube is a tube that is installed in power turbines.

3. The draft tube at the exit of the nozzle increases the _______
a) Temperature
b) Pressure
c) Volume of the flow
d) Density of flow

Explanation: The draft tube at the exit of the nozzle increases the pressure in the fluid. It increases it at the expense of its velocity. This means that the turbine can reduce its pressure without fear of back flow to the tail race.
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4. Efficiency of a draft tube gives __________
a) Temperature difference
b) Pressure difference
c) Kinetic energy difference
d) Density of flow

Explanation: The efficiency of the draft tube gives difference of the kinetic energy between the inlet and the outlet tube losses. The efficiency of the draft tube is defined as the ratio of actual conversion of kinetic energy into the pressure energy.

5. Cavitation in a draft tube occurs when _______
a) Temperature difference
b) Pressure drop
c) Kinetic energy difference
d) Density of flow

Explanation: Cavitation in a draft tube occurs when pressure drop takes place. The absolute pressure falls below the saturated vapour pressure of the water for the given temperature.

6. Which among the following is an important parameter to avoid cavitation?
a) Tail race length
c) Height of draft tube
d) Pump

Explanation: Cavitation in a draft tube occurs when pressure drop takes place. The absolute pressure falls below the saturated vapour pressure of the water for the given temperature. Height of the draft tube is an important parameter to avoid cavitation.

7. The draft tube is situated in the _______
a) Inlet
b) Outlet
c) Tank
d) Nozzle

Explanation: The simple elbow draft tube is placed close to the inlet of the turbine. 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.

8. Which equation is applied to determine the flow?
a) Newtons equation
b) Rutherford’s equation
c) Bernoulli’s equation

Explanation: Bernoulli’s equation is used to determine the flow of the fluid from inlet to the outlet. It also helps in avoiding cavitation. Cavitation in a draft tube occurs when pressure drop takes place.

9. Height of the draft tube is denoted by _____
a) H
b) h
c) z
d) x

Explanation: Height of the draft tube is denoted by ‘z’. It is equated to z2, which is attached at the outlet of the turbine. Thus, the universal symbol to denote height of the draft tube is ‘z’.

10. Draft tube allows turbine to be placed above the tail race.
a) True
b) False

Explanation: The draft tube helps to cut down the cost of excavation. The draft tube is placed close to the tail race. The turbine pressure head is increased by decreasing the velocity at the draft tube. Draft tube allows turbine to be placed above the tail race.

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