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Steam Turbine is a device in which Energy Stored in the Steam (produced by heating the water using a fuel) is converted into Mechanical work (by expanding the steam)

Construction

Turbine Shaft, rotates inside the Turbine Casing

Turbine Wheel is mounted on the Turbine Shaft

Turbine Blades are mounted on the periphery (around) the Turbine Wheel

Nozzles/Nozzle Plate is fixed to the Casing

Guide Blades/Fixed Blades is fixed to the Casing (acts as nozzles)

Two Types of Turbines are of two types

1)IMPULSE 2) REACTION

IMPULSE TURBINE

High Pressure Steam is produced in the Boiler.

High Pr. Steam is passed through Nozzle /Nozzle Plate

Steam Expands in the Nozzle Plate (completely expands in nozzle)

Steam pressure drops (drastically)

Pr. Energy (Heat energy) is converted into Kinetic (Velocity) Energy

Velocity of the Steam is increased (drastically)

High Velocity Steam Jet is created. Steam is directed on the Blades

Blades are of constant cross section. Hence, there is no expansion of steam. So Pressure remains constant as it passes over blades

Hence, Relative velocity of the steam remains constant

However, the Direction of Steam is Changed as it passes over the blades

This results in acceleration of Steam.

Hence, a Impulse Force is produced

This Impulse force is imparted on the blades causing the Turbine wheel to rotate

Basic Marine Engineering Book

Page No. 65 Fig 3.1

Page No. 66 Fig 3.2

Page No. 67 Fig 3.3

Pressure – Compounded Impulse Turbine – pressure is dropped in more than one stage

Each stage consists of a set of nozzles (a nozzle plate) and turbine blades (moving blades)

Velocity – Compounded Impulse Turbine – pressure is dropped in Stage /one set of nozzles (one nozzle plate – but velocity is dropped in more than one stage

Each stage consists of a set of guide blades (fixed blades) and turbine blades (moving blades)

Pressure – Velocity – Compounded Impulse Turbine – It is a combination of two

These turbines are used as

Ahead running Propulsion Turbine
Prime Mover for Cargo Oil Pump (Cargo Oil Pump Turbine (COPT))

For Ahead & Astern running propulsion Turbine – one more set of rotor blades, fixed nozzle plate and fixed guide blades are fitted on the LHS

For Astern running – Ahead Steam (on the RHS) is shut and Astern steam on the LHS is opened

Reaction Turbine

High Pressure Steam is produced in the Boiler

High Pr. Steam is passed through Guide Blades or Fixed Blades (acts as nozzle)

Steam expands in the Guide Blades

Steam pressure drops gradually & continuously as it passes over guide blade

Pr. (Heat energy) is converted into Kinetic (Velocity) Energy

Velocity of the Steam is increased gradually & continuously as it passes over guide blades

Steam is directed on the Blades

Blades are of variable cross section (converging type). Hence there is expansion of steam. So Pressure drops gradually & continuously as it passes over blades

Hence the relative Velocity of the Steam is increased gradually & continuously as it passes over the blades

Steam issues / leaves from the Blades at a high velocity (in a direction opposite to the movement of the blades)

Hence a reaction force is produced

This Reaction force is imparted on the blades causing the turbine wheel to rotate

Basic Marine Engineering Book Page No. 68 Fig 3.4

Difference between reaction and impulse turbine

Difference

Sl. No.	Impulse Turbine	Reaction Turbine
1	Steam completely expands in the nozzle itself. Hence its pressure remains constant as it pass over the moving blades	Fixed blades (guide blades) act as nozzles. Hence steam expands both in fixed and moving blades continuously as it passes over them. Thus the pressure drop occurs gradually and continuously as it pass over both the fixed and moving blades
2	Blade (steam passage) is of constant cross section area, as there is no expansion of steam.	Blade (steam passage) is of variable cross-sectional area (converging type), as there is expansion of steam
3	As pressure remains constant in moving blades, the relative velocity of steam passing over the moving blades remains constant	As Pressure drop occurs continuously in the moving blades (steam expands continuously as it passes over the moving blades), the relative velocity of the steam passing over the moving blades continuously increases
4	Shape of the Blades are symmetrical, hence manufacturing of blade is simple.	Shape of the Blades are non-symmetrical (aerofoil), hence manufacturing of blade is difficult
5	Because of large pressure drop in the nozzle, the steam velocity is high & turbine rpm /speed is also high.	Due to small pressure drop in the fixed blades (guide blades), the steam velocity is low & turbine rpm/speed is also low.
6	Because of large pressure drop in the nozzles, the number of stages is less, for the same pressure drop. Hence the size of the turbine, for the same power output is small.	Because of small pressure drop in each stage, the number of stages is more, for the same pressure drop. Hence the size of the turbine, for the same power output is large. These turbines are multi-stage turbines only.
7	Occupies less space per unit power	Occupies more space per unit power
8	Suitable for small powers	Suitable for medium and higher powers

Difference

Sl. No	Steam Turbine	Gas Turbine
1	Uses high pressure steam as the working fluid	Uses air or some other gas as the working fluid.
2	It is a device consisting of only the turbine	It is a device consisting of compressor, combustion chamber, and turbine
3	It is only a component (executes one step of the Rankine cycle)	It is an engine (executes the whole Brayton cycle
4	Delivers torque as the work output	Delivers torque or thrust as the work output
5	Efficiency is less	Efficiency is more
6	Low Operating Temperatures 550⁰C	High Operating Temperatures 1500⁰C
7	More space is required (for installing boilers and heat exchangers which are connected externally for heat addition	Less Space required
8	Water (Steam) (working Fluid will not be available readily every where	Air (working fluid) is readily available any where