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Lecture 4

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Turbine Components (Part I)

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Turbines Components

Casings or Cylinders

The cylinder has to be extremely stiff in a longitudinal direction in order to prevent

bending and to allow accurate clearance to be maintained between the fixed and

moving parts of the turbine

The working pressure aspects demand thicker and thicker casing and the

temperature aspects demand thinner and thinner casings

Design developments took place to take care of both pressure and temperature

considerations leading to three types of casing design

a. Single shell casing

b. Multiple (double) shell casing

c. Barrel type casing

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Turbines Components

HP Turbine Casing (Single Shell Split Casing)

Horizontal joint permit assembly and dismantling

Thick flanges are for joining the two halves of casing and to withstand the bursting

force caused by steam pressure inside the casing

Severe thermal stresses may generate in thicker sections of casing during machine

startup and load changes

The diaphragms (sets of fixed blades) are usually fitted into the grooves in the

cylinder

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Turbines Components

Double Shell Casing

They are used in HP cylinder of turbines of large output operating at high pressure

and temperatures

The inner shell carries the diaphragms and is subject to full steam pressure

The space between the two shells is subjected to the exhaust steam pressure of

the particular HP cylinder

The advantages of this arrangement are:

Each shell need to be designed for a relatively small pressure difference

This permit reduced shell thickness and allows quicker warming up without

undue stress when starting up

Special expansion joints are provided for the main steam inlet pipes to pass

through the outer shell and to connect to the inner shell

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Turbines Components

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Double Shell Casing

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Turbines Components

Barrel Type Casing

Barrel type casing have no horizontal flange joint

They have uniform wall thickness around the axis of rotation

They can easily be designed to accommodate high pressure whilst remaining

relatively thin. It encloses the inner casing which is lightly stressed.

They can be relatively thin with light flanges for horizontal joint

The barrel type casing also remains constant in shape and leak proof during quick

changes in temperature (e.g., on startup and shutdown, on load change and under

high pressures)

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Turbines Components

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Barrel Type Casing

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Turbines Components

LP Turbine Casing

LP cylinders are often of double casing construction with an inner casing

containing:

Diaphragm supports

Bleed steam

Water extraction belts and

An outer casing directing the exhaust to the condensers and providing

structural support for the inner casing

All casings have bolted horizontal joints

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Turbines Components

9Axial Section of LP Turbine Cylinder

Glands

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Turbines Components

Expansion of Turbine Rotor and Casing

Expansion of rotor and casing during warmup following steam admission takes place

Reverse is true during cool down

The expansion of the casing may not always be the same as that of the rotor creating

This need to be managed effectively so as to ensure that striking of moving blades with

fixed blades does not take place

The arrangements to take care of such expansion are shown in the figure

The collar of the LP rotor is held in place by the thrust bearing (located between HP and

LP cylinders) and forms the anchor point for rotor expansion The rotor is free to expand in either direction of this anchor point due to its expansion

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Turbines Components

Expansion of Turbine Rotor and Casing (contd)

Stationary casings of turbine also expand from their anchor point by keying them

remote from anchor points to allow movement relative to the anchor

The relative expansion is known as differential expansion

When the turbine is warming up during the cold start, the rotor will normally bewarmer than the casing and positive differential expansion will occur

During a hot start, it is possible that the casing may have retained a sufficiently

high temperature to cause negative differential expansion

The method of coping with expected differential expansion for equilibrium

operating temperatures is to allow sufficient clearances between fixed and moving

blades for accommodating these expected changes

However, close monitoring using prescribed procedure is to be ensured during

warmup or cool down

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Turbines Components

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Fixed Points of Shaft and Casings