Turbine Part 5

7/22/2019 Turbine Part 5

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Lecture 5

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Turbine Components (Part II)


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Turbine Components

Labyrinth Seals

Turbine shaft of HP and LP rotors come out of casing at both ends

Labyrinth packings are provided to seal the steam flow from inside to outside and

airflow in LP from outside to inside

Labyrinth packings are sharp edged teeth made in two halves

These are made of babbitt, aluminum, brass, bronze; for high temperature, they

are made of stainless steel or alloy steel

In case of rubbing, the packing wears out and damage to the shaft is minimum

Labyrinth packings are supplied with external steam for sealing

Steam pressure is controlled by a regulator. This is called the steam seal regulator.

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Turbine Components

Labyrinth Gland: Working Principle

The labyrinth gland provides a series of very fine annular clearances in the gap

between the cylinder wall and the shaft

The steam is throttled through this gap and its pressure is reduced step by step

In expanding through each clearance, the steam develops kinetic energy at the

expense of its pressure energy

Due to abrupt changes in volumes, the generated turbulences convert the kinetic

energy into heat with no recovery of pressure energy

In this way, the pressure is progressively broken down and the leakage may be

significantly reduced

Glands must allow for axial expansion of the shaft and casing to take place withoutcausing a rub including those due to shaft vibration

The sector can be replaced readily if they are damaged

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Turbine Components

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Labyrinth Glands


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Turbine Components

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Spring Loaded Sectors of Labyrinth Gland


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Turbine Components

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A Cut-Out View of Labyrinth Gland


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Turbine Components

Moisture Separator and Reheater (MSR)

HP exhaust steam quality is improved in MSR before being fed to LP turbine

Moisture of the wet steam gets separated in moisture separation section of MSR

In NPPs, two stage reheating is done, first stage by steam bled from HP stage and

other directly from the live steam tapped from the main steam line

Working principle of a typical moisture separator and its combination with

reheater as used in nuclear plants is being described further

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Turbine Components

Chevron or Corrugated Plate Separator

Chevron Mist eliminators work on the principle of direct impaction combined with

centrifugal force

In the left illustration (shown in the next slide), the liquid collection troughs (Point

A, B, C) are critically located to remove the majority of entrained moisture droplets

with minimum pressure drop

Performance is good at higher steam velocity

Compact in size

Provision of good channeling protects re-entrainment of moisture with steam

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Turbine Components

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Chevron Separators


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Turbine Components

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Two Stage MSR with Chevron Spacers


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Turbine Components

Journal Bearings

The purpose of a journal bearing in turbine is to:

Retain the rotor system in its correct radial position, relative to the cylinders

Provide a low friction support to withstand the static and dynamic loads of shaft

rotation Two bearing normally support each section of the turbine shaft with solid coupling

Plain white metal journal bearings with hydrodynamic oil lubrication are invariably used

because of their high loading capacity, anti-friction properties, reliability and absence of

wear

The high pressure oil film formed by dragging and wedging of oil by the rotation of shaftat high speed in these bearings is able to support the load and prevent metal-to-metal

contact and damage to bearings and

shaft

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Turbine Components

Shaft Turning (Barring) Gear

Following turbine shutdown, distortion in the stationary rotor and cylinder may

occur due to temperature differential at the upper and lower halves of turbine

cylinder because of steam condensation

When turbine is kept shutdown for long period, sagging of rotor may take place

ETG (Electrical Turning Gear or Barring Gear) is provided to rotate the turbine

generator rotor at low speed for a span of time when the unit is shutdown

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Turbine Components

Shaft Turning (Barring) Gear (contd...)

It circulates the steam air mixture to minimize the temperature difference

between top and bottom halves of the cylinders

Also during startup after a long shutdown, barring operation straightens the rotor

sagging. (The shaft should be rotated for a period of two hours or more prior to

startup on the turning gear.)

Turning the rotor with ETG after shutdown during cooling and maintaining a flow

of cooling oil to the bearings prevent damage to the bearing from heat transferred

along the shaft from high temperature sections of the rotor

ETG may be used to rotate the turbine rotor for maintenance purpose

The turning gear is arranged to engage automatically as the turbine shaft slowsdown and reaches barring speed

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Turbine Components

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Barring Gear and Other Turbo-Supervisory Sensors Arrangements


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Turbine Components

Atmospheric Relief Valve/Diaphragms

In the event of failure of low vacuum trips, the pressure in LP turbine exhaust may

rise to an excessively high level

Due to large size of LP outer casing, the design pressure is generally low

To protect against excessive internal pressure, atmospheric relief valves/rupture

diaphragms are provided in the exhaust hoods

Each valve assembly has approximately 1 mm thick gasket clamped between valve

seat and valve disc

In case of high pressure (say 1.2 bar abs.) the valve disc tries to lift and thereby

rupture the gasket ring, thus allowing the steam to exhaust into the atmosphere

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Turbine Part 5

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