Turbine Part II

8/12/2019 Turbine Part II

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Lecture 2

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Steam Turbine (Part II)


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Steam Turbine: Velocity Diagram for

Impulse Blade

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Steam Turbine: Velocity Diagram

Explanation The steam jet issuing from the nozzle with absolute velocity V1, falls on the moving

blade at an angle with the plane of rotation of moving blades

Relative velocity U1of steam with respect to the moving blade is the vector sum of

steam absolute velocity and reverse of blade velocity

Assuming no frictional resistance to the flow of steam by the moving blades, the

relative velocity at blade outlet will be equal to the relative velocity at blade inlet,i.e., U1= U2

If friction is taken into account, U2will be < U1

As the steam glides over the moving blade, its relative velocity U2at blade exit

follows its outlet angle 2

Absolute velocity at blade outlet V2is the vector sum of relative velocity U2andblade velocity u

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Steam Turbine: Velocity Diagram

Explanation (contd...) The change in momentum of the steam in the direction of blade motion

=(Vu1-Vu2) * (Mass of steam)

Vu1and Vu2denote absolute tangential velocity at inlet and outlet of the moving

blade

This change in momentum provides the required force in the direction of blade

motion

The component of absolute velocity Vf1in axial direction called velocity of flow, is

responsible for the flow of steam in axial direction

The multiplication of the mass of steam and change in the velocity of flow (Vf1

Vf2) is responsible for the axial thrust on the blade

Assuming no friction, U1= U2, and if the blade inlet and outlet angle 1and 2areequal, then Vf1= Vf2. making axial thrust zero. In actual design, angle (2) is made

slightly larger than angle (1) to account for friction between steam and blade.

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Steam Turbine: Velocity Vector

Diagram for Impulse Blade (With

Friction)

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= (Vf1- Vf2)


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Steam Turbine

Impulse Reaction Turbine

In impulse reaction turbine, the moving blades of a reaction stage are shaped so as

to create a nozzle effect in the space between the blades

Thus, pressure gets reduced in both fixed and moving blades

Work is done by the impulse effect due to the change of direction of the high

velocity steam plus a reaction effect due to the expansion of steam through the

moving blades

The relative velocity U2 at the blade outlet is therefore larger than U1 in the

velocity diagram for reaction turbine

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Steam Turbine: Reaction Turbine

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Steam Turbine

Pressure, Velocity or Combination Compounding in Turbine

The turbine speed shall become very high if the steam kinetic energy is absorbed

in single stage, i.e., turbine having single row of nozzles and moving blades

It may lead to the failure of blades due to the centrifugal force

Therefore, division of steam energy is done in multiple steps to keep the velocity

of steam and turbine within practical values

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Steam Turbine

Pressure, Velocity or Combination Compounding in Turbine (contd)

Following are the various types of compounding:

In pressure compounding turbine, pressure drop takes place equally in in each

stage in the nozzle part, the velocity gained at nozzles is subsequently

absorbed in the next moving blades (refer to the figure in the next slide)

In velocity compounding, the full pressure drop occurs in the first nozzle ring

and the pressure remains constant in the moving and fixed blades. The total

velocity gained at the first nozzle is absorbed in each moving blades.

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Steam Turbine

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Example of Pressure Compounding


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Steam Turbine

Pressure, Velocity or Combination Compounding in Turbine (contd)

Pressure and velocity compounding is a combination of both the previous

methods. The merits of the pressure and velocity compounding are as follows:

Less stages are necessary because of bigger pressure drop between the stages

For a given pressure drop, the turbine will be shorter

It may be noted that the diameter of the turbine is increased at each stage to

allow for the increasing volume of steam

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Steam Turbine

Moisture Formation: Causes and Effects on Turbine

As steam passes through the turbine, its heat energy is consumed for providing

kinetic energy

As this occurs, part of the steam gets converted into minute water droplets

These droplets are carried along with steam strike against the moving blades

This phenomenon leads to erosion of the turbine blades

As water droplets do not move as fast as steam, the back side of the blades are

continuously in water droplets

This results in retarding force against the moving blades and loss of turbine

efficiency

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Steam Turbine

Design Features for Addressing Adverse Impact of Moisture in Turbine

Modern turbine incorporates various design features for moisture removal and

against erosion

Wet steam with greater than 10% moisture cannot be tolerated

In case of nuclear plants, with saturated steam entry at the turbine inlet, the

steam reaches 10% moisture content before extraction of its all useful energy

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Steam Turbine

Design Features for Addressing Adverse Impact of Moisture in Turbine (contd)

The turbine unit is spilt into two turbines namely High Pressure (HP) turbine which

receives steam directly from steam generator and Low Pressure (LP) turbine

connected to condenser for exhausting its steam in it

HP turbine is designed so as to provide not more than 10% wet steam at its

exhaust/into a moisture separator

With almost complete removal of moisture from the moisture separator, steam is

superheated in a reheater prior to admission in LP turbine

This reheated steam improves overall turbine efficiency

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Steam Turbine

Simplified Turbine Unit

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Simplified Diagram of NPP Turbine Unit


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Steam Turbine

Axial Thrust and its Management

When steam enters at one end of the turbine and after its expansion, exhausts

through the other, such turbine are called single flow as steam flows in one

direction

Modern reaction turbines contain blading which allows substantial pressure drop

across the moving blading due to steam flow

These pressure drops with large steam flow tend to push the blade wheels from

high pressure side to low pressure side making it difficult to manage such thrust

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Steam Turbine

Axial Thrust and its Management (contd)

To balance out such forces on rotor, the following provisions are used either alone

or in combination:

1. Thrust bearing

2. Double flow turbine

3. Use of only impulse blading

4. Dummy piston

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Steam Turbine

Thrust Bearing

This is the most common method for dealing with axial thrust

The thrust bearing collar rotates with the shaft and stationary shoes on either side

of the shaft absorb the thrust

Oil is supplied between the shoes and thrust collar to lubricate and cool the

bearing

Though thrust bearing is used in all turbines, it may not suffice except for small

turbines

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Steam Turbine

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Sketch of a Thrust

Bearing

Turbine Part II

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