Lecrures of Steam Compounding of Turbine

8/18/2019 Lecrures of Steam Compounding of Turbine

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IUBAT- International University of Business Agriculture and TechnolFounded 1991 by Md. Alimullah Miyan

COLLEGE OF ENGINEERING AND TECHNOLOGY(CEAT)

STEAM TURBINE POWER PLANT (2)

Course Title: Power Plant Engineering

Course Code : MEC 403

Course Instructor: Engr. Md. Irteza Hossain

Engr. Md. Irteza Hossain Faculty, BSME



COMPOUNDING OF

STEAM TURBINE



INTRODUCTION A steam turbine is a device that extracts thermal energy from

pressurized steam and uses it to do mechanical work on a

rotating output shaft. Its modern manifestation was inventedby Sir Charles Parsons in 1884.

The motive power in a steam turbine is obtained by the rate of

change in momentum of a high velocity jet of steam impinging on

a curved blade which is free to rotate.

The steam from the boiler is expanded in a nozzle, resulting in theemission of a high velocity jet. This jet of steam impinges on the

moving vanes or blades, mounted on a shaft. Here it undergoes a

change of direction of motion which gives rise to a change in

momentum and therefore a force

The steam does not strike on the blades but glides on the surface

of the blades to impart the motion to the blade.



TYPES OF STEAM TURBINE

a) Impulse turbine

Steam is expanded only in nozzles, and pressure at the outlet side of

blades is equal to that at inlet side.

Drop in pressure of steam takes place only in nozzles and not in movingblades.

Energy transformation takes place only in nozzles.

b) Impulse-Reaction turbine

Expansion of steam takes place in nozzles(fixed blade) as well as inmoving blades.

This difference of pressure at inlet and outlet result in reaction and

adds to the propelling force.

Both energy transfer and transformation takes place in moving blades.



WHY COMPOUNDING IS REQUIRED?

The steam produced in the boiler has got very

high enthalpy. In all turbines the blade velocity is

directly proportional to the velocity of the steam

passing over the blade.

Now, if the entire energy of the steam is

extracted in one stage, i.e. if the steam is

expanded from the boiler pressure to the

condenser pressure in a single stage, then its

velocity will be very high. Hence the velocity ofthe rotor (to which the blades are keyed) can

reach to about 30,000 rpm, which is pretty high

for practical uses.

Moreover at such high speeds the centrifugal

forces are immense, which can damage the

structure. Hence, compounding is needed.



WHAT IS COMPOUMDING?

Compounding is employed for reducing the rotational speed of the impulseturbine to practical limits.

Compounding of steam turbines is the method in which energy from the steamis extracted in a number of stages rather than a single stage in a turbine.

A compounded steam turbine has multiple stages i.e. it has more than one setof nozzles and rotors, in series, keyed to the shaft or fixed to the casing, sothat either the steam pressure or the jet velocity is absorbed by the turbinein number of stages.

There are three main types of compounded turbines.a) Velocity-compounded impulse turbine.

b) Pressure-compounded impulse turbine.

c) Pressure and velocity compounded impulse turbine.



VELOCITY-COMPOUNDED IMPULSE

TURBINE

This turbine is also termed as

Curtis Turbine.

Velocity drop is arranged in manysmall drops through many moving

blades instead of single row of

moving blades.

High pressure steam is expandedin nozzle which is then transferred

to first set of moving blades wheresteam losses part of its kinetic

energy.

Fixed blade are guide blades that

guide the steam to succeeding rowof fixed blade which further

reduces its velocity.



DISADVANTAGES OF VELOCITY

COMPOUNDING

Due to the high steam velocity there are high friction losses

Work produced in the low-pressure stages is very less.

The designing and fabrication of blades which can withstand such high

velocities is difficult.



PRESSURE-COMPOUNDED IMPULSE

TURBINE

This turbine is also known as Rateau Turbine.

In this pressure drop from chest pressure to the condenser pressure is splitup into smaller pressure drop across several stages of impulse turbine.

It consists of alternate rings of nozzles and turbine blades. The nozzles are

fitted to the casing and the blades are keyed to the turbine shaft.

In this type of compounding the steam is expanded in a number of stages,instead of just one (nozzle) in the velocity compounding.



The steam coming from the boiler

is fed to the first set of fixedblades i.e. the nozzle ring. The

steam is partially expanded in thenozzle ring. This is then passed

over the set of moving blades. As

the steam flows over the movingblades nearly all its velocity is

absorbed. However, the pressure

remains constant during this

process. After this it is passed intothe nozzle ring again to further

reduce the pressure.



DISADVANTAGES OF PRESSURE

COMPOUNDING

The disadvantage is that since there is pressure drop in the nozzles, it

has to be made air-tight.

They are bigger and bulkier in size.



PRESSURE-VELOCITY COMPOUNDED

IMPULSE TURBINE

This turbine is the combination of pressure and velocity compounding.

It consist of set of nozzles each consist of moving and fixed blades.

Pressure compounding occurs in set of nozzles while velocity

compounding occurs in moving and fixed blades.

In first set of nozzles, slight decrease in pressure occurs while the

kinetic energy increases. There is no pressure drop in two rows of

moving blade. However, velocity drops in moving blades.



In second set of nozzles, the remaining pressure drop takes place,but

velocity increases and, the drop in velocity takes place in moving blades of

the second rotor.

This method of compounding is used in Curits and Moore Turbine.

Lecrures of Steam Compounding of Turbine

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