Steam Turbine Design

Impulse Turbine Impulse steam turbine stage consists as

usual from stator which known as the nozzle and rotor or moving blades

Impulse turbine are characterized by the that most or all enthalpy and hence pressure drop occurs in the nozzle.

The rotor blades can be recognized by their shape, which is symmetrical and have entrance and exit angles around 20o. They are short and have constant cross sections.

Single Stage Impulse Turbine

Nozzles Blades

IMPLUSE STAGE It is usually called De-Laval

turbine The steam is fed through one or

several convergent-divergent nozzles

The nozzles do not extend completely around the circumference of the rotor, so that only part of the blades are impinged upon by the steam.

Pressure drop occurs in the nozzle and not in the blades.

Maximum velocity and hence kinetic energy of the steam occurs at the nozzle exit

Velocity change occurs in the rotor blades where the steam gives up its energy to the rotor blades.

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Compounded Steam Turbines Compounded steam turbine means multistage

turbine. Compounding is needed when large enthalpy

drop is available. Since optimum blade speed is related to the

exit nozzle speed. It will be higher as the enthalpy drop is higher.

The blade speed is limited by the centrifugal force as well as needs of bulky reduction gear

Compounding can be achieved either by velocity compounded turbine or pressure compounded turbine.

Velocity Compounded Impulse Turbine The velocity compounded turbine was

first proposed by C.G Curtis. It is composed of one stage of nozzles, as

the single stage turbine, followed by two rows of moving blades instead of one.

These two rows are separated by one row of fixed blades which has the function of redirecting the steam leaving the first row of the moving blades to the second row of moving blades.

Velocity Compounded Impulse Turbine (Contd.)

Velocity Compounded Impulse Turbine (Contd.) In Curtis turbine steam leaving the nozzle is

utilized in both rows of moving blades instead of single raw as in the de-Laval turbine.

The velocity remain almost constant across the fixed blades.

Using an analysis similar to that used for the single stage , The work of the Curtis turbine is as follows:

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23

24

24

23

21

22

22

21 rrssrrss

oo VVVVVVVVm

w

First Row Second Row

Due to friction effect inlet and exist velocities for different rows are related as follows:

33

434

22

323

11

212

vr

rrr

vs

sss

vr

rrr

kV

VVV

kV

VVV

kV

VVV

Velocity Compounded Impulse Turbine (Contd.)

Velocity Compounded Impulse Turbine (Contd.)

Although the Curtis stage is composed of two rows of moving blades, a velocity compounded turbine can be composed of any number of such rows.

All these rows are sharing in the same kinetic energy of the incoming steam.

These stages are usually built with successively increasing blade angles such that they become flatter and thinner blades toward the last row.

Expression for the optimum speed is as follows:

n

CosVV s

optB 211

.,

Three Stages Velocity Compounded Turbine

Velocity Compounded Impulse Turbine (Contd.) The work ratio of the highest-to-lowest

pressure stages in an ideal turbine is 3:1 for two stages turbine and 5:3:1 for the three stage turbine and 7:5:3:1 for four stages turbine.

The lower pressure velocities stages produces little work compared with the added investment. This makes additional stages above two (Curtis) uneconomical.

If blade speeds must be reduced below that afforded by Curtis turbine another type of compounding could follow the Curtis stage.

Pressure Compounding Impulse Turbine Pressure compounding impulse turbine is a multistage

impulse turbine where expansion in the fixed blades (nozzles) is achieved equally among the stages.

This type of turbines is usually called as Rateau turbine Accordingly the inlet steam velocities to each stage is

essentially equal, due to equal drop in enthalpy.

Where n is the number of stages This equal enthalpy drop does not mean equal

pressure drop

n

hVV totss

2...21

Pressure Compounding Impulse Turbine (Contd.)

Two Stages Pressure Compounding Turbine

Three Stages Pressure Compounding Turbine

Pressure Compounding Impulse Turbine (Contd.) In reference to the previous velocity triangle the whirl of

all stages is equal to zero (δ=90o). The kinetic energy from each stage should be

neglected, because the nozzle of each stage must receive the steam discharged by the preceding stage.

The pressure compounding has the advantages of: reduced blade velocities reduced steam velocities (and hence friction). equal work among the stages as desired by the designer.

It suffers from the following disadvantages: Pressure drop across the fixed raw of nozzles which require

leak tight diaphragms. Large number of stages

Accordingly pressure compounding is used for large turbine where efficiency is more important than the capital cost

Comparison Between Velocity and Pressure Compounding Impulse Turbines

Velocity CompoundingPressure CompoundingNot equal velocity drop for each stage

Equal velocity drop for each stage

No pressure drop per stageNot equal pressure drop per stage

Non equal power per stageEqual power per stage

High friction losses due to high velocities

Low friction losses due to reduced steam velocity

Not recommended for more than two stages

Recommended for multistage

No problem with steam leakLarger steam leak

Suitable for small turbines as well as only for the first stage in large turbine

Suitable for large turbines

Advantages of Impulse Turbines No pressure drop in moving blades

low steam thrust low leakage losses at blade extremities and

shaft ends Low consumption of spare parts

spare parts unnecessary for stationary and mobile blades

Compact design High operation flexibility

Reaction Principle Reaction effect results from issuing a fluid at very

high velocity from a nozzle. This results in a reaction which moves the nozzle in the opposite direction.

Pure reaction happens if the flow is accelerated from zero velocity to its exist velocity in the moving blades.

Since this is not the case in turbines, thus there are no pure reaction turbine but it is usually a mix between impulse and reaction. Accordingly the term reaction turbine does not mean a full reaction turbine but a partially impulse and partially reaction.

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Reaction Turbine Reaction turbine has been

invented by C.A. Parson Turbine with 50% reaction is the

turbine where 50% of the enthalpy drop happens in the stator and the other 50% occurs in the rotor. It is important to mention that this does not mean equal pressure drops.

Pressure drop is usually higher for the fixed blades and greater for the high pressure conditions, where the pressure drop per unit of enthalpy drop is higher at the high pressure

The rotor blades of a reaction turbine are not symmetrical as in the impulse turbine, they are similar to those of the stator but curved in the opposite direction.

Reaction Turbines (Contd.) Reaction Ratio “RR”

or (Degree of Reaction): is the ratio of enthalpy drop in the rotor to the total enthalpy drop in the stage.

Accordingly impulse turbine could be considered as reaction turbine with Zero degree of reaction

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)1(*. RRhh stagestat

RRhhrotor *

stage

rotor

h

hRR

Two Stages Reaction Turbine

Analysis of Reaction Stage

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2

2

23

24

2

22

23

21

22

1

rrrotor

ssstator

rrrotor

VVh

VVh

VVh

coscos 21 rro VVmF

Analysis of Reaction Stage (contd.)

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P

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P

CosVVCosVVmP

CosVVCosVmF

2

221

22

22

21

211

211

Optimum Blade Velocity for Reaction Turbine

For the case of similar fixed and moving blades θ=γ

221max

1

1

1

12

022

2

Bo

so

sB

Bso

B

BsBo

sr

VmCosVmP

CosVV

VCosVmdV

dP

VCosVVmP

CosVCosV

Efficiency of the Reaction Turbine

The efficiency of the reaction turbine depends of the efficiency of the fixed and the moving blades.

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sstat

sosstat

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m

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Efficiency of the Reaction Turbine (Contd.)

It is clear that the reaction turbine is an efficient machine

This can be explain in the light of the steam velocity where for the same VB, where:

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sB

sB

VV

pulseforCosV

V

actionforCosVV

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1

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Disadvantages of Reaction Turbine The main disadvantage of the reaction

turbine that it is not suitable for large pressure drop, where ΔP/Δh is high at high pressure, and consequently high potential of steam leak.

The usual design for large turbine at high boiler conditions is to make the first stage of impulse time (velocity compounding) to reduce the pressure and then continue with reaction stages.

Axial Thrust The turbine rotor is subjected to axial thrust due to the

pressure drop as well as the change in the axial momentum.

For impulse turbine and since there is no pressure drop in the rotor blades, the axial thrust is minimum.(Vr1≈Vr2 & Φ=γ).

In the reaction turbines the effect of pressure drop is added to the thrust force.

A technique to reduce the thrust force is the use of double flow steam turbine. This technique has the following advantages:

Canceling the thrust force Reduce the thrust due to the reduction in the blades

height

sinsin 21 rro

axial VVmF

Steam In

Twisted Blades

NDVB

h=1/3Dm

Providing that Vs and θ do not change while Φ increases and γ decreases with height due to the increase in VB. This means that the blade will have a twisted shape.

This makes the degree of reaction changes along the blade height (impulse at the base and maximum reaction at the top

The blade is designed for optimum conditions at the midpoint.

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