Study of Multi Stage Steam Turbines

P M V SubbaraoProfessor

Mechanical Engineering Department

Techno-economical Viable Solution forLow Capacity Mechanical Power ……

Blade Power at Maximum Efficiency COnditions

Ideal Impulse Stage : UVUmP aBladeimpulse 11 cos2

Ideal Parson Stage : UVUmP aParson 11 cos2

2

cos 1

1

aV

U 22 UmP Bladeimpulse

2

cos 122

1 aBladeimpulse

VmP

11

cosaV

U 2UmPParson

122

1 cos aParson VmP

Moderate Capacity of Parson : Same Blade Velocity

At optimum U/Va1, an impulse stage produces TWICE the power

of a 50% reaction stage for same blade speed!

This means that an impulse turbine requires only half the number of stages as a 50% reaction turbine for a given application!

This fact has a major impact on the construction of the turbine

It is also responsible for some of the greatest miss understandings, since people assume that this means that impulse blading is cheaper overall - this is NOT true!

Impulse turbines have fewer stages, but they must use a different form of construction which is expensive

Capacity of Parson : Same Inlet Steam Velocity

So at optimum U/Va1, a 50% reaction stage produces TWICE the

power of an impulse stage for same value of Highest Steam jet velocity.

This means that a 50% reaction turbine requires only half the number of stages as an impulse turbine for a given application!

This fact has a major impact on achieving lower fluid dynamic losses with improved capital cost.

Reaction turbines with fewer stages and less expensive cost are highly preferred in large power plants.

Mechanical Arrangements of Steam Turbines

• Solutions to Turbo-machinery Issues.

• Tandem Reheat Steam Turbine

• Cross Compound Steam Turbine

Tandem Reheat Steam Turbine

Cross Compound Reheat Steam Turbine

Tandem-compound four-flow steam turbine

Large-Capacity Steam Turbines for Fossil Thermal Power Plant

Parson’s concept of multi-stage had produced an additional but marginal thermodynamic advantage.

Enthalpy Entropy Diagram for Multistage Turbine

h

s

Turbine Inlet

Turbine Exit

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

Internal Reheating due to Irreversibilities

3

4s

4IIs

4IIIs

4Is

4Ia

4IIa

4IIIa

4Vs

4IVs

4IVa

4Va

4VIs

4VIa

T

s

Behavior of Superheated Steam

h

T

Well Behavior of Superheated Steam

T

pT

h

Steam Flow Path in a Multi Stage Impulse Turbine

• Global available enthalpy for Power:

3

4s

4IIs

4IIIs

4Is

4Vs

4IVs

4Ia

4IIa

4IIIa

4IVa

4Va

4VIs

4VIa

savg hhh 43,

• Internally available enthalpy for Power:

n

is

ia

Is

avstageav

hhhh

hh

24

1443

,int,

• Total actual stage work output per unit mass:

n

ia

ia

Iaact hhhhw

24

1443

4IIss

4IIIss

4IVss

4Vss

Define Stage Efficiency:

Is

Iast

stage hh

hh

43

431

is

ia

ia

iaith

hh

hhstage

41

4

41

4

n

is

ia

ithstage

Is

ststageact hhhhw

24

1443

1

Global internal efficiency of turbine:

s

actturbine hh

w

43

s

nis

ia

ithstage

Is

ststage

turbine hh

hhhh

43

24

1443

1

s

n

iiss

iss

ithstage

Is

ststage

turbine hh

qhhhh

43

24

1443

1

qi is always positive.

Therefore, istageturbine

•Multistage turbines will increase the possibility of recovering lost availability!•The larger the number of stages, the greater is the heat recovery.•The difference is called heat recovery factor, •General value of is 0.04 to 0.06.

Compounding of impulse turbine

• Compounding is done to reduce the rotational speed of the impulse turbine to practical limits.

• Compounding is achieved by using more than one set of nozzles, blades, rotors, in a series, keyed to a common shaft; so that either the steam pressure or the jet velocity is absorbed by the turbine in stages.

• Three main types of compounded impulse turbines are: • a) Pressure compounded Steam Turbine : The Rateau Design • b) velocity compounded Steam Turbine : The Curtis Design• c) pressure and velocity compounded Impulse turbines : The

Rateau-curtis Design.

Pressure compounded impulse turbine