Large Steam& Gas Turbines

P M V SubbaraoProfessor

Mechanical Engineering Department

Backbones of Modern Nations ……

Advanced 700 8C Pulverised Coal-fired Power Plant Project

The state-of-the-art Gas Turbines

• The newer large industrial gas turbines size have increased and capable of generating as much as 200 MW at 50 Hz.

• The turbine entry temperature has increased to 12600C, and the pressure ratio is 16:1.

• Typical simple cycle efficiencies on natural gas are 35%. • The ABB GT 13 E2 is rated at 164 MW gross output on natural

gas, with an efficiency of 35.7%. • The pressure ratio is 15:1. • The combustion system is designed for low Nox production.

• The dry Nox is less than 25 ppm on natural gas. • The turbine entry temperature is 11000C and the exhaust

temperature is 5250C. • The turbine has five stages, and the first two rotor stages and

the first three stator stages are cooled; • the roots of the last two stages are also cooled.

Fuel  Natural gas

 Frequency  60 Hz

 Gross Electrical output  187.7 MW*

 Gross Electrical efficiency  36.9 %

 Gross Heat rate  9251 Btu/kWh 

 Turbine speed  3600 rpm

 Compressor pressure ratio  32:1

 Exhaust gas flow  445 kg/s

 Exhaust gas temperature  612 °C

 NOx emissions (corr. to 15% O2,dry)  < 25 vppm

GT24 (ISO 2314 : 1989)

Fuel  Natural gas

 Frequency  60 Hz

 Gross Electrical output  187.7 MW*

 Gross Electrical efficiency  36.9 %

 Gross Heat rate  9251 Btu/kWh 

 Turbine speed  3600 rpm

 Compressor pressure ratio  32:1

 Exhaust gas flow  445 kg/s

 Exhaust gas temperature  612 °C

 NOx emissions (corr. to 15% O2,dry)  < 25 vppm

9756 kJ/kWh

0200

21

hh

hh

21

22

22

21

21

22

rraa

rr

VVVV

VV

22

21

22

22

21

2000rraa VVVV

hh

22

21

21

22 1 aarr VVVV

Exact definition of DoR

1021

22 1

hhVV rr

Stage with General Value of Degree of Reaction

Total possible drop in Enthalpy:

Theory of General Reaction Blading

Vr2 > Vr1

U

Vr1Va1

Va1Va2

112 1

1122 coscos rrstage VVUmP

22

21

21

22 1 aarr VVVV

Ideal reaction blade:

1

111 cos

cos

UV

V ar

2

221

2

1

1122 1cos

cosaa

ar VV

UVV

Available power in % Reaction stage :

21

22

21%, 2 rraavialble VVV

mP

22

21

21%, 12 aaaavialble VVV

mP

Stage Sizing

2

cos 1max,

stage

12

cos 1

1max

aV

U

Steam Path

increasing

Selection of Degree of Reaction

132

4

jetV

U

stage,diagram

Definition of Isentropic/adiabatic Efficiency

• Relative blade efficiency is calculated as:

• Internal Relative Efficiency is calculated as:

dropEnthalpy Effective

loss Blade Moving & Nozzle-dropEntalpy Effectiverel

dropEnthalpy Effective

loss profile - loss lakage-loss Blade Moving & Nozzle-dropEntalpy Effectiveint, rel

Typical Distribution of Losses AStages

Structure of Large HP Turbine

Calculations of HP and IP Turbine Efficiencies

• The efficiency of a joined group of turbine stages between two successive bleed points is defined.

• Full loss of the exit velocity in the last stage, for operation on superheated steam is also accounted.

• The statistically generalized expression is

av

gr

avav

gstri

h

vm

120000

6001

5.0925.0 0

where .

avm = average steam flow rate = . .

1 2m m kg/sec.

.

1m = Steam flow rate at entry of group in kg/sec,

.

2m= Steam flow rate at exit of group in

kg/see And similarly

avv = 1 2v vm3/

kg,

0grh is the available enthalpy drop of the group

ev is exit velocity loss coefficient = 2

1

1sin

z

Z = No. of stages in group,

= Nozzle exit angle

Calculations of Last LP & Last Stage Turbine Efficiency

• To calculate the internal relative efficiency for the low pressure cylinder, proper consideration to be given to incorporate losses due to exit velocity and the losses due to moisture.

• The statistically generalized expression is

0

0 0

4000.87 1 1 1

10000

wfLPCLPC ev

ri wf LPC LPC

h hhk

h h

where correction for wetness fraction 1 21

2wf wf

y yk a

= 0.8 for peripheral moisture separation design.wfa

Exit velocity loss is given by

2.310 0.1

12 1

c cev

m vh

i

2 2d l 2m

Axial surface area at the exit from last stage moving blades, and

2

2

d

l

i = No. of flows in LP turbine

Average diameter to blade height ratio is

General Rules for Steam Path Design

• For HP Axial (flow) velocity at the inlet is 40 m/sec and at the outlet 65 m/sec.

•  For IP axial velocity of steam at the inlet is 60 m/sec and at the outlet 80 m/sec

•  For LP axial velocity of steam at the inlet is 75 m/sec and at the outlet of last front stage is 130 m/sec.

• Maximum mean blade speed used so far: 450 m/s

• Generally acceptable range of inlet flow angle(1) : 150 to 200

Stage Loading and Flow Coefficient

Stage Loading Coefficient: Ratio of specific stage work output and square of mean rotor speed.

2,0

2

,0

m

stagestage

r

h

U

h

Flow Coefficient: Ratio of the axial velocity entering to the mean rotor speed.

m

exitstagefexitstagefflow r

V

U

V , ,

flow

Regions of Design

General Rules for Efficient & Economic Flow Path Design

• For HP Axial (flow) velocity at the inlet is 40 m/sec and at the outlet 65 m/sec.

•  For IP axial velocity of steam at the inlet is 60 m/sec and at the outlet 80 m/sec

•  For LP axial velocity of steam at the inlet is 75 m/sec and at the outlet of last front stage is 130 m/sec.

• Maximum mean blade speed used so far: 450 m/s

• Generally acceptable range of inlet flow angle(1) : 150 to 200

Range of turbine Design Parameter

• High Pressure Turbine:

• Maximum AN2 : 2.5×107 – 3.3 ×107 m2.rpm2.

• Stage loading coefficient: 1.4 – 2.0

• Stage Exit Mach Number: 0.4 – 0.5

• Low Pressure Turbine:

• Inlet mass flow rate: 195 – 215 kg/m2.s

• Hub/tip ratio: .35-.5

• Max. Stage loading (based on hub): 2.4

• Exit Mach Number: 0.4 – 0.5

For reaction turbine maximum efficiency occurs at certain loading factor

With known value of U , change enthalpy is obtained .

From change in enthalpy absolute velocity of steam can be obtained

Enthalpy Entropy Diagram for Multistage Turbine

h

s

Turbine Inlet

Turbine Exit

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

Optimal Variable Reaction 3D Blade Designs