Turbine Loss

8/22/2019 Turbine Loss

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Estimation of Losses in Large Turbines

P.M.V. Subbarao

Associate Professor

Department of Mechanical Engineering

Indian Institute of Technology, Delhi

Accounting of Losses is Saving of Losses


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Sequence of Energy Losses

Steam

Thermal

PowerSteam

kineticPower

Blade

kinetic

Power

Nozzle Losses

Moving Blade

Losses

Stage Losses

Isentropic efficiency of

Nozzle

Blade Friction Factor


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Losses in Turbine Stage

Losses in Regulating valves : The magnitude of loss of pressure

due to throttling with the regulating valves fully open is:

Dpv = 3 to 5% of pmax.

Loss in nozzle blades.

pressure loss in moving blades. Loss due to exit velocity.

Loss due to friction of the disc and blade banding

Loss associated with partial admission.

Loss due to steam leakages through clearances.

Loss due to flow of wet steam.

Loss due to exhaust piping.

Loss due to steam leakage in seals.


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Losses in Nozzles

Losses of kinetic energy of steam while flowing through nozzlesor guide blade passages are caused because of

Energy losses of steam before entering the nozzles,

Frictional resistance of the nozzles walls,

Viscous friction between steam molecules,

Deflection of the flow,

Growth of boundary layer,

Turbulence in the Wake and

Losses at the roof and floor of the nozzles.

These losses are accounted by the velocity coefficient, f.


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Loss in nozzle cascade, 2 211 / 2000n th cD kJ/kg

2 221 / 2000m th wD kJ/kgLoss in moving blade cascade,

2

2/ 2000evh cD kJ/kgExit velocity Energy

Theoretical steam velocity at nozzle exit,1 0

2000t nc h m/s.

1.Relative steam velocity at entry to moving blade cascade,

2 2

1 1 1 12 cosw c u uc m/s

Absolute velocity at moving blade exit,

2 2

2 2 2 22 cosc w u uw m/s.


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0

0

m nrb

E h h

E D DBlade efficiency,

Available energy of stage,0 0 ev ev

E h h

D kJ/kg


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Turbine pressure profile at designed condition

0

2

4

6

8

10

12

14

16

MSPr

HP

1(1st

St.)

HP5

HP9

HP13

HP17

HP21

CRHPr IP

3IP

7IP

10

IP

12(Ex5

)IP

16

IPExh

st

LP

4(E

x3)

LP

6(Ex2

)

LP7

(Ex1

)

LPExh

st

Stages

Pr.

(MPa

)

Pr in HBD

Pr from Simulation


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Temperature profiles at designed condition

0

100

200

300

400

500

600

MSLine

HP3

HP7

HP11

HP15

HP19

HP23

CRH

TIP

3IP

7

IP12

(Ex5

)IP

16

IPExh

st

LP4(E

x3)

LP6(E

x2)

LPExh

st

Stages

Temperature

(degC)

Temp in HBD

Temp from Simulation


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HP Turbine per stage Enthalpy drop profiles at

designed condition.

14.4

14.6

14.8

15

15.2

15.4

15.6

15.8

16

16.2

HP1(1st

St.)

HP2HP

3HP

4HP

5HP

6HP

7HP

8HP

9

HP10

HP11

HP12

HP13

HP14

HP15

HP16

HP17

HP18

HP19

HP20

HP21

HP22

HP23

HP24

HP25

Stages

EnthalpyDrop

(kJ/kg)

Stagnation Enthalpy Drop

Static Enthalpy Drop


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Nozzle& Moving Blade Losses for HP Stages at Designed

condition

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

HP1(1st

St.)HP

3HP

5HP

7HP

9

HP11

HP13

HP15

HP17

HP19

HP21

HP23

HP25

Stages

Loss

(kJ/kg)

Nozzle loss

Moving blade loss


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IP Turbine per stage Enthalpy drop profiles at

designed condition.

15

20

25

30

35

40

IP1

IP2

IP3

IP4

IP5

IP6

IP7

IP8

IP9

IP1

0

IP1

1

IP1

2

(Ex5)

IP1

3

IP1

4

IP1

5

IP1

6

IP1

7

IP1

8

IP1

9

IP2

0

Stages

Enthalpydrop(kJ/kg)

Stagnation Enthalpy drop

Static Enthalpy drop


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Nozzle& Moving Blade Losses for IP Stages at

Designed condition

0.4

0.50.6

0.7

0.8

0.91

1.1

1.2

1.3

1.4

IP1

IP3

IP5

IP7

IP9

IP11

IP13

IP15

IP17

IP19

Stages

Loss(kJ/kg)

Nozzle loss

Moving blade loss


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LP Turbine per stage Enthalpy drop profiles at designed

condition.

60

70

80

90

100

110

120

LP1

LP2

LP3

LP4

(Ex3)

LP5

LP6

(Ex2)

LP7

(Ex1)

LP8

Stages

Enthalpydro

p(kJ/kg)

Stagnation Enthalpy drop

Static Enthalpy drop


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Nozzle& Moving Blade Losses for LP Stages at Designed

condition

0

1

2

3

4

5

6

7

8

9

LP 1 LP 2 LP 3 LP 4

(Ex 3)

LP 5 LP 6

(Ex 2)

LP 7

(Ex 1)

LP 8

Stages

Loss(kJ/kg)

Nozzle loss

Moving blade loss


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GLAND Leakage Flows


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Gland leakage losses

The steam leaked out from the system does not work on the blades,

it represents energy loss

1.Diaphragm leakageIt takes place in stages through the radial clearance between the

stationary nozzle diaphragm and the shaft or drum.

2.Tip leakage

It occurs in stages through the clearance between the outer peripheryof the moving blades and the casing due to the pressure difference

existing across the blade.

3.Shaft leakage

Shaft leakage occurs through radial clearance between the shaft andcasing at both high and low pressure ends of turbines.

At the high pressure end , steam leaks out to the atmosphere,

whereas at the LP end, the pressure being less than the atmospheric ,

air leaks into the shell


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Loss by leakage through diaphragm gland,

1

1 1

g g g rbd k F

F z

Loss by leakage through banding gland,

1

1

1

1.8eq rbb

d l

F d


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Leakage Losses for Turbine Stages

0

0.005

0.01

0.015

0.02

0.025

0.03

HP1(1stSt.

)HP5HP9HP13HP17HP21HP25 IP4 IP8

IP12(Ex5

)

IP16IP20

LP4(Ex3

)LP8

stages

Losses(kJ/kg)

Gland loss


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Turning loss

These occurs as the steam turns in the blade passage.

Disc friction loss

When the turbine disc rotates in the viscous steam, there is surface friction loss

due to relative motion between the disc and steam particles. Due to centrifugalforce , steam thrown radially outward.

The moving disc exerts a drag on the steam, sets it in motion from root to tip,

and produces a definite circulation.

Some part of Kinetic energy of steam is lost due to this friction.


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Friction loss,

32

1

fr fr

f

d uk

F c

Relative Internal efficiency, 1 1d b

ri rb fr

Effective enthalpy drop, 0i rih E

Power of the stage,.

i iP m h


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Profile Losses for Turbine Stages at Designed

Condition

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

H

P1

(1stSt.)

HP5

HP9

HP1

3

HP1

7

HP2

1

HP2

5

IP4

IP8

IP1

2(Ex5)

IP1

6

IP2

0

LP4

(Ex3)

LP8

Stages

Loss

(kJ/kg)

Profile Loss


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Losses associated with partial admission of steam

Partial admission of steam to turbine stages is employed in cases when thevolume flow rate of steam is not high (ie. Turbine of low capacity)

In turbine with partial admission ,steam is fed onto the moving blades only

an arc of length , rather than along the entire circumference.

Along the arc , there is no active flow of steam , and the blade passage

opposing this arc are filled with stagnant steam from the disc chamber.

Owing to the rotation of the disc , the steam filling this passage is entrained

by centrifugal force and moves from the roots to tips of moving blade;


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Steam can even flow from one side of the blade to the opposite side

Diagram of windage currents

in a partial admission turbine

stages

The work associated with this motion of the steam in bladepassages of the inactive portion of the arc of moving blades, is lost-

Usual energy of the turbine stage is decreased by the energy loss

associated wit this motion ( windage) of steam in blade passages


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Residual velocity loss

Steam leaving the last stage of the turbine has certain velocity,

which represent the amount of kinetic energy that cannot beimparted to the turbine shaft and thus it is wasted

Exhaust end loss

1. Exhaust end loss occur between the last stage of low pressureturbine and condenser inlet.

2. Exhaust loss depends on the absolute steam velocity.

Turbine Exhaust end loss = Expansion-line -end point - Used energy endpoint.

Typical exhaust loss curve :


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Turn-up loss

Total Exhaust

Loss

Gross hoodloss

Actual leaving

loss

Annulus

restriction loss

(1 0.01 )

3600

a

an

an

Q v YV

A

Annulus velocity

(m/sec)

Condenser flow

rate

Annulus area

Percentage of Moisture at

the Expansion line endpoint

Typical exhaust loss curve :

Annulus Velocity (m/s)

ExhaustLossofdr

yflow

0 120 240 180 240 300 360

10

20

30

40

50

SP.Volume


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Problems in Low pressure turbine

In the case of condensing turbines the last few stages operate

under wet steam conditions. This results in the formulation of minute droplets of water.

These droplets under the influence of centrifugal force are thrownout towards the periphery.

At the same time these droplets of water receive an accelerating

force from the steam particles in the direction of flow . Thus some of the kinetic energy of the flowing steam is lost in

accelerating these water droplets.

The absolute velocity of the steam is considerably greater thanthat of the water droplets into the moving blade passages.

The water droplets are deflected onto the back of the movingblades as a result of which the moving blades experience animpact force caused by impingement of the moving blades.

As a result of this moving blades experience an impact forcecaused by the impingement of water droplets on their backs.


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The practical investigations that the blade tips are subjected towear from one side water droplets present in the last few stagescan also result in erosion damage of turbine blades and nozzles .

One of the loss mechanisms in the steam turbine is the kineticenergy of the steam as it leaves the last stage blade.

The lower the kinetic energy, the higher the steam turbineefficiency will be.

The magnitude of loss is proportional to the square of the ratio of

the volume flow rate of the steam through the last stage of thesteam turbine and the annulus area of the turbine exit.

To decrease the loss, a larger turbine exit annulus area is needed.

An increase in the last stage blade annulus area can be

accomplished by either using shorter blades mounted on a largerdiameter rotor (largerhub) or

by using longer blades mounted on a smaller diameter rotor.


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The low-pressure turbine exhaust end is one of the important factors

affecting the turbine performance.

The size of the exhaust end is determined by the number of exhaust

flows and the length of the last stage blades.

In general, the larger the exhaust ends, the lower the full load net heat

rate. Under the part-load conditions.

Turbines with a large exhaust end will deteriorate more rapidly in

performance.

C l ti L f T bi St t D i d


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Cumulative Loss for Turbine Stages at Designed

Condition

0

1

2

3

4

5

6

7

89

10

HP1(1stS

t.)HP

5HP

9

HP13

HP17

HP21

HP25 IP

4IP

8

IP12(Ex

5)

IP16IP

20

LP4(Ex3

)LP

8

Stages

Loss

(kJ/kg)

Cumulative loss

Turbine Loss

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