Irreversibilities : Turbine to Condenser-II

P M V SubbaraoProfessor

Mechanical Engineering DepartmentI I T Delhi

Loss in Capacity of Turbine & Increase in Capacity of Condenser….

Irreversible Adiabatic Flow Through Turbine : SSSF

h0

sexitiso

Ideal work wiso = h0in – h0exitiso

Actual work wact = h0in – h0exitact

Internal Efficiency of a turbine

exitisoin

exitactinturbineiso hh

hh

,0,0

,0,0,

exitactual

actexit

exitin

in wV

hV

h 22

22

in

actexitin whh 00

Losses in Turbine Stage

• Losses in Regulating valves : The magnitude of loss of pressure due to throttling with the regulating valves fully open is:

pv = 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.

Losses in Nozzles

• Losses of kinetic energy of steam while flowing through nozzles or 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, .

Stage Internal Power at Designed Condition

02000400060008000

1000012000140001600018000

Stage

Inte

rnal

Pow

er (k

J/kg

)

Internal Power

500 MW

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

Stages

Loss

es (k

J/kg

)

Nozzle loss

Moving blade loss

Nozzle & Moving Blade Losses for HP Stages : 500 MW

0

0.5

1

1.5

2

2.5

3

3.5

Stages

Loss

es (k

J/kg

)

Nozzle loss

Moving blade loss

Nozzle & Moving Blade Losses for IP Stages : 500 MW

0123456789

10

Stages

Loss

es (k

J/kg

)

Nozzle loss

Moving blade loss

Nozzle & Moving Blade Losses for LP Stages : 500 MW

H-s Diagram of Turbine Exhaust Steam

Irreversible Flow Turbine Exit to Condenser

P M V Subbarao Professor

Mechanical Engineering DepartmentI I T Delhi

Irreversibilities due to Closed Cycle Policy …..

The Last Stage of LP Turbine

LP Turbine Exhaust System • In a condensing steam turbine, the low-pressure exhaust hood,

consisting of a diffuser and a collector or volute!, connects the last stage turbine and the condenser.

• The function of the hood is to transfer the turbine leaving kinetic energy to potential energy while guiding the flow from the turbine exit plane to the condenser.

• Most of exhaust hoods discharge towards the downward condenser. • Flow inside the hood therefore must turn about 90 deg from the

axial direction to the radial direction before exhausting into the condenser.

• The 90-deg turning results in vortical flow in the upper half part of the collector and also high losses.

• The exhaust hood is one of the few steam turbine components that has the considerable aerodynamic losses.

• It is a challenge for engineers to operate a hood with high pressure recovery and low total pressure loss in a compact axial length.

H-s Diagram of Turbine Exhaust Steam

exhaustturbinep ,

exhaustturbine,p ,0

2

2,lastaeV

condenserp

,condenserp0

Exhaust Hood

Exhaust Diffuser For L P Turbine

Steam Turbine Exhaust Size Selection

• The steam leaving the last stage of a condensing steam turbine can carry considerably useful power to the condenser as kinetic energy.

• The turbine performance analysis needs to identify an exhaust area for a particular load that provides a balance between exhaust loss and capital investment in turbine equipment.

Path Lines in Exhaust Hood

Residual velocity loss

• Steam leaving the last stage of the turbine has certain velocity, which represent the amount of kinetic energy that cannot be imparted to the turbine shaft and thus it is wasted

Exhaust end loss

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

2. Exhaust loss depends on the absolute steam velocity.

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

Turn-up loss

Total Exhaust Loss

Gross hood loss

Actual leaving loss

Annulus restriction loss

Annulus Velocity (m/s)

Exh

aust

Los

s, k

J/kg

of

dry

flow

0 120 240 180 240 300 360

10

20

30

40

50Annulus velocity (m/s)

Condenser flow rate

Annulus area

Percentage of Moisture at the Expansion line end point

Typical exhaust loss curve showing distribution of component loss

SP.Volume

an

steamexan A

xvmV

3600

01.01.

Optimal Design of Exhaust Hood

H-s Diagram of Turbine Exhaust Steam

exhaustturbinep ,

exhaustturbine,p ,0

2

2,lastaeV

condenserp

,condenserp0