TURBINE EFFICIENCY

Sankar BandyopadhyayEmail : sankarbando1956@gmail.com

Heat Rate - concept• Common term used for indicating Power Station efficiency

• Heat rate = Heat input in Kcal / Power output in KWH

Defined : Heat required in Kcal to generate one KWH of Power

Heat Rate

UHR= TG HR/ BOILE EFFY= 2000/0.85 = 2235 kcal/kWh

NHR (Net Heat rate)= UHR /(1-apc%/100)= UHR/0.93 (Assumed APC %= 7 %)= 2235/0.93= 2403 kcal/kWh

Net Unit Thermal Efficiency= 860/2403 * 100 = 35.8 %

Efficiency and Heat Rate

• Efficiency (%) = Power generated in KWH*860* 100 / Heat Input in Kcal

= 860*100/Heat rate •Gross Turbine cycle Heat rate

Heat input to Turbine cycle in KCalGTCHR =

Power generated in KWh

Sensitive Analysis of Turbine Efficiency on Heat Rate

1 % change in HP or IP Turbine Efficiency in a 500 MW unit leads to change in HR by about

4.5 kcal/kWh and having cost implication of about Rs 57 lakhs per year (rail fed station)

• A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work.

Impulse turbine Reaction turbine

Based on Compounding:

Pressure compounded

Velocity compounded

Classification

Impulse Turbines

• An impulse turbine uses the impact force of the steam jeton the blades to turn the shaft. Steam expands as it passesthrough the nozzles, where its pressure drops and itsvelocity increases. As the steam flows through the movingblades, its pressure remains the same, but its velocitydecreases. The steam does not expand as it flows throughthe moving blades.

Impulse Turbines

Velocity compounded impulse turbine

Pressure compounded

Reaction Turbines

In the reaction turbine, the rotor blades themselves arearranged to form convergent nozzles. This type of turbinemakes use of the reaction force produced as the steamaccelerates through the nozzles formed by the rotor.

Reaction Turbines

Velocity Triangles • Basic analysis of the effect of the blade rows on the steam

flow can be done through velocity triangles

Impulse- Reaction Comparison • Three significant differences (nature of the expansion process)

• Number of stages, • Bucket design, • Stage sealing requirements

• Peak efficiency is obtained in an impulse stage with more work per stage than in a reaction stage, assuming the same bucket diameter.

• Relative to an impulse turbine, a reaction turbine requiring either 40% more stages, 40% greater stage diameters, or some combination of the two to obtain the same peak efficiency.

• Reaction stage has a higher aerodynamic efficiency than an impulse stage. • Leakage losses are higher on the reaction stages • As the blade height increases, the influence of leakage losses decrease and a

point is reached where the reaction stage is more efficient

RANKINE CYCLE

1% HPT Efficiency 0.16% 0.3%

1% IPT Efficiency 0.16% 0.16%

1% LPT Efficiency 0.5 %

Output Sharing by Turbine Cylinders

210MW 500MW

HPT 28% 27%

IPT 23% 34%

LPT 49% 39%

0.5 %

Impact of Turbine Efficiency on HR/Output

Description Effect on Effect on TG HR KW

Gross Turbine cycle Heat rate

Fms( H1 - Hf ) +Frhs( H3 - H2 ) + Fss ( Hf - Hs ) + Frs ( H3 - Hr ) = ----------------------- ------------------------------------------------

PgWhere,

Fms = Main steam flow (T/Hr) Hs = Enthalpy of S/H spray water Frhs= Hot reheat steam flow Hr = Enthalpy of R/H spray water Fss= Superheater spray flow Pg = Power generated Frs= Reheater spray flow H1 = Enthalpy of Main steam Hf= Enthalpy of feed water H3 = Enthalpy of Hot reheat steam H2= Enthalpy of Cold reheat steam

Heat Added to cycle :

Heat Added MS = Flow MS * (hMS - hFW), kcal/hr

Heat Added CRH = Flow CRH* (hHRH - hCRH),kcal/hr

Heat added by SH Attemp= Flow SH Attemp* (hMS-hSHATT) Kcal/hr

Heat added by RH Attemp= Flow RH Attemp * (hHRH-hRHATT) Kcal/hr

Turbine Losses1.External Losses

2. Internal Losses

Turbine External Losses1.Shaft gland leakage Losses

2. Journal & thrust bearing losses

3.Governor & oil pump losses

Turbine Internal Losses

• Inter stage gland leakage loss

• Wetness loss

• Leaving Loss

• Exhaust loss

•Pressure drop losses

•Control valves

•Pipes

Turbine Stage Efficiency P1

P2

P3

T1

h

s

H

X YZ

W X’

Z’

Due to friction the relative velocity of steam gets reduced and hence the heat drop across the blade gets shifted from X to Z where HX is frictionless heat drop.

Stage efficiency = (Heat drop HZ / Heat drop HX) x 100 %

Turbine Cylinder efficiency

• HP cylinder efficiency

• IP cylinder efficiency

Cylinder efficiency =

Actual enthalpy drop *100/ Isentropic enthalpy drop

h

s

P1 P2P3 P5P4

P6

Saturation line

h1

h2h3h4

h5

h6h7

HP eff. =(h1-h2)*100/(h1-h4)

IP eff.=(h5-h6)*100/(h5-h7)

IP cylinder exhaust

HP exhaust

1 Gross Load 13 FW Press HPH Inlet 2 MS Pressure before ESV 14 FW Temp HPH Inlet 3 MS Temp before ESV 15 FW Press HPH Outlet 4 HPT Exhaust Pressure 16 FW Temp HPH Outlet

5 HPT Exhaust Temp. 17 Main Steam Flow (Q1) 6 HRH Steam Press. before IV 18 Feed Water Flow (Qf) 7 HRH Steam Temp. before IV 19 CRH Flow (Q2) 8 FW press after top heater 20 S/H Spray Flow (Qs) 9 FW Temp at Eco inlet 21 R/H Spray Flow (Qr) 10 HPH Ext. Steam Temp 22 S/H Spray Temp.

11 HPH Shell Pressure 23 R/H Spray Temp. 12 HPH Drip Temp 24 Leak Off Flow

Parameters required For efficiency calculation

Turbine Efficiency – Measurement Points

6 5

6 7

6 9

71

73

75

77

79

8 1

8 3

170 18 0 19 0 2 0 0 2 10 2 2 0

Gross Generator Output (MW)

HP

Turb

ine

Effic

ienc

y (%

) HP Turbine Ef f iciency at CPO (%)HP Turbine Ef f iciency at VPO (%)

Station "A" H P Turbine Efficiency vs LoadTurbine Cycle heat Rate Tests

Major energy losses in steam turbine

Blading part of flow pathNon bladed part :Inlet & Exhaust sections of turbine

casing & valvesShaft seals

OTHERS6%

SHAFT SEALS15%

INTER STAGE

27%

TIP SEALS

52%

Turbine Seals Loss break up

• Surface finish degradation:

- Deposits

- Corrosion

- Solid Particle Erosion

- Mechanical damage

• Roughness up to 0.05 mm can lead to decrease in efficiency by 4%

Turbine Surface Roughness

DIAPHRAGM

TIPSPILL STRIPS

BALANCE HOLE

WHEELPACKING

SHAFT

STEAM FLOW

ROOTSPILL STRIPS

ROTATINGBLADE

STATIONARYBLADESTAGE

PRESSURE

DOVETAIL

INTERSTAGE PACKING LEAKAGE

BALANCE HOLEFLOW

ROOT LEAKAGE

TIPLEAKAGE

COVER ORSHROUD

TENON

Impulse Wheel and Diaphragm

Construction

Seal Leakage

ROTOR

TRAILINGEDGE

BLADECARRIER

LEADINGEDGE

INTERSTAGEPACKING

TIP SPILLSTRIPS

STATIONARYBLADE

ROTATINGBLADE

TENONTIPLEAKAGE

COVER

Reaction Drum Rotor Construction

Seal Leakage

• Seal leakage is important as it is the largest singlecause of performance reduction in HP turbines.

• – Interstage seals. These include seals to preventleakage around the rotating and stationary stage.

• – End seals or packing glands are used to minimizeleakage at the ends of cylinders. They are intendedto prevent air injection into the LP and condenser

Turbine Sealing

Damaged Seals

Inter stage seals and peak seals

Diaphragm profile damage

SEALING GLANDS

• Steam is supplied to the sealing chamber at 1.03 to 1.05Kg/sq.cm abs and at temperature 130 deg.C To 150deg.C from the header.

• Air steam mixture from the last sealing chamber issucked out with the help of a special steam ejector togland steam cooler.

• Provision has been made to supply live steam at thefront sealing of H.P. and I.P. rotor to control thedifferential expansion, when rotor goes undercontraction during a trip or sharp load reduction.

Labyrinth seal

Typical Gland Seal in an HP Turbine

Rotor Shaft Area with Gland Seals Exposed

Seal Steam System

Summary of Losses

585.1

3473.0

601.3

9287.8

4486.7

47.794.00

2000

4000

6000

8000

10000

Inte

rsta

ge P

acki

ngs

Tip

Spill

Stri

ps

End

Pack

ings

Mis

cella

neou

s Le

akag

es

Flow

Pat

h D

amag

e

Flow

Cha

nge

Impa

ct

Sur

face

Rou

ghne

ss

Cov

er D

epos

its

Han

d ca

lcul

atio

ns

Trai

ling

Edge

Thi

ckne

ss

Tota

l

Pow

er L

oss

(kW

)

Followings are the reasons for error in computation of efficiency.

HPT efficiency test not done at VWO

Measurement points are not representative

Steam Turbine gas plant (Exhaust point after mixing of LP steam)

Steady conditions of Unit is not achieved.

Necessary corrections like ambient pressure, water leg not taken care

Measuring instruments are not accurate

Efficiency Assessment & Issues

HP/IP Turbine EfficiencyImpact of Measurement error on Turbine efficiency

Impact on HPT

Efficiency

Main Steam HPT Exhaust

PressureKg/cm2

TempDeg C

PressureKg/cm2

TempDeg C

1 1 1 1

0.6 % 0.6 % 2.0 % 0.7 %

Impact on IPT

Efficiency

IPT Inlet IPT Exhaust

PressureKg/cm2

TempDeg C

PressureKg/cm2

TempDeg C

1 1 1 1

1.2 % 0.3 % 6.0 % 0.4 %

FACTORS EFFECTING TURBINE EFFICIENCY

Effect of load

Terminal condition i. MS and RH P

&T ii. Effect of vacuum

Effect of heater efficiency Feed pump power

Factors affecting Turbine cycle Heat rate

• Unit Load

• Main steam temperature

• Main steam pressure

• Hot reheat temperature

• Condenser back pressure

• Final feed water temperature

• Make up water flow

• Reheater pressure drop • Superheater spray flow • Reheater spray flow • HP cylinder efficiency • IP cylinder efficiency • Generator hydrogen pressure • Grid frequency

Factors affecting Turbine cycle Heat rate

Efficiency Tests for the Assessment of Turbine Cycle Efficiency

1. Turbine Heat consumption test 2. Condenser Performance Test 3. HP Heaters Performance Test 4. Turbine Pressure Survey 5. HP / IP Cylinder Efficiency Tests 6. Estimation of Unaccounted Losses

Turbine heat consumption test • To determine the heat input into the turbine for 1 KWh of output at a particular loading. • During the test • The plant condition should be as steady as possible • RH spray flow should ideally be zero. • All the flow measurement of water and steam to be corrected for amount of in-leakage.

Turbine heat consumption test • When heat consumption at different load are plotted on a graph,

it is supposed to lie on a straight line called ‘Willans Line’. • At lower load the heat rate increases because the prominence

of fixed heat component on total heat consumption. • The slope of the curve is known as incremental heat rate.

Unaccountable Losses

High Energy drain Passing Instrument Error / Uncertainty System Water Loss L.P. Turbine Performance L.P. Heaters

High Energy Drains

Passing of High Energy drain valve affects in 3 ways

• Loss of High Energy steam

• Deterioration in Condenser Vacuum

• Damage to the valve

Methodology to reduce Unaccountable High energy drains passing

• Listing of all the drains/steam traps • Temperature mapping of drains • Action plan for repair replacement of valves • Installation of thermocouples on down stream • Progressive replacement of High energy drain valves • Attending valves during opportunity shut down. • Checking of valve of valve passing before O/H • Joint checking by Operation & TMD after unit startup

Methodology to reduce Unaccountable

Instrument Error

• Use of accurate & calibrated Instrument

System Water Loss

• D/A drop test to be conducted periodically.

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