TURBINE
EFFICIENCY
Sankar Bandyopadhyay
Email : [email protected]
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 KCal
GTCHR =
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.
Classification
Impulse turbine
Reaction turbine
Based on Compounding:
Pressure compounded
Velocity compounded
Impulse Turbines
• An impulse turbine uses the impact force of the steam jet
on the blades to turn the shaft. Steam expands as it passes
through the nozzles, where its pressure drops and its
velocity increases. As the steam flows through the moving
blades, its pressure remains the same, but its velocity
decreases. The steam does not expand as it flows through
the moving blades.
Impulse Turbines
Velocity compounded impulse
turbine
Pressure compounded
Reaction Turbines
In the reaction turbine, the rotor blades themselves are
arranged to form convergent nozzles. This type of turbine
makes use of the reaction force produced as the steam
accelerates 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
Impact of Turbine Efficiency on HR/Output
Description
Effect on
TG HR
Effect on
KW
1% HPT Efficiency
0.16%
0.3%
1% IPT Efficiency
0.16%
0.16%
1% LPT Efficiency
0.5 %
0.5 %
Output Sharing by Turbine Cylinders
210MW
500MW
HPT
28%
27%
IPT
23%
34%
LPT
49%
39%
Gross Turbine cycle Heat rate
Fms( H1 - Hf ) +Frhs( H3 - H2 ) + Fss ( Hf - Hs ) + Frs ( H3 - Hr )
= ----------------------- -----------------------------------------------Pg
Where,
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 Losses
1.External Losses
2. Internal Losses
Turbine External Losses
1. 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
T1
H
P2
X Y
h
Z
P3
W X
’
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.
Z’
s
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
P3
h5
P1
P2
P4
P5
h1
HP eff. =(h1-h2)*100/(h1-h4)
HP exhaust
IP eff.=(h5-h6)*100/(h5-h7)
h2
h3
h4
IP cylinder exhaust
h
Saturation line
P6
h6
h7
s
Parameters required For efficiency calculation
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
Turbine Efficiency – Measurement Points
Station "A" H P Turbine Efficiency vs Load
Turbine Cycle heat Rate Tests
83
HP Turbine Efficiency (%)
81
79
HP Turbine Ef ficiency at CPO (%)
HP Turbine Ef ficiency at VPO (%)
77
75
73
71
69
67
65
170
18 0
19 0
200
Gross Generator Output (MW)
2 10
220
Major energy losses in steam turbine
Blading part of flow path
Non bladed part :
Inlet & Exhaust sections of turbine
casing & valves
Shaft seals
Turbine Seals Loss break up
INTER
STAGE
27%
SHAFT
SEALS
15%
OTHERS
6%
TIP
SEALS
52%
Turbine Surface Roughness
• Surface finish degradation:
- Deposits
- Corrosion
- Solid Particle Erosion
- Mechanical damage
• Roughness up to 0.05 mm can lead to decrease in
efficiency by 4%
Seal Leakage
DIAPHRAGM
TIP
SPILL STRIPS
TENON
TIP
LEAKAGE
COVER OR
SHROUD
STAGE
PRESSURE
ROTATING
BLADE
STATIONARY
BLADE
STEAM FLOW
ROOT LEAKAGE
DOVETAIL
ROOT
SPILL STRIPS
BALANCE HOLE
FLOW
BALANCE HOLE
PACKING
WHEEL
INTERSTAGE PACKING LEAKAGE
SHAFT
Impulse Wheel and
Diaphragm
Construction
Seal Leakage
BLADE
CARRIER
TIP SPILL
STRIPS
TIP
LEAKAGE
TENON
COVER
ROTATING
BLADE
STATIONARY
BLADE
TRAILING
EDGE
Reaction Drum Rotor
Construction
LEADING
EDGE
INTERSTAGE
PACKING
ROTOR
Turbine Sealing
• Seal leakage is important as it is the largest single
cause of performance reduction in HP turbines.
• – Interstage seals. These include seals to prevent
leakage around the rotating and stationary stage.
• – End seals or packing glands are used to minimize
leakage at the ends of cylinders. They are intended
to prevent air injection into the LP and condenser
Damaged Seals
Inter stage seals and peak seals
Diaphragm profile damage
SEALING GLANDS
•
•
•
Steam is supplied to the sealing chamber at 1.03 to 1.05
Kg/sq.cm abs and at temperature 130 deg.C To 150
deg.C from the header.
Air steam mixture from the last sealing chamber is
sucked out with the help of a special steam ejector to
gland steam cooler.
Provision has been made to supply live steam at the
front sealing of H.P. and I.P. rotor to control the
differential expansion, when rotor goes under
contraction 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
0
10000
Total
Trailing Edge Thickness
Hand calculations
94.0
Cover Deposits
Surface Roughness
Flow Change Impact
Flow Path Damage
Miscellaneous Leakages
585.1
End Packings
4000
Tip Spill Strips
Interstage Packings
Power Loss (kW)
Summary of Losses
9287.8
8000
6000
4486.7
3473.0
2000
601.3
47.7
Efficiency Assessment &
Issues
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
HP/IP Turbine Efficiency
Impact of Measurement error on Turbine efficiency
Main Steam
Impact on
HPT
Efficiency
Pressure
Temp
Pressure
Temp
Kg/cm2
Deg C
Kg/cm2
Deg C
1
1
1
1
0.6 %
0.6 %
2.0 %
0.7 %
IPT Inlet
Impact on
IPT
Efficiency
HPT Exhaust
IPT Exhaust
Pressure
Temp
Pressure
Temp
Kg/cm2
Deg C
Kg/cm2
Deg 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
Factors affecting Turbine cycle Heat rate
•
•
•
•
•
•
•
Reheater pressure drop
Superheater spray flow
Reheater spray flow
HP cylinder efficiency
IP cylinder efficiency
Generator hydrogen pressure
Grid frequency
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.
THANK YOU