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.
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