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7/29/2019 61259769 Turbine Efficiency
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TURBINEEFFICIENCY
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THERMAL CYCLE EFFICIENCY The thermal power station works on the principle of modified Rankine
Cycle
In the ideal cycle, the steam expands in the turbine and the expansion is
assumed to be frictionless and adiabatic. The expansion of steam
continues until some reduced pressure. Condensation at a constanttemperature takes place until all the latent heat has been removed
There are 2 ways to improve the basic Rankine efficiency:
i) Reduce the rejected heat component
ii) Increase the useful heat component
The rejected heat component is dependent primarily upon the
condensation temperature and this in turn is determined by the cooling
water (CW) temperature (usually is a bit controllable)
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THERMAL CYCLE EFFICIENCYThe useful heat is determined largely by the steam temperature.
The cycle efficiency can, therefore, be improved by reheating and feed heating
Rankine cycles efficiencies:
Rankine cycle efficiency (Ideal) : 41.4%Rankine cycle efficiency with super- heating(S/H) : 45.7%
Rankine cycle efficiency with S/H & R/H : 47.5%
Rankine cycle efficiency with S/H, R/H & Reg. feed heating : 53.2%
Therefore, incorporation of reheating increases the total heat input and
incorporation of feed heating reduces the amount of heat rejected, therebyincreasing the cycle efficiency.
In general, the entire cycle efficiency of a power station depends upon theefficiency of its components i.e. boiler, turbine, generator, pumps etc.
Cycle efficiency = Boiler efficiency x Turbine efficiency x Gen. efficiency
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TURBINE EFFICIENCYThe turbine efficiency depends upon the following factors:
INTERNAL LOSSES
EXTERNAL LOSSES
INTERNAL LOSSES: Nozzle friction: The effect of nozzle friction is to reduce the
effective heat drop of the steam as it passes over the nozzle
Blade friction: Its effect is the same as that of the nozzle friction.As friction increases, steam expansion tends to be more irreversible i.e.its not ideal isentropic expansion.
Stage efficiency= (Actual heat drop/ Isentropic heat drop) x 100
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TURBINE EFFICIENCY Disc friction: The discs on impulse turbine shafts rotate
in an atmosphere of steam. The disc surface friction causes some drag
and produces eddies of steam causing loss of power.
Tip leakage: In impulse/ reaction turbines, there is pressure
drop across each stage or blade; thus there is steam flow around tips of
all fixed and moving blades. Seals at the tips in radial & axial directions
are provided. Because of wear & tear of these seals, leakage loss can
amount from 0.55% to 1.0%.
Partial admission loss: In nozzle- governed machines in particular,
and in throttle governed machines at part load conditions, steam is
subjected to throttling. The throttling process causes loss of efficiency.
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TURBINE EFFICIENCY Exhaust loss: The kinetic energy of steam as it leaves the last LP
stage cannot be gainfully employed to do some useful work, hence it is aloss. This loss varies with the turbine back pressure/ vacuum.
Wetness loss: The wetness of steam goes on increasing towardsthe last stages of a turbine. Condensation of steam causes wetness orformation of water droplets on blades which lose some mechanical workin throwing off the drops. Apart from that, severe erosion is also causedto the blade tips of last stages. Generally, 1% increase in wetnesscauses 1% loss in efficiency.
EXTERNAL LOSSES
The external losses are due to shaft gland leakage, journal & thrustbearing, Governor and Oil pump etc.
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TURBINE CYLINDER EFFICIENCYThe HP and IP cylinder efficiencies can be calculated byaccurately measuring the temperature & pressure of steam beforeand after the respective turbine cylinders.
The LP cylinder efficiency cannot be calculated as the steam is
wet there and exact state point is not known.
Turbine stage efficiency=Actual enthalpy drop/ Isentropic enthalpy drop
The design values of HP cylinder efficiency & IP cylinder efficiency for 210MW Unit-6 are 85.8% and 90.26 % respectively
Common causes of cylinder efficiency deterioration are:
Damage to blades caused by debris past the strainers
Damaged seals and glands
Deposition on blades
Increased roughness of blade surfaces
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Turbine Stage EfficiencyP1
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 isfrictionless heat drop.
Stage efficiency = (Heat drop HZ / Heat drop HX) x 100 %
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Turbine Heat RateIt is the amount of heat supplied in TG cycle to generate one
kWh of Power
Heat rate = Heat input in Kcal / Power output in KWH
The design value of TG cycle heat rate for 210 MW Unit-6 is 1979.33
kCal/ kWh
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Impact of Turbine Efficiency on HR/Output
Description Effect on Effect onTG HR MW
_____________________________________________
1% HPT Efficiency 0.16% 0.3%
1% IPT Efficiency 0.16% 0.16%
1% LPT Efficiency 0.5 % 0.5 %
Output Sharing by TurbineCylinders
210MW 500MW
HPT 28% 27%
IPT 23% 34%
LPT 49% 39%
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EFFECT OF OPERATING
PARAMETERS ON CYCLE EFFICIENCYThe following operating parameters directly affect the cycle efficiency:
TURBINE BACKPRESSURE/ VACUUM
ESV STEAM PRESSURE
ESV/ RH STEAM TEMPERATURE AMOUNT OF ATTEMPERATION SPRAY
FINAL FEED WATER TEMPERATURE
BOILER EXCESS AIR
COMBUSTION IN ASH
APH GAS OUTLET TEMPERATURE DM WATER MAKE-UP RATE
AUX. POWER CONSUMPTION
UNIT LOAD
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EFFECT OF TURBINE BACK PRESSURE
This is the most important parameter that severely affects the efficiency of apower plant cycle. Improving the backpressure improves the amount of workdone by steam but up to a limit.
The turbine vacuum is dependent upon:
1. Condenser air tightness
2. CW inlet temperature
3. Condenser tube fouling
4. Performance of ejectors/ vacuum pump
5. CW flow in condenser
Increasing the turbine vacuum beyond the optimum value also increases somelosses which are listed as under:
CW pumping power
Leaving/ exhaust loss
Reduced condensate/ feed water temperature
Wetness losses
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Condenser About 50 % heat is lost in condenser
For a 210 MW unit about 28000 TPH of CW water is required
Slight deterioration in the performance of condenser leads to huge
financial loss. According to a study, an improvement of 1 mm (Hg) (.001
kg/cm2) reduces the Heat Rate by 2 kCal. Recently, condenser vacuum
of a 210 MW Unit increased after chemical cleaning by approx. 0.14
kg/cm2 which resulted in an annual savings of approx. Rs. 15 Crores at
80% annual PLF
Normally design value of condenser pressure is 70 mm Hg abs (0.1 Kg
abs) (0.9 kg/cm2)
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THERMAL PROCESSES OCCURRING IN
CONDENSERS
The condenser never receives pure steamfrom the turbine.
A mixture of steam and non-condensable
gases (Air-steam mixture) enters thecondenser.
The ratio of the quantity of gas that enters the
condenser to the quantity of steam is called
the relative air content.
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FACTORS AFFECTING THE PERFORMANCE
OF THE CONDENSER
CW inlet temperature
CW flow Presence of non-condensable gases
Ejector/ vacuum pump performance
Dirty tubes
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TEMPERATURE PROFILE IN CONDENSER
CW Temperature
Condensing Steam Temp.
TTDITD
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Feed water Heaters
Feedwater heaters increases the cycle efficiency by
increasing the average temperature of heat addition.
Typically 6 heaters ; 2 HP, 3 LP and 1 Deaerator used in
a 210 MW Unit.
Heaters can be either open or close
Deaerator is an open feedwater heater
Feedwater heaters are either horizontal or vertical
Mostly shell and tube type heaters are used
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EFFECT OF FEED WATER TEMPERATURE
The feed water temperature at boiler inlet is another main
important factor determining cycle efficiency. It is mainly
dependent upon
Heater performance Feed flow through heaters
Terminal temperature difference (TTD)
Bled steam pipe pressure drop
Steam temperature at heater inlet
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FEED WATER HEATER PERFORMANCE
Two important important parameters used in assessing heater performance areTTD (Terminal temp. difference) and Drain Approach.
TTD is the difference between the outlet feed water temp. and the steamsaturation temperature at the extraction pressure. 1 C rise in TTD leads to
0.027% drop in efficiency.
Drain Approach is defined the difference between feed water temperature atinlet to heater and drain outlet temperature after the heater.
Factors causing deterioration ofHeater Performance
Air accumulation
Water side contamination
Steam side contamination
Drainage defects
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FEED WATER HEATER PERFORMANCE
Air accumulation: Air is a superb thermal insulator, hence, highlyundesirable. Proper vents are provided on heaters body to prevent accumulationof air. Air can get into heaters when extraction pressure in them is reducedbelow atmospheric pressure i.e. when the machine load is reduced or themachine is off-loaded.
Steam side fouling: Cupro- nickel (70/30) alloys were generallyused as heater tube material. This material has a tendency to exfoliate i.e. itflakes off like dead skin. Due to this, the space between the tubes in the clusterbecomes blocked with debris and heat transfer is progressively reduced. Now adays, 90/10 cupro-nickel is being used to reduce problems of exfoliation.
The effects of exfoliation include: Progressive increase of TTD
Reduced feed water temperature rise
Eventual tube failure due to weakening
Accumulation of debris in the heater shell
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FEED WATER HEATER PERFORMANCE
Water side fouling: Most common cause of water side fouling is oil. Oil can
get into the system from leaking bearings and gland seals of LP turbine.
Deposition of oil occurs in HP heaters thereby affecting heater performance.
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EFFEC
T OFH
EATER OUT OF SERVIC
EAnyone heater being out of service considerably affects the cycle
efficiency. Feed water temp. is lowered and the next heater has to do
extra work. If the final (highest pressure) heater is taken out, the feed
water to boiler is at lower temp. and has to have extra heat given in the
boiler.Further, bled steam, which is now not being bled, can do extra work
in the turbine, significantly improving the Unit output although at the
expense of lower thermal efficiency.
The cycle efficiency reduces by about 0.5% when a LP heater is
kept out of service and by 1.5% when the last HPH is kept out ofservice.
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