61259769 turbine efficiency

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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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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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.

61259769 turbine efficiency

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