April 10, 2023 1

TURBINE EFFICIENCY

April 10, 2023 2

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 constant temperature 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)

April 10, 2023 3

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, thereby increasing the cycle efficiency.

In general, the entire cycle efficiency of a power station depends upon the efficiency of its components i.e. boiler, turbine, generator, pumps etc.

Cycle efficiency = Boiler efficiency x Turbine efficiency x Gen. efficiency

April 10, 2023 4

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

April 10, 2023 5

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.

April 10, 2023 6

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 a loss. This loss varies with the turbine back pressure/ vacuum.

• Wetness loss: The wetness of steam goes on increasing towards the last stages of a turbine. Condensation of steam causes wetness or formation of water droplets on blades which lose some mechanical work in throwing off the drops. Apart from that, severe erosion is also caused to the blade tips of last stages. Generally, 1% increase in wetness causes 1% loss in efficiency.

EXTERNAL LOSSESThe external losses are due to shaft gland leakage, journal & thrust bearing, Governor and Oil pump etc.

April 10, 2023 7

TURBINE CYLINDER EFFICIENCYThe HP and IP cylinder efficiencies can be calculated by accurately measuring the temperature & pressure of steam before and 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 210 MW 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

April 10, 2023 8

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 is frictionless heat drop.

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

9

Turbine Heat Rate

It 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

April 10, 2023 10

Impact of Turbine Efficiency on HR/Output

Description Effect on Effect on TG 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 Turbine Cylinders

210MW 500MW

HPT 28% 27%

IPT 23% 34%

LPT 49% 39%

April 10, 2023 11

EFFECT OF OPERATING PARAMETERS ON CYCLE EFFICIENCY

The 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

April 10, 2023 12

EFFECT OF TURBINE BACK PRESSURE

This is the most important parameter that severely affects the efficiency of a power plant cycle. Improving the backpressure improves the amount of work done by steam but up to a limit.The turbine vacuum is dependent upon:1. Condenser air tightness2. CW inlet temperature3. Condenser tube fouling4. Performance of ejectors/ vacuum pump5. CW flow in condenser

Increasing the turbine vacuum beyond the optimum value also increases some losses which are listed as under:

• CW pumping power• Leaving/ exhaust loss• Reduced condensate/ feed water temperature• Wetness losses

April 10, 2023 13

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)

April 10, 2023 14

THERMAL PROCESSES OCCURRING IN CONDENSERS

• The condenser never receives pure steam from the turbine.

• A mixture of steam and non-condensable gases (Air-steam mixture) enters the condenser.

• The ratio of the quantity of gas that enters the condenser to the quantity of steam is called the relative air content.

April 10, 2023 15

FACTORS AFFECTING THE PERFORMANCE OF THE CONDENSER

• CW inlet temperature • CW flow• Presence of non-condensable gases • Ejector/ vacuum pump performance• Dirty tubes

April 10, 2023 17

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

April 10, 2023 18

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

April 10, 2023 19

FEED WATER HEATER PERFORMANCE

Two important important parameters used in assessing heater performance are TTD (Terminal temp. difference) and Drain Approach.

TTD is the difference between the outlet feed water temp. and the steam saturation 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 at inlet to heater and drain outlet temperature after the heater.

Factors causing deterioration of Heater Performance• Air accumulation• Water side contamination• Steam side contamination• Drainage defects

April 10, 2023 20

FEED WATER HEATER PERFORMANCE

Air accumulation: Air is a superb thermal insulator, hence, highly undesirable. Proper vents are provided on heaters body to prevent accumulation of air. Air can get into heaters when extraction pressure in them is reduced below atmospheric pressure i.e. when the machine load is reduced or the machine is off-loaded.

Steam side fouling: Cupro- nickel (70/30) alloys were generally used as heater tube material. This material has a tendency to exfoliate i.e. it flakes off like dead skin. Due to this, the space between the tubes in the cluster becomes blocked with debris and heat transfer is progressively reduced. Now a days, 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

April 10, 2023 21

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.

April 10, 2023 22

Performance monitoring

• Individual Feed water outlet temperature as per heat balance diagram.

• Heater drip level as recommended.• TTD and DCA within limit.

April 10, 2023 23

EFFECT OF HEATER OUT OF SERVICEAnyone 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 of service.