Calculated reasons for increase in Turbine specific steam consumption

1. Lower vacuum

Turbine consumes more steam, if vacuum in condenser is maintained on lower side.

Example:  Consider a 20 MW Steam Turbine having Inlet steam parameters 65 kg/cm2 & 490 Deg C & Vacuum maintained in condenser is -0.9 kg/cm2.

Calculate the steam consumption of turbine at vacuum -0.9 kg/cm2 & -0.85 kg/cm2

A-Steam consumption Q at -0.9 kg/cm2 to develop 20 MW power

P =Steam flow X( Enthalpy of inlet steam-Enthalpy of exhaust steam)/ 860

Enthalpy of inlet steam at inlet steam parameters =810 kcal/kg

Exhaust steam enthalpy at -0.9 kg/cm2 vacuum = 619 kcal/kg

Then, 20 = Q X (810-619)/860

Q1 = 90 MT

B- Steam consumption Q at -0.85 kg/cm2 to develop 20 MW power

Exhaust steam enthalpy at -0.85 kg/cm2 vacuum= 623 kcal/kg

Then, 20 = Q X (810-623)/860

Q 2= 90.9 MT

It is clear that, Turbine operating at -0.9 kg/cm2 vacuum consumes lesser steam as compared to turbine operating at vacuum-0.85 kg/cm2

2. Lower inlet main stream pressure& temperature

Turbine operating at higher main steam pressure consumes lesser steam as compared to turbines operating at lower pressure

Example: Consider a 20 MW Steam Turbine having Inlet steam temperature 490 Deg C & Vacuum maintained in condenser is -0.9 kg/cm2.

A-Inlet steam parameters: Pressure: 65 kg/cm2 & temperature 490 deg C , Enthalpy = 810 kcal/kg

Exhaust steam parameters P = 0.9 kg/cm2 & Enthalpy = 619 kcal/kg

Steam consumption of Turbine Q = P X 860 / (Enthalpy of inlet steam-Enthalpy of exhaust steam)

Q = 20 X 860 / (810-619)

Q1 = 90.05 MT

B-Inlet steam parameters: Pressure: 87 kg/cm2 & temperature 515 deg C , , Enthalpy = 818 kcal/kg

 

Steam consumption of Turbine Q = P X 860 / (Enthalpy of inlet steam-Enthalpy of exhaust steam)

Q = 20 X 860 / (818-619)

Q2 = 86.43 MT

It is clear that, Turbine operating at pressure 65 kg/cm2 & temperature 490 deg C consumes more steam as compared to turbine operating at 87 kg/cm2 & temperature 515 deg C

3. Higher extraction/bleed steam flow

Steam turbines consume more steam to develop same power on higher steam extraction as compared to lower extraction.

Example: A condensing & extraction steam turbine having Inlet steam flow 105 TPH at pressure 65 kg/cm2 & 490 Deg C & Vacuum maintained in condenser is -0.9 kg/cm2.

Here we can cross check the power generation by steam turbine by increasing the extraction flow keeping inlet steam constant.

A-Extraction pressure = 2 Kg/cm2 & Temperature = 150 Deg C, flow = 75 TPH, Exhaust steam to condenser = 30 TPH

Enthalpy of inlet steam, H1 = 810 kcal/kg

Main steam flow Q1 = 105 TPH

Enthalpy of extraction steam = H2 =660 kcal/kg

Extraction steam flow Q2 = 75 TPH

Enthalpy of exhaust team = 620 kcal/kg

Exhaust steam flow Q3 = 30 TPH

Power developed by steam Turbine P = (Q2 X (H1-H2) / 860) + (Q3 X (H1-H3) / 860 )

P = (75 X (810-660) / 860) + (30 X (810-620) / 860) = 19.7 MW

B- Extraction pressure = 2 Kg/cm2 & Temperature = 150 Deg C, flow = 65 TPH, Exhaust steam to condenser = 40 TPH

Enthalpy of inlet steam, H1 = 810 kcal/kg

Main steam flow Q1 = 105 TPH

Enthalpy of extraction steam = H2 =660 kcal/kg

Extraction steam flow Q2 = 65 TPH

Enthalpy of exhaust team = 620 kcal/kg

Exhaust steam flow Q3 = 40 TPH

Power developed by steam Turbine P = (Q2 X (H1-H2) / 860) + (Q3 X (H1-H3) / 860 )

P = (65 X (810-660) / 860) + (40 X (810-620) / 860) = 20.16 MW

It is clear that, Turbine power generation at same inlet main steam flow will increase as extraction flow gets decrease & vice versa

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4. Higher pressure/temperature of extraction & bleed steam

Higher pressure/temperature of extraction & bleed steam leads to increased steam consumption to generate same power or power consumption reduces at same inlet flow.

Example: A condensing , extraction & bleed steam turbine having Inlet steam flow 105 TPH at pressure 65 kg/cm2 & 490 Deg C & Vacuum maintained in condenser is -0.9 kg/cm2

 A-Bleed steam 10 kg/cm2 & Temperature 200 Deg C, flow =25 TPH, Extraction pressure = 2 Kg/cm2 & Temperature = 150 Deg C, flow = 60 TPH, Exhaust steam to condenser = 25 TPH

Enthalpy of inlet steam, H1 = 810 kcal/kg

Main steam flow Q1 = 105 TPH

Enthalpy of bleed steam = H2 =674 kcal/kg

Bleed steam flow Q2 = 25 TPH

Enthalpy of extraction steam = H3 =660 kcal/kg

Extraction steam flow Q3 = 60 TPH

Enthalpy of exhaust team H4= 620 kcal/kg

Exhaust steam flow Q4 = 20 TPH

Power developed by steam Turbine P = (Q2 X (H1-H2) / 860) + (Q3 X (H1-H3) / 860 ) +(Q4 X (H1-H4)/860)

P = (25 X (810-674) / 860) + (60 X (810-660)/860) + (20 X (810-620)/860)

P = 18.82 MW

B-Bleed steam 14 kg/cm2 & Temperature 260 Deg C, flow =25 TPH, Extraction pressure = 2.5 Kg/cm2 & Temperature = 170 Deg C, flow = 60 TPH, Exhaust steam to condenser = 25 TPH

Enthalpy of inlet steam, H1 = 810 kcal/kg

Main steam flow Q1 = 105 TPH

Enthalpy of bleed steam = H2 =704 kcal/kg

Bleed steam flow Q2 = 25 TPH

Enthalpy of extraction steam = H3 =669 kcal/kg

Extraction steam flow Q3 = 60 TPH

Enthalpy of exhaust team H4= 620 kcal/kg

Exhaust steam flow Q4 = 20 TPH

Power developed by steam Turbine P = (Q2 X (H1-H2) / 860) + (Q3 X (H1-H3) / 860) + (Q4 X (H1-H4)/860)

P = (25 X (810-704) / 860) + (60 X (810-669)/860) + (25 X (810-620)/860)

P = 18.43 MW

It is clear that, Turbine power generation reduces at higher extraction or bleed steam pressure &temperature

Note: Steam consumption of turbine increases if,

1-Bleed steam & extraction steam pressure increases

2-Bleed steam & extraction steam temperature increases

3-Bleed steam flow & extraction steam flow increases

4. Increase of exhaust steam temperature due to more clearance in labyrinth seals

Turbine steam consumption increases if exhaust steam temperature to condenser increases.

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