Most Steam Turbines in India are designed considering Cooling Water Inlet Temperature of 33 degC and temperature rise of 9 degC through the Condenser.
Assuming Terminal Temperature Difference (TTD) of 4 degC the saturation temperature at Turbine exhaust works out as follows
CW Inlet Temperature + Temperature rise through Condenser + TTD
=33+9+4 = 46 degC
Turbine Back Pressure corresponding to 46 degC is 0.101 Bar and 0.103 Kg/cm2
Most Thermal Power Stations are located at Mean Sea Level (MSL) between 200 to 250 Metres and the Barometric Pressure corresponding to these elevations are 1.003 Kg/cm2 and 1.009 Kg/cm2.
If you subtract 1.003 from 0.103 you get -0.9 Kg/cm2.
That is why most people think that -0.9 Kg/cm2 gauge is the correct value of Condenser Vacuum.
There is one more problem. The above are the design parameters for NTPC's flagship Thermal Power Station at Singrauli even when it has once through Cooling Water System where the Cooling Water Temperature hardly touches 27 degC. Therefore the actual Condenser Vacuum used to be much better than -0.9 Kg/cm2 not because of any achievement by the flagship station but because of once through Cooling Water System.
People not only think that -0.9 Kg/cm2 is the correct Condenser Vacuum they also think that they need to add 1 Kg/cm2 to get the absolute value of Back Pressure i.e. -0.9 + 1 = 0.1 ata assuming Barometric Pressure as 1 Kg/cm2.
Today I am dealing with one person at 2x500 MW Durgapur Steel Thermal Power Station at DVC who is getting 0.1077 ata by adding 1 Kg/cm2 and thinks that it is correct and apparently there is no vaccum problem although the LP Turbine exhaust and Hotwell Temperatures are indicating 50 to 51 degC.
The Mean Sea Level at the Power Station is 73 Metres and the Barometric Pressure corresponding to it is 1.024 Kg/cm2.
Therefore the correct Turbine Back Pressure would be
= 0.1077-1.0+1.024 = 0.1317 ata which is very poor vacuum.
When I am delivering lectures on Turbine Efficiency my first lesson is to show the variation of Barometric Pressure with altitude of the place.
There is another interesting incidence of JSW Energy at Vijaynagar (Torangallu).
The Mean Sea Level at Torangallu is 520 M and the Barometric Pressure corresponding to it is 95.22 Kpa. The design back pressure is 10.05 Kpa but the station people are using 100 Kpa as Barometric Pressure and saying that design value of Condenser Vacuum is 10.05-100 = -89.95 Kpa.
In fact they were not believing what I was teaching till I pointed out to them that the Barometric Pressure recorded in the PG Test Report was 95.193 Kpa so close to the value I told.
One should say that the design value of Turbine Back Pressure is 10.05 Kpa and if you convert it to Condenser Vacuum it will be -85.17 Kpa valid only for Torangallu and not for Ratnagiri the other Power Station of JSW Energy where the Barometric Pressure is 100.7 Kpa and the Condenser Vacuum should be -90.65 Kpa (Notice the difference of 5.48 Kpa).
Update on 14th July 2014
Although the Condenser Vacuum should be generally -83 to -87 kPa at Torangallu it was actually in the range of -87 to -91 kPa.
Today I checked in the DCS Engineering Room and found that DCS was adding -4 kPa to what was coming from the transmitter as follows:
The transmitter has the range 0 to -100 kPa for 0 to 4 ma but the DCS was converting 0 to 4 ma into -4 to -104 kPa thereby adding -4 kPa.
When I asked whether Chinese advised to add -4 kPa the Engineer replied that BHEL had set it like this in 130 MW Unit and same was adopted in 300 MW Units for uniformity.
So that is one example of how BHEL cheats. Off course I can write many articles about how BHEL cheats in various ways.
Instead of teaching that the Vacuum indication will be different considering the Barometric Pressure of Torangallu they have simply made the DCS to add -0.04 Kg/cm2 (BHEL still uses MKS units) to the value coming from the transmitter.
Assuming Terminal Temperature Difference (TTD) of 4 degC the saturation temperature at Turbine exhaust works out as follows
CW Inlet Temperature + Temperature rise through Condenser + TTD
=33+9+4 = 46 degC
Turbine Back Pressure corresponding to 46 degC is 0.101 Bar and 0.103 Kg/cm2
Most Thermal Power Stations are located at Mean Sea Level (MSL) between 200 to 250 Metres and the Barometric Pressure corresponding to these elevations are 1.003 Kg/cm2 and 1.009 Kg/cm2.
If you subtract 1.003 from 0.103 you get -0.9 Kg/cm2.
That is why most people think that -0.9 Kg/cm2 gauge is the correct value of Condenser Vacuum.
There is one more problem. The above are the design parameters for NTPC's flagship Thermal Power Station at Singrauli even when it has once through Cooling Water System where the Cooling Water Temperature hardly touches 27 degC. Therefore the actual Condenser Vacuum used to be much better than -0.9 Kg/cm2 not because of any achievement by the flagship station but because of once through Cooling Water System.
People not only think that -0.9 Kg/cm2 is the correct Condenser Vacuum they also think that they need to add 1 Kg/cm2 to get the absolute value of Back Pressure i.e. -0.9 + 1 = 0.1 ata assuming Barometric Pressure as 1 Kg/cm2.
Today I am dealing with one person at 2x500 MW Durgapur Steel Thermal Power Station at DVC who is getting 0.1077 ata by adding 1 Kg/cm2 and thinks that it is correct and apparently there is no vaccum problem although the LP Turbine exhaust and Hotwell Temperatures are indicating 50 to 51 degC.
The Mean Sea Level at the Power Station is 73 Metres and the Barometric Pressure corresponding to it is 1.024 Kg/cm2.
Therefore the correct Turbine Back Pressure would be
= 0.1077-1.0+1.024 = 0.1317 ata which is very poor vacuum.
When I am delivering lectures on Turbine Efficiency my first lesson is to show the variation of Barometric Pressure with altitude of the place.
There is another interesting incidence of JSW Energy at Vijaynagar (Torangallu).
The Mean Sea Level at Torangallu is 520 M and the Barometric Pressure corresponding to it is 95.22 Kpa. The design back pressure is 10.05 Kpa but the station people are using 100 Kpa as Barometric Pressure and saying that design value of Condenser Vacuum is 10.05-100 = -89.95 Kpa.
In fact they were not believing what I was teaching till I pointed out to them that the Barometric Pressure recorded in the PG Test Report was 95.193 Kpa so close to the value I told.
One should say that the design value of Turbine Back Pressure is 10.05 Kpa and if you convert it to Condenser Vacuum it will be -85.17 Kpa valid only for Torangallu and not for Ratnagiri the other Power Station of JSW Energy where the Barometric Pressure is 100.7 Kpa and the Condenser Vacuum should be -90.65 Kpa (Notice the difference of 5.48 Kpa).
Update on 14th July 2014
Although the Condenser Vacuum should be generally -83 to -87 kPa at Torangallu it was actually in the range of -87 to -91 kPa.
Today I checked in the DCS Engineering Room and found that DCS was adding -4 kPa to what was coming from the transmitter as follows:
The transmitter has the range 0 to -100 kPa for 0 to 4 ma but the DCS was converting 0 to 4 ma into -4 to -104 kPa thereby adding -4 kPa.
When I asked whether Chinese advised to add -4 kPa the Engineer replied that BHEL had set it like this in 130 MW Unit and same was adopted in 300 MW Units for uniformity.
So that is one example of how BHEL cheats. Off course I can write many articles about how BHEL cheats in various ways.
Instead of teaching that the Vacuum indication will be different considering the Barometric Pressure of Torangallu they have simply made the DCS to add -0.04 Kg/cm2 (BHEL still uses MKS units) to the value coming from the transmitter.
