Wednesday, June 12, 2013

Effect of Atmospheric Pressure on measurement of Condenser Vacuum.

I have following measurements of Condenser Vacuum for 210 MW units in two locations:

Tuticorin TPS -662 mmHg CW Inlet 33.15 degC outlet 43.3 degC
Nasik TPS -658 mmHg CW Inlet 28.3 degC outlet 36.15 degC

It appears that there is not much difference between -662 and -658 mmHg in the two measurements the difference is actually much bigger considering that Tuticorin is located at sea level and Nasik at a height of 599 metres from MSL. The barometric pressures at these locations are as follows:

Tuticorin 760 mmHg
Nasik 712 mmHg

The absolute pressure for the measurement would be:
Tuticorin -662+760 = 98 mmHg = 130 mbar
Nasik -658+712 = 54 mmHg = 72 mbar

The mercury gauges are not found in the modern plant so let me convert the values to Kg/cm2 and Kpa for understanding by the new generation of engineers:

Tuticorin -662 mmHg = -0.8998 Kg/cm2 = -88.23 Kpa
Nasik -658 mmHg = -0.89436 Kg/cm2 = -87.7 Kpa

It appears that the difference is only 0.5 Kpa but the real difference when converted to absolute pressure would be 130 - 72 = 58 mbar = 5.8 Kpa

The best way to get the atmospheric pressure of your place is to use a Barometer. In case you don't have it you can know the atmospheric pressure approximately from the altitude of the place.
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Altitude metres Atmospheric Pressure mbar
50 1007.27
100 1001.29
150 995.38
200 989.48
250 983.57
300 977.67
350 971.86
400 966.08
450 960.3
500 954.54
Source of the above data.

Please note that altitude of a place town/city also varies a lot e.g. if you read Wikipedia page about Nasik it gives the altitude as 560 m and it is true that the area around Godavari River is at 560 m but the Thermal Power Station is situated at 599 m. To get the correct altitude you should use the data of your power station or you can use Google Maps on this link.