Saturday, November 10, 2012

Accuracy of Condenser Vacuum indication in Control Room

Modern power stations have pressure transmitters and digital display of Condenser Vacuum in the Control Room. Although the measurement is adequate for monitoring on day to day basis for the same unit there is a problem when you compare the Condenser Vacuum of one unit with another identical unit in the power station. Because when you compare the difference in the two values it is small e.g. Unit no 1 showing -89 Kpa and Unit no 2 showing -90 Kpa and you are making a statement that Unit no 2 is having better vacuum than Unit no 1. In addition some people have a fascination for the magic figure of -0.9 or -90 Kpa and hate any value below the figure.

I generally suspect the measurements when dealing with small differences in the parameters and try to cross check with other parameters and very often I have found that Unit showing -90 Kpa is actually worse than the Unit showing -89 Kpa. My favourite parameter is Condensate Temperature at the suction of Condensate Extraction Pump and my memory takes me to good old 210 MW units.

The sizing of the Hotwell was inadequate in old 200/210 MW units and the  Deaerator level control valve could not work at low Condensate Flows. It was almost impossible to maintain Hotwell level unless you raise the level to full and also use part of the Condenser where area of cross section increases many fold and the storage volume increases beyond the capacity of the Hotwell.

When I looked at the Condensate Temperature in such Unit having small Hotwell capacity I suspected under cooling of Condensate and asked the Desk Engineer to reduce the Hotwell level. When the Desk Operator brought the level within Hotwell the Condensate Temperature increased by more than 2 degC. The under cooling of Condensate was happening because the Hotwell level was being maintained in the Condenser submerging some tubes.

By the time NTPC designed the 200 MW units for Singrauli the above problem was well known and the cross section of Hotwell was made much bigger than the original design but again it was same old Hotwell at Vindhyachal 6x210 MW since it came from USSR.

I hope those reading this blog post are having adequately sized Hotwell and are not maintaining the level in Condenser in their power station and in case the Hotwell is small they can always bring down the level to avoid under cooling of Condensate.

When you are sure of that compare the Condensate Temperature of two units as well and your statement that Unit no 2 is having better vacuum than Unit no 1 may be wrong.