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. 

Comparison of Condenser Vacuum in Cooling Tower versus once through Cooling Water System.

I am going to mostly write about condensing, reheat turbines with regenerative cycle. Condenser Vacuum is the vital parameter of such machine and let us talk about what is achievable today compared to old power stations.

Once again my memory takes me to Obra. On local panel we use to have a mercury tube indicating as much as 712 mmHg vacuum in December/January. In summer it use to be around 690 mmHg.

Our competitor was Badarpur where it hardly touched 690 mmHg in winter.

I asked the Engineering Department in BHEL Haridwar and they told me that Badarpur Station had Cooling Tower whereas Obra was having once through Cooling Water System and our Cooling Water temperature was very low compared to what the Cooling Tower would give.

After joining Operation Services Department at NTPC Corporate Centre in 1983 and studying the Acceptance Test Codes of Steam Turbine as well as Cooling Tower I realised the real difference.

The temperature of Cooling Water in once through system can be as low as the Wet Bulb Temperature but the Cooling Tower has to be infinitely large to equal that temperature. We had Induced Draft Cooling Towers designed to deliver 33 degC temperature at 27 degC Wet Bulb Temperature. The difference in the two temperature is called approach. The approach was 6 degC at design point but it was increasing as much as 12 degC at 15 degC wet bulb temperatures.

Without the technical jargon it meant that the max temperature in once through system was 27 degC and in Cooling Tower it was 33 degC. In winter the once through system could be lower than 12 degC but in Cooling Tower it hardly went below 25 degC.

The flagship station of NTPC is Singrauli Super Thermal Power Station (SSTPS) and it has once through Cooling Water system. In modern times there is hardly any station with once through system, therefore, it is my advice that you don't compare the Condenser Vacuum of your station with SSTPS unless you have similar Cooling Water System.

At Jharsuguda we have Natural Draft Cooling Towers. This tower is also built to deliver 33 degC at design point but its performance depends upon both the Dry Bulb and Wet Bulb temperatures. The reason being variation in air flow which is constant in Induced Draft Cooling Towers unless you touch the fan.

In worst weather i.e. both Dry and Wet Bulb Temperatures on higher side the Natural Draft Cooling Tower delivers higher temperature compared to Induced Draft Cooling Tower but in Winter it delivers lower temperature due to increased air flow.

But the most important aspect of Cooling Tower System is fouling in Condenser Tubes unless you religiously maintain residual Chlorine.

As Turbine Maintenance In-charge at NTPC Vindhyachal Super Thermal Power Station (VSTPS) Stage 1 (6x210 MW) I could not maintain the Chlorination because it was 2 Km long underground pipe failing frequently.

This resulted into slime formation in Condenser Tubes which initiates fouling. I introduced tube cleaning with High Pressure Jet Pumps and got very good Condenser Vacuum but it used to deteriorate within three months in the absence of residual chlorine.

At Jharsuguda we have Sodium Hypo-chloride dozing system which is also having underground piping but thanks to the Chemistry Department which maintains residual chlorine by manual dozing if there are leakages in underground pipes.

Update on 27th May 2013
Since I had seen very good Vacuum at Obra and Singrauli I was under the impression that once through Cooling Water Systems provide better Vacuum but after visiting Tuticorin the belief is broken.

I was expecting better measured value of Condenser Vacuum at Tuticorin because at seashore the atmospheric pressure is maximum. What you see in the gauge or indication in Control Room is the difference between the Turbine Back Pressure and atmospheric pressure. Other things being equal i.e. for same absolute back pressure (say 0.09 ata) what you read at Singrauli (for atmospheric pressure of 1.0126 ata) will be -0.9226 Kg/cm2 and at Tuticorin (for atmospheric pressure of 1.033 ata) it should be -0.943 Kg/cm2.

Although I found my familiar mercury in glass Vacuum Gauge at Tuticorin and expected better than 690 mmHg, it was disappointing to see 660 mmHg.

On analysis I found CW Pump assisted Siphon formation in Condensers Water Box and the practice of not running Water Box Priming Ejector periodically to remove liberated dissolved gases.

Zero of Axial Shift Indicator.

What I learnt in the year 1977 while boxing up bearings of India's first 200 MW unit at Obra (Unit 11) was as follows:

You move the rotor towards the Generator side and the thrust pads which get tightened are called working pads. If the Millwright Fitter had no brains and scrapped the parting plane of the bearing and made it non perpendicular to vertical plane against which the pads are pressed the two halves of the pads will be in different planes, therefore, you must ensure that bottom half as well as top half pads get tightened.How? Measure the axial float with bottom half pads, press the rotor against working pads, put a dial indicator to fix the axial position of the rotor to zero, assemble the top half pads, measure the axial float again, press the rotor against the working pads and see whether you get the zero in the dial indicator and nearly the same axial float.

The activities in the previous paragraph are supposed to be done before the box up of the turbine casings because the reference point of steam flow path is  when the rotor is pressed against working pads. Since I was not present before box up I re-checked to confirm and to give clearance for final checking and calibration of Axial Shift Indicator which should also indicate zero with rotor pressed against working pads.

My curiosity was what position the rotor takes while the machine is on load. The unit was synchronized on 31st December 1977 and it was fulfilled. The rotor gets pressed towards the working pads and the axial shift indicator indicates negative value corresponding to the thickness of the oil film which also varies. What happens when the unit trips? The rotor moves towards the non working pads (also called surge pads) and indicates higher negative value. While rotating on Turning Gear (also called Barring Gear) sometimes it almost touches the surge pads and indicates a negative value equal to the axial float in the bearing and this is the acid test of the erection work.

My next curiosity was whether the Turbine trips on axial shift. The answer was it may never trip in 30 years life. Then why the axial shift indicator? To indicate:

• wear of thrust pads if it indicates zero or positive value and
• axial float in thrust bearing sometimes when on Turning Gear.

After many years I am associated with 600 MW Steam Turbines at Jharsuguda and the same holds good but people who have come from various power stations have doubts.

One person confidently says that my knowledge is obsolete and the steam flow path is adjusted with the rotor kept in the middle of the axial float and the axial shift indicator is set to zero at this position of the rotor. When I ask that person which way the rotor goes when the unit is on load he has no answer. He thinks it is the job of operation to see that. When I tell him that the axial shift indicator is not an important parameter for operation because the Turbine is not going to trip on axial shift for its entire lifetime he says then how does it matter what way he sets the zero.

It matters because if you set the steam flow path and the axial shift indicator zero in the middle of the float the Turbine will come on working pads on load and not operate at the intended position corresponding to steam flow path. In addition the axial shift indicator will not serve the purpose of indicating the wear of working pads accurately.

My only advice to such people is to observe what the axial shift indicates while the machine is on load and on Turning Gear and speak accordingly.