Why most people think that -0.9 Kg/cm2 gauge is the correct value of Condenser Vacuum?

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.

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.

-->

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.

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.