SPECIAL PRINT (2021) - PowerPlant Chemistry 2010, 12(4

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PPCHEM® Journal ▪ www.ppchem.com ▪ SPECIAL PRINT (2021) ▪ PowerPlant Chemistry 2010, 12(4), 246–251

The Journal of All Power & Plant Chemistry Areas

S P E C I A L P R I N T

ANALYTICAL INSTRUMENTS

Water Steam Cycles

Conductivity monitor with pH value and alkalizing reagent concentration calculation

AMI Deltacon DGAutomatic and continuous measurement of total, cation and degassed cation conductivity. Re-boiler according to Larson-Lane (ASTM D4519-94).

Swan Analytische Instrumente AGCH-8340 Hinwil ∙ [email protected]

Sampling, Monitoring, Analytics

Effects of Steam Sample Degassing on CCGT Station Start-up ProfilePeter J. ClarkPowerPlant Chemistry 2010, 12(4), 246–251



© 2021 by PPCHEM AG. All rights reserved.

246 PowerPlant Chemistry 2010, 12(4)

PPChem Effects of Steam Sample Degassing on CCGT Station Start-up Profile

INTRODUCTION

This paper is designed to address the effect of carbon

dioxide upon steam cation conductivity and to ascertain

whether or not the actual conditions of the steam entering

the steam turbine can be assessed more reliably when

using the degassed cation conductivity monitoring. The

theory and conclusions gained from this study can be

applied to both base and peak load power plants,

although peak load power plants shut down and start up

their steam turbines more often than base load stations,

so the conclusions may be of more benefit to those partic-

ular sites.

The research and practical investigation was carried out at

Centrica Energy South Humber Bank (SHB), a 1 260 MW

combined cycle gas turbine (CCGT) power station, utiliz-

ing the start-up of the second phase of the plant after a

planned outage.

The current method of analysing steam sample conductiv-

ity in operation at the station is cation (after-cation-

exchange) conductivity monitoring. This technique is not

capable of taking into account the contribution of carbon

dioxide dissolved in the sample, so during start-up, the

time delay to wait for steam cation (after-cation-exchange)

conductivity to go below a certain level (0.2 µS · cm–1) is

elevated due to the presence of carbon dioxide in the

sample. In removing the dissolved carbon dioxide, the

degassed cation (after-cation-exchange) conductivity

measurement should supply more reliable information

about the actual steam quality and possibly allow the

steam to be transferred to the steam turbine earlier. The

scope of the project was to identify the levels of cation

(after-cation-exchange) conductivity that account for dis-

solved carbon dioxide and to determine whether these

levels have a significant impact upon the start-up profile of

a CCGT power station.

ABSTRACT

Many power stations dose feedwater with oxygen scavengers such as carbohydrazide; these compounds remove

the dissolved oxygen but release inorganic carbon dioxide into the water. The effect of carbon dioxide upon corro-

sion levels is a controversial subject and as such is not within the scope of the work discussed in this paper. The

effect of carbon dioxide upon conductivity measurements is the major consideration.

Degassed cation conductivity (DGCC) is a widely used technique to remove dissolved gases from high purity water. A

typical DGCC instrument consists of a reboiler which raises the temperature of the sample water above its saturation

temperature, thus reducing the solubility of gases, such as carbon dioxide, effectively boiling the gas out of the water

sample stream.

The present method used for measuring water or steam purity is cation (or acid) conductivity, often denominated as

after-cation-exchange conductivity. This technique should indirectly assess levels of anions such as chloride,

sulphate, formate and acetate for corrosion avoidance purposes. However, due to the presence of carbon dioxide

dissolved in the sample, the monitoring results are not appropriate for this purpose. The degassed cation conductiv-

ity technique can be applied to power station start-ups when the steam conditions have to be monitored closely. By

removing the dissolved carbon dioxide from the sample stream, more accurate information about the actual purity of

the water or steam is given. This paper will give the results and economic benefits when this monitoring technique is

applied to a cold start on a combined cycle gas turbine (CCGT) power station.

Effects of Steam Sample Degassing on CCGT Station

Start-up Profile

Peter J. Clark

© 2010 by Waesseri GmbH. All rights reserved.

SAMPLING, MONITORING, ANALYTICS

1


PPCHEM SPECIAL PRINTSAMPLING, MONITORING, ANALYTICS

247PowerPlant Chemistry 2010, 12(4)

PPChemEffects of Steam Sample Degassing on CCGT Station Start-up Profile

There are different methods of degassing water (or con-

densed steam) samples; the most popular ones are pre-

dominantly heating and stripping (gas and membrane) [1].

Boiling was chosen for this investigation as it is a reliable,

proven technology for this application. Membranes such

as those used in total organic carbon monitoring technol-

ogy do not provide such a simple, robust and cost effec-

tive analysis.

South Humber Bank is a triple steam pressure plant pro-

ducing

– high pressure (HP) steam at 91.6 bar

– intermediate pressure (IP) steam at 19.3 bar

– low pressure (LP) steam at 3.8 bar

The degassed cation (after-cation-exchange) monitoring

equipment was put onto the high pressure steam sample

line at a point before the HP steam enters the steam tur-

bine.

INSTRUMENT USED

The instrument used throughout this investigation was the

Swan AMD Degassed Cation Conductivity Monitor. A

schematic of the monitor is shown in Figure 1.

The unit has a reboiler positioned after a cation exchange

column. There are three measurements obtained from the

unit: specific, cation (after-cation-exchange) and degas -

sed cation conductivity. Specific conductivity is the meas-

urement of the condensed steam sample straight from the

sample line, i.e., before a cation exchanger. The cation

conductivity measurement is taken after the ion exchange

column, and degassed conductivity is taken after the

reboiler positioned downstream of the ion exchange col-

umn.

PRELIMINARY INVESTIGATION

Several tests were conducted to test the resilience of the

instrument against differing flow rates and water qualities.

Manual calibration of the monitor was conducted to

establish a baseline of results that data taken from the

monitor could be compared against. The baseline for

degassed cation (after-cation-exchange) conductivity was

established at 0.18 µS · cm–1, in line with the purity of the

feedwater (0.2 µS · cm–1) required for the steam conditions

at plant start-up.

The results of the preliminary investigations were conclu-

sive in that the degassed cation (after-cation-exchange,

Figure 1:

Instrument schematic (courtesy of Swan UK Analytical Instruments).

2





ACE) conductivity returned to the baseline result after 15

minutes for a cold start and 5 minutes for a hot start. The

reboiler has proven its high efficiency.

PRIOR INVESTIGATIONS

A similar experiment was conducted by Pedro Wuhrmann

of Swan Analytical Instruments to prove the effects of

using degassed cation (ACE) conductivity as compared to

cation (ACE) conductivity as a method of monitoring start-

up water purity.

The results from this investigation provide the ideal pat-

tern that the results from the project at South Humber

Bank should mirror and are shown in Figure 2 [2].

The chart shows the marked difference between

degassed cation (ACE) conductivity and classic cation

(ACE) conductivity during start-up. These results were

taken from a newly built power station in Portugal on the

first day of commissioning.

PRIMARY INVESTIGATION

The primary investigation was centred on the cold start of

two of the 160 MW gas turbines after a 2-month planned

outage. The degassed cation (ACE) conductivity monitor

was linked into the high pressure steam sample line corre-

sponding to that of the shut-down HRSG's and inline

results were established throughout the start-up period.

The serial tags of the two turbines monitored were GT21

and GT22 and they will be referred to in this way for the

remainder of the paper.

GT21 Start-up

The commissioning profile for GT21 was in two parts, the

first consisting of a full speed no load followed by three

days of stepped load tests. The second period occurred a

week later and was a ramped profile until sustained high

load (110 MW). Figure 3 shows the start-up profile for the

gas turbine combined with the data from the degassed

cation (ACE) conductivity monitor.

The steam turbine was synchronized at 15:58:00 BST, with

the steam conditions shown to be ready hours before the

steam turbine synchronization. The dashed box shows the

time frame when the degassed cation (ACE) conductivity

(DGCC) instrument was recording results, and this period

is shown in greater magnification in Figure 4. The readings

taken by this instrument clearly give more reliable informa-

tion than the classic cation (ACE) conductivity instrument

with respect to the actual steam purity. (Cation conductiv-

ity levels in the presence of carbon dioxide are sometimes

more than twice the levels measured after carbon dioxide

removal.) The cation (ACE) conductivity readings are also

affected by the addition of dosing chemicals such as car-

bohydrazide that do not affect degassed readings; this

effect is shown on Figure 3 just after 13:12:00 BST when

the cation (ACE) conductivity reading takes a sudden

increase. An assumption can be made that an increase in

the levels of dissolved carbon dioxide (due to decomposi-

tion of carbohydrazide) is the cause as the degassed

cation (ACE) conductivity readings do not report a similar

pattern.

The difference in conductivities seems slight but the

degassed cation (ACE) conductivity reaches the pre-

determined steam condition set-point of 0.5 µS · cm–1 by

2.5

2.0

1.5

1.0

0.5

0

–1

Co

nd

uctivity [

µS

cm

]·

Time [h:min]

09:30

250

200

150

100

50

0

Lo

ad

[M

W]

10:00 10:30 11:00 11:30 12:00 12:30 13:00

10:08 Gas turbine

synchronized 13:03 Degasser off

9:30 Flame on

full speed no load

Acid Degassed Gas turbine load

Figure 2:

Ideal results (courtesy of Swan UK

Analytical Instruments) [2].

3





12:51:00, whereas the classic cation (ACE) conductivity

measurement does not even show such low levels of con-

ductivity. Furthermore, degassed cation (ACE) conductiv-

ity values show fewer fluctuations in the results than the

cation (ACE) conductivity values do.

The large initial spike in both conductivities is the

response to pressurization of the water-tubes in the boiler

and the high concentrations of ions present in these tubes

before start-up.

The steam conditions are at their pre-determined level

much sooner, so theoretically HP steam can be sent to the

steam turbine earlier; however, as discussed at the start of

the this paper, the economic benefit lies with the gas tur-

bine and the analysis of this will be shown below.

GT22 Commissioning

Although sufficient results were obtained from the start-up

profile of GT21, results from GT22 prove the reliability of

both the degassed cation conductivity instrument design

and the technology used. The GT22 commissioning profile

didn't include steam turbine synchronization so only the

patterns of conductivity during the gas turbine start could

be monitored.

Figure 3:

GT21 start-up profile.

ST steam turbine

Figure 4:

GT21 – Difference between cation

(ACE) and degassed cation (ACE)

conductivity of the steam sample.

–1

Co

nd

uctivity [µ

Scm

]·

20

18

16

14

12

10

8

6

4

2

0

Lo

ad

[M

W]

120

100

80

60

40

20

0

Time [h:min]

10:48 12:00 13:12 14:24 15:36 16:48

Degasser switched off

ST28

synchronized

at 15:58

Acid Degassed ST28 GT21

–1

Co

nd

uctivity [

µS

cm

]·

20

18

16

14

12

10

8

6

4

2

0

Lo

ad

[M

W]

60

50

40

30

20

10

0

Time [h:min]

11:31 11:45

Time taken for DGCC to show

lower conductivity than after-

cation techniques

12:00 12:14 12:28 12:43 12:57 13:12 13:26 13:40 13:55 14:09

GT21 set to load

Acid Degassed GT21

4





This data was collected from Day 3 of the commissioning

program for GT22 so the profile and results are more suit-

able for a warm start-up.

It is observable that the levels of degassed cation conduc-

tivity reach a required steam purity at much earlier times

than the classic cation (ACE) conductivity levels (Figure 5).

Using this in conjunction with Figure 4 gives the time

saved by the use of the degassing technique to be on

average 1 hour after a gas turbine (GT) start. However,

steam conditions are not the only property that deter-

mines the time difference between GT start-up and steam

turbine (ST) synchronization. At South Humber Bank sta-

tion, the other conditions normally take around two hours

to be at the correct specification, but this time frame dif-

fers between sites. Using the degassed cation conductiv-

ity helps in reducing the time gap between GT start-up

and ST synchronization.

BENEFITS AND COST ANALYSIS

A cost analysis for this project is very difficult to determine

as there are differing factors that affect a start-up and as a

consequence the costing. Initially it seemed viable to set

the ST to base load earlier, thus trading power at base

load for a longer time. On closer inspection of the start-up

profile it is only the amount of time the GT is held at

80 MW that can be decreased. This has a direct effect on

the EOH (estimated operating hours) of the gas turbine as

the number of gas turbine operating hours producing

80 MW decreases so the amount of gas turbine operating

hours at base load increases. At 160 MW (base load) the

power station is trading electricity for a higher price for

each operating hour than at 80 MW, increasing the profit

margin.

Another cost analysis method would be to look at the dif-

ferences in efficiency between running the gas turbine for

longer at 80 MW than for a shorter time at 160 MW. At

160 MW the CCGT uses more natural gas but the spark

price (the difference in profit between selling the gas and

converting it into electricity and selling the power) is

higher.

Savings in carbon credits can also be accomplished. One

credit is equivalent to one tonne of carbon; if an industrial

producer is below its credit quota, the credits can be sold

as a commodity to other producers who may have pro-

duced more than their credit quota. By running a gas tur-

bine for a shorter time at start-up the amount of carbon

produced per year decreases so the excess carbon cred-

its can be sold for a profit; this benefit may also be applied

to the planned NOx credits. From an environmental per-

spective a decrease in the amount of carbon produced

every year by reduced running of the gas turbine during

start-up is a large advantage of this monitoring technique.

This economic analysis gives a qualitative perspective on

the benefits of using degassed cation conductivity moni-

toring. The data in this paper shows that this technique

can save operating time during a gas turbine start-up,

which as shown in the economic analysis has several

other benefits when the number of gas turbine start-ups

performed every year is taken into account. A quantitative

–1

Co

nd

uctivity [

µS

cm

]·

Lo

ad

[M

W]

120

100

80

60

40

20

0

Time [h:min:s]

08:24:00 08:38:24 08:52:48 09:07:12 09:21:36 09:36:00 09:50:24 10:04:48

6

5

4

3

2

1

0

Acid Degassed GT22

Figure 5:

GT22 start-up profile.

5





analysis depends upon the type of turbine and differs

between stations and load profiles, so this has not been

discussed in this report.

CONCLUSION

The application of a degassed cation conductivity monitor

allows an earlier start-up of the steam turbine. However,

whilst the steam conditions are ready for steam turbine

synchronization earlier, this doesn't mean the steam tur-

bine can be started earlier; conversely, the gas turbine can

be fired up later preserving its life and allowing trading for

a shorter time at an output of 80 MW. This is a very broad

conclusion gained from the investigation, but it gives an

overall experimental outcome.

Looking more closely at the results, it is clear that the

degassed cation conductivity is a more reliable method of

indicating the purity of the steam sample. In the start-up

graphs, Figures 3–5, the degassed cation conductivity

curves not only show a large decrease in the time taken to

reach acceptably low levels of conductivity, but they also

indicate that degassing of the sample after cation

exchange reduces the amplitude of the oscillations in the

curves. This reduction in amplitude signifies that the use

of a degassed cation conductivity monitor also has the

effect of reducing extreme values in the start-up profile.

The economic benefits of decreasing the time spent wait-

ing for correct steam conditions in a station start-up pro-

file are astounding. Not only is the gas turbine primary

mover held at lower power output for a shorter time,

increasing the time spent with the primary mover held at

higher efficiencies, but the risk of corrosion of plant cycle

components is reduced due to more reliable and precise

determination of the possible presence of corrosive con-

taminants in the plant cycle. The annual turbine operating

hours are also reduced as the gas turbines may be ignited

later in the start-up profile.

REFERENCES

[1] Drew, N., PowerPlant Chemistry 2004, 6(6), 343.

[2] Wuhrmann, P., Cation and Degassed Cation Con -

ductivity, 2008. Paper presented at the Second

International Conference on the Interaction of

Organics and Organic Cycle Chemicals with Water,

Steam and Materials, November 4–6, 2008 (Lucerne,

Switzerland). PowerPlant Chemistry GmbH,

Neulussheim, Germany.

[3] Jonas, O., Machemer, L., Proc., Eighth International

Conference on Cycle Chemistry in Fossil and

Combined Cycle Plants with Heat Recovery Steam

Generators, 2006 (Calgary, Alberta, Canada). Electric

Power Research Institute, Palo Alto, CA, U.S.A.,

1014831, 9-2.

ACKNOWLEDGEMENTS

The author would like to thank Paul Kelk, the South

Humber Bank Station chemist, and the BIAPWS (in partic-

ular Richard Harries, BIAPWS Secretary) for the appoint-

ment as the undergraduate award student 2009, as well as

for the invaluable help and support during and after the

work placement. Thanks also go to Swan Analytical

Instruments (Joern Boedeker, Swan UK) for technical sup-

port during the work period and consent for the use of all

Swan information given in this paper.

THE AUTHOR

Peter J. Clark is a third year undergraduate studying

Chemical Engineering at the University of Birmingham. He

has been in higher education since 2007. He was awarded

the BIAPWS Undergraduate Award placement 2009 in co-

operation with Centrica Energy, the main focus of the

placement being on degassed after-cation conductivity

with a side-interest in total organic carbon measurement

techniques. Peter Clark has been awarded the University

Nash Prize 2009 for excellence in both academia and

extra-curricular activities in the second year. He is a cur-

rent student member of the Institute of Chemical

Engineers.

CONTACT

Peter J. Clark

The Knoll

Thornbury Hill

Alveston (Bristol)

BS35 3LG

United Kingdom

E-mail: [email protected]

6



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SPECIAL PRINT (2021) - PowerPlant Chemistry 2010, 12(4