C O R R O S I O N M O N I T O R I N G O F T O P F L O W IN W E T G A S E N V I R O N M E N T

U S I N G E L E C T R O C H E M I C A L N O I S E M E A S U R E M E N T ( E C N )

H. B. Wang, K. George, T. Hong and W. P. Jepson

Institute for Corrosion and Multiphase Technology,

340 '/2 West State Street

Ohio University, Athens, OH, 4570 !

H. J. Chen

Chevron Petroleum Technology Company

1300 Beach Boulevard La Habra, CA 90631-6374

A B S T R A C T

The corrosion of the top flow in wet gas environment was studied in this work by using

Electrochemical Noise Measurement (ECN). All the tests were carried out in a 3-inch I. D, 28-ft long

acrylic pipeline. A CO2 or N2 wet gas environment was generated by using high-velocity gas to entrain

low-velocity seawater, which formed a annular-mist flow in the pipeline. The ECN results show less

current and voltage noise fluctuation in a wet gas environment, which suggests a low corrosion activity.

The slope of voltage and current amplitude during low frequency domain in Fast Fourier Transform

(FFT) plots indicates the corrosion type as uniform. The spectral noise impedance, Rsn, obtained from

FFT plots is high because the liquid film is very thin, therefore the corrosion activity of top flow in wet

gas environment is relatively low. The results obtained from N2 and COs wet gas environment give a

further proof that ECN can be used to monitor the corrosion rates since the difference of the current and

Copyright ©2000 by NACE International.Requests for permission to publish this manuscript in any form, in part or in whote must be in writing to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Pdnted in U.S.A.

voltage between N2 and COz wet gas environment is evident. The increase of current and voltage noise

fluctuation with increase in gas velocity shows that the corrosion activity increases with increase in gas

velocity.

Keywords: wet gas environment, electrochemical noise, top flow, corrosion activity

I N T R O D U C T I O N

With the increasing demand of natural gas motivated by its clean-burning characteristics, deep

offshore reserves will play a key role in the future natural gas supply scenario. This means that

produced natural gas, which may be wet and often times accompanied by liquid water, will be

transported over substantial distances in pipelines. Prior to processing, a typical natural gas is composed

of several hydrocarbon compounds, water, carbon dioxide, nitrogen and sometime other compounds

such as hydrogen sulfide. Much of the flow in the wet gas transportation pipeline takes place in the

annular and annular-mist flow. The presence of seawater and CO2 in a wet gas environment will cause

corrosion problems due to the extensive use of carbon steel as pipeline material in the oil and gas

industry. The previous studies ~' 2 show that the corrosion rate in the top of the line increases with COz

partial pressure, gas flow rate and condensation rate, and is significantly reduced when inhibitor is

added to the gas. However, the corrosion problem under wet gas environments is still not well

investigated and understood especially at the top of the pipes.

Electrochemical Impedance Spectroscopy (EIS) cannot be used as a reliable corrosion

monitoring technique in a wet gas environment because a liquid phase/liquid film may not be present at

all times. Thus, Electrochemical Noise Measurement (ECN) is used in this work. Fluctuation of the

electrical quantities (electrode potential and current) in electrochemical systems is commonly referred

to as electrochemical noise. 3 It originates, in part, from natural variations in electrochemical rate

kinetics during a corrosion process. So electrochemical noise is often regarded as a random (stochastic)

phenomenon coupled to deterministic kinetics. The most important data are the current and potential

transients which are related to metal dissolution or cathodic processes (or both) that constitute a

corrosion process. Noise data are typically acquired by monitoring the evolution of a corrosion process

on two coupled electrodes without the application of an external signal.

The present article is aimed at determining whether or not electrochemical noise measurement

can be used to monitor corrosion rates in a wet gas environment and studying the corrosion activity in a

CO2 wet gas environment. The corrosion activity in a N2 wet gas environment is carried out for the

purpose of comparison. The influence of gas velocity on the corrosion activity in a CO2 wet gas

environment is also studied.

E X P E R I M E N T A L S E T U P A N D P R O C E D U R E

All the experiments were carried out in a 3-inch (7.62 cm) I.D., 28-f~ (8.53 m) long acrylic

pipeline. The overall flow loop schematic is shown in Figure 1. Electrochemical noise measurements

were taken in the test section shown in Figure 2.

By using high-velocity CO2 or N2 gas to entrain low-velocity seawater, a CO2 or N2 wet gas

environment was simulated. An annular-mist flow in the pipeline was obtained. Test solution was

prepared from de-ionized water dissolved with ASTM synthetic sea-salt. The liquid velocity used was

0.08 m/s for all the different gas velocities. Before the tests, the solution was deoxygenated with CO2

for 5 hours in order to reduce the concentration of dissolved oxygen to less than 20 ppb.

At the beginning of each experiment, the noise probe was polished with Grit 600 and 1500

sandpaper, and then washed with acetone. The probe was then flush mounted on the top of the pipe. In

each experiment, the probe was held for 30 minutes in the pipe for surface stability before collecting

current and voltage noise data. Noise data under various deferent gas velocities were measured using

different sampling rates successively.

Three nominally identical C-1018 carbon steel electrodes were used as working electrodes and

reference electrode in this work. The surface area of each working electrodes is 0.07 cm 2. The current

and voltage measurements across the noise probe were measured with an ACM Instrument Fast

AutoAC ® and analyzed by an in-built software. Zero resistance ammeter (ZRA) mode, which maintains

a negligible potential difference between two working electrodes, was selected to measure the current

between these two working electrodes.

In present study, the following tests as shown in Table 1 were carried out in order to determine

the validity and comparability of using ECN as method to study corrosion rates in a CO2 wet gas

environment.

D A T A A N A L Y S I S

The data analysis is the key of ECN application. Both statistical analysis in time domain and

spectral analysis in frequency domain were used in present work. In order to get a direct view of real

current and voltage noise fluctuation, the MAP, (Moving Average Removal) method 4 was used in this

work. The main idea behind this method is to remove the DC trend from raw noise data, and get the

transient noise components. The extraction of real noise fluctuation (Mi,noise, M represents voltage or

current) from the raw noise-time record (Mi) is the key step of noise analysis as shown in the Equation

(1) and (2).

Mi,noise = M i - Mi,DC (1)

{ i-p< (2 p + M, PC ~ Mi ' " = 2 ) (2)

where p=3 in this work which means that 8 data points were used to get the DC limit.

The noise data obtained by simultaneous collection of potential and current noise were also

analyzed in the frequency domain using Fast Fourier Transform (FFT). 5 To evaluate the frequency

dependence of the electrochemical noise data, the spectral noise response (noise impedance) Rs,(f) 6

was calculated at each frequency f as:

Rsn(tf) = [ VFFT(O [ / [ IFFT(O I (3)

where Vvm-(f) and IFFT(f) are complex numbers obtained from FFT by using the ACM software.

Extrapolation to the DC limit (f-->O) allows the determination of the spectral noise impedance R°sn,

which was suggest to be compared to polarization resistance. 7, 8

E X P E R I M E N T A L R E S U L T S A N D D I S C U S S I O N

Val id i ty o f E C N in a C a r b o n D i o x i d e W e t Gas E n v i r o n m e n t

The results obtained with successive collection of potential and current noise for the corrosion in

a CO2 wet gas environment at a gas velocity of 10 m/s are displayed in Figure 3 and Figure 4 for

sampling rates of 10 Hz, 60 Hz and 100 Hz. The comparison &voltage or current curve in Figures 3 or

4 shows that the level of voltage or current activity is almost the same with a voltage range of 145 mV

to 155 mV for 10 Hz and 60 Hz sampling rates, and a current range of 0.06-0.09 mA/cm 2 for 10, 60 and

100 Hz. However, the voltage range with 100 Hz sampling rate is slightly lower than that of 10 Hz and

60 Hz. This is probably due to small differences on the metal surface.

It is noticed that, for a wet gas environment, the fluctuations in both current and voltage are

random about a mean value, similar to those in full pipe flow at steady state. This may be caused by the

small amount of liquid in contact with the probe at wet gas conditions. Lower fluctuation suggests that

there is less activity on the probe surface.

When the MAR method of analysis is used, i.e., when the DC has been removed, the magnitude

of the noise fluctuations can be more clearly seen. This is illustrated in Figures 5 and 6. The current

noise fluctuations are given in Figure 5 and show that the magnitude of the fluctuation is almost the

same for each sampling frequency. However, much more detail is noted at 60 and 100 Hz. For these

two sampling rates similar results are found. This would suggest that a frequency of 60 Hz or greater

would capture more of the detail of the corrosion processes. For the voltage fluctuations, Figure 6

shows similar results except that at the 10 Hz frequency, slightly larger fluctuations are noticed at some

isolated locations. The spontaneous current and voltage noise fluctuations are shown in Figures 7. In

this Figure, electropositive potential transitions were always accompanied by equivalent electropositive

shifts in the current at the same instant. This is an indication of uniform corrosion.

Figures 8 and 9 show the noise amplitudes of electrochemical current and voltage data obtained

from FFT at sampling rate 10 Hz and 60 Hz respectively. The noise amplitudes of current can be related

to the general corrosion rate, whereas the slopes of the noise amplitudes may indicate the different types

of corrosion in the low frequency domain. 9 In both plots, there are no obvious differences or changes in

the shape and slope in the low frequency domain between 0.1 Hz and 10 Hz. The slope in low

frequency domain is around -40 dB/decade for voltage or a little higher than -40 dB/decade for current,

which may indicate the corrosion type as uniform corrosion. 10 For 10 Hz and 60 Hz sampling rate,

similar slopes for current or voltage are obtained during data collection procedure.

Figure 10 shows the spectral noise responses (R~n) calculated from Equation (3) as a function of

frequency for two experiments at a sample rate of 10 Hz. As we have seen, the noise resistance R°sn,

obtained by extrapolating the trend line of R~n to a frequency equal to zero, is high in a wet gas

environment. This high impedance suggests a very low corrosion activity since there is only a thin

liquid film at the top of the pipe. Along the low frequency domain, the noise resistance trend lines do

not shift much in log scale. These frequency-independent Rsn values at low frequency in these plots

indicate that the corrosion type in a wet gas environment is mostly a uniform corrosion process.

ECN in a Nitrogen Wet Gas Environment

In order to get a direct view of the magnitude of the corrosion activity in a CO2 wet gas

environment, experiments were carried out in a neutral solution environment of N2 wet gas for the

purpose of comparison. Figures 11 and 12 show the current and voltage noise fluctuation obtained by

MAR method in a N2 wet gas environment at sampling rate of 1 Hz. This sampling rate is chosen

because of the very low corrosion activity here. Again, less random fluctuations are seen. In a N2 wet

gas environment, the lack of hydrogen ions can significantly suppress the cathodic reaction. Hence,

little corrosion is expected. This can be verified by the current and voltage noise fluctuation in the time

domain as shown in Figures 11 and 12. The voltage noise fluctuation range is less than _+0.15 mV and

the current fluctuation range is less than _+0.0001 mA/cm 2. The noise fluctuation values in Figures 11

and 12 indicate that the level of both voltage and current noise activity is relatively very small as

compared to that in a CO2 wet gas environment, where the range of fluctuations is _+1.0 mV and _+0.002

mA/cm 2 at 1 Hz sampling rate.

Influence of Gas Velocity in a Carbon Dioxide Wet Gas Environment

Table 2 shows the current and voltage noise fluctuation for different gas velocity for 1 Hz

sampling rate. It is shown that both voltage and current noise fluctuation obtained by using MAR

method increased when the gas velocity is increased from 10 m/s to 17 rn/s. This suggests an increase in

corrosion activities. Compared to the corrosion activity in a N2 wet gas environment, current noise

activity at 17 m/s gas velocity is almost 100 times higher. This is fiarther exemplified in Figures 13 and

14. Figure 13 shows the current noise fluctuation at a gas velocity of 10 m/s, 15 m/s and l 7 ln/S

respectively using 1 Hz sampling rate. Figure 14 shows the equivalent voltage noise fluctuation. These

behaviors can be considered as follows: at the lower gas velocity, there would be a thin liquid film at

the top of the pipe may be thin. With the increase of gas velocity, more liquid is presented and liquid

film will be completely spread around the pipe, resulting in increases in corrosion activity.

The above results

conclusions:

1.

.

C O N C L U S I O N S

of electrochemical noise experiments and analysis indicate the following

Current and voltage noise fluctuations in a wet gas environment suggest a low corrosion activity.

Both the frequency-independent spectral noise resistance Rsn and the slope of voltage and current

amplitude during low frequency domain in FFT plots indicate the corrosion type as uniform. The

noise resistance R°sn is high because the liquid film is very thin, so the corrosion activity is

relatively slow.

The results obtained from a N2 and CO2 wet gas environment shows that ECN can be used to

monitor the corrosion rates since the difference of the current and voltage noise fluctuation between

N2 and CO2 wet gas environment is evident.

. The increase of current and voltage noise fluctuation with the increase in gas velocity indicated that

the corrosion activity increases with the increase in gas velocity.

In a summary, ECN is a practical technique for continuously monitoring the corrosion activity

of carbon steel pipelines in present work. The quantitative analysis still requires further

investigation.

.

.

,

4.

5.

.

7.

8.

.

10.

R E F E R E N C E S

R, Nyborg, A. Dugstad, and L. Lunde, "Top-of-Line Corrosion and Distribution of Glycol in a

Large Wet Gas Pipeline," CORROSION/93, NACE Paper No. 77, 1993

T. R. Andersen and A. Valle, "Prediction of Top of the Line Corrosion in Pipelines Carrying

Wet CO2 Containing Gas", CORROSION/93, NACE, Paper No. 73, 1993

Bertocci, U., Huet, F., Corrosion Science, Vol.51, 131, 19952.

Tan, Y. J., Bailey S., Kinsella, B., Corrosion Science, vol.38, 1681, 1996

Bendat, J. S., Piersol, A. G., Random Data: Analysis and Measurement procedures, JohnWiley

& Sons, New York, 1986

Mansfeld, F., Han, L. T., Lee, C. C., Corrosion Science, vol.39,255, 1996

Xiao, H., Mansfeld, F., Electrochem. Soc., 141, 2332, 1994

Mansfeid, F., Xiao, F., Proc. 12 th Int. Corrosion Cong., NACE, Houston, TX, Sept.,

1388, 1993

Dawson, J.L., "Electrochemical Noise Measurement: The definitive In-Situ TeChnique

for Corrosion Applications?," Electrochemical Noise Measurement for Corrosion

Application, ASTM STP 1277, Jefery R. Kearns, John R. Scully, Peirre R. Roberge,

David L. Reichert, and John L. Dawson, Eds., American Society for Testing

and Materials, 1996, pp. 3-5

Searson, P.C., Dawson, J.L., J. Electrochem. Soc., 135, 1908, 1988

TABLE 1

TEST MATRIX

Parameter Condition

CO2 partial pressure, MPa 0.136

Temperature, °C 40

Water cut, % 100

Test gas CO2 & N2

Superficial liquid velocity, m/s 0.08

Superficial gas velocity, m/s 10, 15, 17

Sampling rates, Hz 1, 10, 60, 100

TABLE 2

C O M P A R I S O N OF NOISE FLUCTUATION UNDER DIFFERENT GAS VELOCITIES

Voltage noise fluctuation (mV)

Current noise fluctuation

(mA/cm 2)

10 m/s 15 m/s 17 m/s

_+1.0 _+5.0 _+15

+0.002 __+0.005 +0.01

• - - - - ~ " Gas Vent

I Back Pros,wry Regulator

75 mm I.D., 10 m Plexlglass Pipe

Gas Flow Meter

Source GasTaak J I - - -~- -1 ~ ° ° ' ~0 " " °

$talnlmz Steel Storase Tank

0.~ m 3 I

4, Trot ~ t l o n

Gate Mixing Tank with Baffle

0.03 m 3

Liquid Drain

Staltfless StNi Pump 2.3kW

so mm PVC P|i~ I

¥ Orifice Meter

Figure 1. Schematic layout of 7.5 cm I. D. experimental flow system

C

I Flange A Flange B

D

Flow direction

Flange A

......l....

Flange B

C: ECN probe (3.2 cm diameter) D: Digital thermometer probe

Figure 2. Schematic of test section

2 0 0

180

~ ' 1 6 0

140

120

100

10 H z 60 H z

L---..----""~

100 H z

20

i i i i J i i i i I i i i i i i d i i I i i i i i i r J I i i i ~ i i i i i I i i r I I I i i I

21 22 23 24

Time (s)

F i g u r e 3. No i se response of wet gas for 10 m / s C O 2 f low rate S a m p l i n g ra te=10 , 6 0 , 1 0 0 Hz

25

1 . 0 0 E - 0 1

9 . 0 0 E - 0 2

d""

8 . 0 0 E - 0 2 <

~- 7 . 0 0 E - 0 2 I -

6 . 0 0 E - 0 2

5 . 0 0 E - 0 2

- - 1 0 H z

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . 6 0 H z .......

........ 1 0 0 H z

. _ . / " - ' - ' J ' ~ " '-" "." ', ,,x _, ' , : '~' , , . .z - - . ,~ . -~ , , . , ~'/,~,-",_,",,~_.,,,_.¢.,./.; : 'X, " \ r ' ' " " x . -.e,~.,-- r,.,,,. " : ~ ' ~ ' ' ' ' ~

20

I I I f I I i I I I I I I I i r I i I i i I i

2 l 22 23 24

Time (s)

F igure 4. Noise re sponse of wet gas for 10 m/s CO2 f low rate S a m p l i n g ra te=10 , 6 0 , 1 0 0 Hz

25

4.0E-03

@

Ca t_ t_

¢9

2 .0E-03

"4 0 .0E+00 £

-2 .0E-03

-4 .0E-03

1 0 H z

. . . . . 6 0 H z

............................................................................. , ~OOH__~ : . . . . . . )

AAZ~ ,~ ' '~ ' : ~ i ' F'~'~ d

~ q , " - : ! " ! ; : ' ~ , u :; ~ : : ~ f ' : ~ ' : ~ l ~ ' . _ ' . ' " ~ ' i ~ ' ~ ' ~ b . ' , , : . ~ ' ~ j ~ 5 ~ 4 ~ ! ; ; ~ l ' : i i " ~ '1" . . : :!' ~ C / ~ " : ~ : ' ' I ~ '

' "! k /

20 21 22 23 24 25 Time (s)

Noise response of wet gas for 10 m/s COs f low rate Sampl ing rate=10, 6 0 , 1 0 0 Hz

Figure 5.

C ° m

c a

>

0.5

0

-0.5

- 1

. . . . . . 6 0 H z

. . . . . . 1 0 0 H z

10 H z

• 1, ~4" :{;! ° '7": t,~,,~ ~ l i : t ~ ~ ~:'; :'~'~i ' ~ : 7 ";'~

20 21 22 23 24 T i m e (s)

Noise response of wet gas for 10 m/s CO2 f low rate Sampl ing rate=10, 6 0 , 1 0 0 Hz

Figure 6.

2 5

5"

@

d @

@

>

1 . 0 E + 0 0

5 , 0 E - 0 1

0 . 0 E + O 0

- 5 . 0 E - 0 1

- 1 . 0 E + 0 0

v o l t a g e

. . . . . . c u r r e n t A

,, ? i z

i i : ' ; ' :) : ~i , i ~ ~\ .:: !~

~, : ; ' • : : : : : ' ; . : :~ : i 5 : :

,:~. • : • .

: : .' . , : : ; ; , : : : : :

20

i i ~ J i i i

21 22

T im e (s)

F i g u r e 7. N o i s e r e s p o n s e o f w e t g a s f o r 10 m / s C O 2 f l o w r a t e

S a m p l i n g r a t e = 60 H z

2 . 0 E - 0 3

1 .5E-03

1.0E-03

@

5 . 0 E - 0 4 =

2 d-" O.OE+O0 ~

- 5 . 0 E - 0 4 ~ g

l , i , .t...

- I . 0 E - 0 3

- 1 . 5 E - 0 3

- 2 . 0 E - 0 3

1 . 0 E + 0 5

1 . 0 E + 0 3

,-.,

• -r I . O E + O I

¢o >

'~ 1 . 0 E - 0 1 >

1 . 0 E - 0 3

1 . 0 E - 0 5

v o l t a g e ] .... 1

0 . 0 1

i i i i i , i i i i i i i i i r i i

0 .1 1

F r e q u e n c y ( H z )

F i g u r e 8. N o i s e r e s p o n s e f r o m F F T f o r C O 2 w e t g a s

( g a s v e l o c i t y = 1 0 m / s , s a m p l i n g r a t e = 1 0 H z )

r i i i i i

10

1 . 0 E + 0 3

1 . 0 E + 0 2

1 . 0 E + 0 1

1 . 0 E + 0 0 ~ ~

o .Ol

1 . 0 E - 0 2

1 . 0 E - 0 3

1 . 0 E - 0 4

1 . 0 E + 0 5

= ~, 1.0E+03

g 1.0E+01

>

1.OE-01

1 . 0 E - 0 3

0.001

.......................... ~ ~ ....................... I vo~ta~o 1 .......

i ii>,, --- , '~:~ .if '"INL.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ ' i . . . . . . . . _ . . ' . ~ 2 . 1 . * l ! , ! l l l ; I m , , , . . . . . . . . . . . . . .

i i i i i i i i i i i i i r i i l l i i i i i i i i i i i i i i i i i I i !1 i i i i i

0 . 0 1 0.1 1 10 1 0 0

Frequency (Hz)

Figure 9. Noise response from FFT for CO2 wet gas (gas velocity=10 m/s, sampling rate=60 Hz)

1.0E+04

1.0E+03

1.0E+02

1.0E+01 ~..'- 2~

I.OE+00 ~ .~

, . g 1.0E-01

l . 0 E - 0 2

1.0E-03

1 . 0 E + 0 4

1.0E+03

& 1.0E+02

1 .0E+01

1 . 0 E + 0 0

_.:-, . . . . . . . . ~ . . . . . . . . - - . , .~ :, !: ~: ~i IA ! ! : ~ - . ' 7 - - ~ ' - - - i " . . . . . . . . . . . . . . . . . . ~ ; " 2 ' v , ~ ; " 7 ~ ~ . . . . . . . ' , . ~ : : ' : 1

. . . . . . . . . . . . . . . . . . . . . . )i::i

Trend line of Rsn

¢

- I . . . . . . . . . . . . . |

0.1

i i i ~ i i i i I i i i I i i i i

1 Frequency (Hz)

Figure 10. Noise resistance R~n for wet gas (gas velocity=10 m/s, sampling rate=10 Hz)

10

0.3

0.2

0.1

.¢2 0

-0.1

"~ -0.2

= -0.3

-0.4

-0.5

400

i i n i i i n n * I n n n n I n n I n I i n n I I I I n J I n I I n I n n r J I i I n I t i u n n

410 420 430 440 450 460 470 480 490 500

T i m e /s

F i g u r e l l . No i se r e s p o n s e o f we t gas f o r 1 0 m / s N 2 f l o w r a t e

S a m p l i n g r a t e = l l t z

2.0E-04

1.5E-04

1.0E-04

5 . o E - o 5

°i O.OE+O0

-5.0E-05

-1.0E-04

-1.5E-04

-2.0E-04 ,

400

i i I i i i i i i i i i i i i i i I

420 440 460 480 T i m e (s)

F i g u r e 12. N o i s e r e s p o n s e o f w c t g a s f o r 10 m / s N z f l o w r a t e

S a m p l i n g r a t e = l H z

500

2 . 5 E - 0 2

.=

1 . 5 E - 0 2

5 .0E-03

-5 .0E-03

- 1 . 5 E - 0 2

- 2 . 5 E - 0 2

400

1 5 m / s

i m /S

" : ~ : ' ' ~ v . , : : i l i ~. -:i.i'.:,~.:?:, ,f~.~'i ~:i j:: ' "-..4: i~2~:~: :i?:i' :': ...... , ! ...... ~:~:,2 i: ........ i?' \ ; "~:I

',. ' : ; ', . . . . . . ' , , [ .... ~ , ,.:~.. :7 : :T : : '.~',~ . - ' , : : ' - : . : ;i 'i :: : : i :: ::!" ' :' ::" '",: :: " :: : '

: ' :: .... :.-::,- .~ --:~ : : . ; : ~ :; '

, :,

I J i ~ I I I

420 440 460 480 500 520 540 560 580 600

T i m e (s)

F i g u r e 13. N o i s e r e s p o n s e o f C O 2 w e t g a s

( 1 0 , 15 , 17 m / s C O z f l o w r a t e , S a m p l i n g r a t e = l H z )

30

8

° ~

~ 0 _=

20

l0

0

-10

- 2 0

- 3 0

------15 m/s "1 . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . / l o / ..... i::/

; : ?. L t i ' ~ '.

! ! :: : ; ~ . . . . . 1 7 m / s ~ :/ . ' t ' ~ ; : ~ ; ; ~ • :~ .,. ~ - - ~ - - ; ' . ' : -

• . ~ . j ' . . . , . . . . . .~ • . . . . . . . . . , . . . . . . . . . . . . . ~,= . . . . . . ~ . . . . . . . . . ~ , , ~ • . . . . ? . . . ; . , | , ,

: ::~:, :: :. ::.: .::: :: :: :.~ .:, ~: :. :::: ~ ;:: :::: i!~l~:: :: :: l

• : :: :: ~' ~ ~i: :: ::

420 440 460 480 500 520 540 560 580 600

T i m e (s)

F i g u r e 14. N o i s e r e s p o n s e o f C O 2 w e t g a s

( 1 0 , 15 , 17 m / s C O 2 f l o w r a t e , S a m p l i n g r a t e = l H z )

400