Ultrasonic Meter Diagnostics.pdf

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Ultrasonic Meter Diagnostics

John Lansing

.

ABSTRACT

This paper discusses both basic and advanceddiagnostic features of gas ultrasonic meters (USM), identify problems that often may not have beenidentified in the past. It primarily discusses fiscal-quality, multi-path USMs and does not cover issuesthat may be different with non-fiscal meters as they areoften single path designs. Although USMs basicallywork the same, the diagnostics for each manufacturerdoes vary. All brands provide basic features asdiscussed in AGA 9 [Ref 1]. However, some provide

more advanced features that can be used to helpidentify issues such as blocked flow conditioners andgas compositional errors. This paper is based uponthe Westinghouse configuration (also knows as achordal design) and the information presented heremay or may not be applicable to other manufacturers.

INTRODUCTION

During the past several years there have beennumerous papers presented which discuss the basicoperation of USMs [Ref 2]. These papers discuss themeaning of the five basic diagnostic features.Following is a summary of the five features availablefrom all USM manufacturers.

Individual path velocities Individual path speed of sound Gains for each transducer Signal to noise for each transducer Accepted pulses, in percentage, for each

transducer pair

Although these features are very important, little hasbeen written on how to interpret them. Part of thereason is analysis varies by manufacturer.

Some manufacturers provide additional diagnosticfeatures such as swirl angle, turbulence, AGA 10

[Ref others.

Graphs shown in this paper are from Excelspreadsheets based on data generated by softwarethat is used to communicate with the meter. Note thatthese graphs were not individually developed butrather automatically generated from the data collectedduring calibration or maintenance procedures.

Obviously it is important for users to collect periodic -

Many utilize some of the data for entry into theircompany database for tracking over time. However, alarge numb trending of data.

BASIC DESIGNS OF ULTRASONIC METERS

Before discussing diagnostics it might be helpful toreview some of the basic designs that are used today.Figure 1 shows 5 types of velocity integrationtechniques [Ref 4]. The various meter configurationsin Figure 1 provide different velocity responses toprofiles, and are thus analyzed differently. This is

particularly true when trying to perform comparisonson velocity and SOS. Looking at differences in SOSbetween the various paths may require somewhatdifferent analysis. This is primarily the case when ameter is operated at very low velocities as thermalstratification can occur (more on this later). Analysis inthis paper will be applicable to design D in Figure 1.

A B DC E

Figure 1 Ultrasonic Meter DesignsBASIC DIAGNOSTIC INDICATORS

One of the principal attributes of modern ultrasonicmeters is the ability to monitor their own health, and todiagnose any problems that may occur. Multipathmeters are unique in this regard, as they can comparecertain measurements between different paths, as wellas checking each path individually.

diagnostics. Internal diagnostics are those indicatorsderived only from internal measurements of the meter.

External diagnostics are those methods in whichmeasurements from the meter are combined withparameters derived from independent sources todetect and identify fault conditions. An example of thiswould be to compute the gas SOS, based uponcomposition, and compare to the meters measuredSOS.



Gain

multipath USMs have automatic gain control on allreceiver channels. Transducers typically generate thesame level of ultrasonic signal time after time. Theincrease in gain on any path indicates a weaker signal

at the receiving transducer. This can be caused by avariety of problems such as transducer deterioration,fouling of the transducer ports, or liquids in the line.However, other factors that affect signal strengthinclude metering pressure and flow velocity.

Figure 2 shows gains from a 16-inch meter at the timeof calibration. These were taken when the meter wasoperating at approximately 20 fps.

Average AGC at 22.7 ft/s

32

37 37

3132

37 37

31

10

20

30

40

50

Path 1 Path 2 Path 3 Path 4

Path

A v e r a g e A G C ( d B )

Transducer A Transducer B

Figure 2 Gain at 20 fps 16 inch Meter

Note that the gains on each of the pairs are verysimilar, and the gains by path are higher in the middletwo paths. This is due to the increased path lengthrequiring additional amplification. Figure 3 shows thesame meter at 155 fps.

Average AGC at 153.6 ft/s

35

41 41

3536

40 40

35

10

20

30

40

50


Path

A v e

r a g e A G C ( d B )


Figure 3 Gain at 155 fps 16 inch Meter

Figure 3 shows the gains for all pairs have increased.This is normal when a meter is operating at much

higher velocities due to signal attenuation. However,notice both graphs have the same look in that thecenter pairs have higher gains than the outer ones.

Again, this is due to the longer path length.

Signal Quality Transducer Performance

This expression is often referred to as performance(but should not be confused with meter accuracy). Allultrasonic meter designs send multiple pulses acrossthe meter body to the opposing transducer in the pair,before updating the output. Ideally all the pulses sentwould be received and used. However, in the realworld, sometimes the signal is distorted, too weak, orthe received pulse does not meet certain criteriaestablished by the manufacturer. When this happensthe electronics rejects the pulse rather than usesomething of questionable quality that might distort theresults.

The level of acceptance (or rejection) for each path isgenerally considered as a measure of performance,

and is often referred to as signal quality. Unless thereare other influencing factors, the meter will normallyoperate at 100% performance until it reaches theupper limit of the velocity rating. Here the transducersignal becomes more distorted and some of thewaveforms will ultimately be eliminated since they less.

Figure 4 shows the performance of a 16 meter at avelocity of about 20 fps.

Average Performance at 22.7 ft/s

100.0 100.0 100.0 100.0

50

60

70

80

90

100

110


Path

A v e r a g e P e r f o r m a n c e ( % )

Figure 4 Transducer Performance at 20 fps

Figure 5 shows the same meter operating at 155 fps. As we can see the performance has fallen from 100percent on all paths to the 90+% range. This is normalfor high velocities as signal distortion will have someimpact on waveforms at these high velocities.



Average Performance at 153.6 ft/s

92.8 91.789.6

91.7

50

60

70

80

90

100

110


Path

A v e r a g e P

e r f o r m a n c e ( % )

Figure 5 Transducer Performance at 155 fps

Signal-to-Noise Ratio

Signal to noise (SNR) provides information that is also

user of possible impending problems. Eachtransducer is capable of receiving noise informationfrom extraneous sources (rather than its oppositetransducer). In the interval between receiving pulses,meters monitor this noise to provide an indication of same ultrasonic frequency spectrum as thattransmitted from the transducer itself.

The measure of signal strength to the level of or SNR for short. Typically this is not monitored nearlyas often as gains and performance. SNR is generallynot an issue unless there is a control valve or other

noise generating piping component present. Whenthat occurs, the SNR values will drop. The magnitude methodology of expressing the value.

Figure 6 shows the SNR from a 16-inch meter flowing20 fps at the time of calibration. As can be seen theSNR is about 40 dB.

Average SNR at 22.7 ft/s

40 40 40 4040 40 40 40

10

20

30

40

50


Path

A v e r a g e S N

R


Figure 6 SNR at 20 fps Meter Velocity

Figure 7 show the same meter at about 155 fps. TheSNR values have decreased by about 5-15 dB,depending upon whether they are upstream ordownstream. This is due to ultrasonic noise beinggenerated inside the piping. As the downstreamtransducers face the upstream direction, the increasedlevel of noise has more impact on the downstreamtransducers. Also note that the SNR for the middlepairs has decreased more than the outer pairs. This isdue to the path length being longer and thusattenuating the signal more.

Average SNR at 153.6 ft/s

35

32 31

36

30

26 27

31

10

20

30

40

50

Path 1 Path 2 Path 3 Path 4Path

A v e r a g e S N R


Figure 7 SNR at 155 fps Meter Velocity

Noise levels can become excessive if a control valve isplaced too close to the meter and the pressuredifferential is too high. When this happens the metermay have difficulty in differentiating the signal from thenoise. By monitoring the level of noise, when no pulseis anticipated, the meter can provide information to theuser, via the SNR, warning that meter performance(signal quality) may become reduced. In extreme signal to the point that the meter becomes inoperative.

w generation of transducers can handlesignificant levels of control valve noise. By usingtransducers that have a higher frequency, combinedwith higher efficiency and stronger sound pressurelevels, the affects of control valve noise have beensignificantly reduced as compared to past generationsof USMs. Figure 8 shows a picture of a meter and acontrol valve located immediately downstream of theUSM.



Figure 8 Control Valve near 2-inch USM

In the test shown in Figure 8, the meter was beingoperated at 600 psig and the regulator was producingabout 200 psig differential pressure. went from a normal of 40 dB to 24 dB. For this meterwhen the SNR approaches 13 the meter would beginto reject waveforms. Figure 9 shows the waveformduring this test. Figure 10 shows the same pair oftransducers when there is no regulator noise.

Figure 9 Waveform with Control Valve Noise

Baseline withRegulator Noise



Figure 10 Waveform with no Control Valve Noise

From Figure 9 we can see there is a little noise on thebaseline preceding the major waveform. The baselinein front of the received signal is not perfectly flat as it isin Figure 10. The SNR values are above 24 dB for thiscondition on the upstream transducers (one that facesthe source of the noise). The downstream transducerhas a SNR of 30 because it is facing away from thenoise source. Figure 10 shows the waveform when

there is no noise from the regulator.SNR can also be low if the electronics has a problemor there is a poor connection between the transducerand the electronics. Figure 11 shows the SNR graphedwhen there is a problem with the electronics.

Average SNR [dB]

0

5

10

15

20

25

30

35

40


SNR AB SNR BA

Figure 11 Poor SNR on Path 4

Here we can see that the SNR from upstream todownstream is not consistent. All of the SNR values of

AB are lower than BA. This is due to a problem withthe electronics. Figure 12 shows the results of thesame meter after the electronics was replaced.

Average SNR [dB]

0

5

10

15

20

25

30

35

40


SNR AB SNR BA

Figure 12 Good SNR on all Paths

In Figure 12 we can see that all the SNR values arenow close to 40 dB. This is the normal for this meter.Even though the SNR was poor in Figure 11, the normal. Thus, it is possible to have low SNR and allother diagnostic indicators are normal.

Baseline with noRegulator Noise



Speed of Sound

Probably the most discussed and used diagnostic toolof an ultrasonic meter is the speed of sound (SOS).The reader may recall that speed of sound on anindividual path is basically the sum of the transit timesdivided by their product, all then multiplied by one halfof the path length. A more detailed discussion on this

can be found in a previously presented paper [Ref 5].There are at least 2 ways of looking at SOS. The first SOS calculated by the meter. Figure 13 shows agraph of the SOS of a 10 inch meter at the time ofcalibration.

Speed of Sound at 22.7 ft/s

1374

1375

1376

1377

1378

1379

1380

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

Time (sec)

S O S

Average Path 1 Path 2 Path 3 Path 4

Figure 13 SOS by Path at Calibration

This data was taken from the meter operating at 23 fpsand showing a very stable reading. Here we can see re very close. Butperhaps an easier way of looking at the SOS values is value. Doing this is makes it easier to spot problems.

Figure 14 shows the percent difference of each path reported average SOS.

SOS Difference from Average at 22.7 ft/s

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

Time (sec)

S O S D i f f

e r e n c e ( % )


Figure 14 Path Percent Difference in SOS

Figure 14 shows the SOS by path in percentage ach indicates good correlation between each path and alsono temperature stratification within the meter.

When a meter is operated at lower velocities, typicallyless than 3 fps, and there is a large difference between

the gas and atmospheric temperature, heat transfercan occur. As the heat transfer occurs, internaltemperature gradients can develop. When thishappens the hotter gas inside the pipe rises to the topof the meter. Since the speed of sound in the gas isrelatively sensitive to temperature, this will be seen asa SOS difference between the paths. This is oftencalled thermal stratification.

Figure 15 shows the SOS values of the same 10 inchmeter when it is operated at 1.8 fps at the calibrationlab.

SOS Difference from Average at 1.8 ft/s

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

Time (sec)

S O S D i f f e r e n c e ( % )


Figure 15 Thermal Stratification Effects

From Figure 15 it can be seen that the averagepercent difference in SOS compared to the meter hasincreased a little. This is due to a slight thermalgradient within the meter. That is the gas at the top ofthe meter the gas is slightly warmer than that at thebottom. Path 1 color is blue, path 2 is red, path 3 isgreen and path 4 is gold in color. Figure 15 showsupper paths have increased and the ones at thebottom decreased.

This difference in SOS may be thought to impact the

accuracy of the meter. In extreme cases this can bethe case. However, for this example the impact isvirtually on-existent. Figure 16 shows the results of this10-inch at the time of calibration.



10-inch As Found and As Left Results

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0 20 40 60 80 100 120

Meter Vel ocity (ft/sec)

%

E r r o r

As Found As Left Verification Points

Figure 16 10-inch As-Found Calibration Performance

he very little impact in the performance of the meter eventhough there was some thermal stratification.

Velocity Profile

Monitoring the velocity profile is possibly one of themost overlooked and under-used diagnostic tools of to the condition of the metering system, as well as themeter. AGA Report No. 9 requires a multipath meterprovide individual path velocities.

Once the USM is placed in service, it is important tocollect a baseline (log file) of the meter. That is,record the path velocities over some reasonableoperating range, if possible. These baseline logs can

also be obtained at the time of calibration. However,as the piping in the field will likely be different than thatat the calibration facility, there could be some minorchanges in profile. Good meter station designsproduce a relatively uniform velocity profile within themeter. The baseline log file may be helpful in theev at a laterdate.

Figure 17 shows the velocity ratio of each path relative average velocity. This ratio is computedby taking each path velocity during a periodof time and dividing it by the average velocity asreported by the meter over the same period of time.

Since the ratio for each path remains essentially operation are easier to detect than by looking at theactual velocity on each path.

Flowing Velocity Ratios at 22.7 ft/s

0.917

1.018

1.021

0.912

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4

Average Flowing Velocity (ft/sec)

Figure 17 Path Ratios at 23 fps

Typically the ratio for a chordal design meter is about91% (ratio = 0.91) for paths 1 and 4, and about 102%(ratio = 1.02) for paths 2 and 3. The difference inratios is due to the fact that the outer paths are closer

to the pipe wall, and thus the velocity of the gas thereis less than the gas that is closer to the center of thepipe. When the velocity falls below approximately 3feet per second, depending upon meter size andstation design, the velocity profile may change. Figure18 rofile when thevelocity is at 2.8 fps.


0.842

0.997

1.053

0.961

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4


Figure 18 Path Ratios at 2.8 fps

When comparing Figure 17 and 18 it is very clear thatthe velocity profiles are very different. Both of thesewere taken from a 16-inch meter at the time ofcalibration. Even with the difference in path ratios, the 9 from the calibration.



As Found Results for 16-inch Meter

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0 10 20 30 40 50 60 70 80

Pipeline Velocity (ft/sec)

% E

r r o r

As Found

Figure 19 16 inch As-Found Results

Figure 19 shows even though path ratios were different as shown inFigures 17 and 18. This is the same meter discussedearlier that was calibrated to 155 fps, but the x-axishas been adjusted to better show the low end

performance.Figures 20 and 21 show velocity profiles for the 10-inch meter discussed earlier at 20 and 1.8 fpsrespectively. Figure 20 shows the baseline at about23 fps and the velocity profile is very symmetrical. InFigure 21 the profile has become a little distorted. Thisis in part due to the minor thermal stratification.However, as figure 16 shows, there was very little -found linearity (open circles).


0.908

1.022

1.021

0.912

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4


Figure 20 10-inch 20 fps Path Ratios


0.930

1.037

1.005

0.894

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4


Figure 21 10-inch 1.8 fps Path Ratios

Even though this meter had some thermal significantly. There is a difference, but once again it isnot as significant as the blocked flow conditioner

example in Figure 26.ADVANCED DIAGNOSTIC INDICATORS

The basic diagnostic parameters that are provided by section. They are gain, performance, signal to noiseratio (SNR), speed of sound (SOS) and velocity profile.Of these the most difficult for most to understand is theVelocity Profile. This is due in part to the various USMpath configurations and different methods ofpresenting path velocity information by themanufacturers.

For the chordal type of meter (see path layout D inFigure 1), most manufacturers talk about path velocityratios. This method of displaying the velocity profile is change significantly over the majority of velocities theUSM is operated at today.

However, there are easier ways to analyze the variousdifferent profiles that can occur in the field. Thesevariations occur due to the wide range up upstream minimize the distortion of the gas velocity profile eliminate them.Even though the USM can handle a wide range of

distortion with minimal impact on accuracy, the idealsituation is to reduce the (distortion of the gas velocity profile) to a minimum.

In order to understand if the velocity profile haschanged over time, advanced methods of diagnosingthe USM have been developed. Since the velocityprofile is the most difficult, due to the wide range ofpossible scenarios, a simpler method of summarizingthese profiles would be beneficial.



Profile Factor and Symmetry

Looking at four path ratios takes understanding whythe velocities are different. Since these can change bysmall amounts, a simpler method of identifyingchanges in profile is desired. A single value would bemuch easier to understand, and also easier to quicklyanalyze. One of these methods is called Profile

Factor .The Profile Factor is computed by adding the velocityratios of paths 2 and 3 together and dividing by thesum of the ratio of paths 1 and 4. The equation lookslike this: Profile Factor = (2 + 3)/(1 + 4). Assuming thatpaths 1 and 4 are 0.91, and the path 2 & 3 values are1.02, the Profile Factor is about 1.12. This value doesvary a little from meter to meter due to pipinginstallation effects, and to some degree, the type offlow conditioner and its distance from the meter.

Another method used to analyze path velocities is tocompare the sum of paths 1 & 2 to the sum of paths 3

& 4. This provides a look at the symmetry of theprofile from top to bottom, and is called Symmetry. symmetrical resulting in a value close to 1.000.Figures 13 and 14 show both the Profile and theSymmetry in a single graph.

In Figure 22, when the meter was flowing at 23 fps, theProfile Factor was 1.115 (average of the magentacolored line). As the velocity dropped to 2.8 fps(Figure 23) the Profile Factor increased to 1.137. Thisis about a 2% change in profile when compared to theProfile Factor at 23 fps.

Profile Factor and Symmetry at 22.7 ft/s

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

Time (sec)

P r o f i l e F a c t o r

0.85

0.90

0.95

1.00

1.05

1.10

1.15

S y m m e t r y

[ ( P 1 + P 2 ) / ( P 3 + P 4 ) ]

Profile Factor

Symmetry

Profile Factor Std. Dev. = 0.0111

Figure 22 Profile Factor and Symmetry at 20 fps

The other diagnostic worth reviewing is the Symmetryvalue. Figure 23 shows a significant change (on theorder of 10%) in the Symmetry at the lower velocities.This can be seen by comparing the blue line if Figure22 to the blue line in Figure 23. However, there was nosignificant impact on meter performance (seeFigure 16). These graphs indicate there was a change

and this is to be expected atlower gas velocities.


1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

Time (sec)


0.85

0.90

0.95

1.00

1.05

1.10

1.15

S y m m e t r y

[ ( P 1 + P 2 ) / ( P 3 + P 4

) ]

Profile Factor

Symmetry



The Profile Factor can be a valuable indicator of

abnormal flow conditions. The previous discussionshowed what happens to the Profile Factor andSymmetry due to low velocity operation. This profilechange is typical when the meter is operated at lowervelocities.

Figure 24 shows an ideal profile from a 12-inch meter.This was based on the log file collected at the time ofcalibration. Users have often asked what impact partial accuracy. This meter was used to show what happensnot only to the profile, but to quantify the change inaccuracy.


0.919

1.020

1.017

0.915

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4


Figure 24 12-Inch Meter Profile Normal

The Profile Factor for this meter is 1.118. For thesecond test, the flow conditioner was modified to haveabout 40% of the holes blocked with duct tape. Ducttape was used to ensure repeatability. Figure 25shows the flow conditioner just before it was installedin the pipeline.



Figure 25 40% Blocked Flow Conditioner

Figure 26 shows the velocity ratios during the time theflow conditioner was blocked. This was taken at a

velocity of 66 fps. The profile at two other velocities,22 and 45 fps, looked the same.

Path Velocity Ratios at 66.5 ft/s

0.885

1.000

1.039

0.945

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4

Path Ratios

Figure 26 12-inch Meter Profile 40% Blocked

The Profile is obviously distorted with higher-than-normal readings on path 3 and 4, and lower thannormal on paths 1 and 2. The flow conditioner wasinstalled with the blockage at the bottom of the pipe.

As the gas flowed through the open holes, there was alow-pressure created just downstream of the blockedarea causing the gas to then accelerate downward,

thus causing the higher velocity at the bottom of themeter than at the top.

Figure 27 shows the graphical results of the ProfileFactor and Symmetry with no blockage.


1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

Time (sec)


0.85

0.90

0.95

1.00

1.05

1.10

1.15

S y

m m e t r y

[ ( P 1 + P

2 ) / ( P 3 + P 4 ) ]

Profile Factor

Symmetry



From Figure 27 the average Profile Factor is 1.111and the average Symmetry is 1.003. These are justabout the ideal values for both. Figure 28 shows theProfile Factor and Symmetry graph with 40%

blockage.


1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

Time (sec)


0.85

0.90

0.95

1.00

1.05

1.10

1.15

S y m m e t r y

[ ( P 1 + P 2 ) / ( P 3 + P 4 ) ]

Profile Factor

Symmetry


Figure 28 Profile Factor and Symmetry at 66 fps 40% Blocked Flow Conditioner

Figure 28 shows the results of the Profile Factor andSymmetry with 40% blockage of the flow conditioner.The average of the Profile Factor is 1.053 and theSymmetry is 0.936. This is about a -6% change inProfile Factor and about a -7% change in Symmetry.Both of these would be considered significant andshould be treated a cause for investigation.

After installation in the field a meter typically willgenerate a Profile Factor that is repeatable to ±0.02(or about 2%). However, this does depend upon thepiping, and makes the assumption that there are noother changes like flow conditioner blockage.

The next question is what was the impact on accuracywith this distorted velocity profile? Figure 29 showsthe result of the three test velocities and the impact onmetering accuracy.



68.4 -0.02

45.3 -0.12

22.9 -0.10

Velocity

(fps)

% Diff. with

Blocked CPA

Baseline vs. 40%

Blocked CPA

Figure 29 Blocked CPA Results

As can be seen the meter was affected by an averageof about 0.15% for all flow rates. In this case themeter slightly under-registered with this distortedprofile. Later in this paper a more advanceddiagnostic feature will also show the meter hasblockage, but for now one can see the Profile Factorhas indicated a significant change.

In the past many have thought that looking at theProfile Factor alone would be a good indication if there

was any contamination or flow conditioner blockage.This may not always be true. Figure 30 shows apicture of a flow conditioner with 3 holes blocked at thebottom.

Figure 30 3 Holes Blocked Flow Conditioner

In this test the three blocked holes in the flowconditioner were located at the bottom of the meterrun. This is the same 12-inch meter and testing asdiscussed with the 40% blockage. Figure 24 shows agraph of the Path Ratios during this test with the 3blocked holes located at the bottom of the meterpiping.


0.885

1.000

1.039

0.945

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4

Path Ratios

Figure 31 Path Ratios with 3 Holes Blocked - Bottom

Comparing Figure 31 with Figure 24 (the normal PathRatio profile) it is obvious that the two graphs of pathratios do not look the same. However, whencomputing the Profile Factor, the average value for

Figure 31 is 1.114 (1.11 is the ideal factor). This isalmost the perfect number for this meter.

Upon further investigation Figure 32 shows that theSymmetry has changed significantly.


1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

Time (sec)

P r o

f i l e F a c t o r

0.85

0.90

0.95

1.00

1.05

1.10

1.15

S y m m e t r y

[ ( P 1 + P 2 ) / ( P 3 + P 4 ) ]

Profile Factor

Symmetry


Figure 32 Profile Factor and Symmetry 3 HolesBlocked (at Bottom of Pipe)

In Figure 32 the average for the Profile Factor is 1.114,and is almost normal, but that the Symmetry valueaverage is about 0.95, or approximately a -5% shift

from normal (1.00 being normal). Thus, it is possiblefor the Profile Factor to be normal even though thevelocity profile in the meter is not. This is the reasona combination of Profile Factor and Symmetry isboth require to fully analyze the velocity profileentering the

Figure 33 accuracy was with this blockage was located at thebottom and also rotated 90 degrees so that it wasblocking paths 2 & 3 more significantly (than when itwas at the bottom of the pipe.



12-inch, 4-Path Meter - 3 Holes Blocked Results

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

0 10 20 30 40 50 60 70 80

Meter Velocity (ft/sec)

% E r r o r

Un-Blocked CPA 3 Holes Blocked Results 3 Holes Blocked - Rotated 90 Degrees

Figure 33 As-Found Results 3-Holes Blocked

When the blockage was at the bottom of the meterrun, there was very little impact on accuracy (onaverage -0.024%). When the blockage was rotated 90degrees to the side, the meter responded with a shiftof about +0.22%. All other blockage tests to date hadshown the meter responded with a negative shift in

error, but for the first time the meter now measuredfast with this blockage.

The question might be asked as to why this has ity profile when thisblockage occurred on the side of the flow conditioner.

Flowing Velocity Ratios at 4 4.6 ft/ s

0.968

0.989

0.994

0.987

0.80 0.85 0.90 0.95 1.00 1.05 1.10

Path 1

Path 2

Path 3

Path 4

Average Flow ing Velo cit y (ft /sec)

Figure 34 Path Ratios with 3 Holes Blocked - Side

Figure 34 shows that the velocity profile is much flatterthan normal. That is the center two paths (paths 2 & 3)are reading much less than normal and are almost thesame as the outer paths (2 & 3). This makes sense

since the blockage was in direct line with the middletwo paths when it was rotated 90 degrees vs. blockingprimarily the bottom path when at the bottom.

When analyzing the Profile Factor and Symmetry arather interesting result is apparent. Figure 35 shows agraph of the Profile Factor and Symmetry for the 3-hole blockage on the side.


1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

Time (sec)

P r o f

i l e F a c t o r

0.85

0.90

0.95

1.00

1.05

1.10

1.15

S y

m m e t r y

[ ( P 1 + P

2 ) / ( P 3 + P 4 ) ]

Profile Factor

Symmetry 2


Figure 35 Profile Factor and Symmetry 3-HolesBlocked (at Bottom and Rotated 90 Degrees)

Figure 35 shows that the average Symmetry is almostnormal (0.985, or about 1.5% change from ideal), butthe average Profile Factor is 1.011, or about a 10%

change. If only Profile Factor alone were monitored, ashift of 1.5% would not be considered significant. Thusa problem might be over-looked.

This is why both the Profile Factor and Symmetry needto be computed, and monitored, in order to state the . If bothare the same, then there is no combination of velocityprofile ratios that can produce a distorted profile andstill provide the same average values of 1.11 forProfile Factor and 1.00 for Symmetry.

Sometimes, reviewing path ratios as shown by theUSM software, it is difficult to see if everything is

normal. By reviewing a Maintenance Report, wherethe average value is generally presented, it is easier tosee. Figures like 26, 28, 31 and 34 show the path significant changes occur, it is easy to see. However,what is needed is a simpler way to present this. Thefollowing graph is from the USM software and it showsall of the typical diagnostics.

Average: +0.22%

Average: -0.024%



Figure 36 Summary of All Diagnostics Normal Velocity Profile

Figure 37 Summary of All Diagnostics Abnormal Velocity Profile

In Figure 36 we see a very symmetrical velocity profile(upper left corner). In Figure 37 we see a distortedvelocity profile. The Profile Factor and Symmetry areshown in the graph called Profile Indication which isthe 3rd graph from the left on the second row. In Figure36 we can see there are two dots inside the red boxand in Figure 37 we see that one of the dots is outsidethe red box and the line is yellow. This is telling us theSymmetry is outside of normal tolerances.

The Profile Indication box is really a summary of boththe Profile Factor and Symmetry. The dot in the middleof the box is established when commissioning themeter and is the average value when first started up.The second dot is the current, or live, value of theProfile Factor value (represented by the X axis) andthe Symmetry value (represented by the Y axis). Wheneither of these values causes a change of more than5% ( an adjustable value but just a baseline for thisexample), then the line turns yellow indicating there is

a significant change. Figure 38 shows a close-up of anormal value, and Figure 39 show a problem.

Figure 38 Normal Profile Factor and Symmetry



Figure 39 Abnormal Profile Factor and Symmetry

Figure 38 shows a dot just to the right of the centeredone (which is configured in the meter) and itrepresents the current reading of both Profile Factorand Symmetry. These values are show to the right of

the graph.When a problem occurs that causes a shift in eitherProfile Factor or Symmetry, by more than theprogrammed limit in the meter (shown here as 5%),then the dot will move outside of the box and turnyellow. Such is the case for Figure 39 where we seethe Symmetry is 0.895, or more than 10% from normal(1.00 being normal). Thus the technician can veryeasily see there is a problem with the profile.

Turbulence

During the past several years an additional diagnosticfeature has been studied by Engineering. This

ghly ina previous paper [Ref 6]. Essentially Turbulence is ameasure of the variability of each paths velocityreadings during the time the meter was sampling, andis provided each time it updates the velocityinformation. This gives the technician an idea of thesteadiness of the flow as seen by the meter.

Typically the level of turbulence on a Westinghousedesign shows paths 1 and 4 to have around 4%turbulence, and paths 2 and 3 around 2%. This isbased upon the history of many meters. The outerpaths 1 and 4, being closer to the pipe wall, alwaysexhibit higher turbulence because they are moreaffected by the surface friction of the upstream piping.Turbulence can be computed from the maintenancelog file for older meters. With the advent of moreadvanced electronics, it is now computed real-time inthe meter and reported on the maintenance log files.This greatly reduces the time for analysis since it is notonly stored in the log file, it is graphed outautomatically for quick review.

Recently viewing Turbulence has solved severalmetering problems. Distorted velocity profiles oftencause concern about metering accuracy. If thevelocity profile, as shown in Figure 24, now appearslike that in Figure 26, the cause needs to bedetermined. Some might feel this is just due toupstream affects and may not believe there is anyobject blocking the flow conditioner.

The 12-inch meter in Figure 40 shows a veryconsistent level of Turbulence during the period of thetest. It was collected at the time of calibration and thevelocity was about 66 fps. The average for these is2.44% and this is considered normal.

Turbulence at 66.4 ft/s

2

4

6

8

20

40

60

80

1

10

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Time (sec)

T u r b u l e

n c e ( % )


Figure 40 Normal 12-inch Meter Turbulence

Figure 41 show the Turbulence with a 40% blockedflow conditioner as shown in Figure 25.


2

4

6

8

20

40

60

80

1

10

100

0 1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

1 4 0

1 5 0

Time (sec)

T u r b u l e n c e ( % )


Figure 41 High 12-inch Meter Turbulence

It is clear that the turbulence in Figure 41 is about 3times higher, or an average of 7.03%. Certainly thevelocity profiles for this meter, shown in Figures 24and 26, look different. Anyone looking at the blockedprofile would immediately recognize there is aproblem.



It is possible, however, to have a complete blockage ofa flow conditioner with something like a porous bag, orpiece of carpet, and have a relatively symmetricalprofile. In this situation the turbulence would beexcessive, indicating there is a problem with blockage.This has been observed in the field and withoutTurbulence it would have gone un-detected.

The following figures show the Turbulence with the 3holes blocked. The first is with the holes located at thebottom of the piping, and the second with the blockedholes rotated 90 degrees.


2

4

6

8

20

40

60

80

1

10

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Time (sec)



Figure 42 Turbulence w/3 Holes Blocked - Bottom

Figure 42 shows the average Turbulence is higherthan normal (the average is 4.02%) as the 40% blocked, but it is significant. Figure 41shows the Turbulence when the blocked holes wererotated 90 degrees.


2

4

6

8

20

40

60

80

1

10

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Time (sec)



Figure 43 Path Ratios w/3 Holes Blocked - Side

Figure 43 shows the average Turbulence is 4.33% andalso higher than the average with no blockage of2.44%. Both tests with 3 holes blocked indicate aboutthe same and approximately 60% higher than normal.Thus Turbulence is generally the best method ofidentifying flow conditioner blockage.

CONCLUSIONS

During the past several years the industry has learneda lot about USM operational issues. The traditional 5diagnostic features, gain, signal-to-noise,performance, path velocities and SOS have helped theindustry monitor the USM. These 5 features provide alot of information about the meter

initial baseline on the meter at the time of installation,and monitoring these features on a routine basis cangenerally identify metering problems in advance offailure.

More advanced diagnostic indicators, such asTurbulence, are paving the way to allow the meter tobecome virtually maintenance-free. In the future it islikely that a meter will have enough power andintelligence to quickly identify potential measurementproblems on a real-time basis.

As the industry learns more about not only the USM,and the operation of their own measurement system,

the true value of the ultrasonic meter will berecognized. The USM industry is still relatively youngand technology will continue to provide more tools to ment problems.

REFERENCES

1. AGA Report No. 9, Measurement of Gas byMultipath Ultrasonic Meters, June 1998, AmericanGas Association, 1515 Wilson Boulevard,

Arlington, VA 22209

2. John Lansing, Basics of Ultrasonic Flow Meters, American School of Gas MeasurementTechnology, 2000, Houston, Texas

3. AGA Report No 10, Speed of Sound in NaturalGas and Other Related Hydrocarbon Gases, July2002, American Gas Association, 1515 WilsonBoulevard, Arlington, VA 22209

4. BSI 7965:2000, Guide to the Selection,Installation, Operation & Calibration of TransitTime Ultrasonic Flowmeters for Industrial Gas

Applications

5. Klaus Zanker, Diagnostic Ability of the DanielFour-Path Ultrasonic Flow Meter , Southeast AsiaFlow Measurement Workshop, 2003, KualaLumpur, Malaysia

6. John Lansing, Solve Metering Problems, North Sea FlowMeasurement Workshop, 2005, Tonsberg, Norway

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