A NOVEL DESIGN OF A 12-CHORD ULTRASONIC GAS FLOW METER
Paper 9030
Jan G. Drenthen, Martin Kurth & Marcel Vermeulen
KROHNE New Technologies
Kerkeplaat 14
Dordrecht, The Netherlands
1
Introduction
Over the past 15 years, thousands of ultrasonic flow meters have been successfully employed in the
natural gas industry and in particular in the upstream and transmission segments. Here, the users take
the full advantage of the non-intrusive measurement technology, the absence of pressure drop and
the virtually maintenance-free operation; almost to the level of install-and-forget.
However, as you look to the technical data sheets of the different manufacturers you find different
specifications like:
Measurement accuracy
Uncertainty
Repeatability
≤±0.5% of measured value, uncalibrated
≤±0.2% of measured value, high-pressure flow calibrated
(relative to calibration laboratories)
≤±0.1% of measured value, calibrated and linearized
≤±0.1%
But this specification is relative to the conditions at the calibration laboratory and relative to the
uncertainty of the laboratory. Most of the high pressure natural gas flow laboratories have an
uncertainty of about 0.2% which is the uncertainty relative to the meter and kilogram in Paris. What all
of the different manufacturers don't mentioned in their datasheet is the effect of transferring the
obtained calibration certificate at the calibration facility (fig.1) into the field at actual conditions (fig.2).
This effect is also called “installation effect”.
Fig.1 Ideal conditions at test rig (courtesy of TCC)
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Fig.2 Actual conditions in the field
A novel design of a 12-chord ultrasonic gas flow meter
The typical uncertainty from transferring the calibration conditions into the field is approximately 0.5 %;
much higher than the calibrated uncertainty of the meter itself.
(See for instance the paper by T Grimley, AGA conference, Denver 2000 [4])
Another uncertainty not mentioned in the manufacturer’s datasheet is the effect of possible
contamination during operation. During flow calibration (fig.3) the meter is clean but during operation,
after 3 months or 6 months, the meter might be contaminated. The additional uncertainty introduced
can easily reach 0.3% - 0.5% depending on the path configuration [1], the same magnitude as
installation effects.
Figure 3
A clean ultrasonic meter prior to installation
Figure 4
A dirty meter during visual inspection
The result is a total uncertainty which is much larger than the ones stated in the datasheet.
Uncertainty of UFM
Conv. UFM
Direct chord
Conv. UFM
Reflective chords
KROHNE V12
Meter (Repeatability / Non-Linearity)
0.20%
0.20%
0.15%
Installation effects
0.50%
0.50%
0.15%
Effect of typical contamination
0.50%
0.30%
0.10%
Overall uncertainty rel. to calibration
0.75%
0.6%
0.25%
Table 1: Uncertainty comparison between conventional USM and KROHNE V12
In order to reduce the total uncertainty, the impact of both the installation effect as well as the
contamination has to be reduced; consequently this has been one of the prime targets in the
development of the ALTOSONIC V12.
In this design, the main attention was given to immunity against installation effects, performance
monitoring and the optimization of the diagnostic capabilities. The end result is a state-of-the-art
measurement concept that combines high level performance monitoring with the best attainable
accuracy and unsurpassed performance monitoring.
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A novel design of a 12-chord ultrasonic gas flow meter
2
Variations in fouling
Fouling is one of the major concerns for additional uncertainty. Moreover, one of the main problems
with fouling is the variety in manifestations. In figures 5 to 7 some examples of excessive fouling are
presented. The first one is the result of a production problem, the second one is an example of an offshore installation and the third one is an on-shore installation.
Figure 5
Fouling as a flow on the bottom of the pipe
Figure 6
Fouling intermittently stuck to the pipe wall
Figure 7
Fouling evenly distributed over the pipe wall
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A novel design of a 12-chord ultrasonic gas flow meter
As already described in one of our previous papers “The use of ultrasonic meters at M&R stations”,
presented at the AGA Operations Conference 2006, fouling can be classified into 5 categories which
all have a different impact on the measurement.
These Fouling Categories are:
A
B
C
D
E
Small flow on the bottom of the pipe
Intermittent sticking to the pipe wall
Evenly distributed coating on the inside of the pipe.
Dirt build-up on the transducers (especially on those facing upstream)
Liquid build-up in the transducer pockets
For ultrasonic flow meters, the effects of fouling on the measurement are:
•
•
•
•
•
•
A reduction of the cross sectional area
An increased wall roughness
The shortening of the acoustic path
The attenuation of the acoustic signal through the reduction of the reflection coefficient
The absorbance of the ultrasonic signal due to the layer of fouling on the transducer
Increased cross-talk when liquid is present in the transducer pockets
In order to cope with this, the design first focused on the ability to detect the various forms of fouling.
In the second phase, detection had to be converted into correction; but in this, the manufacturers and
users need to work closely together to collect and investigate experiences from real life applications.
After that, the algorithms are further developed to improve the measurement accuracy under fouling
conditions; protecting the customer’s investment and making the meter future proof.
3
Meter design
Looking at the current meter designs on the market, in general terms, it must be concluded that
despite some lucky occasions, none of them are sufficiently able to cope with highly distorted flows
and / or fouling conditions:
•
•
The conventional parallel chord designs lack the interrogation of pipe wall but are able to
measure close to the pipe wall.
The reflective chord design currently available on the market is able to measure wall built-up,
but as a result as a result of the design, its triangular path cannot get closer to the wall than the
0.5R position. For b- and c-categories of fouling, where the wall roughness is increased, this
path is too far away to deal effectively with the changes in the flow velocity profile close to the
pipe wall.
Hence the optimal solution is the combination of both technologies.
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A novel design of a 12-chord ultrasonic gas flow meter
3.1
Optimization of the chord position
Various designs have been considered as part of the research into possible meter design; 3 of these
configurations are shown in figure 8.
Figure 8 Analyzed path configurations
In evaluating these designs, both analytical flow models and an extended library with flow patterns
based on the laser-Doppler measurements from the PTB were used. This library contains hundreds of
real-life flow patterns ranging from straight pipes to flow patterns behind single and double-out-ofplane bends as well as expanders and reducers. Some examples are shown in figures 9 and 10.
These show both the calculated results and the Laser Doppler Anemometer results from the PTB.
Figure 9 Distorted flow pattern
Figure 10a Flow profile behind a reducer
Position x: 0R
Position x: 0R
1.5
1.5
Disturbed profile 5.5 D after a single 45° bending
measured in a 135° plane
Disturbed profile 5.5 D after a double bending
measured in a 0° plane
1
v/v gem [-]
v/vgem [-]
1
Measured LDA
0
-1
Measured LDA
Theory (30% and 0.6R)
Theory (20% and 0.68R)
0.5
0.5
-0.8
-0.6
-0.4
-0.2
0
r/R [-]
0.2
0.4
0.6
0.8
1
0
-1
Figure 10b Flow profile behind a single bend
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-0.8
-0.6
-0.4
-0.2
0
r/R [-]
0.2
0.4
0.6
0.8
1
Figure 10c Flow profile behind a double-out- ofplane bend
5
A novel design of a 12-chord ultrasonic gas flow meter
Based on all this, a solution was found, whereby:
1.
2.
3.
The path configuration was optimized to detect the various ways of fouling.
The combination of path positions and associating weighting factors and algorithms was
designed in such a way that the meter is robust to wall build-up.
The number of paths was optimized to attain the lowest uncertainty under a large variety of
possible flow profile distortions, including all the ISO 17089 perturbations.
The final and winning configuration is shown in figures 11 & 12:
Figures 11 & 12 Winning configuration of the 12 chords in the ALTOSONIC V12
In this configuration:
•
Each path consists of 2 chords in a V-arrangement (in total the meter is equipped with 12
chords, hence the name ALTOSONIC V12).
•
The vertical reflecting path is exclusively used to detect the presence of a liquid layer on the
bottom.
•
All paths are reflecting paths, whereby four of them use small acoustic mirrors at the opposite
side of the pipe.
The path through the center is essential for the meter’s ability to deal with flow velocity patterns behind
single and double-out-of-plane bends. Behind bends, one of the major impacts on the flow velocity
profile is the velocity defect in the middle of the pipe (as shown in figures 5, 6b and 6c). This defect is
easily detected by looking at the ratios between the path through the middle and that of the outer
paths.
3.2
Dealing with fouling
Performance monitoring and especially the detection of fouling has been one of the key elements in
the design. The description on how to deal with the fouling categories follows the sequence A-E from
chapter 2.
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A novel design of a 12-chord ultrasonic gas flow meter
3.2.1
Handling the A-category of fouling, bottom fouling
(valid for all velocity-area meters such as vortex, turbine, ∆P and ultrasonic meters)
As described, the vertical reflecting path is used exclusively to detect the presence of a liquid layer on
the bottom. The stability and ability to detect this depends on the viscosity, the density of both the fluid
and the gas, as well as the flow velocity. For more information see [2].
In the example below the following is assumed: line size 12” (300mm) and a solid layer of 0.5mm with
an angle β of 70º. By using the same conditions as in the example in paragraph 6 the potential loss or
profit (depending whether you are a buyer or seller of the gas) amounts to $ 561,632. Such a deposit
cannot be detected by any other meter including the ultrasonic if they do not have the vertical path.
However with a reflecting layer of 0.5mm the error in the SOS is 0.15%. As the sensitivity of the SOS
is 0.03% it would have been easily detected with a V12. Actually at a layer of only 0.2mm an alarm
would have been given as the alarm level is set to twice the sensitivity = 0.06% error.
Fig. 13 Calculation of reflecting layer effect on cross-sectional area and SOS
3.2.2
Handling B and C categories of fouling (asymmetrical and symmetrical)
The increased wall roughness due to fouling intermittently stuck to the wall and fouling which is evenly
distributed over the wall primarily affects the flow velocity pattern close to the wall and can be detected
by trending the velocity ratios of the 2 outer paths.
3.2.3
Handling D category of fouling, fouling on the transducers
The fouling on top of the transducer shortens the travel time and lowers the signal strength. By looking
at the combination of these two parameters as well as the change over time, this category of fouling
can be detected and a first order estimation of the thickness of the layer can be made. In practice,
most of the fouling will be found on the transducer facing downstream.
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A novel design of a 12-chord ultrasonic gas flow meter
3.2.4
Handling E category of fouling, contamination of the transducer pockets
This category of fouling affects the shape of the acoustic waveform, the signal-to-noise ratio and
results in an offset at the lower flow velocities. This category of fouling is more difficult to detect and
has to be done in the signal processing part.
3.3
Swirl elimination
Conventional parallel path meters are prone to swirl, resulting in a measurement error. To reduce this
sensitivity, manufacturers employ different path configurations and have the paths either in a single
plane or in a criss-crossed configuration. They and combine the measurements of the individual paths
to cope with the impact of swirl. These path configurations are shown in figures 14 a & b
Figure 14a Parallel path in plane configuration
Figure14b Parallel path criss-crossed configuration
However these configurations are a trade off:
•
•
•
The criss-crossed arrangements perform better after a single bend where there is a combination
of two opposite rotating vortices as in figure 15 a.
Single plane configurations on the other hand perform better after a double-out-of-plane bend
where one single large swirl component dominates (see figure 15 b)
None of them perform well under a-symmetrical swirl conditions. In this case all paths are
subjected to a different swirl strength and therefore the meter is not able to cancel out swirl
completely.
Fig.15.Cross flow a: behind a single bend. b: behind a double-out-of-plane bend c: a-symmetrical swirl
The only way to overcome this is to eliminate the effect of the swirl in each of the measurement
planes. (See also paper 2.2 of the NSFMW 2006; “Investigations on an 8 path ultrasonic meter”).
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A novel design of a 12-chord ultrasonic gas flow meter
The elimination of swirl in each of the measurement planes can be done through either an X or Vconfiguration. Due to the fact that swirl is stable over hundreds of diameters, measurement-wise there
is no difference between the two. This is illustrated in figure 16.
Vline1 and 2
Vaxial
Vswirl (unwanted)
Figure 16, Swirl elimination in each of the measurement planes
The various path configurations and their ability to deal with swirl are presented in table 2.
Parallel path configuration and
swirl
In plane
Criss-crossed
V12
Single bend
-
+
+
Double
Out-of-plane
bend
+
-
+
asymmetrical
swirl
-
-
+
Configuration
Flow pattern
Table 2: Designs with parallel and direct path configuration are prone to swirl
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A novel design of a 12-chord ultrasonic gas flow meter
3.4
Effect of pipe size reduction or expansion
Manufacturers who have concentrated on solving swirl effects in a better way than mentioned in the
designs described above are lacking information close to the pipe wall and cannot compensate for
axial flow profile changes. These path configurations are shown in figures 13 a & b.
Figure 13a
2 Double reflection chords combined with 3 center
chords
Figure13b
3 parallel X- direct chords
Although as design in figure 13a appears to cover the full cross sectional area of the pipe in reality it
can be split in 3 separate parallel path configurations at the position of 0º (green chords), 60º (blue
chords), and 120º (red chords), see Figure 14 below
Figure 14
3 parallel chords positioned at 0º (green
chords), 60º (blue chords) and 120º (red chords)
Figure 15: The so-called 5-path design behaves as a 3-chord parallel design
As a matter of fact the above mentioned designs lack information close to the wall and appear to be
blind to wall roughness changes and pipe size reduction or expansion.
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A novel design of a 12-chord ultrasonic gas flow meter
Fig. 16 Typical pipe size reduction
at a flow calibration lab
(photo courtesy of NMi)
But the real problem comes if a meter based on such a design is calibrated with a pipe size reduction
upstream which it does not recognize and operates in the field with its line size. Consequently the
meter factor as calibrated does not represent the actual conditions and causes an measurement error.
2x double refl.
3x single refl.
3X parallel
V12
reducer
-
-
+
expander
-
-
+
Swirl compensated designs
Configuration
Flow pattern
Table 3: Designs with 3 parallel measuring planes lack information close to the pipe wall
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A novel design of a 12-chord ultrasonic gas flow meter
4
Measurement accuracy
Estimated Uncertainty (%)
In figure 17, the accuracy of the various path configurations is shown as determined by the NEL and
presented at the 5th South East Asia Hydrocarbon Flow Measurement Workshop 2006. In this graph,
the ALTOSONIC V12 performance has been added based on the results of the CFD calculations
regarding the various path configurations.
0,9%
0,8%
0,7%
0,6%
0,5%
ALTOSONIC V12
0,4%
0,3%
0,2%
0,1%
0,0%
3 crisscrossed
chords
4 crisscrossed
chords
3 parallel
chords
5 crisscrossed
chords
4/5 parallel
chords
5-path
triangle
8 chords
12-V
crossed inchords
plane
crossed inplane
Figure 17 Various path configurations in comparison
From these, the 8 chords crossed in plane and the 12 V-chords in plane eliminate the swirl in each of
the individual measurement planes. From this figure it is clear that the swirl compensation in each of
the measurement planes results in an substantial improvement of the measurement uncertainty. The
5-path triangle arrangement has also the capability to cancel out swirl, but performs less than optimal
due to the relatively large distance of the outer paths to the pipe wall.
5
Test results based on ISO17089 and OIML R137
In autumn 2008 the ALTOSONIC V12 was tested at the E-ON Ruhrgas test facility in Lintorf,
Germany. The tests performed comply with requirements based on ISO17089 for USM and OIML
R137 with very high swirl conditions. It is the first time an USM has been tested under such severe
conditions and results are documented. The tests were witnessed by the Dutch NMi and have been
used to obtain the MID (Measurement Instruments Directive) type approval for the European Union.
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A novel design of a 12-chord ultrasonic gas flow meter
Figure 18 Upstream conditions for V12 testing
Figure 19 V12 tested at 90º rotated with a double bend out of plane at 10D distance
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A novel design of a 12-chord ultrasonic gas flow meter
Figure 20 Base line tests
Figure 21 Distortion tests in compliance with ISO17089 at 10D
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A novel design of a 12-chord ultrasonic gas flow meter
Figure 23 Diameter steps +/- 3%
Figure 24 Pipe reduction and expansion at 5D distance
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A novel design of a 12-chord ultrasonic gas flow meter
Figure 25 OIML R137 distortions at 10D
Figure 26 OIML R137 distortions at 5D with KROHNE flow conditioner at 3D
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A novel design of a 12-chord ultrasonic gas flow meter
Figure27 OIML R137 distortions at 5D without flow conditioner
The tests described above have all been conducted at the Ruhrgas Lintorf test facility. This test facility
was chosen based on the excellent flexibility; its traceability to the European national standards and its
very good short term reproducibility. With a row of 5 parallel orifice meters being used as reference
meters the only weak point of the facility is its low flow characteristic and performance. Hence to
investigate that as well, additional tests were conducted at the Bergum facility of the NMi which has an
outstandingly low flow capability. There the same 8” meter, as well as a 4” meter, was tested (see
figure 28). The results show the outstanding capability of the meter design to deal with very low flow
velocities.
1
8" ALTOSONIC V12 @ 20barg
Natural Gas
4" ALTOSONIC V12 @ 20barg
Natural Gas
0,8
0,6
0,4
% error
0,2
0
0
100
200
300
400
500
600
700
800
-0,2
-0,4
-0,6
-0,8
-1
Lintorf low flow range
Figure 28 Low flow test
results at NMi Bergum
calibration facility
m3/h
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A novel design of a 12-chord ultrasonic gas flow meter
8” ALTOSONIC V12 @ 20 barg Natural Gas
Error
Repeatability
Flow rate % Qmax
[%]
[%]
[m3/h]
1600
53
-0,10
0,16
500
17
0,02
0,06
150
5
0,05
0,06
75
2,5
0,13
0,03
40
1,3
0,39
0,14
28
0,9
0,28
0,20
20
0,7
0,92
0,12
15
0,5
0,58
0,13
Weighted mean error: -0.03%
Table 4 NMi Bergum results 8” V12
4” ALTOSONIC V12 @ 20 barg Natural Gas
Flow rate % Qmax
Error Repeatability
[m3/h]
[%]
[%]
750
94
-1,06
0,08
525
66
-0,05
0,05
300
38
-0,12
0,01
188
24
-0,09
0,10
40
5
0,38
0,04
20
3
0,30
0,18
10
1
0,95
0,22
Weighted mean error: -0.09%
Table 5 NMi Bergum results 4” V12
It can be concluded that the V12 performs excellently under all conditions. The V12 is the first
ultrasonic meter to be installed with a 5D straight upstream inlet length where uncertainties better than
±0.2% are required.
Table 6 Conclusion Uncertainty
Installation effects
For further illustration the authors recommend to refer to [3] and [4]. Both are from independent test
laboratories or organisations and were published in 2000 [4] and 2004 [3]. As the path configurations
of those manufacturers have not changed the results remain valid.
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A novel design of a 12-chord ultrasonic gas flow meter
6
Uncertainty expressed in US$
Let us assume we have one metering run with a 12” line size operating at 75bar (1125psi) with a gas
velocity of 25m/s (83ft/s). This meter operates 12 hours a day and 6 months a year. As recently
published during the Russian – Ukrainian gas war Gazprom wanted Neftegas to pay 400$ for each
1000m3 (35000cf) of natural gas. Using that price it results in a bill of $ 867,240.000 on a yearly basis.
Consequently, an uncertainty of 0.1% is already equivalent to $ 867,240.
Figure 29 Potential Loss Calculation
7
Conclusion
With the ALTOSONIC V12 a new era in ultrasonic flow measurement has begun:
•
•
•
•
•
•
•
The ALTOSONIC V12 is the first meter designed to targets the reduction of the installation
effects as well as the optimization of the performance monitoring in combination with diagnostic
capabilities.
It is the first meter design that employs as standard a dedicated acoustic path to detect liquid
contamination at the bottom of the pipe.
It is also the first meter where the diagnostic data can be separated from the meter’s fiscal data.
The swirl is completely eliminated in each of the 5 parallel measurement planes.
Through the use of 5 measurement planes, more information on the shape of the velocity profile
is gained than any other existing meter resulting in the highest attainable accuracy.
The V12 is the first USM to obtain OIML R137 measuring class 0.5.
The V12 is the first USM measuring within an uncertainty of ±0.2% with a 5D straight inlet
length.
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A novel design of a 12-chord ultrasonic gas flow meter
Figure 30 Family concept of ALTOSONIC V12
The ALTOSONIC V12: Reflecting tradition ----- Challenging perfection
The authors like to thank all members of the KROHNE Altometer and KROHNE New Technologies
R&D teams for their contributions.
References:
[1] Diagnostics for reflective multipath ultrasonic meters
Jim Robertson, Senior Measurement Engineer, Pacific Gas and Electric Company, AGA Operations
Conference, Dallas 2007
[2] A novel design of a 12 chords ultrasonic gas flow meter with extended diagnostic functions
Jan G. Drenthen, Martin Kurth & Jeroen van Klooster. KROHNE New Technologies, AGA Operations
Conference, Dallas 2007
[3] Evaluation of Flow Conditioners – Ultrasonic Meters Combinations
Bruno Delenne, Gaz de France, NSFMW 2004
[4] Ultrasonic Meter Installation Configuration Testing
T. Grimley, Southwest Research Institute, AGA Conference Denver 2000
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A novel design of a 12-chord ultrasonic gas flow meter