Verification of ultrasonic gas flow meters Jan G. Drenthen, Martin ...

CEESI ultrasonic seminar June 2009 1 Verification of ultrasonic gas flow meter

Verification of ultrasonic gas flow meters

Jan G. Drenthen, Martin Kurth & Marcel Vermeulen KROHNE Oil and Gas

Minervum 7441, 4817 ZG Breda, 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 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, measuring most accurately under flow lab conditions is not a guarantee for maintaining accuracy over time or independency of upstream conditions. This paper describes the insensitivity of a new meter design, describes the different types of fouling and gives recommendations on the re-calibration period with respect to verification level and volume measured.

2 Test results based on ISO17089 and OIML R137

AGA report no.9 chapter 7.2.2 Piping Installations says the following:

To ensure that the UM, when installed in the operator’s piping system, will perform within the specified flow rate measurement accuracy limits defined in Sections 5.1, 5.1.1 and 5.1.2, the manufacturer shall do one of the following, as directed by the designer/operator: Recommend at least one upstream and downstream piping configuration, without a flow conditioner or one configuration with a flow conditioner, as directed by the designer / operator that will not create an additional flow rate measurement error, after calibration adjustment, of more than ±0.3% due to the installation configuration. This error limit should apply for any gas flow rate between qmin and qmax. This recommendation shall be supported by test data.

In autumn 2008 the ALTOSONIC V12 was tested at the E-ON Ruhrgas test facility in Lintorf, Germany. The tests performed comply with requirements in ISO17089 for USM and OIML R137 with very high swirl conditions. It is the first time that 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.


Figure 1: Upstream conditions for V12 testing

Figure 2: V12 tested at 90º rotated with a double bend out-of-plane at 10D distance


Figure 3: Base line tests

Figure 4: Distortion tests in compliance with ISO17089 at 10D


Figure 5: Diameter steps +/- 3%

Figure 6: Pipe reduction and expansion at 5D distance


Figure 7: OIML R137 distortions at 10D

Figure 8: OIML R137 distortions at 5D with KROHNE flow conditioner at 3D


Figure 9: 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 outstanding low-flow capability. There the same 8” meter, as well as a 4” meter, was tested (see Figure 10). The results show the outstanding capability of the meter design to deal with very low flow velocities.

Figure 10: Low flow test results at NMi Bergum calibration facility

-1

-0,8

-0,6

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

0 100 200 300 400 500 600 700 800

m3/h

% e

rror

8" ALTOSONIC V12 @ 20bargNatural Gas4" ALTOSONIC V12 @ 20bargNatural Gas

Lintorf low flow range-1

-0,8

-0,6

-0,4

-0,2

0

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0 100 200 300 400 500 600 700 800

m3/h

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rror

8" ALTOSONIC V12 @ 20bargNatural Gas4" ALTOSONIC V12 @ 20bargNatural Gas

Lintorf low flow range


8” ALTOSONIC V12 @ 20 barg Natural Gas 4” ALTOSONIC V12 @ 20 barg Natural Gas Flow rate

[m3/h] % Qmax Error

[%] Repeatability

[%] Flow rate

[m3/h] %

Qmax Error [%]

Repeatability[%]

1600 53 -0.10 0.16 750 94 -1.06 0.08 500 17 0.02 0.06 525 66 -0.05 0.05 150 5 0.05 0.06 300 38 -0.12 0.01 75 2,5 0.13 0.03 188 24 -0.09 0.10 40 1,3 0.39 0.14 40 5 0.38 0.04 28 0,9 0.28 0.20 20 3 0.30 0.18 20 0,7 0.92 0.12 10 1 0.95 0.22 15 0,5 0.58 0.13

Weighted mean error: -0.09% Weighted mean error: -0.03% Table 1: NMi Bergum results 8” V12 Table 2: 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 3: Conclusion on Uncertainty due to 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.

3 Meter verification

The question that people are still most frequently asking is whether an ultrasonic flow meter has to be re-calibrated or not. The second most frequently asked question is how often a meter has to be re-calibrated. Table 4 gives an example of variously sized metering stations and the amount of money billed per annum. Depending on size, pressure and load the amounts vary from $16 billion per year for the largest metering station to $49 Million for the smallest station.


Table 4: Annual billing for various sizes of stations

4 Factory Acceptance Test (FAT)

AGA Report No.9 chapter 6.3 requires a zero flow verification test which is performed during the FAT. For that purpose the meter is covered with blind flanges and the air is extracted. The meter body is filled with 100% nitrogen composition and the meter is pressurized between 150 – 250psi. Taking simultaneous measurements of pressure and temperature permits calculation of the speed of sound (SOS) according to AGA Report No.10. Consequently, the SOS calculated can be compared with the one measured. As measured and calculated SOS usually match within 0.03% alignment of transducers can be verified. As an SOS comparison verifies the path length only but not the transducer angle, KROHNE uses an optical 3D measurement to guarantee highest quality. It is clear that the FAT enables a complete functional check of the meter itself.

Figure 11: Flow meter undergoing FAT


5 High pressure flow calibration

It is a requirement of AGA Report No.9 chapter 6.4 that all custody transfer metering packages be flow calibrated in a flow calibration facility or by a calibration traceable to a recognized national / international standard such as TCC and CEESI in Canada/US, or Westerbork and Pigsar in Europe. Typically a 6 flow rate calibration is performed.

Figure 12: Testing at NMi, Westerbork

6 In-situ verification by the meter itself

Although availability of a custody transfer meter is of utmost importance, electronics or transducer may fail once a while. Fortunately compared to turbine meters or orifice plates the ultrasonic flow meter has a large amount of diagnostic tools available. By combining several parameters as shown in the relational diagnostic diagram in Table 5 below, the reason for failure can be located and reported. The ALTOSONIC V12 measures in 5 horizontal measuring planes where according to the Dutch NMi two may fail without violating custody transfer measurement.

Per

form

ance

pp

AG

C le

vel p

p

AG

C li

mit

pp

SO

S p

p

Flow

vel

ocity

pp

Transducer failure, detection problems X X X XUltrasonic noise X X XProcess conditions (pressure) XProcess conditions (temperature) XFouling of meter X X X XFlowprofile changes XHigh velocity X X X

Table 5: Relational diagnostic diagram


7 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. Figures 5 to 7 present some examples of excessive fouling. The first one is the result of a production problem, the second one is an example of an off-shore installation, and the third one is an on-shore installation.

Fouling as a flow on the bottom of the pipe

Fouling intermittently stuck to the pipe wall

Fouling evenly distributed over the pipe wall

Figure 13: Various types of fouling


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 Small flow on the bottom of the pipe B Intermittent sticking to the pipe wall C Evenly distributed coating on the inside of the pipe. D Dirt build-up on the transducers (especially on those facing upstream) E 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. Table 7 shows the probability of detecting fouling for various chord designs.

8 Calculation example of fouling at the bottom of the pipe

Imagine there is a layer the thickness of a credit card at the bottom of the pipe - just below the 0.8 radius position where the horizontal chord cannot detect the fouling. In the example of a 2 metering line city gate station from Table 6: Potential loss calculation with bottom fouling such a credit-card thick layer would cause a difference of 0.06% in the cross-sectional area. This amounts to a potential loss of $ 748,842 on an annual basis. However, it is possible to detect a bottom layer that is as little as1/5th of the thickness of a credit card: that is only 0.02”. This is done by the vertical path which has a different SOS to the horizontal chords. Figure 14: Artistic impression of fouling as thin as a credit-card at bottom of pipe


Table 6: Potential loss calculation with bottom fouling

fouling on transducersurface

symmetricallydistributed wall build-up

asymmetrically stuck to the pipe wall

fouling on bottom of pipe

6 chordscrossed in 3 planes

Single & double

reflection

Direct chorddesign

KROHNEtype of fouling

fouling on transducersurface

symmetricallydistributed wall build-up

asymmetrically stuck to the pipe wall

fouling on bottom of pipe

6 chordscrossed in 3 planes

Single & double

reflection

Direct chorddesign

KROHNEtype of fouling

Table 7: Probability of detecting different types of fouling

9 Verification using a gas chromatograph

The next step in verification is using an independent external source to verify the meter. As gas chromatographs are wide-spread the gas composition is known and can be used to calculate the SOS according to AGA Report No.10. The procedure is similar to what is done during the FAT. Modern flow computers are capable of performing AGA 10 calculations.


gas composition

TP

gas composition

TP TP

Figure 15: SOS calculation based on gas composition, pressure and temperature

10 Verification with 2 meters in series

A better comparison than using the SOS is of course comparing volume with volume. In Europe the comparison of two different measuring principles is popular. In most such cases an ultrasonic meter is compared to a turbine meter. Supposedly this is to avoid common-mode errors which may occur if two meters with the same measuring principle are put in series. Norwegian operators have been successfully using ultrasonic meters in series for many years.

Figure 16: 2 meters in series

11 Verification with master/duty meter

Using the master-duty meter concept offers the highest level of measurement integrity. As there is no wear-off in full bore meters the only concern is contamination. And as the master meter is only used for verification of the duty meter once in a while it is assumed that the master meter is clean. Visual inspection should be carried out from time to time. The advantages of this concept are:

o Volume to volume comparison o Clean versus potentially contaminated meter verification o If required duty meter can be calibrated against master meter o Meter line redundancy

For maintenance For break down For temporary capacity shortage

o Recalibration of master meter without interruption of service This concept is widespread in the Middle and Far East as calibration labs are far away.


Figure 17: Master/duty configuration (Z-bridge)

12 Validation period

Today there is no international standard or regulation that defines the recalibration period of ultrasonic meters for custody transfer. There are a few exceptions such as the UK where all CT meters have to be recalibrated every 2 years; or Germany where the recalibration period is 8 years if the meter has not been installed in series with another meter. In the Netherlands the large ultrasonic meters (≥20”) are recalibrated every 5 years. In no case is reference made to the level of verification such as SOS verification, 2 meters in series or the master/duty concept. Table 4 shows the potential loss expressed in Dollars for 0.1% uncertainty in different types of metering stations. But it is obvious that with an increase in verification level the recalibration period should be extended. In the first row of Table 4 a metering station with 6x 24” meters is calculated resulting in a potential loss of $16,241,040 (=US$ 16.2 million) for an uncertainty shift of 0.1% which is equal to a symmetrical wall built-up of 0.012” (360º) or 0.059” (70º arc) at the bottom of the pipe. The recommended recalibration period is based on conventional ultrasonic meters whereby Table 7 shows the recommended recalibration period for a KROHNE V12. The longer period of the V12 is based on much better diagnostic capabilities as outlined in Chapter 8.


Table 8: Recalibration period for conventional USM

Table 9: Recalibration period for ALTOSONIC V12

13 Conclusion

With the ALTOSONIC V12 a new era in ultrasonic flow measurement has begun: • The ALTOSONIC V12 is the first meter specifically designed to target installation effects as well

as optimize 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 by any other existing meter resulting in the highest attainable accuracy.

• The V12 is the first USM to obtain OIML R137 Accuracy Class 0.5. • The V12 is the first USM that measures within an uncertainty of ±0.2% with a 5D straight inlet

length.

Figure 18: Family concept of ALTOSONIC V12 The ALTOSONIC V12: Reflecting tradition ----- Redefining perfection The authors would like to thank all members of the KROHNE Altometer and KROHNE Oil and Gas 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

Verification of ultrasonic gas flow meters Jan G. Drenthen, Martin ...

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