THE PERFORMANCE OF A TWO PLANE MULTI-PATH ULTRASONIC FLOWMETER. D. Augenstein & T. Cousins. Caldon Inc.

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1 THE PERFORMANCE OF A TWO PLANE MULTI-PATH ULTRASONIC FLOWMETER By D. Augenstein & T. Cousins Caldon Inc. SUMMARY The two plane multi-path ultrasonic meter is a very different meter from the usual multi-path ultrasonic meter. The two plane multi-path meter is composed of two separate four path meters, with the path s plane orthogonal to each other. This configuration allows the meter to be considered as two separate single plane meters. However, when the two meters are combined, they form a single meter with powerful extra features. This two plane meter has the ability to withstand very poor installation conditions, to provide extra self-diagnostic capabilities, to provide path redundancy without loss of uncertainty, and to improve flow measurement accuracy. This paper shows the meters immunity to installation, their ability to be able to predict the change in uncertainty by the use of diagnostic checks and their measurements of velocity profiles quantified as swirl (%) and flatness ratio. The flatness ratio is introduced as a metric of the velocity profile s flatness and is used to predicted and correct for changes in calibration. Data from meter calibrations are used to determine the influence factors relating the changes in flatness ratio, and hence the potential errors due to profile change. The meter operates in difficult installations with minimal effect on the calibration and may be considered a high quality master meter. 1.0 INTRODUCTION Currently, there is a growing need to be able to install meters for oil custody transfer without the use of on-site proving. This need has come about for a number of reasons, including: Space and weight for certain applications, for example offshore drilling platforms, has become a premium asset. The capacity of many applications requires either a number of conventional meters, turbine or positive displacement meters with the consequent valving and piping costs, or a single large ultrasonic flow meter. In such cases the potential size and cost of a prover would be prohibitive. The potential flow range of an ultrasonic flow meter allows for much higher flow rates than for conventional meters, but ends up requiring much larger and consequently expensive provers. The standard multi-path ultrasonic meters available at present do not have the stability of calibration with varying installation conditions to allow for complete confidence in the transfer of a meter s calibration at an external facility to the operating conditions. Currently, the solutions involve the use of master meters, often of the same type as the operational meter or checks against tanks or inventory balances. These methods are not usually of the same quality as a true volumetric prover, and thus it is essential that there is maximum confidence in the meter design and its ability to carry the calibration through variations in the installation; whether the variations are between the TP90 Page 1

2 calibration facility hydraulics or changes due to process changes (e.g., valve positions and pump line-up). The two plane multi-path meter, described in this paper, has been shown to be an extremely stable meter design, having the capability of dealing with a wide range of varying flow conditions, from high level swirl to severely distorted profiles with a minimal change in the calibration. Further, with the use of the extra diagnostics from such meters, the performance can be monitored in detail, including monitoring changes in profile and swirl as well as monitoring the quality of the ultrasonic signals. The latter indicates any degradation or changes in the received signals that may influence the measurement accuracy of the meter. 2.0 METER OPERATION The two plane multi-path meter works on the transit time (time of flight) principle described in many previous papers. For this particular design, two planes, each with four paths, are used as subsystems to the total meter. Each four path meter subsystem uses the basic Gaussian quadrature method to integrate the velocities obtained from its paths. As effective as Gaussian quadrature integration method is, a single plane cannot successfully remove all the effects of asymmetric cross-flows, hence leaving potential uncertainties in its flow measurement due to the presence of asymmetric swirl. Swirl in general, almost always in practice, is asymmetric to some extent. The answer has been to combine two planes of four path meters at the same measurement position, but orthogonal in orientation, as shown below. FF Figure 1: Two Plane Multi-Path (Eight Paths) Chordal Configuration Meter Each set of four paths acts as an independent meter and is capable of giving a low uncertainty indication of volumetric flow in its own right. The combined calculations for obtaining the volumetric TP90 Page 2

3 flow through two plane meter are shown in Figure 2. The calculation is treated as the mean of the two separated four path meter sections. V1 V2 V3 V4 Q F = MF F a (ID/2) 8 2 Wi Lffi ( ti) i = 1 tanϕi(ti + ti/2 τ ) i 2 Shown is a single four path plane of the complete eight path Where: Q F = mass flow rate through the LEFM MF = Meter factor (calibration coefficient) F a = thermal expansion factor ID = internal diameter of the spool piece W i = integration weighting factor ϕ i = angle between path i and normal to spool L ffi = transducer face-to-face dimension t i = total time of flight of path i t i = up minus downstream transit times τ i = non-fluid time delays Figure 2: Multi-Path Equations The improvement due to the two planes is due to the orthogonal planes cancellation of the transverse velocity components due to swirl. The method ensures the complete cancellation of these components as they affect the orthogonal paths in the same magnitude, but in opposite directions, as shown in Figure 3. For this cancellation to operate properly, however, the planes have to be at the same measuring point. If this is not the case, for example, if the second plane is further downstream (e.g., in the form of a second meter), there is a high probability that the asymmetry will have changed, and the cancellation will be not be complete or exact. TP90 Page 3

4 Swirl Vc Vc,error (minus) Vc,error (positive) Axial Flow Direction Vs Vc,1 Vc,2 Plane 1 Plane 2 Vc,error net = 0 = Vc,1 error + Vc,2 error Figure 3: Method of Swirl Removal 3.0 FLATNESS RATIO The eight path arrangement delivers low and predictable sensitivity to changes in axial profile. Figure 4 shows raw 1 calibration data versus Reynolds Number, a major hydraulic factor that can affect profile. Each point represents a collection of weigh tank (calibration) runs taken at a specific flow rate in a specific hydraulic configuration. The graph also shows the theoretical sensitivity between the UFM calibration and Reynolds number. The data shows that the sensitivity of this flow element to Reynolds number is consistent with theory. The axial profile, as it affects chordal multi-path meters, is best characterized by its flatness, which Caldon quantifies by the ratio of the axial velocities measured on the shorter (outside) chords of the UFM to the axial velocities measured on the longer (inside) chords. Referring to Figure 2, the flatness ratio for a four path meter is given by, (V1+V4)/(V2+V3). With normal symmetrical profiles the ratio is less than 1. Typically, a fully developed profile at high Reynolds number the ratio will be around 0.85, for peaky profiles at low Reynolds numbers or downstream of an expansion the ratio is around 0.7, and downstream of a contraction, or in the presence of swirl it will be closer to The flatness ratio is extended further for the eight path meter, a combination of the two 4 path meters, as: (V1+V4+V5+V8)/(V2+V3+V6+V7). V5, V6, V7, V8 are the equivalent velocities on the second plane. The additional paths make the ratio a more representative indication of profile changes. Figure 5 shows the calibration data plotted against the flatness measured by the chordal meter. The data represent a wide range of hydraulic conditions. For this calibration series, the character of the axial and transverse velocity profiles seen by the flow element was varied by changing the fraction of the total flow delivered by the three non-planar feeders to a header and also by performing a set of calibration runs in straight pipe (test done 8 diameters downstream of a Mitsubishi Flow straightener is considered as straight pipe ). Figure 5 also shows the theoretical relationship of calibration to flatness the dotted line as well the flatness encountered on site. The response of this flow element to changes in flatness is, as with Reynolds Number, generally in accordance with theory. 1 In this paper, raw calibration is referred to the meter s calibration prior to any internal software corrections or algorithms. TP90 Page 4

5 But, unlike the Reynolds Number case, extrapolation is required here, and the uncertainty of that extrapolation must be (and is) accounted in the overall uncertainty of the flow element Profile Factor Reynolds Number (x10 6 ) 100 Percent Line 1-Cal 147C 100 Percent Line 3-Cal 147B Straight Pipe-Cal 147A Theoretical Sensitivity to RN Plant data Log. (Theoretical Sensitivity to RN) Log. (Plant data) Figure 4: Meter Factor vs. Reynolds Number PF straight pipe Theoretical Sensitivity Implemented PF Flatness Figure 5: Meter Factor vs. Flatness Ratio TP90 Page 5

6 4.0 PERFORMANCE UNDER DIFFERENT INSTALLATION CONDITIONS Caldon has carried out extensive testing on the effects of calibration changes due to installation variation. Some of this data, described in Reference 1, is related to meters used in the nuclear power industry, where on site proving is largely impossible. The referenced paper shows that the uncertainty requirements are similar to those for custody transfer in the petroleum industry and that the data and concepts could be transferred. Two further examples of test data that demonstrate the performance of eight path meters are shown next Meter for the Beaver Valley Nuclear Plant In the nuclear industry, each meter is tested in a model of the application and compared to an ideal calibration in a straight meter run. In many cases, further tests are carried out to simulate possible changes in the installation conditions with time. An example of the tests and the effects of installation variations are those conducted for the Beaver Valley Nuclear plant. Preliminary tests were carried out on a straight, unobstructed spool piece, ensuring that there was no swirl and a symmetrical profile. These produced the base meter factor consistent with prediction. Tests were then carried out on a full scale model of the installation, see Figure 6. The upstream configuration is very complex. The spool piece was mounted approximately 6 diameters downstream of a tee union that merged two streams into one. The tee union is preceded by several out of plane bends. The complex combination of fittings creates a swirling flow that can persist well downstream from the fittings, as well as produce distorted flow profiles. A series of tests were carried out on the model, including the introduction of extra installation effects, including a Mitsubishi flow conditioner close upstream, and a partial blockage of one of the inlet legs. The results are summarized below. TP90 Page 6

7 Figure 6: Model at Alden Laboratory MF= MF= MF= MF= MF= Figure 7: Variation of Meter factor with Different Piping Configurations (Test Description, Clockwise from Top Left: Model with Meter Installed Horizontally, Model with Mitsubishi Flow Straightener added, Model with Eccentric Orifice added, Model with Eccentric Orifice with Meter Installed Vertically, Model with Meter Installed Vertically) TP90 Page 7

8 As can be seen from the data shown, that for all the model tests the meter factors are within +/- 0.02%. The overall uncertainty of this meter, including all the above tests for the installed condition, is shown in the table below. Measured Uncertainty Percent Facility 0.12% Test Uncertainty (All variations) 0.11% Hydraulic Model 0.03% Observational Uncertainty 0.02% RMS Total 0.17% Operational Uncertainty Measured Uncertainty 0.17 Geometry 0.12 Time 0.09 Total Volumetric Installed Uncertainty Two 10 Petroleum Meters- Calibration Site Comparison The data presented here are for a pair of 10 two plane multi-path meters 2 that were calibrated in the following manner: The meters were calibrated at two different facilities to demonstrate the transfer of calibration from one installation to the next A set of calibrations to demonstrate the performance of the meters in different installation conditions The two meters, shown in Figure 8, are at Alden Laboratory, the location of the first calibration. Note that the meters are bolted back to back; this is acceptable because the meters are of the same bore as the pipe. They are mounted at 45 degrees in the axial plane. 2 Caldon s two plane multi-path meter is referred to as the 280C model. The 280C has its eight paths as illustrated in Figure 1. TP90 Page 8

9 Figure 8: Test Configuration at Alden Laboratory The meters were then tested at NMIJ, the Japanese Weights and Measures calibration facility. Caldon supplied the same upstream pipe for both of the tests 3. The tests were carried out over equivalent Reynolds number ranges and over two different orientations. Details of the flow profiles and swirl are shown in the table below. The data show that there is negligible swirl in both installations and the profiles are all very close to each other, and equal to essentially a symmetrical fully developed high Reynolds number profile. Meter/Test Alden Flatness Ratio 4 NMIJ Flatness Ratio Alden Swirl NMIJ Swirl Upstream meter 30 degrees % -0.3% Downstream meter 30 degrees % -0.2% Upstream meter 90 degrees % -0.3% Downstream meter 90 degrees % -0.1% A summary of the results is as follows, the comparison calibration is Alden: 3 Upstream pipe was 12D straight pipe followed by a Mitsubishi flow conditioner, followed by 12D of straight pipe. It is noted that at Alden, the site provided upstream pipe was approximately 10D while at NMIJ the site provided upstream pipe was over 100D. 4 Flatness ratio and Swirl at Alden are for the same Reynolds numbers as the NMIJ data. TP90 Page 9

10 Upstream 10 Meter 30 Degree Orientation Calibration Difference 0.00% 90 Degree Orientation Calibration Difference -0.06% Downstream 10 Meter 30 Degree Orientation Calibration Difference 0.04% 90 Degree Orientation Calibration Difference 0.03% 4.3 Two 10 Petroleum Meters- Installation Effects The following installations were chosen to demonstrate the potential effects on the meter: Two 280C meters installed 17 diameters downstream of in-plane multiple bends with a Mitsubishi flow conditioner installed ~12 diameters upstream of the meters (e.g., the Mitsubishi flow conditioner 5 diameters downstream of the compound elbows). Two 280C flow meters installed 17 diameters downstream of in-plane multiple bends without the Mitsubishi flow conditioner. 5D to Conditioner Flow 15D to Meter Fig 9 Double In-Plane Bends (with and without Mitsubishi Flow Conditioner) Two 280C flow meters installed 17 diameters downstream of two 90 degree out of plane bends (without a flow conditioner). TP90 Page 10

11 17D Figure 10: Double Out of Plane Bends TP90 Page 11

12 The flow conditions for each installation are shown in the table. Installation Meter 1 Meter 2 Meter 1 Meter 2 Meter 1 Meter 2 Flatness Flatness Meter Meter Swirl Swirl Ratio Ratio Factor Factor Double In plane bend Double In plane bend with Conditioner Two bends out of Plane 0% 0% % 1% % 28% The results from these tests are: The calibration curves for both meters over the range tested show a linearity of better than 0.14% for all installation configurations (10:1 turndown). The tests with the Mitsubishi flow conditioner showed a more skewed velocity profile than that when the Mitsubishi flow conditioner was removed. This skewed velocity profile is likely due to one of the following: o Either the Mitsubishi flow conditioner was not manufactured correctly. o This conditioner may have frozen the distorted velocity profile produced by the compound elbows. The observed velocity profile distortion was oriented in the correct plane for the bend. For all the tests, the flatness ratios (FRs) varied from a low value of 0.86 to 0.94 (very flat). In general the repeatability is poorer with the Mitsubishi conditioner tests than the other tests. This is almost certainly due to the introduction of turbulence by the conditioner. The variation in meter factor for the eight path configuration is far less than for the single four path meters. The maximum meter factor difference over the range of installations was 0.26% (+/-0.13%), including the case of very severe swirl. The variation in meter factor when compared to the flatness ratio behaves as expected based on a theoretical slope derived from a combination of semi-empirical data, and theoretical derivation of flow profiles (see Figure 11). TP90 Page 12

13 Meter Factor Theoretical Variation Flatness Ratio Figure 11: Meter Factor vs. Flatness Ratio (Theoretical Variation Shown as Well) 5.0 DIAGNOSTICS 5.1 Overview There are a number of features that make Multi-path Ultrasonic flow meters attractive. No moving parts Small pressure loss No obstruction to the flow Good Flow Range However, there is one feature that really sets them apart, that is, the ability through diagnostics, to determine and validate the status of the meter in terms of its physical operation. With the two plane meter, this can be combined with the ability to view the performance in terms of potential uncertainty changes due to profile variations. In general, providing that the paths represent a sensible cross-section of the profile, the more paths, up to a limit, the more useful the diagnostic data. Again, the use of Gaussian integration theory allows the prediction of effects due to profile, such as flatness ratio and the amount of swirl, giving the ability to track variations from the calibrated set-up. TP90 Page 13

14 5.2 Auditing General Auditing is an essential feature of all custody transfer systems. Normally, auditing is a combination of a continuous health check of the meter and its systems, along with an attempt to determine the continuing calibration stability. The health check is essentially the functionality of the meter, and dependent on the meter type, different methods must be used to determine the state of the meter. In liquid measurement, the calibration stability is historically checked by using some form of on-line proving, which is only feasible up to a certain size, or meter capacity. Above such limits site calibration becomes increasingly a problem. The proving is combined with rigorous checking of the system to ensure the meter station integrity. While continuous auditing is useful, it is often essential to have snapshot audits. Snapshot audits impose a proper discipline on the system and ensure that the correct procedures for data validation are being carried out, and that there is no relaxation in the metering quality Calibration It is conceivable that the meters can be checked by comparison of the meter totals against a tank measurement. Unfortunately, the method is fraught with practical difficulty to obtain good results. Comparison with other flow meters at other locations may be feasible, but that method comes with its own potential uncertainties due to line packing, different flowing conditions, etc. It may also be possible to compare the meter readings with process measurements, again this is often fraught with errors, and rarely giving an uncertainty within custody transfer limits. The obvious method is to calibrate the meter at an approved facility and to transfer this calibration to the operational installation. The weakness of this method is obvious, how do we know that the calibration at a facility can be transferred to the operational conditions with a minimal increase in uncertainty? It is well known that most meters have calibration variations, some substantial, with changing installation conditions. Further, these changes are generally unquantifiable, other than by reference to similar data, always a hit or miss concept, and sometimes may not even be identifiable. Flow conditioners, may alleviate the problem, but experience is that they can make the matters worse. For example, when a flow conditioner installed too close to a single bend some flow conditioners appear to interfere with the natural mixing of the fluid (due to the contra-rotating vortices) and freeze the asymmetric profile over longer distances than would occur without the flow conditioner. Thus a meter, such as an orifice or turbine meter, where there is no indication of the profile variation, will give answers that are different to those expected. Total immunity to the effects of installation, is of course a good basis for confidence, however this does not leave room for a great deal of choice. Where does this leave us with Multi-path Ultrasonic flow meters? A well designed multi-path meter has a good level of immunity to installation effects. The amount is dependent on the design and number of paths, for example the Caldon 8 path meter is better at dealing with general flow disturbances than the 4 path meter. More importantly the multi-path meter can give an indication of the changes in profile, and installation conditions, from the calibration site to the operational site. This analysis is carried out within the diagnostics. A four path meter can give a reasonable indication, while the eight path provides a detailed indication. TP90 Page 14

15 If, as described in Reference 1, a reasonable model of the meter installation is built and tested as part of the calibration, this, in conjunction with the diagnostics, ensures a high level of confidence in the calibration on site. Changes in the process, such as drag reducing agent variations or profile variations due to process changes, can be continuously tracked. The two plane multi-path meter allows the implementation of preventative and predictive maintenance regimes, particularly using control chart techniques. 5.3 What Data can be Obtained From Diagnostics? The Caldon meter, like many other multi-path meters, offers a wide range of data relating to the performance of the meter. Some of the data, such as flight times and time differences are less relevant to the user than others, such as gains and signal to noise ratios. The diagnostic data do, however, all add together to make a complete picture of the meter health and performance. We will however, for brevity, here concentrate on the major functions: Signal Gain- The amount of amplification of the received signal required to reach a predetermined value. Signal to Noise ratio (SNR)- The ratio of the main part of the received signal to the surrounding noise. Standard Deviation- The variation in the calculated velocity from sample to sample Individual Path Velocities- The velocity along each path, which together give an indication of flow profile. Flatness ratio- The shape of the flow profile ( Also the swirl and skew-ness of the profile can be determined) Velocity of Sound- The calculated velocity of sound for the fluid passing through the measurement section. Rejects- The number of measurements rejected due to a failure of one or more of the major pieces of data, for example SNR being too low. Cable checking Nominal Fluid Viscosity Density The data from these functions typically help with the identification of gas presence, second liquid phase, high viscosity fluids, connection and cable problems, wax deposition, and variations in profile compared to calibration, or with process changes. Any of this data may not be directly point to a solution, but in combination with knowledge of the installation solutions can be readily found The result of using the diagnostics is a complex matrix of actions that point down to final solution. A typical diagnostic output is shown in Figure 12. TP90 Page 15

16 Figure 12: Typical Diagnostic Output 6.0 THE COMPLETE PACKAGE Individually the concepts described above do not solve the problem of non-proving on site. However, combining the data obtainable from the diagnostics, model calibration and knowledge that the meter is relatively immune to installation variations, gives a high degree of confidence in the performance of the meter under operational conditions. First, a calibration, testing the variations in flatness ratio, to demonstrate the possible changes that may be expected in the meter factor for the application. The diagnostics then confirm the quality of the measurement, both in terms of the meter operation and the flow conditions on a continuous basis. The combination in conjunction with the ability of the meter to withstand changes in flow profile with minimal variation in meter factor, fit together as pieces of a jigsaw, to give confidence in the installed performance of the meter. 7.0 CONCLUSIONS The introduction of a second plane to the multi-path meter adds significantly to the performance of the ultrasonic flow meter. The second plane allows for the complete removal of the effect cross flows induced by rotational flows, even non-symmetrical swirls. The second plane adds significantly to the self diagnostic ability of the meter, which in turn gives: o Greater reliability o A better understanding of the meter uncertainty o A more detailed fault finding ability TP90 Page 16

17 Because of the preceding features, confidence in the calibration of the meter and its transfer to operational conditions is much higher than for other meter designs. The extra diagnostics gives the ability to detect significant changes in the process flow conditions that may affect the meter performance. Calibration, using models of the installation allow the production of a series of Flatness ratios that will bound the potential meter operational conditions, with the consequence of improving the installed performance A combination of immunity to installation effects, good calibration and diagnostics allows high level confidence that the installed calibration of the two plane ultrasonic meter can be predicted to within custody transfer uncertainty limits. 8.0 REFERENCES 1.) INSTALLATION EFFECTS AND DIAGNOSTIC INTERPRETATION USING THE CALDON ULTRASONIC METER by H. Estrada Caldon, Inc. T. Cousins, Caldon, Inc. D. Augenstein, Caldon, Inc. NSWS 2004 St. Andrews 2.) INSTALLATION EFFECTS ON C - EIGHT PATH METERS By T. Cousins Caldon Report ) Profile Factor Calculation and Summary for the Alden/NMIJ Comparison Tests By D. Augenstein Caldon Report ) Profile Factor Calculation and Accuracy Assessment for the Beaver Valley LEFMCheck+ Spool Piece By F. Weber Caldon Report No.175 TP90 Page 17