Achieving Custody
Transfer Accuracy

Using an Ultrasonic Flowmeter With
Five Diameters of Upstream Straight Run

Sentinel LCT8

Abstract
Hydrocarbon liquids, such as crude oils and refined products,
have both high economic value and a hazardous nature.
An accurate and repeatable measurement system is vitalfor both custody transfer and leak detection monitoring
applications. Historically, the use of ultrasonic flowmeters has
been hampered by the need for long, straight runs of pipe
mainly upstream of the flow measurement point. This paper
provides a description of the testing performed using ourlatest meter suitable for custody transfer applications, alongwith a look at the diagnostic capabilities of the flowmeter.
Several meter configurations, including a single-planefour-path and double-plane eight-path flowmeters in sizesof 6 and 10 inches, we tested, and the results are presented
here. By performing numerical studies across a wide range
of Reynolds numbers, and through actual flow testing using
several different viscosity oils, we were able to demonstrate
the superiority of an eight-path meter when the only
available option is a shortened straight run between meterand upstream disturbances. At higher Reynolds numbers,
a flowmeter such as the Sentinel LCT8 meter (dual-plane,eight-path) shows a marked advantage over single-plane,
four-path meters in conditions typically encountered inreal-life situations. At lower Reynolds numbers, however,other fluid effects can adversely affect meter performance.
Because the orthogonal planes of the eight-path meter candetermine the actual axial and transverse profiles, the use ofthis type of flowmeter can further aid in diagnosing potential
pipeline problems.

Sentinel LCT8

Introduction“Custody transfer” in the oil and gas industry refers to thetransfer of hydrocarbons from one owner to another. Given
the economic value and hazardous nature of these assets,
the industry requires high accuracy and confidence in themeasurement system used. The flowmeter performancerequirements are often governed by industry standards setby OIML (the International Organization of Legal Metrology)
or API (the American Petroleum Institute), or by national
regulations (such as INMETRO in Brazil), and throughcontracts between the two parties. Ultrasonic custodytransfer flowmeters are often used in pipeline leak detection
applications as well due to their ability to maintain accuracy
through viscosity changes and maintain overall leakdetection system uncertainty below ±1%.
In the past 10 years, ultrasonic flowmeters have seen one of
the largest market share growths for custody transfer andleak detection metering. This growth has been largely driven
by the advanced diagnostics capability of the meters, which
can be used to identify when potential issues surface, infer
other measurement parameters, and feed the AI (artificialintelligence) algorithms for the industry internet. Ultrasonicflowmeters tend to have higher capital expense costs
compared to other technologies, but lower operational
expenses because they often do not need to be recalibrated
or re-proved when fluid properties change, such as at thestart of unloading a new oil batch.
One historical weakness of the use of ultrasonic flowmeters
for custody transfer has been the need for long straightruns. Typically, uninterrupted pipe run requirements havebeen twenty diameters upstream of and five diameters
downstream of the meter itself. In some situations, thestraight run could be reduced to ten diameters with the useof a flow conditioner. The associated pressure drop of aflow conditioner may be undesirable considering that oneof the benefits of ultrasonic flowmeters is a low inherentpressure drop. Long straight runs have been typicallynecessary to ensure a fully developed flow profile with aslittle swirl as possible, which allows for a more accurate
measurement of the fluid velocity at each acoustic chord
location within a meter. With such well-conditioned flow,
a four-path ultrasonic meter provides enough flow fieldvelocity resolution to appropriately measure the flow
meeting custody transfer accuracy expectations andholding performance through viscosity changes (within thecalibrated Reynolds number range).
In some applications, especially skid-mounted meters forpipeline pumping stations or offshore platforms, the skid
footprint is limited and may not have sufficient space forlong, straight runs. In such cases, an eight-path ultrasonicflowmeter, such as the Panametrics Sentinel LCT8, can offer
custody transfer accuracy with only five diameters of straightrun between the meter and any upstream disturbances andmay be the only available option.

Less sensitve to swirl
Eight-path meters, with orthogonal planes—where a pairof perpendicular, crisscrossing paths are both at the sameradial distance from the pipe centerline—are less sensitive
to the potential errors that flow swirl can cause because
the transducer paths are symmetrical about the axial
center line of the pipe, both from top to bottom and fromone side of the pipe to the other. When the paths are crossoriented this way, velocity swirl components generated bycertain upstream disturbances, such as pipe elbows, areautomatically cancelled out of the flow-rate calculations.

Corrections andadjustments not needed

Figure 1 shows results from a numerical analysis, which werecombined with test observations from early prototypes.
More optimal chord locations were realized that could better
handle both swirl flow and allow the meter to perform overa wider range of Reynolds numbers (Re), covering a muchbroader range of fluid viscosities.

Figure 1. Computational Numerical Simulations Including Transducer Port
Effects

Corrections and adjustments not needed

Figure 2. Performance Results of a 6-inch Sentinel LCT8

Figure 2 is the performance plot of a 6-inch Sentinel LCT8
initially calibrated and verified in a long, straight-pipe setup,and subsequently installed only five diameters downstreamof a double elbow in plane pipe configuration (DEIP), andfive diameters downstream of a double elbow out of plane(DEOP) pipe configuration. Figure 3 shows similar results fora 10-inch Sentinel LCT8, including a test of the meter five
diameters downstream from a fully opened globe valve(more disruptive to flow than a full port ball valve). The
testing was carried out across a wide range of Reynolds
numbers using several oils of different viscosities. For bothmeters, the correction factor tables were derived strictly fromthe straight-pipe configurations. No additional corrections
or adjustments were applied to either meter during thesubsequent cases of disturbed pipe flow to achieve theplotted accuracy.
Figure 3. Performance Results of a 10-inch Sentinel LCT8The results show the insensitivity of the Sentinel LCT8 tothe standard upstream disturbances that are typicallyencountered in actual installations. While the performanceof the meter tails off at lower Reynolds numbers, the benefitsof an eight-path meter are most notable above Re of 10,000.
While maybe not the tightest accuracy, the meter stillshows good performance below Re of 10,000. At these lower
Reynolds numbers, other fluid effects, such as transition flow,
begin to dominate the flow behavior, which can adversely
affect meter performance.

More accurate composite volumetric flow rate
One of the benefits of the cross-paths of the Sentinel
LCT8 meter is the ability of the meter to use the velocity
information from the two individual transducer planes,
A and B, to resolve the true flow profiles both in the axial(down the pipe) and transverse (swirl and cross-pipe)
directions. The individual plane velocity profiles of Figure
4 are manipulated mathematically into the bulk axial andtransverse flow profiles of Figure 5. The plots are from the6-inch DEOP configuration data of Figure 2. The pipe flow rateis calculated based on this real resolution of the axial flow
velocity, rather than being calculated based on an erroneous
single-plane “axial” profile that is influenced by the swirl
component of the flow. Using the resolved axial flow velocity
in this manner yields a composite volumetric flow rate that ismore accurate. With a correction factor based on straightpipe calibration applied to the composite volumetric flow
rate, the most accurate flow rate can be calculated for thegiven properties of the fluid.

Figure 4. Velocity Profiles Measured by Individual A and B Planes

Figure 5. Axial and Transverse Velocity Profiles as Resolved From LCT8Cross-PlanesTwo planes improve
performance

Figure 2 shows that an eight-path meter can perform wellonly five diameters downstream of a DEOP disturbance, atleast above an Re of 10,000, though the performance does
degrade as the Reynolds number decreases. Also of interest
is investigating how a single-plane, four-path meter would
handle the same disturbance. Are eight paths necessary or
can a less-complex four-path meter do the job? Figure 6
breaks down the composite performance of the eight paths,
as well as the performance of each four-path plane takenon its own for the 6-inch Sentinel LCT8 downstream of theDEOPs.

Figure 7. Performance Comparison of the 10-inch LCT8 Against Separated
Four-Path Planes

Immediately evident in both Figures 6 and 7 is that a singleplane four-path meter, taken on its own, performs muchworse than when the velocities of all eight paths of the twoplanes are taken together. The improved performance ofthe two planes is a direct result of the cancelling out theasymmetry of axial flow profile.

Figure 6. Performance Comparison of the 6-inch LCT8 Against Separated
Four-Path PlanesFigure 7 is the similar test regiment data plotted as in Figure
6, but for the 10-inch Sentinel LCT8 meter, also downstreamof the DEOPs.

Conclusion
In conclusion, the Panametrics Sentinel LCT8 UltrasonicFlowmeter shows good insensitivity to common upstream
flow disturbances. The performance data presented wasgathered from actual flow testing of two typical meter sizes:
6 inches and 10 inches. The performance improvement ofa dual-plane, eight-path meter over a single-plane, fourpath meter in the same disturbed flow condition is apparent
when comparing the calculated composite flow rate of either
meter against the test reference meter. A secondary benefitof meters that utilize orthogonal paths, such as the Sentinel
LCT8, is the ability of the meter to resolve the velocity profile
into the axial and transverse profiles. Such diagnostics canbe used to identify pipeline problems that may develop overtime.

About the author
Nicholas J. Mollo, Principal
Engineer
Nick has been working with thePanametrics ultrasonic flowmeter brandfor fifteen years, specializing in meter
flow performance testing, calibration,metrology, and certification.

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