Figure 2. Meter proving data

These experiences, in combination with Micro Motion’s accu-racy specifications, triggered interest in testing a flowmeter ina LACT unit.

LPG LACT unit

In 1999 Total Austral signed a contract to deliver LPG to ENAP(the Chilean National Oil Company). ENAP’s previous experi-ence with Coriolis had been very good, so both ENAP and TotalAustral considered Coriolis a suitable technology for the LACTunit.

Facility and design

Total Austral and ENAP agreed to build a LACT unit with twometers in series. The measurement objectives were to meterLPG quantities and split mass units, in propane and butane, toproduce commercial-quality propane and butane as per GPA2140, propane content approximately 60%, typical density0.535 g/cm3.

The LACT unit is located north of Tierra del Fuego Island, onthe Argentina-Chile border. This area experiences frequenthigh winds (above 100 km/h), and the ambient temperaturevaries from -20 °C in winter to +25 °C in summer. The installa-tion uses a 6” pipeline with centrifugal pumps, for an applica-tion flow of 700-780 tons/day at a pressure of 70 bar. Fluidtemperature is 20-30 °C. The site is supervised remotely viatelemetry.

The LACT unit contains two 3” ELITE sensors from MicroMotion, connected to RFT9739 transmitters. The sensors aremounted in series. Fiscal figures are obtained by averaging thedata from both meters. The transmitters send mass and densi-ty data to the flow computer. The frequency/pulse outputs areconfigured to send volume totals to the flow computer, whichwill use them for proving input for calibration.

MICRO MOTION WHITE PAPER

Coriolis Flowmeters in an LPG Custody Transfer Application

This paper discusses the use of Coriols mass flowmeters in aLeased Automated Custody Transfer (LACT) application involv-ing Liquid Petroleum Gas (LPG). It contains a description ofTotal Austral’s facilities, calibration strategy, proving results,and application problems. Finally, this paper presents TotalAustral’s conclusions concerning the appropriateness ofCoriolis technology for this application.

Previous experiences with Coriolis

At Total Austral, we first used Coriolis technology in 1991 tometer the liquid outlet of our test and production separators.Coriolis was a solution to maintenance problems because welltest figures were very close to actual oil production. The avail-ability of a density measurement allowed us to estimate BasicSediment and Water (BSW), and after some time separatorswere moved from tri-phase to bi-phase operation.

In 1997, a Micro Motion Coriolis meter was mounted down-stream in series with the fiscal positive displacement (PD)meters in a crude oil application. Typical flow in this applica-tion is around 50% of mass nominal range of the sensor.During six years of uninterrupted operation, a number of PDmeters were replaced. However, the Coriolis meter neverrequired maintenance, and its meter factors remained within0.1%, with no drift since startup.

Figure 1 shows the solid mounting of the meter; Figure 2shows data from seven months of weekly proving.

Figure 1. Mounting of Micro Motion meter

KNOWLEDGEBY RUBEN MOLDES, ENGINEER, TOTAL AUSTRAL

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Other equipment on the LACT unit (see Figure 3) includes:

· Danalyzer 500-2350 automatic sampler and on-line chro-matograph

· Auxiliary transmitters for conventional volume proving

· Bi-directional volume prover of 570 liters (1140 liters perround trip)

The LACT unit has several known design weaknesses:

· The flowmeters are slightly oversized. This was intentional,to allow for future expansion and to minimize pressure drop.The typical mass flow of this application is around 25% ofnominal range.

· For simpler skid design, the sensor is mounted in an invertedposition. This carries the risk of spurious measurement dur-ing a stop, if fluid in the sensor is partially vaporized. Toovercome this, the meters are maintained under pressureduring stops, to avoid two-phase flow through the sensor.

· Pipe fixing elements are not rigid enough for a high-pressureapplication with severe stress during pump start.

These weak points are reflected in higher zero shifts, com-pared with our other fiscal application, and the meter factorspresent a bigger spread, but accuracy is still well within APIsuggested limits.

Figure 3. Proving equipment

Calibration strategy

Because Total Austral places top priority on safety, the select-ed calibration strategy could not involve manipulation of dan-gerous liquids.

Considering manufacturer accuracy specs (0.12% at our typicalflow) and the sensitivity of LPG density to pressure and tem-perature (0.5 °C change in temperature means 0.15% changein liquid density), it is difficult to improve uncertainty figureswith a field prover. The LACT unit was implemented to obtaintraceability to national standards, while at the same time guar-anteeing that no systematic errors would remain undetected.Reduction of measurement uncertainty was not the primaryobjective.

Installation of a picnometer or a master densitometer was con-sidered and discarded. Instead, density measurements fromthe Coriolis meter were validated against a calculated densitywhich we obtained from composition, pressure, and tempera-

ture data using the COSTALD method. This procedure isdescribed later in this paper, in the section entitled “DensityValidation”.

Because the flowmeter density values and the calculated den-sity values were in perfect agreement, we felt confident aboutboth the Coriolis density performance and the on-line chro-matograph and temperature transmitters.

The meter factor for mass can be derived from the volumemeter factor and the density meter factor using the followingformula:

Volume proving

Volume proving is performed using the conventional method-ology of API MPMS Cap 4.8. The meter factor is the average ofall valid runs. According to policy, any run that is more thanthree standard deviations from the average is discarded, butthat did not happen during volume proving.

In the first several procedures, twenty runs were performed.However, due to the consistency of results, only ten runs wererequired for later procedures as long as random uncertaintyremained below 0.027%.

Density self-calibration

Density is read several times to minimize random effects ofrepeatability, and both averages are used to obtain densitymeter factors with the following equations:

During the first two years, density data were collected withmeters at operating pressure, operating temperature, andzero flow. Approximately sixteen measurements for eachflowmeter were averaged in the flow computer. This methodallowed data collection with both meters in the same condi-tion. However, a slight drift was observed between the twoflowmeters, probably originating in a different cool-down ofmeters due to ambient temperature.

To overcome the drift, density data were collected during theentire day, with both meters at normal flow and pressure com-pensation applied as per API MPMS Chap 11.2.2.

When this correction was applied, the average daily densityvalues reported by the two meters were within 0.01% of eachother.

Total Austral compared these results to the results predictedby manufacturer specifications. Micro Motion’s specificationsfor density measurement are 0.0005 g/cm3 for accuracy and0.0002 g/cm3 for repeatability (assuming a 95% confidencelevel). A density product of 0.53 g/cm3 corresponds to 0.1%accuracy. The average of both meters should have an expand-ed uncertainty around 0.07% (95%).

On the other hand, if both instruments are operating normally,

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Coriolis Flowmeters in an LPG Custody Transfer Application

density spread should be within . This is the expect-ed density meter factor (DMF) limit of each meter:

Proving results

The Coriolis meters have been calibrated fifteen times sincestartup. Average meter factors for the two meters are 0.9988and 0.9985, with a standard deviation of approximately 0.1%.Figure 4 shows data from thirteen procedures. One abnormalvalue was observed in one of the proving meters. This valuewas assumed to be an outlier, and was discarded.

Figure 4. Proving results

Measurement spread between meters

At typical operational flow, Micro Motion’s specifications formass flow measurement are 0.12% for accuracy and 0.05% forrepeatability. Gross differences in mass flow measurementsbetween the two meters should be less than 0.16% (assuminga 95% confidence level) with a rate of change of the gross dif-ference value between one day and the next of approximately0.07%.

Data on meter performance for calendar year 2002 are shownbelow. Figure 5 shows the difference between meters. MicroMotion’s specifications are verified. Figure 6 shows an esti-mate of repeatability, using a 24-hour time period.

FIgure 5. Gross differences between mass flow meters, CY 2002

Figure 6. Repeatability estimates

Density validation

The correlation between the density values from the Coriolismeters and the on-line chromatograph is used to confirm theaccuracy of the flowmeter density measurements, the tem-perature transmitters, and the on-line chromatograph.Accordingly, this procedure was implemented very carefully.

The flow computer measures density and reads and calculatesaverage values for pressure and temperature. Every five min-utes (the approximate time between chromatograms), it cal-culates meter density at standard conditions and comparesthese values with a calculated density obtained from composi-tion and the COSTALD method.

Coriolis density values are transformed to standard conditionsusing API MPMS Chap 11.2.2, pressure correction, and GPA TP25 for temperature compensation. An iteration technique isrequired for maximum accuracy, with an estimated value atstandard conditions. Meter density is first corrected in pres-sure to calculate a value at fluid temperature and vapor pres-sure, then TP 25 converts the value to 60 °F (there is no TP 25for international units yet). This iteration is necessary if esti-mated and calculated values are different.

FIgure 7. Density validation data from a typical day

Using the 5-minute data, a report and graph are generated inthe SCADA system. Figures 7 and 8 compare data from thetwo flowmeters and the chromatograph. Figure 7 shows datafrom a typical day; Figure 8 shows data from a day with some

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Coriolis Flowmeters in an LPG Custody Transfer Application

MICRO MOTION WHITE PAPER

process variation. The chromatograph data show a slight delaydue to sample time. Both graphs demonstrate the high agree-ment of both density measurements. Daily differences areapproximately 0.05% to 0.1%.

Figure 8. Density validation from day with process variation

Flowmeter operation problems

On two occasions, a Micro Motion transmitter reported a sen-sor failure alarm. Both times, the alarm disappeared when thetransmitter was reset. This was diagnosed as a defect in thetransmitter software, which has since been corrected. In threeyears of operation, no other incidents have been observed.

Conclusion

Performance of the Coriolis technology in this application hasbeen highly satisfactory, in terms of both reliability and lowmaintenance. Micro Motion’s specifications have proved accu-rate. Total Austral has confirmed its experience against severalreports from recognized independent institutions such as the

South West Research Institute (SWRI) and the NationalEngineering Laboratory of Great Britain (NEL).

In conclusion, in Total Austral, Coriolis flowmeters are one ofthe preferred technologies for liquid hydrocarbon custodytransfer.

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WP-00695 ©2004. Micro Motion, Inc. All rights reserved.

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The contents of this publication are presented for informational purposes only and,

while every effort has been made to ensure their accuracy, they are not to be construed

as warranties or guarantees, expressed or implied, regarding the products or services

described herein or their use or applicability. We reserve the right to modify or improve

the designs or specifications of our products at any time without notice.Micro Motion supports PlantWeb field-based architecture, a scalable way to use open and interoperable devices andsystems to build process solutions of the future.