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LIQUID METER PROVING TECHNIQUESClass # 4095

Dan ComstockBusiness Development

Measurement & InstrumentationSGS North America Inc.

12621 Featherwood Drive

Houston, Texas 77034 USA

Introduction

Liquid Meter Proving is the physical testing of the performance of a meter, in a liquid service, that is measuringthe flow or volume throughput. The meter "proof" or test, is performed by placing a meter in series with a meterprover, which has a known "base" volume at standard conditions, in such a way that during any given test run, allthe product measured by the meter is also measured by the prover, and equally important, only the productmeasured by the meter is measured by the prover. Then the meter indication is compared to the known provervolume. Meters can provide more precise measurement of the liquids handled, if they are proved regularly and inactual operating conditions. A meter, which has been tested on a given product, with a given density andviscosity, at a given flow rate, temperature and pressure will need to be retested when any one of the aboveconditions change significantly.

The density of liquids is affected by temperature and pressure deviations from base conditions (60F and 0 psigare the US Customary Units). This fact is apart from the points above. However, since the temperature andpressure in the meter are not necessarily equal to the temperature and pressure in the prover, the volumedifferences occurring must be adjusted for, in the results obtained. The magnitude of these volume changes issuch that accurate thermometers with increments of 0.2 degrees F and accurate pressure indicators withincrements no greater than 1% of scale, should be used on crude oil and refined products. Adjustable mercurythermometers, pressure gauges, digital thermometers and digital pressure indicators, should all be routinelychecked to ensure the integrity of the measurement system. Traceability to an appropriate national agency (e.g.,NIST in the USA) must be maintained for all temperature, pressure and volumetric measurements. API MPMSChapter 12.2 Part 3 provides an excellent guide to the calculation of meter factors.

Types of Meters

There are a number of types of flow meters that are used in liquid service depending upon the application andoperator preference. These meters include among others: displacement, conventional turbine, helical turbine,Coriolis-Effect types and ultrasonic types of meters. All of these meters have demonstrated their ability tomeasure liquids effectively if they are properly installed and properly operated in a system that has been properlydesigned. When any of these meters are used special care must be given to both design and installation. Ingeneral terms, the proving process is essentially the same regardless of the type of meter being proved. Forexample, all meters should be proved at a uniform flow rate condition during any given meter factor determination.However care must be taken to understand the differences so that good results are achieved. For example,conventional turbine meters in theory at least have the most uniform pulse train of discrete pulses. This assumesthat the blades are equidistant and of uniform orientation. For this reason they lend themselves especially well topulse interpolation when needed to achieve a 1 part in 10,000 resolution of the pulses per proving pass (in a bi-directional prover) or run (in a unidirectional prover) and the provers can be quite small if equipped with highprecision detectors. Displacement meters sometimes have too much gear play to effectively use pulse

interpolation although special designs of displacement meters taking pulses from a location closer to the primaryelement of the meter have proved to work just fine. Helical turbines can effectively make use of pulseinterpolation but the prover might need to be somewhat larger because there are fewer discrete pulses perrevolution than in a conventional turbine. Ultrasonic and Coriolis type meters have so called manufacturedpulses which may require that the proving runs be somewhat longer because of an uneven output. In somecases this might mean using a displacement prover to prove a turbine meter for the purpose of making mastermeter runs that are larger than the displacement volume of the prover being used to prove the master meter.Trouble shooting each type of meter in the face of unsatisfactory proving results requires knowledge of the type ofmeter itself in addition to the general considerations of leaking valves and meter prover performance.

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Conventional Pipe Provers

When proving a meter with a pipe prover, a pulse counter is used to gather the pulses generated at the meter(displacement or PD meters have high resolution pulsers attached while turbine meters generate pulsesdirectly). The counter is gated "on" and "off" by the displacer detectors. This means that the meter indicationused in custody transfer (be it electronic totalizer or mechanical register head) is displayed by equipment separatefrom the prover counter/pulser system. For this reason it is important to check the integrity of the prover andmeter totalizer system each time a meter is proved. This check should be made in terms of pulses read by the

prover totalizer, per unit volume indicated by the custody transfer totalizer.

In the case of PD meters, there is often an internal prover gate switch in the photopulser which gates the provertotalizer "on" and "off" each revolution. So if you have a photopulser which by design generates 1000 pulses perrevolution, you can easily test the electronics with the "test" switch. By internally gating the counter "on" and "off"the prover counter will accumulate successive sums by 1000 pulse increments.

The above internal test is not sufficient however, to fully test the system before proving a given meter. It isessential that you know, by separate test, what the ratio is of revolutions of the pulser, per unit volume of thecustody transfer totalizer. This can be done by manually gating the prover totalizer "on" while simultaneouslyreading the custody transfer totalizer. After several minutes have passed, gate the prover totalizer off in thesame manner. Tests of less time duration can successfully be made by means of a switch on the register headitself which would automatically gate the prover counter "on" and "off" at whole unit volume increments. The test

must be of long enough duration to smooth out any gating error to within 0.01 percent. It is highly recommendedthat this method be automated.

By following the above procedure you are assured that the prover totalizer system being used to prove the meteris in agreement with the meter totalizer system, used in the custody transfer. The essential point is that it is notenough to know that the prover electronic system is working properly. One gear erroneously changed ordamaged in the register head of a PD meter, or bad information on hand regarding the ratio of pulses per unitvolume on a custody transfer turbine meter totalizer, could result in a sizeable error in the meter factor determinedby the proving, unless a pulse check had been made.

Proving Procedures

1) Record the delivery flow rate and pressure at the meter.2) Direct the flow of the meter being proved into (through) the prover and open vents at all high points to purge

the air or gas while filling.3) Check all valves (the use of double block and bleed valves are required in measurement stations) to be surethat none of the flow through the meter being proved is in any way bypassing the prover and that none of theflow through the prover is in any way bypassing the meter being proved. To check a double block and bleedvalve in crude oil or refined products service:

a) Partially open the double block valve.b) Open bleeder to discharge product in order to verify that the bleeder is not plugged.c) Close the double block and bleed valve.d) Open the bleeder again at which time the bleeder flow should cease, thereby proving the integrity of

the double block and bleed valve.Some double block valves are checked by means of a pressure gauge. When the valve is closed the innercavity space is increased allowing the trapped fluid to expand and the pressure to drop. In another design thecavity space might be reduced which would raise the pressure. To check a double block and bleed valvedesigned for the pressure gauge method:

a) Read the cavity pressure when valve is partially open.b) Read pressure gauge at the cavity again when valve is closed.c) There should be a significant differential pressure.d) In the case of automated and/or remote control operations, it is important to ascertain what a "seal"

light means. Often it is simply a valve position indicator. It might be some other type of indirect sealindication which the operator must understand fully in order to know whether to believe the indication,or to check for seal directly at the valve site.

e) This discussion includes 4-way valves and sphere interchanges, both of which must affect a sealbefore the displacer actuates the first detector encountered in a pass.

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Small Volume Provers and Displacement Provers with Captive Displacers

By definition, a small volume prover is simply a prover which is being used to prove a meter that does notgenerate enough pulses to totalize at least 10,000 pulses on any given pass of the displacer in a mechanicaldisplacement type prover. However, the desired resolution can be obtained through the use of pulseinterpolation. There are several techniques, but double chronometry is the only one discussed in Chapter 4.6 ofthe API Manual of Petroleum Measurement Standards. When pulse interpolation is used, extra care must be

taken to ensure and verify the integrity of the meter proving results. However, the routine field checks areessentially no more difficult than the pulse check described earlier.

Many small volume provers have a shaft on one end that is attached to the displacer. Thus, the effective volumewhen proving a meter downstream is different than the calibrated volume when proving a meter upstream,because a portion of the shaft enters and leaves the prover chamber from the outside. Therefore, it is essentialthat the correct choice of calibrated volumes be used in the calculations of the meter factor. When a smallvolume prover of this type is calibrated, it is recommended that it be waterdraw calibrated both upstream anddownstream. If the actual waterdraw calibrated volumes do not exhibit the same bias as would be predicted bythe displacement of the shaft on one end, at least one of the volumes is in doubt. Therefore, the system shouldbe inspected and the calibration should be performed again to restore confidence.

Master Meter Methods

Variations of master meter proving include the "indirect pipe prover" method and the "common master meter"method. Both utilize a test meter to prove another meter, and are thus one step removed from the prover.However, there are substantial differences in the levels of accuracy that can be expected.

In the "indirect displacement prover" method, a master meter is installed in series with both a displacement proverand the line meter being proved. The master meter is first proved against the pipe prover and then immediatelyused as the standard against the line meter, at the same operating conditions. This method can be extremelyaccurate and if properly executed can achieve results comparable to direct meter proving with a displacementprover under certain conditions.

In the "common master meter" method, a meter is proved at one given location by a volumetric tank or pipeprover. Then it is transported to another site to be used as a standard against a line meter. The weakness of thismethod lies in the difficulty in duplicating the conditions at the prover site (i.e. flow rate, density, viscosity,temperature, pressure, etc.) to those at the line meter site. In the case of any meter sensitive to the flow profileeven the installation into the test loop and later re-installation into the field location can be a great source of error.Furthermore, something might happen to the meter while being transported. Nonetheless, the common mastermeter method is sometimes used in low volume applications.

Although the direct proving with a displacement prover would normally be preferred, the "indirect displacementprover" method can be very effective in the proving of meters that do not have a means of generating pulses.They can also be beneficial in the proving of line meters that have cyclic type temperature compensators, withrelatively small prover volumes, since longer runs can be made (master meter vs. line meter) to prove the linemeter. An example of this situation would be the proving of a meter that drives a cyclic type calibrator with a gearratio of one (1) revolution per five (5) gallons, with a pipe prover that has a calibrated volume of less than five (5)barrels but is not an even multiple of five (5) gallons. Another example might be a meter that generatesmanufactured pulses in a somewhat uneven train. Finally, it must be recognized that when proving a meter

using the indirect pipe prover method, the pulse check becomes less critical (assuming the custody transferindicator is also the indicator used for the meter proving). Any errors in gear ratios on the master meter would becancelled out in the calculations of the line meter factor. However, the electronic system check should always bemade to verify the integrity of the pulse train.

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Conclusion

Liquid meter proving is the physical testing of the performance of a meter in a liquid service that is measuring theflow or volume throughput. The purpose of the test is to assure accuracy in the measurement of liquids beinghandled and transported in some way. Good measurement is vital, whether the primary purpose is processcontrol, in-house inventory, scheduling a pipeline, or custody transfer. Ultimately, it is important in dollars andcents. It costs money to make "off spec" product (or to produce good product inefficiently). Good liquid meter

proving techniques are therefore vital, and are one important link in the overall program of wisely managing yourcompany's resources.

Acknowledgement

The author wishes to thank Harold Gray for his good insight through the years and who as the speaker will bring aspecial dimension to the presentation of this paper and class session.

Reference Material

API Manual of Petroleum Measurement Standards Chapter 1 Vocabulary

API Manual of Petroleum Measurement Standards Chapter 4 Meter Provers

API Manual of Petroleum Measurement Standards Chapter 5 Meters

API Manual of Petroleum Measurement Standards Chapter 11 Petroleum Measurement Tables API Manual of Petroleum Measurement Standards Chapter 12 Calculations

Paper Presented in May 2007SPEAKER: HAROLD GRAY of ALYESKA PIPELINE SERVICE COMPANY

International School of Hydrocarbon Measurement (ISHM)Oklahoma City, Oklahoma USA

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