Verifying Flowmeter Accuracy

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Verifying Flowmeter Accuracy

Calibration techniques to ensure measurement performance

Greg LivelliSeptember 26, 2010

Many factors can cause a flowmeter to lose calibration, including: buildup of deposits, minerals, oils, and solvents;

wearing, breakage, or failure of internal mechanical parts; damaging impact; improper installation; and modified piping configurations.

A flowmeter calibration, usually carried out by the manufacturer, adjusts the output of the meter to bringit back to a value within the specified accuracy tolerance. This article discusses the pros and cons ofseveral calibration techniques.

Flowmeter Calibration Relativity

Flowmeter calibrations are not absolute operations. A calibration compares a flowmeter measurementrelative to a standard. The comparison establishes a relationship between what the flowmeter measuresand what the standard measures. The standard consists of a system of pumps, pipes, fluids,instrumentation, quantity reference measurement, calculations, and operators all combined to measurethe quantity of fluid passing through the flowmeters in a unit of time.

The relationship between the flowmeter under test and the standard must be expressed in a way that givesa meaningful expectation of how the flowmeter will perform in use. In practice, accuracy is the term that

most users can relate to and that can usefully express an expectation and general specification. Accuracyis a qualitative term, and the number associated with it must be taken in the spirit of this concept. Itindicates how close the flowmeter measurement agrees with the true measured flowrate.

READ ALSO: Flowmeter Piping Requirements

The standard must be able to reproduce the measurement that it claims to make with some degree ofconfidence. To this end, all the measurements in the system have to show traceability to higher-levelmeasurements, and ultimately to national and international standards. Traceability must be through anunbroken chain of comparisons with stated uncertainties.
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The uncertainties of each calibration higher in the chain should be smaller at each step. Note, however,that providing or claiming traceability makes no statement regarding the quality or uncertainty of thefinal calibration; it only satisfies one aspect of the quality requirements for an accredited calibration.

The uncertainty quoted for a calibration or a standard depends on a detailed examination of all thecomponents of the system, the use of the system, and its history. The quote will specifically state which

parameters underlie the uncertainty. This may be the quantity measured by the standard or the quantitypassed through the flowmeter. This quoted uncertainty is not that of the calibration result. The resolution

of the meter, influencing factors, and finally the repeatability and linearity of the calibration results mustall be included to provide the uncertainty of the calibration.

Why Bother with Flowmeter Calibration?

For one, manufacturers want to establish the quality of their management systems, as spelled out in ISO9001 of the International Standards Organization (www.iso.org). Third-party auditors and regulators ofthis standard require documentation to verify the quality of these manufacturing management systems.Obviously a manufacturing process that depends on an accurate flowmeter for maintaining product

quality will require documentation relating to its calibration.

Often a flowmeter measures the amount of fluid transferred by pipeline from one company to anotherentity or division, sometimes known as fiscal metering. This is the case when you purchase gasoline. Theflowmeter measurement determines the cost of the transfer and sometimes involves taxation. Flowmeteraccuracy in these cases is obviously of paramount importance. Companies and governments will mandatethe calibration frequencies to check on flowmeter accuracy.

Another reason for flowmeter calibrations is better management of processes. With time, flowmeterperformance may slowly degrade, negatively affecting quality and/or costs. Timely calibrations helpmanagement keep operating equipment functioning properly and efficiently.

But what is a timely calibration? For most applications, users must examine the operating conditions anddefine their own calibration frequency. In other cases a third party or standard may mandate thecalibration frequency. The idea is to determine a calibration interval that minimizes the risk of anincorrect meter reading that makes a significant impact on the process. Keeping a good history of pastcalibrations helps to spot trends for predicting when calibrations become necessary.

Unfortunately, calibrating a flowmeter with goodconfidence in the result is usually costly and difficult.

Practical calibration techniques do not exist, andmany methods depend heavily on operator skill.Locating good testing points in the pipeline is usuallydifficult. And flowmeters experiencing highflowrates often cannot be calibrated.

Flowmeter Calibration In a Test
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Figure 1.In the drop test, calibration engineersdetermine the amount of liquid collected in atank within a certain time interval.

Figure 2.Transit-time ultrasonic flowmeters measure the timedifference between ultrasonic beams moving with and againstthe fluid flow.

Rig

This technique requires removing the flowmeter andshipping it to a calibration facility having a test rig traceable to the National Institute of Standards andTechnology (www.nist.gov). These facilities generally consist of a reservoir, pumps, meter runs, andweigh tanks. The system operates as a constant-flow facility. It uses timed collections of water tocompute the average flow through the meter being calibrated. The relative expanded uncertainty for thesefacilities is between 0.2 percent and 0.5 percent.

The calibration report typically includes an uncertainty value for the calibration factor of the flowmeter.Uncertainty depends on the reproducibility of the meter under test and the uncertainty of anyinstrumentation associated with the flowmeter output. A flow calibration often includes five differentaverage flowrates and the standard flow made at each setpoint. Today, calibration in a test rig willtypically run about $5,000 per meter.

Drop Test or Volumetric Method

Calibrations using this technique determine the amount of liquid collected in a tank within a certain timeinterval. The amount collected can be measured by weight or volume. The uncertainties tend to be large,typically 5 percent to 10 percent. For example, suppose the diameter of the tank is 10 feet +/- two inches,and the level changes three feet +/- one inch. The dimensional uncertainties compute to a difference of7,040 to 7,500 gallons, or 6.1 percent. In addition, the tank may not have a perfectly circular crosssection or exactly plumb walls. Undetected leaks will further degrade accuracy.

The drop test diagram (Figure 1),typically involves volumes that are

too large to be practical. Smalluncertainties in the tank internaldiameter or level can have asignificant effect on calibrationaccuracy. Such tests are also timeconsuming.

Ultrasonic Clamp-On

Meters

The user can install clamp-onultrasonic transducers to the outsidewall of a pipe and take measurementsof flowrate to compare with readingsof a flowmeter to be calibrated(Figure 2). These transit-timeflowmeters measure the timedifference between ultrasonic beamsmoving with and against the fluid
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flow. This time difference, combined with knowledge of the pipe''s internal diameter and the distancebetween the two ultrasonic transducers, permits a calculation of the volumetric flowrate through the pipe

The best measurement accuracies possible with clamp-on ultrasonic flowmeters are 2 percent to 5percent. But many other unknown factors generally result in lesser accuracies5 percent to 10 percent.The three major sources of error include the pipe''s internal diameter, the flow velocity profile, andacoustic interference.

Nonlaminar profile uncertainties, amounting to 1 percent to 10 percent of the measured flow value, canbe corrected by determining the appropriate K factor from calibration at specific flow conditions, fromempirical calculations, or by sampling a greater fraction of the cross-sectional flow area. Acoustic short-circuit interference can cause errors exceeding 7 percent if the signal/noise ratio is 10-to-one or less, orerrors greater than 0.6 percent for signal/noise ratios below 100-to-one. Beam path changes caused bytemperature, pressure, composition, or mechanical effects can be compensated for or eliminated by

positioning each transducer with permanent mounting pads in a positive manner, by empiricallycalibrating the flowmeter at particular intervals of temperature, pressure, and composition, and bymodifying the pipe interior.

Errors relating to the pipe''s internal diameter can cause significant measurement errors. For example, ifthe pipe''s nominal ID is 78.85 inches, and the maximum ID is 81.79 inches, the difference produces ameasurement uncertainty of 3.7 percent.

To improve the calibration accuracy, install the ultrasonic transducers at a location that minimizes thediscontinuities between the meter to be verified and the clamp-on meter. Discontinuities would include

pipe fittings and open branches. To ensure a well-developed flow profile, the straight-pipe sectionupstream of the clamp-on meters should be at least 30 pipe diameters in length. Since uncertaintyincreases if the cross-sectional area calculation depends on single measurement of pipe diameter, youshould average two perpendicular diameters.

Insertion Probes

Insertion probes, which measure fluid velocity at apoint within a pipe''s cross-section, can check theperformance of an installed full-bore meter. Aninsertion flowmeter (Figure 3), measures the fluidvelocity at a point. It is unaware of surrounding flowvelocities outside of the immediate location of the

probe tip. The user or a secondary device must

calculate the volumetric flowrate based on knowledgeof the flow profile within the pipe. (For moreinformation on flow profiles, see Part III of this series- Flow Control, May 2007, page 14.) Measurementaccuracy ranges from 2 percent to 5 percent. Thistechnique works best for a fully developed flow

profile at the measuring location, usually achieved byinstalling the probe after a long length of straight

pipe. The proper straight length depends on thenature of the upstream disturbances to the flow.


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Figure 3.Insertion probes, which measure fluidvelocity at a point within a pipe''s cross section,can check the performance of an installed full-bore meter.

Figure 4.An insertion probe inserted one-quarter into the pipe having this flow velocity

profile will measure a fluid velocity that isabout 30% too high.

Attempting calibration in a location without a well-developed flow profile can lead to large errors.Figure 4 shows a fluid flow profile following anelbow fitting. An insertion probe tip sitting at a pointone-quarter of the pipe diameter will measure a fluidvelocity that is about 30 percent too high. To developthe flow profile, the engineer can make multiplevelocity measurements across the pipe''s diameter

a time-consuming operation.

Other sources of inaccuracy with insertion probes include: errors in internal pipe diameter, cross-sectional area, and pipe ovality; pulsating and unstable flows; varying flowrates between point measurements while determining profiles;

errors and uncertainties in associated instrumentation; and particulate material in the fluid.

Tracer Methods

Tracer techniques for calibrating flowrates include the transit-time and the dilution methods. Attainablemeasurement accuracies range from 2 percent to 5 percent.

Using the transit-time method, engineers inject a pulse of tracer fluid into the main flow stream and


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Figure 5.Transit-time tracer calibrations measure the time theinjected tracer fluid passes between two detectors.

measure the time taken for the tracer to pass between two detection points (Figure 5). Since the volumeof the pipe between the detectors is known, they can determine the volumetric flowrate. Somedisadvantages include: not suitable for sluggish or slow moving flows; difficulties in determining the volume between detectors; and often requires many measurements, which can be time consuming.

For the dilution method (Figure 6), engineers use a tracer fluid that is detectable in low concentrationsand inject it into the flow at a known rate. They then sample the mainstream flow downstream of theinjection point, far enough to allow homogeneous mixing. The downstream detector measures the tracerfluid concentration. Since the tracer fluid flowrate q is comparatively small, they can derive the mainflowrate Q via the equation: Q = q/C, where C is the measured tracer concentration.

The primary source of error occurs in accurately determining the tracer concentration. Additionally, thetechnique also requires many measurements and can be quite time consuming.

Hydraulic Model TestingFor some piping flow situations, engineers may find it difficult to calibrate the flow measurement systemusing either the dilution or volumetric tracer technique. For example, they may be unable to reproducethe full operating flow range in the system. In some cases, testing would potentially result in a release ofan unacceptable contaminant loading to the environment. (For example, test flowrates may be limited byseasonal downstream receiving water restrictions.) If the site handles only emergency overflows, testingmay be ruled out by water quality limitations.

For these situations, engineers may be

able to construct a hydraulic model ofthe flow system and then runcalibration tests on the model underlaboratory conditions. They woulddesign the hydraulic model based onthe principle of hydraulic similitude.With this approach, the modelrepresents a geometric reduction ofthe actual flow measurement system.The model is scaled down via a fixed

ratio between the model and actualflow system for all homogeneouslengths, velocities, and forcesinvolved in motion. Engineers should pick a scale factor that provides model flows as close as practicalto actual flows. Of course the model must be consistent with pumping capacity available at the testingfacility.

The hydraulic model should be constructed based on field-measured dimensions that are confirmedbefore construction. Common construction materials for a hydraulic model are wood and steel. Inlaboratory testing facilities, flowrate through the model is usually determined by applying the volumetrictracer method. Measurement accuracy ranges from 10 percent to 15 percent.


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Figure 6.In the dilution tracer calibrationtechnique, the fluid flowrate is a function of thetracer injection rate and its downstreamconcentration.

Figure 7.In the case of the reference meter inseries, one flowmeter verifies another.

A detailed description of hydraulic similitude and hydraulic model studies may be found in Hwang(1981) and Streeter and Wylie (1985) [1, 2].

Reference Meter in Series

Another way to verify the calibration of a flowmeter is to install two or more in a single pipeline (Figure7). In this case, one meter verifies another. For example, one flowmeter may be used as the pay meterand the other as a check meter. The pay meter serves for billing purposes and the check meter ensuresthat the pay meter is still within calibration. The meters are checked against one another on a regular

basis. Good practice calls for proving the pay meter on an annual basis.

To minimize discrepancies, the meter readings must be taken at the same time every reporting period. Ifpossible, it is best to record the inventory readings from both meters simultaneously. The longer thereporting period, the smaller the errors associated with recording the inventory readings will be.Measurement accuracies for reference meters in series typically range from 0.5 percent to 1 percent.

Obviously having two flowmeters in series for a single measurement can be quite expensive and is oftennot practical.

Flowmter Calibration Applied:

CalMaster Magmeter Verification

In the case of electromagnetic flowmeters, ABBInstrumentation (www.abb.com) offers averification system that can check calibration ofABB''s Magmaster flowmeters without access to
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Figure 8.The CalMaster verification systemfrom ABB Instrumentation compares keymagmeter parameters to those measured by the

factory at the time of meter manufacture.

the pipe or the sensing electrodes. Called theCalMaster system, it permits in-place verificationand certification of the magmeter to ensure that itremains within its specified calibration.

When connected to a MagMaster transmitter and apersonal computer (Figure 8), the portableCalMaster system performs a complex series of

tests over the course of 20 minutes. It compares keyflowmeter parameters to those measured by thefactory at the time of meter manufacture. The testsevaluate the status of the complete system,including the sensor coils, electrodes, cables, andtransmitter. The flowmeter will require servicingonly when it fails the calibration check.

The CalMaster system also serves as a diagnosticand condition-monitoring tool. It automaticallystores all the measured values and calibration

information in its own database files for each meter. It maintains a calibration history log, making iteasy to undertake long-term trend analysis. Trends can give early warning of possible system failure,enabling the maintenance engineer to anticipate problems and take remedial action in advance.

Greg Livelliis a senior product manager for ABB Instrumentation, based in Warminster, Pa. He hasmore than 15 years experience in the design and marketing of flowmetering equipment. Mr. Livelliearned an MBA from Regis University and a bachelors degree in Mechanical Engineering from New

Jersey Institute of Technology. Mr. Livelli can be reached [email protected] 215 674-6641.

www.abb.com

References1.Hwang, N.C. 1981,Fundamentals of Hydraulic Engineering Systems, Prentice-Hall Series inEnvironmental Sciences.2.Streeter, V.L., E.B. Wylie. 1985,Fluid Mechanics, Eighth Edition, McGraw-Hill.
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Verifying Flowmeter Accuracy