verifying flowmeter accuracy.pdf

8/14/2019 Verifying Flowmeter Accuracy.pdf

1/8

What is your company's position on mobile solutions for plant and/or field-based data

access?

What is your company's position on mobile solutions for plant and/or field-based data access?

We are currently using mobile solutions

We are considering/evaluating mobile solutions

We are not using or considering mobile solutions

View ResultsPoll ArchiveHome Verifying Flowmeter Accuracy

Verifying Flowmeter Accuracy

By Greg LivelliSeptember 26, 2010No Comments

Email/ Print/ Reprints/ |

More

/ Text Size+

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 bring it backto a value within the specified accuracy tolerance. This article discusses the pros and cons of several calibration

techniques.

Calibration RelativityFlowmeter calibrations are not absolute operations. A calibration compares a flowmeter measurement relative toa standard. The comparison establishes a relationship between what the flowmeter measures and what thestandard measures. The standard consists of a system of pumps, pipes, fluids, instrumentation, quantity referencemeasurement, calculations, and operators all combined to measure the 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 gives ameaningful expectation of how the flowmeter will perform in use. In practice, accuracy is the term that mostusers can relate to and that can usefully express an expectation and general specification. Accuracy is aqualitative term, and the number associated with it must be taken in the spirit of this concept. It indicates how

close the flowmeter measurement agrees with the true measured flowrate.

The standard must be able to reproduce the measurement that it claims to make with some degree of confidence.To this end, all the measurements in the system have to show traceability to higher-level measurements, andultimately to national and international standards. Traceability must be through an unbroken chain of

comparisons with stated uncertainties.

The uncertainties of each calibration higher in the chain should be smaller at each step. Note, however, thatproviding or claiming traceability makes no statement regarding the quality or uncertainty of the final 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 the componentsof the system, the use of the system, and its history. The quote will specifically state which parameters underlie

fying Flowmeter Accuracy http://www.flowcontrolnetwork.com/articles/verifying-flowmeter-accuracy

12 13/11/2013 20:22


2/8

the uncertainty. This may be the quantity measured by the standard or the quantity passed through theflowmeter. This quoted uncertainty is not that of the calibration result. The resolution of the meter, influencingfactors, and finally the repeatability and linearity of the calibration results must all be included to provide the

uncertainty of the calibration.

Why Bother?For one, manufacturers want to establish the quality of their management systems, as spelled out in ISO 9001 ofthe International Standards Organization (www.iso.org). Third-party auditors and regulators of this standard

require documentation to verify the quality of these manufacturing management systems. Obviously amanufacturing 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 another entity ordivision, sometimes known as fiscal metering. This is the case when you purchase gasoline. The flowmetermeasurement determines the cost of the transfer and sometimes involves taxation. Flowmeter accuracy in thesecases is obviously of paramount importance. Companies and governments will mandate the calibration

frequencies to check on flowmeter accuracy.

Another reason for flowmeter calibrations is better management of processes. With time, flowmeter performancemay slowly degrade, negatively affecting quality and/or costs. Timely calibrations help management keep

operating equipment functioning properly and efficiently.

But what is a timely calibration? For most applications, users must examine the operating conditions and definetheir own calibration frequency. In other cases a third party or standard may mandate the calibration frequency.The idea is to determine a calibration interval that minimizes the risk of an incorrect meter reading that makes asignificant impact on the process. Keeping a good history of past calibrations helps to spot trends for predicting

when calibrations become necessary.

Unfortunately, calibrating a flowmeter with good confidence in the result is usually costly and difficult. Practicalcalibration techniques do not exist, and many methods depend heavily on operator skill. Locating good testing

points in the pipeline is usually difficult. And flowmeters experiencing high flowrates often cannot be calibrated.

Calibrating In a Test RigThis technique requires removing the flowmeter and shipping it to a calibration facility having a test rig traceableto the National Institute of Standards and Technology (www.nist.gov). These facilities generally consist of areservoir, pumps, meter runs, and weigh tanks. The system operates as a constant-flow facility. It uses timedcollections of water to compute the average flow through the meter being calibrated. The relative expanded

uncertainty for these facilities 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 any instrumentationassociated with the flowmeter output. A flow calibration often includes five different average flowrates and the

standard flow made at each setpoint. Today, calibration in a test rig will typically run about $5,000 per meter.

Drop Test or Volumetric MethodCalibrations using this technique determine the amount of liquid collected in a tank within a certain time interval.The amount collected can be measured by weight or volume. The uncertainties tend to be large, typically 5percent to 10 percent. For example, suppose the diameter of the tank is 10 feet +/- two inches, and the levelchanges three feet +/- one inch. The dimensional uncertainties compute to a difference of 7,040 to 7,500 gallons,or 6.1 percent. In addition, the tank may not have a perfectly circular cross section 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. Small uncertainties

in the tank internal diameter or level can have a significant effect on calibration accuracy. Such tests are alsotime consuming.

Ultrasonic Clamp-On Meters


12 13/11/2013 20:22


3/8

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 flowmetersmeasure the time difference between ultrasonicbeams moving with and against the fluid flow.

The user can install clamp-on ultrasonic transducers to theoutside wall of a pipe and take measurements of flowrate tocompare with readings of a flowmeter to be calibrated (Figure2). These transit-time flowmeters measure the time differencebetween ultrasonic beams moving with and against the fluidflow. This time difference, combined with knowledge of thepipe''s internal diameter and the distance between the twoultrasonic transducers, permits a calculation of the volumetric

flowrate through the pipe.

Thebest

measurement accuracies possible with clamp-on ultrasonic flowmeters are 2 percent to 5 percent. But many

other unknown factors generally result in lesser accuracies 5 percent to 10 percent. The three major sourcesof error include the pipe''s internal diameter, the flow velocity profile, and acoustic interference.

Nonlaminar profile uncertainties, amounting to 1 percent to 10 percent of the measured flow value, can becorrected by determining the appropriate K factor from calibration at specific flow conditions, from empiricalcalculations, or by sampling a greater fraction of the cross-sectional flow area. Acoustic short-circuitinterference can cause errors exceeding 7 percent if the signal/noise ratio is 10-to-one or less, or errors greaterthan 0.6 percent for signal/noise ratios below 100-to-one. Beam path changes caused by temperature, pressure,composition, or mechanical effects can be compensated for or eliminated by positioning each transducer withpermanent mounting pads in a positive manner, by empirically calibrating the flowmeter at particular intervals of

temperature, pressure, and composition, and by modifying the pipe interior.

Errors relating to the pipe''s internal diameter can cause significant measurement errors. For example, if thepipe''s nominal ID is 78.85 inches, and the maximum ID is 81.79 inches, the difference produces a measurement

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 pipefittings and open branches. To ensure a well-developed flow profile, the straight-pipe section upstream of theclamp-on meters should be at least 30 pipe diameters in length. Since uncertainty increases if the cross-sectionalarea calculation depends on single measurement of pipe diameter, you should average two perpendicular

diameters.

Insertion ProbesInsertion probes, which measure fluid velocity at a point within a pipe''s cross-section, can check theperformance of an installed full-bore meter. An insertion flowmeter (Figure 3), measures the fluid velocity at apoint. It is unaware of surrounding flow velocities outside of the immediate location of the probe tip. The user or


12 13/11/2013 20:22


4/8

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

Figure 4. An insertion probe insertedone-quarter into the pipe having this flowvelocity profile will measure a fluid velocitythat is about 30% too high.

a secondary device must calculate the volumetric flowratebased on knowledge of the flow profile within the pipe. (Formore information on flow profiles, see Part III of this series -Flow Control, May 2007, page 14.) Measurement accuracyranges from 2 percent to 5 percent. This technique worksbest for a fully developed flow profile at the measuringlocation, usually achieved by installing the probe after a longlength of straight pipe. The proper straight length depends on

the nature of the upstream disturbances to the flow.

Attempting calibration in a location without a well-developedflow profile can lead to large errors. Figure 4 shows a fluidflow profile following an elbow fitting. An insertion probe tipsitting at a point one-quarter of the pipe diameter willmeasure a fluid velocity that is about 30 percent too high. Todevelop the flow profile, the engineer can make multiplevelocity measurements across the pipe''s diameter a

time-consuming operation.

Othersources ofinaccuracywithinsertionprobes

include: errors ininternalpipediameter,

cross-sectionalarea, andpipe

ovality; pulsatingandunstable

flows; varyingflowrates

between point measurements while determining profiles; errors and uncertainties in associated instrumentation; and

particulate material in the fluid.

Tracer MethodsTracer techniques for calibrating flowrates include the transit-time and the dilution methods. Attainable

measurement 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 measure thetime taken for the tracer to pass between two detection points (Figure 5). Since the volume of the pipe between

the detectors is known, they can determine the volumetric flowrate. Some disadvantages 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.


12 13/11/2013 20:22


5/8

Figure 5. Transit-time tracer calibrationsmeasure the time the injected tracer fluidpasses between two detectors.

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

For thedilutionmethod(Figure 6),engineersuse a tracerfluid that isdetectable

in low

concentrations and inject it into the flow at a known rate. They then sample the mainstream flow downstream ofthe injection point, far enough to allow homogeneous mixing. The downstream detector measures the tracer fluidconcentration. Since the tracer fluid flowrate q is comparatively small, they can derive the main flowrate 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, the

technique also requires many measurements and can be quite time consuming.

Hydraulic Model TestingFor some piping flow situations, engineers may find it difficultto calibrate the flow measurement system using either thedilution or volumetric tracer technique. For example, theymay be unable to reproduce the full operating flow range inthe system. In some cases, testing would potentially result in arelease of an unacceptable contaminant loading to theenvironment. (For example, test flowrates may be limited byseasonal downstream receiving water restrictions.) If the sitehandles only emergency overflows, testing may be ruled out

by water quality limitations.

For these situations, engineers may be able to construct ahydraulic model of the flow system and then run calibrationtests on the model under laboratory conditions. They woulddesign the hydraulic model based on the principle of hydraulicsimilitude. With this approach, the model represents a geometric reduction of the actual flow measurementsystem. The model is scaled down via a fixed ratio between the model and actual flow system for allhomogeneous lengths, velocities, and forces involved in motion. Engineers should pick a scale factor thatprovides model flows as close as practical to actual flows. Of course the model must be consistent with pumping

capacity available at the testing facility.

The hydraulic model should be constructed based on field-measured dimensions that are confirmed beforeconstruction. Common construction materials for a hydraulic model are wood and steel. In laboratory testingfacilities, flowrate through the model is usually determined by applying the volumetric tracer method.

Measurement accuracy ranges from 10 percent to 15 percent.

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 SeriesAnother way to verify the calibration of a flowmeter is to install two or more in a single pipeline (Figure 7). Inthis case, one meter verifies another. For example, one flowmeter may be used as the pay meter and the otheras a check meter. The pay meter serves for billing purposes and the check meter ensures that the pay meter isstill 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.


12 13/11/2013 20:22


6/8

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

Figure 8. The CalMaster verification systemfrom ABB Instrumentation compares key

magmeter parameters to those measured bythe factory at the time of meter manufacture.

To minimize discrepancies, the meter readings must be takenat the same time every reporting period. If possible, it is bestto record the inventory readings from both meterssimultaneously. The longer the reporting period, the smallerthe errors associated with recording the inventory readingswill 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 often notpractical.

Calibration Applied:CalMaster Magmeter Verification

In the case of electromagnetic flowmeters, ABBInstrumentation (www.abb.com) offers a verification systemthat can check calibration of ABB''s Magmaster flowmeterswithout access to the pipe or the sensing electrodes. Calledthe CalMaster system, it permits in-place verification andcertification of the magmeter to ensure that it remains withinits specified calibration.

When connected to a MagMaster transmitter and a personalcomputer (Figure 8), the portable CalMaster systemperforms a complex series of tests over the course of 20minutes. It compares key flowmeter parameters to thosemeasured by the factory at the time of meter manufacture.The tests evaluate the status of the complete system,including the sensor coils, electrodes, cables, and transmitter.The flowmeter will require servicing only when it fails the

calibration check.

The CalMaster system also serves as a diagnostic andcondition-monitoring tool. It automatically stores all the measured values and calibration information in its owndatabase files for each meter. It maintains a calibration history log, making it easy to undertake long-term trendanalysis. Trends can give early warning of possible system failure, enabling the maintenance engineer toanticipate problems and take remedial action in advance.

This is the fourth article in a five-part series on flowmeter technology. Part V will appear in the August issue.

Greg Livelli is a senior product manager for ABB Instrumentation, based in Warminster, Pa. He has more than15 years experience in the design and marketing of flowmetering equipment. Mr. Livelli earned an MBA fromRegis University and a bachelors degree in Mechanical Engineering from New Jersey Institute of Technology.

Mr. Livelli can be reached at [email protected]

215 674-6641.

www.abb.com

References1. Hwang, N.C. 1981. Fundamentals of Hydraulic Engineering Systems. Prentice-Hall Series in Environmental

Sciences.

2. Streeter, V.L., E.B. Wylie. 1985. Fluid Mechanics. Eighth Edition. McGraw-Hill.Email

Tweet 0 0Like 1 ShareShare 1


f 12 13/11/2013 20:22


7/8


8/8

Media KitJob ListingsDirectory ListingsAdvertising Contacts

MOREEditorial Submissions GuidelinesEditorial CalendarRequest Article Reprints

Contact UsPrivacy Policy

Facebook

Twitter

LinkedIn

YouTube

Copyright

2013 EBSCO Industries, Inc. All rights reserved. Grand View Media Group is a subsidiary of EBSCOIndustries, Inc..You may not download or reproduce any images on this site without written permission from GVMG admin foranything other than personal use.Design, CMS, Hosting & Web Development :: ePublishing.


verifying flowmeter accuracy.pdf

Documents