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WITNESSING ORIFICE METER CALIBRATIONS AND FIELD TESTINGClass # 4160

Allen Chandler Training and Development Coordinator 

Atmos Energy14032 Distribution Way

Farmers Branch, Texas, USA, 75234

Introduction

The need for witnessing gas measurement equipment testing has probably been around sincethe dawn of the gas “custody transfer” age. That is, where the gas is physically changingownership from one entity to another. In the modern age, the old handshake is no longer theequivalent of a valid contract as it once was and rightfully so, each producer, transmissionpipeline operator, distribution system owner or transportation broker is concerned that their product is bought and/or sold in as accurate an environment as is humanly possible.

Modern day gas prices are volatile to say the least. The market bounces around on an almostdaily basis and the price per thousand standard cubic foot of gas seems to steadily increase.

Coupled with generally milder temperatures in many regions of the United States and the hugegas volumes that change hands daily, it becomes ever more critical that natural gas is measuredwith greater accuracy and a witness to the testing procedure is one more positive step towardthat end.

The Witness’s Role

The witness’s role is more important than ever to ensure that contractual obligations are met. Allparties to the contract must be satisfied that the best effort is being exercised in achieving qualitymeasurement standards. Electronic flow computers and other highly technical devices are nowbeing utilized to achieve greater accuracies and yet the human factor is and always will be acontributing input to the process.

The witness of a field calibration test is exactly what the word implies; He or she bears witness tothe test. They may be considered an auditor or “onlooker” to ensure that the testing equipmentused and procedures performed are of the highest standards satisfying the demands of all partiesinvolved including assurance that accepted gas measurement standards are met.

The Mark of a Good Witness

The person chosen to function as a witness to a testing procedure should have an extensiveknowledge and background in the gas measurement field and possess personal experience atperforming testing procedures. They should be familiar with their individual company standardsfor gas measurement, following their company’s respective Operation and Safety PracticesManuals. They should be familiar with and understand the guidelines set forth by the AmericanGas Association, Report 3 and API 2530, which are the industry accepted standards for quality

gas measurement practices.

Further, a witness should understand that they are on site to witness the testing procedure as an“agent” representing their company, keeping their company’s best interests in mind. They shouldexhibit a professional and businesslike attitude while being friendly and cordial. They should beprepared to be diplomatic and sometimes assertive when disagreements arise during a test. Agood witness should know that gas measurement is essentially the “cash register” of thecompany and errors can have a huge impact on the bottom line profits of the company, thusimpacting virtually every aspect of the company, even to their own paycheck.

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Testing Equipment

There are numerous choices in today’s market for testing equipment and about as many opinionsassociated with which ones to purchase. There are various opinions throughout the industryabout which test equipment is the best to use. One guideline that may be helpful is the A.G.A.reference that states that any test equipment should have twice the stated accuracy as theinstrument it is being utilized to test.

Example: If a transducer has a stated accuracy of .1%, the test device used to test it should haveat least a stated accuracy of .05%. A typical rule is the higher the accuracy, the higher the cost.For that reason, testing equipment can be very costly considering the higher stated accuracies inthe modern market. The type of equipment is not specified by the A.G.A. Therefore a decisionmay be made between pneumatic/hydraulics and an electronic equivalent as long as theaccuracy is adequate and meet N.A.S.T. Standards.

Generally, some testing equipment types are considered to be “primary standards”. That meansthat a certain type is utilized as a reference for other equipment to relate to. An example of aprimary standard could be a dead weight tester in which the laws of physics are in effect andcannot be violated if maintained and used properly. Therefore, other types of equipment such aselectronic pressure testers may be acceptable as long as they have the ability to mimic or 

represent the physical quality of the dead weight tester and have a proven acceptable accuracy.Since electronic devices are subject to deviate from physical laws due to internal circuitryproblems, internal voltages, or in some cases, external radio frequency distortion, manycompanies opt for the primary standard devices for testing purposes. However, new generationelectronic testers have addressed most of these anomalies and are very reliable.

A typical set of test equipment for electronic measurement should include the following:

Adequate hand tools to set up and install the testing equipment A liquid soap or other substance to check for leakage A certified volt/amp meter  A dead weight tester, approved deadweight gauge, or digital electronic equivalent for the

static pressure test

A dead weight or comparable electronic device for differential pressure testing in inchesof water column

A pressure supply source when applicable (typically nitrogen) A caliper and/or micrometer (digital micrometers are acceptable) A straight edge for checking orifice plate flatness A mercury or electronic digital thermometer  A water content testing device Gas sampling cylinder  Laptop computer with required software(s)

Digital Caliper 

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 The Testing Schedule

Most contractual agreements provide for how the test schedule is to be performed; monthly,quarterly or some other frequency. The frequency of testing is usually dependent on the companyrepresentative who owns the “custody” meter. They are responsible for setting the testingschedule and notifying the other parties to the contract when a test is scheduled to be performed.The test schedule should be flexible enough to allow for last minute alterations if one or moreparties cannot attend as witnesses. In the event that a witness cannot be present but tells thetester to proceed with the test, the tester should send the comprehensive test results in a reportto the absent party after the fact. The report should state any and all results including anychanges that were incurred during the testing procedure.

If a measuring device or recorder is found to have problems, a follow up telephone call to theabsentee witness may be necessary to discuss the how the corrections are to be applied. Thecontractual agreement will usually state very specifically how adjustments are to be handled for auditing purposes. If a problem is found it is not unusual to make a volume adjustment to one half of the elapsed time period since the last test was performed.

The report should clearly state:

The name, location, and station number  The steps taken to perform the verification or calibration “As found” and ”as left” conditions If a gas sample was collected, and if a gas chromatograph is present for live inputs to

the flow computer, how it was verified to be correct. Many companies use a gas sampleto verify the chromatograph. That data has to be followed up since the sample is takento a laboratory to be run. A chromatograph analysis must be run at the time the sampleis gathered.

The results of a comprehensive inspection of the primary element (meter tube and orificeplate) and the recording device (secondary element).

Note: The gas volume is only as accurate as the primary element regardless of the accuracyof the secondary element.

The gas volume before and after the test was performed Any additional remarks pertinent to the condition of the station

Recorder Verification/Calibration

Although it may sound rhetorical, there is a distinct difference between a “verification” procedureand a “calibration” procedure, particularly for a flow computer. The tester should check the staticpressure up and down the full range of the device as well as the differential verification of the“zero” value while both under operating pressure and at atmospheric pressure. Depending on thecontract, the static pressure zero may be either set on “psia” or “psig” and the witness should beaware of the difference and the implications. The witness should be alert to any “as found”conditions which may reveal problems with the device and be knowledgeable of the acceptableparameters for which the device can be in error. These problems must be recorded before any

attempt is made to correct them; else a meaningful quantitative adjustment is lost.

If the flow computer passes the verification procedure, a calibration should not be arbitrarilyperformed. Field calibrating a flow computer re-characterizes the transducers and should only bedone when an error is found. The test procedure may be considered completed if the verificationproves the instrument to be correct in the “as found” condition.

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Meter Tube and Orifice Plate Inspection

The meter tube and orifice plate are the “heart” of quality orifice measurement and any errors thatexist are transmitted and usually multiplied to the secondary element. The meter tube shouldremain clean and free of liquids and other debris that can alter the flow pattern. Although it is notfeasible to internally inspect the meter tube at each testing interval, periodic inspections shouldbe performed. When the meter tube is inspected the witness should be on site to verifymicrometer readings and any other conditions which need to be noted. Some companiesschedule routine meter tube inspections and cleaning on an ongoing basis. Internal observationsmay be done by disassembling the meter tube or by the use of a bore scope which allows internalinspection without the disassembly of the meter tube. In either case, the witness should be on sitewhen the procedure is performed and know what to look for.

Included in the meter tube inspection is a full inspection of the orifice fitting, its operationalcondition and verification as to what type of fitting is installed. Dual chambered orifice fittingsshould be inspected to be sure that all shafts are in good working order and the isolation device isin good working order. The witness should be familiar with the internal parts and their function.

At each testing interval, the orifice plate should be removed and inspected for: Dirt, grease, any other build up Orifice bore direction if beveled Bore diameter using a round-tipped micrometer  Verification of the bore diameter with the stated record, the stamped size on the plate,

and the bore diameter entered in the flow computer program Bore condition, looking for nicks, gouges, dullness, or sharpness Flatness Thickness Eccentricity Seal ring condition

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Typical Orifice Plates Seal Ring

Inspection Report

The inspection test report should be considered a mini contract in and of itself. Once the witnesssigns the report, it finalizes all of the data on the report to be correct and acceptable and any

follow up disputes could be very difficult to argue once the signature is in place. Following thecompletion of the inspection, the witness should verify that all information is correct and properlyrecorded on the test document before signing. The witness and all other parties present shouldreceive a copy of the report before leaving the site.

Conclusion

As natural gas changes hands in a volatile and high priced market, errors in gas measurementtranslate to possible huge revenue losses for either the buyer or seller. These losses impact thebottom line success or failure of any gas company.

The witness serves a very important role and is a key component in the process to ensure thatthe gas measurement devices are in the best possible working order and meet industry standards

and contractual agreements by all parties.

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