By Michael J. Scelzo

Figure 1This two-channel configuration measures gas

flow on two separate pipes.

Field testing anew technology.

The DigitalFlow™ GC868 Clamp-On Ultra-sonic Flowmeter for Gas discussed in this ar-ticle is a winner in this year’s Flow Controlmagazine’s Innovation Awards. This is thefirst in a look behind the technology of thewinning products.

Behind the scenes:

A Clamp-On UltrasonicFlowmeter for Gases

A Clamp-On UltrasonicFlowmeter for Gases

A Clamp-On UltrasonicFlowmeter for Gases

Despite the industrial success of clamp-on ultrasonicflowmeters for liquid measurement, it has long beenaccepted as if it were a fundamental limit imposed bynature, that this technology could not be used to

measure the flow of gases in metal pipes.The incorrect conclusion, that clamp-on gas flow metering is

impossible, developed because the acoustic impedance of gas-es, even at pressure, is much less than the acoustic impedanceof metals. This fact, reinforced with the knowledge that attenu-ation in most gases is much greater than in common liquids at

the same frequency, led to a belief that virtually no usefulamount of ultrasonic energy can be transmitted from the pipeinto the gas, and then back out of the gas through the metal pipeand into the receiving transducer. Along with this assessment ofwhat was thought to be a negligible amount of sound intensitysurviving to the receiver, is the further problem of a relativelystrong coherent interfering wave traveling through the pipewall into the receiver.

Together, these factors led many to conclude that the signal-to-noise ratio would be so low as to be useless, especially if it is

tion was long enough to mount the trans-ducers and clamp, but the amount ofstraight pipe run was much less thanneeded for the flow profile to developfully for best measurement accuracy.Once installed and adjusted for super-compressibility at a pipeline pressure of1,300 psi, however, the clamp-on flowme-ter closely tracked the field meter readingof approximately 34 MMSCF/D. Later, theclamp-on meter’s stability was moni-tored for one-and-one-half hours, andthen the flow rate was varied to test theclamp-on meter’s response. In all tests,the clamp-on meter closely tracked thefield orifice meter (see Figure 3).

At another major petrochemical pro-duction facility in the southern UnitedStates, the clamp-on ultrasonic flowme-ter was tested from November 2000 toFebruary 2001 on a 6-inch, carbon steelnitrogen pipeline at 400 psig, with a flowrange up to 5,300 standard cubic feet perminute (see Figure 4). Flow rate data wascompared with data from an orifice me-ter located downstream. Over the dura-tion of the test, the average correlation ofthe ultrasonic meter with the orifice me-ter was better than ±2 percent of reading.

Calibrating the MeterTo determine the accuracy of the de-

vice compared to that of a reference me-ter, a calibration of two DigitalFlowGC868 flowmeters was scheduled at theGTI Metering Research Facility at theSouthwest Research Institute (SwRI) forthe week of May 21, 2001. The two me-

erlands. These initial field tests were per-formed on air flowing in a large numberof steel pipes ranging in size from 3-inchto 8-inch at pressure from 6 to 12 bar. Thecustomer needed a measurement surveyto determine compressed air usagethroughout the facility. Nearly 100 meas-urements were successfully made in totalon different pipes.

This early trial provided results toshow the viability of the technology inan uncontrolled field environment. Thefacility was rather old, and the pipeswere in relatively poor condition and atlower than expected pressure. Despitethe difficult conditions, the prototypemeter worked as desired, and an estima-tion of the accuracy of the meter wasabove expectations and well within thecustomer’s requirements.

Another test was performed in Novem-ber 2000 on a natural gas transmissionpipeline at a customer location in thewestern United States (see Figure 2). Thesour natural gas in this case, containing12 percent H2S, was corrosive enough to re-quire an Inconel liner inside the 10-inch,Schedule 40 carbon steel pipeline. TheInconel liner posed a problem because itwas not mechanically bonded to the car-bon steel pipe wall. The interface be-tween the carbon steel pipe and theInconel liner was too acoustically reflec-tive to pass the ultrasonic signals through.

Located near a well head were an ex-pansion loop and a short section of solid,straight 6-inch Inconel pipe, approximate-ly 12 inches long. The straight pipe sec-

understood that for a transit-timeflowmeter to succeed, not only must thesignal be detectable, it must be timed to afew nanoseconds or a few microseconds,in order to compute the flow velocity tohigh accuracy.

Even if it is correctly recognized thatsince the signal-to-noise ratio, while verysmall, is nevertheless greater than zero, itis another matter to figure out how to de-tect the gas borne signal and time its ar-rival accurately in both upstream anddownstream directions.

Existing technical methods could notovercome these obstacles. A multiyearprogram to develop a clamp-on ultrason-ic flowmeter that could be used to meas-ure gas flows at high and low pressure inmetal pipes resulted in the PanametricsDigitalFlow™ GC868 flowmeter. The so-lutions to the above mentioned problemsinvolved developing new patent-pendinghardware and software (preventing atthis time, unfortunately, a detailed dis-cussion of how it works).

The new flowmeter system (see Figure1) consists of an electronics console,clamp-on ultrasonic transducers, con-necting cables, and a clamping fixture tohold the transducers solidly in place onthe pipe. A couplant is applied to thetransducer faces to couple them acousti-cally to the outside pipe wall.

Field TestingA prototype flowmeter was built and

tested in August 2000 at a major petro-chemical production facility in the Neth-

Figure 2A short section of solid, straight 6-inch Inconel pipe approxi-

mately 12 inches long was found on this natural gas transmis-sion pipeline to provide a site for the transducer installation.

Figure 3The clamp-on meter’s tracking compared to that of a field

orifice meter in a natural gas pipeline.

252FC, Panametrics, from which this pa-per is excerpted.

About the AuthorMichael J. Scelzo received his B.S. ChE

from WPI in 1969, and his M.B.A. from theSouthern New Hampshire University in1988. At Panametrics since 1973, he workedin the applications engineering, sales andmarketing departments of the PCI Division.He is currently the Vice President ofMarketing for the PCI Division.

of volumetric flow reading for two-pathinstallations, with a time averaged accu-racy of ±1 percent of reading. The re-peatability specification is ±0.2 to ±0.5percent of reading. Minimum requiredgas pressure depends on pipe size andfluid density. For example, 90 psig is theminimum pressure requirement for in-strument air in a 6-inch carbon steelpipeline. The meter will operate on un-lined pipes of all metal and plastic mate-rials, from 3- to 36-inches in diameter.The temperature range for the transduc-ers is –40°to 300°F. Tests are now under-way at up to 450°F.

AcknowledgmentsThe work reported here includes im-

portant contributions from Dr. ShirleyAo, Dr. Scott Li, Lawrence Lynnworthand Peter Kucmas. For more details, seewww.panametrics.com, PCI or PCI R&Dpages, Ultrasonic Report UR-252 or UR-

ters were calibrated simultaneously on 8-inch and 12-inch, Schedule 40 pipe sec-tions, both containing natural gas. Theinstallation point of the meter on the 8-inch pipe section was 40 feet down-stream of an elbow and a reducer. The in-stallation point of the meter on the12-inch section was 10 feet upstream ofan elbow. The pressure and temperaturetaps were another 30 feet downstream.To establish thermal equilibrium, naturalgas was allowed to flow through thepipeline calibration section for about 30minutes before data collection began.

Two calibration runs were performedon May 24, one in the morning at 500psig and one in the afternoon at 700 psig.As neither meter had any initial calibra-tion prior to arrival at the site, the firstcalibration run was used to determinethe meter factor. Data for the second runwas obtained at the GTI MeteringResearch Facility at SwRI, and is present-ed in Figures 5 and 6. The range of flowwas 50 to 1,380 actual cubic feet perminute. The data shows the correlationbetween the DigitalFlow GC868 flowme-ters and the reference meter.

Performance SpecificationsPerformance specifications have been

derived from data obtained during betatesting and calibration runs. The statedaccuracy specification of the meter is:above 5.5 feet per second velocity, ±2percent of volumetric flow reading forsingle-path installations and ±1.4 percent

Figure 4This test at a petrochemical produc-

tion facility was on a 6-inch, carbon steelnitrogen pipeline at 400 psig with a flowrange up to 5,300 SCFM.

Figure 5: SwRI Calibration on a 12-Inch Natural Gas Pipeline @720 psig on 5/24/01(V~=21ft/s, 12 ft/s, 5 ft/s, 2.4 ft/s, 1.25 ft/s, 29 ft/s and 7 ft/s)

Figure 6: SwRI Calibration on Natural Gas @720 psig T&P probes 30 ft downstream

TimeGC868 on 8-in. pipe — Wetted reference

Volum

etric

(acf/

min)

Reprinted by permission of the publisher from the September2001 issue of Flow Control. For subscription information call908-788-0343, ext. 132 or visit www.witterpublishing.com.

©2001 Witter Publishing.

Until now, clamp-on flow metering was limited to liquids. Many thought it was impossible for gases. Existing methods couldn’t be used, so through extensive research...

PANAMETRICS DEVELOPED NEW, ADVANCEDTECHNOLOGY TO CONQUER THE OBSTACLES.Use the new DigitalFlow GC868 Flowmeter for air, hydrogen or natural gas at high or low pressure in pipes of nearly any material including metal. Use it for erosive, corrosive, toxic, highpurity or sterile gases, or anywhere there is a reason not to break the pipe wall boundary.

ACCURACY OF THE DIGITALFLOW GC868 IS SUPERB AT BETTER THAN ±2% OF READING AND±1% OF READING WITH TIME AVERAGING. REPEATABILITY IS ±0.5%.All the benefits of clamp-on ultrasonic flow metering are now available for gas measurement. The DigitalFlow GC868 Flowmeter significantly reduces installation costs with no pipe wall penetration or wetted parts. It causes no pressure drop and requires no regular maintenance.

ONCE AGAIN, PANAMETRICS’ ADVANCED TECHNOLOGY MEANS MORE PERFORMANCE AND VALUE.

LEADING INNOVATION. ADVANCED TECHNOLOGY. WORLDWIDE SERVICE.

221 Crescent Street, Suite 1, Waltham, Massachusetts 02453-3497

Toll free: 800-833-9438 • Fax: 781-894-8582 • www.panametrics.com

ISO 9001CERTIFIED

DigitalFlow™ is a trademark of Panametrics, Inc., USA. © 2001.

EXCITING NEW FLOWMETER TECHNOLOGY BREAKTHROUGH

FROM PANAMETRICSINTRODUCING THE DIGITALFLOW TM GC868

THE WORLD’S FIRSTCLAMP-ON ULTRASONICFLOWMETER FOR GASES.