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Some ultrasonic flowmeters, calledhybrids, incorporate both transit timeand Doppler technology. Which tech-nology is used depends on the flow-stream. Hybrid meters are used to han-dle varying application conditions,including both clean and dirty liquids.

How It All BeganThe story of ultrasonic flowmeters

began in 1963, when Tokyo Keiki intro-duced them in Japan for industrial use.Since that time, Tokyo Keiki hasbecome Tokimec (www.tokimec.co.jp),and the company still offers ultrasonicflowmeters for sale. Tokimec is locat-ed in Tokyo, Japan. However, muchhas changed in the days since ultrason-ic flowmeters were first introduced.

Controlotron (www.controlotron.com)became the first U.S. manufacturer tomarket ultrasonic flowmeters in theUnited States in 1972. In the late 1970sand early 1980s, both Ultraflux(www.ultraflux.com) and Panametrics(www.panametrics.com) experimentedwith the use of ultrasonic flowmetersto measure gas flow. These were theearly days in the development ofultrasonic flow technology.

Initially, ultrasonic flowmeters weresometimes misapplied, and were notwell understood. This gave the metersa black eye in the minds of some peo-ple. Many technological improve-ments have been made in the past 10years, and the limitations of ultrason-ic meters are now better understood.Advances in transit time technologyhave broadened the types of liquidsthat transit time flowmeters can beapplied to. And even though Dopplerflowmeters still do not measure withthe same degree of accuracy as transittime, improvements have also beenmade in Doppler technology.

Advantages of Ultrasonic Ultrasonic flowmeters have specific

advantages over other types offlowmeters, including new-technolo-gy meters. Unlike Coriolis meters,ultrasonic flowmeters do very well inlarge pipe sizes. Over 85 percent ofCoriolis meters are sold for pipe sizes

of two inches or less. While Coriolismeters have successfully been used infour-inch and six-inch lines, theybecome unwieldy and quite expensivein those larger sizes. Size is actually anadvantage for ultrasonic flowmeters,since larger pipes provide the ultra-sonic signal more room to cross. Forpipes six inches and larger, ultrasonicflowmeters in most cases perform bet-ter than Coriolis meters.

Ultrasonic flowmeters have anadvantage over magnetic flowmetersin that ultrasonic meters can measurethe flow of nonconductive liquids,gases, and steam. Magnetic flowme-ters have very limited use in the oil &gas and refining industries becausepetroleum-based liquids are noncon-ductive. For the most part, magneticflowmeters cannot be used to meterhydraulic fluids, oil, or natural gas,flare gas, or process gas. They cannotbe used to measure steam flow. This isone of the most important reasonswhy the ultrasonic flowmeter marketis growing faster than the market formagnetic flowmeters. Magneticflowmeters are unable to participatein the fast-growing gas and steamflow measurement markets.

Ultrasonic flowmeters have anadvantage over vortex flowmeters inthat ultrasonic flowmeters are moresuccessful than vortex meters inmeasuring low flows. Vortex metershave a difficult time with low flows

becausevortices are not generated if theflow is particularly low. Some vortexflowmeters also have a difficult timeregistering zero flow. Ultrasonicmeters are also nonintrusive, with theexception of insertion meters. Whilevortex meters are not as intrusive asorifice plate meters, their bluff bodiescan get knocked out of position byimpurities in the flowstream.

Ultrasonic flowmeters have anadvantage over differential pressure(DP) flowmeters in that they are non-intrusive, with the exception of inser-tion meters. DP flowmeters require aprimary element to make the flowmeasurement. The intrusiveness ofDP flowmeters varies with the pri-mary element used. However, orificeplates cause permanent pressuredrop, and are very intrusive. Orificeplates can get knocked out of position,and DP flowmeters with orifice platesneed to be recalibrated periodically toensure that they are still making anaccurate measurement.

Ultrasonic flowmeters have severaladvantages over turbine meters. First,they are either less intrusive or non-intrusive, depending on the model.Also, they do not have moving partsthat are subject to wear. Turbineflowmeters use a spinning rotor thatis subject to wear over time. Whilesuppliers have improved the durabil-ity of turbine flowmeters, rotors are

32 February 2005 Flow Control

Ultrasonic flowmeters havegained a lot of attention overthe past five years, primarily

because of their ability for measuringcustody transfer of natural gas. Theyare replacing differential pressure(DP) and turbine meters in many nat-ural gas applications. But ultrasonicflowmeters are also widely used to

measure liquids. And they are not lim-ited to clean liquids either; a specialtype of ultrasonic flowmeter can alsoaccurately measure the flow of slur-ries and liquids with many impurities.

How They WorkUltrasonic flowmeters are one of the

most interesting types of meters used

to measure flow in pipes. The mostcommon variety, transit time, containboth a sending and a receiving trans-ducer. Both sending and receivingtransducers are mounted on eitherside of the flowmeter, or on the pipewall. The sending transducer sends anultrasonic signal at an angle from oneside of the pipe to the other and back.

Some ultrasonic flowmeters sendmore than one signal and have morethan one pair of transducers.

The flowmeter measures the time ittakes the ultrasonic signal to travelacross the pipe and how long it takesthe signal to travel back the other way.When the signal travels with the flow,it travels more quickly than it wouldin conditions of no flow. On the otherhand, when the signal travels againstthe flow, it slows down. The differencebetween the “transit times” of the twosignals is proportional to flowrate.

Transit time flowmeters work bestwith clean fluids. However, muchprogress has been made in adaptingtransit time flowmeters to fluids thatcontain some impurities. Doppler-based meters, for example, actuallyrequire impurities in the liquid tooperate properly.

Doppler flowmeters send an ultra-sonic signal across a pipe, but the sig-nal goes only part way. Instead ofsending the signal to the other side ofthe pipe, a Doppler flowmeter relieson having the signal reflected by par-ticles in the flowstream. These parti-cles travel at the same speed as theflow. As the signal is reflected, its fre-quency changes in proportion to theflowrate. The reflected signal is detect-ed by a receiver, which measures itsfrequency. The meter calculates flowby comparing the frequencies of thetransmitted and reflected signals.Doppler ultrasonic flowmeters areused with dirty liquids or slurries.They are not used to measure gas flow.

Ultrasonic vs. Other Meter Types

Type of Flowmeter Advantages of Ultrasonic

Coriolis Can meter flow in medium and largepipe sizes

Magnetic Can meter the flow of gas, steam, and non-conductive liquids

Vortex Can meter low flows; Are less intrusive

DP Either less intrusive or nonintrusive, depending on model; Little or no pressuredrop; Less need for recalibration

Turbine Either less intrusive or non-intrusive, depending on the model; No moving partsthat are subject to wear

ULTRASONIC FLOWMETERS

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flowmeters for measuring gas flow.Each supplier of multipath ultrasonicmeters has a different number of pathsin their ultrasonic meters. EmersonDaniel’s meters have four paths,Instromet’s (www.instromet.com) metershave five paths, and FMC MeasurementSolutions’ (www.fmcmeasurementsolutions.com) meters have six paths.

A different group of companiesmanufacture multipath ultrasonicflowmeters for liquid applications.Faure Herman (www.faureherman.com),based in France, has an 18-path ultra-sonic meter for liquid petroleummeasurement. So far, Faure Herman ismainly selling this meter in Europe.

Caldon (www.caldon.com) makes aneight-path ultrasonic flowmeter for liq-uid applications. Caldon is mainlyfocused on selling into the nuclearindustry. However, the company hasdeveloped an ultrasonic flowmeter thatis designed for use in the petroleumindustry. Krohne (www.krohne-mar.com)manufactures a five-path ultrasonic

flowmeter for liquid applications, andmore recently a three-path meter.

The jury is still out on the extent towhich having a greater number of pathsyields better performance. It is clear thatflowmeters with four, five, and sixpaths perform better than single-pathor dual-path flowmeters. What is notclear is where the upper bound of bene-fit lies, in terms of the number of paths.

More Growth AheadMuch of the rapid growth in the ultra-

sonic flowmeter market comes fromgrowth in the gas flow measurement mar-ket. This growth is due in part to industryapprovals given from groups such as theAmerican Gas Association (AGA) for theuse of ultrasonic flowmeters for custodytransfer. Now suppliers are obtainingapprovals from groups such as theAmerican Petroleum Institute (API) forthe use of ultrasonic flowmeters in cus-tody transfer of liquids. These approvalsare likely to boost the use of ultrasonicflowmeters for measuring liquid flows.

Ultrasonic flowmeters offer many paths tosuccessful flow measurement.

Jesse Yoder, Ph.D.,is a regular contributorto Flow Control maga-zine. He has been aleading analyst in theprocess control indus-try since 1986. Hespecializes in flowmeters and otherfield devices, including pressure, level,and temperature products. Dr. Yoderhas written 60 market research studiesin industrial automation and processcontrol and has published numerousjournal articles. His latest study, TheWorld Market for Gas FlowMeasurement, covers eight of the mostprominent flowmeter types in the gasmeasurement field. Currently, Dr. Yoderis the president of Flow Research Inc.He can be reached at jesse@flowresearch.com or 781 245-3200.

34 February 2005 Flow Control

still subject to wear over time.

Mounting TypesThere are several different

mounting types available forultrasonic flowmeters:

• Clamp-On• Spoolpiece• Insertion

Many people think of theclamp-on style when they thinkof ultrasonic meters. Clamp-onultrasonic flowmeters have transduc-ers that are mounted onto the outsideof a pipe. Some clamp-on meters arefixed and others are portable. Fixedclamp-on meters are also referred to asdedicated meters. One disadvantage ofclamp-on ultrasonic flowmeters is thatthe ultrasonic signal has to passthrough the pipe wall. This is an addi-tional factor to take into account whencomputing flow.

Instead of clamping onto a pipe,spoolpiece meters actually replace a

section of pipe when they areinstalled. Spoolpiece ultrasonicflowmeters include a pipe sectionwhere the transducers are mounted.Spoolpiece meters come in eitherflanged or wafer models. If the trans-ducers are in contact with the fluid,they are called “wetted.”

Insertion ultrasonic flowmeters areused to measure fluid flow in largepipes. Insertion meters are also called“hot tap” or “cold tap” meters.Insertion ultrasonic flowmeters are

used to measure flue gasand flare gas, as well asliquid flows in largepipes.

How Many Paths Does it Take?

The ultrasonic flowmetermarket is the fastest grow-ing of any flow technology.As a result, new demandsare regularly placed onultrasonic meters. Suppliers

are meeting these demands with inno-vations that improve product and per-formance.

In the area of gas flow measure-ment, Emerson Daniel (www.emersonprocess.com/daniel/) has redesigned itsfour-path ultrasonic flowmeter toextend the range of the meter. Theinnovation was done by changing theangle at which the four meter chordsintersect the pipe axis.

Emerson Daniel is one of the mainsuppliers worldwide of ultrasonic

Circle 23 or Request Info Instantly at www.FlowControlNetwork.com Circle 24 or Request Info Instantly at www.FlowControlNetwork.com

150

120

90

60

30

02003 2004 2005 2006 2007 2008

Total Shipments of Ultrasonic Gas Flowmeters Worldwide

(Millions of Dollars)

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