www.FlowControlNetwork.com July 2004 29

of gas, water, or other liquids, are con-sidered to be industrial meters. Theyare typically sold to the owners of theplant or building itself, rather than to autility company.

Natural gas plays an especiallyimportant role in the flowmeter indus-try. Natural gas is a vital source of fueland energy. Like coal and petroleum,it is a fossil fuel. Like air, natural gas isa mixture of gases. These includemethane, ethane, propane, butane,and other alkanes. Natural gas isfound in the ground, along with petro-leum. It is extracted and refined intofuels that supply a significant portionof the world’s energy.

Compressed natural gas is a sourceof fuel for alternative energy vehicles.Vehicles powered by natural gas arehighly efficient and emit low amountsof carbon monoxide, nitrogen oxide,and pollutants harmful to the ozonelayer. CNG-powered vehicles arefueled at filling stations that are verymuch like gasoline stations and sever-al companies have created flowmetersspecifically designed for these CNGfilling stations.

Positive Displacement Positive displacement flowmeters

are widely used for utility gas meas-urement. One main type is thediaphragm meter. Diaphragm metershave several diaphragms that capturethe fluid as it passes through the meter.Differential pressure across the metercauses one diaphragm to expand andone to contract. A rotating crank mech-anism helps produce a smooth flow ofgas through the meter. This mecha-nism is connected via gearing to theindex, which registers the amount offluid that passes through the meter.

Another type of PD meter for gasflow measurement is the rotary meter.Rotary flowmeters have one or morerotors that are used to trap the fluid.With each rotation of the rotors, a spe-cific amount of fluid is captured. Flowrate is proportional to the rotationalvelocity of the rotors. Rotary metersare used for both liquid and gasindustrial applications.

TurbineTurbine flowmeters have a rotor

that spins in proportion to flow rate.There are many types of turbinemeters, but many of those used for gasflow are called axial meters. Axial tur-bine meters have a rotor that revolvesaround the axis of flow. Most flowme-ters for oil measurement and for meas-uring industrial liquids and gases areaxial flowmeters. Axial meters differaccording to the number of blades andthe shape of the rotors. Axial metersfor liquids have a different designfrom axial meters for gas applications.Like PD meters, turbine meters areused as a billing meter to measure theamount of gas used at commercialbuildings and industrial plants.

One main difference between tur-bine and positive displacementflowmeters is that turbine meterscompute flow based on a velocitymeasurement, while positive dis-placement meters actually capturethe flow and measure it volumetri-cally. Turbine meters excel at meas-uring clean, steady, medium tohigh-speed flow of low-viscosityfluids. Positive displacementmeters, by contrast, excel at measur-ing low-speed flows and high-vis-cosity fluids. While there are someapplications in which they compete,turbine and positive displacementflowmeters are more complementa-ry than competing.

Positive displacement meters forgas applications also face some com-petition from turbine flowmeters.However, PD meters are mainly usedfor the smaller pipe sizes, and mostPD meters for commercial and indus-trial gas applications have sizessomewhere between 11⁄2 inches and 10inches. Turbine meters, by contrast,perform best with steady, high-vol-ume flows. For this reason, turbinemeters are more likely to be used forpipe sizes above 10 inches. While tur-bine and PD meters overlap in thefour-to-10 inch size range, PD metersstill have an advantage in the lowersizes. Low flow rates are not an obsta-cle for PD meters.

UltrasonicThe use of ultrasonic flowme-

ters to measure natural gas flowgained momentum following thepublication of AGA-9 in June 1998.The report, issued by the AmericanGas Association (www.aga.org), out-lines criteria for using ultrasonicflowmeters for custody transfer ofnatural gas. Prior to the release ofAGA-9, the association had previous-ly issued reports on differential pres-sure flowmeters (AGA-3) and turbineflowmeters (AGA-7). Since the publi-cation of AGA-9, the AGA has alsoissued a report on the use of Coriolisflowmeters (AGA-11).

The use of ultrasonic flowmeters iscontinuing to grow, both for custodytransfer and process gas measurement.Unlike PD and turbine meters, ultra-sonic flowmeters do not have movingparts. And pressure drop is muchreduced with an ultrasonic meter whencompared to PD, turbine, and DPmeters. Installation of ultrasonic metersis relatively straightforward, and main-tenance requirements are low.

Ultrasonic flowmeters come as bothinline and clamp-on configurations.Some have one ultrasonic beam, whilethose with higher accuracy use multiplebeams. These are known as multipathultrasonic flowmeters. Meters used forcustody transfer purposes are inlinemultipath meters. Most of these cus-tody transfer meters use four, five, or sixpaths, depending on manufacturer, tomake a highly accurate measurement.Manufacturers include Instromet(www.instromet.com), Emerson Daniel(www.emersonprocess.com/daniel/), andFMC Measurement Solutions(www.fmcmeasurementsolutions.com).

Differential Pressure DP flowmeters consist of a dif-

ferential pressure transmitter and aprimary element. The primary ele-ment places a constriction in the flowstream, and the DP transmitter meas-ures the difference in pressureupstream and downstream of the con-striction. The transmitter or a flowcomputer then computes flow, using

28 July 2004 Flow Control

Matter exists in three differentstates: solid, liquid, and gas.Flow measurement is prima-

rily concerned with measuring theflow of liquids and gases. Gas, whichis the primary focus of this article,takes many forms. Types of gasinclude natural gas, fuel gas, atmos-pheric gas, compressed natural gas,and many others. Individual gasesthat are especially important includehydrogen, oxygen, nitrogen, and car-bon dioxide. Air is a gas, or, more pre-cisely, a mixture of gases, includingnitrogen, oxygen, argon, etc. Steam,

which is water vapor, can also be con-sidered a form of gas.

Gas flow measurement can be divid-ed into three broad categories: indus-trial, commercial, and residential.Industrial gas flow measurementincludes flow measurement thatoccurs in manufacturing and processplants, including chemical plants andrefineries. Commercial gas flow meas-urement occurs at businesses and com-mercial buildings such as restaurants,office buildings, and apartment com-plexes. This is a form of utility meas-urement, since these flowmeters typi-

cally measure the amount of naturalgas used by the business or in the com-mercial building. Residential gas flowmeasurement refers to flowmeters thatmeasure the amount of gas used atindividual homes and apartments.

Utility meters are meters used tomeasure gas or water entering a build-ing or plant for the purpose of billingthe plant for its use of gas or water.These meters are typically sold to a gasor water utility and installed at thebuilding or plant by the utility compa-ny. Meters used within a building orplant for internal allocation purposes

By Jesse Yoder

www.FlowControlNetwork.com July 2004 31

Are you maximizing operating efficiencies? Are youwasting energy heating a building with all thedoors open? Are you adjusting for peak usage?

Are you providing your gas broker with an accurateassessment for your next gas allocation? With gasprices rising as fast and as high as they are, these are allvery important, top-of-mind questions to ask.

Under current market conditions, the accurate mon-itoring of gas flow rate and consumption should behigh on the industrial priority list. No longer is it suffi-cient to depend solely on the billing meter, as mostfacilities demand tighter control of their gas usage. Assuch, it may be time to consider placing NIST-trace-able thermal mass flowmeters at both the billing meterand the entry of the plant, thus providing the capabili-ty to track gas usage within your facility each minute,each hour, and each day.

In addition, to get an even better handle on plantefficiency, thermal mass flowmeters can be configuredto monitor the gas lines going into different buildinglocations or departments. By submetering, you canassess departmental inefficiencies, assign costs to dif-ferent operating areas, and institute conservationmeasures as appropriate.

Taking it one step farther, you can optimize yourcombustion process by monitoring the natural gas (orbackup fuel of propane) into a furnace or burner (alongwith the air or oxygen line). Proper control of the air/fuelratio can improve efficiency, lower fuel consumption,improve product quality, and increase product yields.

Thermal mass flowmeters are a good fit for measur-ing natural gas at the plant for a number of reasons.They are affordable (typically $1,500 to $3,000), andthey measure mass flow directly (SCFM, SCFH,LBS/Min, etc.) without the need for temperature andpressure corrections. This eliminates ancillary equip-ment, such as temperature and pressure gauges andtheir associated wiring and installation. Further, ther-mal mass flowmeters are rangeable over a 100-to-1turndown (i.e., their operating range is from 1 percentof specified full scale to 100 percent of full scale).Orifice plates and venturis, on the other hand, general-ly have a turndown in the range of 3-to-1 to 5-to-1.Turbine meters and vortex meters typically have a 10-to-1 turndown.

The wide turndown offered by thermal mass metersis useful in natural gas applications due to the largeswings in demand throughout the day, especially atoff-hours when production is at its lowest capacity.Also, seasonal variations can be quite severe. On burn-er applications, the low-end sensitivity inherent in athermal mass flowmeter is particularly pronounced. Infact, many manufacturers can resolve flow rates as lowas 0.1 of an SCFM in a one-inch pipe (approximately 15SFPM).

Thermal mass meters also have negligible pressuredrop, which is particularly important in conserving natu-ral gas usage. With a linear 4-20 mA output (proportion-

al to mass flow of the gas), ther-mal mass meters are suitable fora DCS, PLC, or Data Logger.

Also, the meters have no mov-ing parts, are dirt insensitive (andcleanable if necessary), and havehigh accuracy and repeatabilitythat is typically 0.25 percent.Furthermore they are easy toinstall, as all manufacturers offerboth in-line styles with NPT fit-tings or flanges for the smallerlines and insertion styles formedium-to-large lines. The inser-tion styles can be installed with ahalf coupling and some mounting hardware, such as a compres-sion fitting or valve assembly. Most manufacturers offer both inte-gral and remote styles in either general-purpose enclosures (typi-cally NEMA 4X) or explosion-proof enclosures (Class I, Div 1,Groups B, C, and D).

Fundamentally, thermal mass flow meters operate on the princi-pal of heat transfer. In the case of the “constant temperaturemethod,” there are always two reference grade platinum sensors.One of the sensors is self-heated and the other is the reference andmeasures the gas temperature. As gas flows by the heated sensor,the gas molecules carry heat away from the surface of the sensorand the sensor cools down as it loses energy. The sensor drive cir-cuit replenishes the lost energy by heating the flow sensor up untilit is a constant temperature differential above the reference sensor.The electrical power required to maintain constant temperature dif-ferential is directly proportional to the gas mass flow rate.

Because of this methodology, thermal mass flowmeters pro-vide an affordable means to monitor natural gas at the plant, atthe department, or at the burner. As a result, users have anopportunity to realize improved operating efficiencies, promoteconservation, and monitor and improve plant processes.

About the AuthorBob Steinberg is the president of Sage Metering Inc., a manu-

facturer of thermal gas mass flowmeters for industrial and munic-ipal applications. Mr. Steinberg has started three successfulenterprises and has over 20 years of management, sales, andmarketing experience in the process industry. In addition, he hashad extensive experience managing, supporting and trainingindustrial rep organizations, specifically in the thermal massflowmeter industry. Mr. Steinberg can be reached atbob@sagemetering.com or 866 677-7243.

30 July 2004 Flow Control

Bernoulli’s theorem.Types of primary elements include

orifice plates, venturis, flow nozzles,pitot tubes, wedges, and others.Venturis are especially suited to high-speed flows. Orifice plates are still themost widely used type of primary ele-ments. However, they can be disad-vantageous because they can generatea pressure drop and can be knockedout of position by impurities in theflow stream. Orifice plates are alsosubject to wear over time.

DP flowmeters are used to meas-ure the flow of liquid, gas, andsteam. Like ultrasonic and turbinemeters, they are used for the custodytransfer of natural gas. In manycases, end-users buy their pressuretransmitters and primary elementsfrom different suppliers. However,several vendors have integrated thepressure transmitter with the pri-mary element to form a completeflowmeter that can be calibratedwith the primary element and DPtransmitter already in place.

Coriolis Coriolis flowmeters are known

for their accuracy. However, Coriolismeters are primarily used for liquidflow measurement because gases areless dense than liquids and are some-what more difficult to measure. Still, anumber of suppliers have developedCoriolis meters for gas flow measure-ment, and this is a growing focus area.One application that Coriolis hascome to excel in is in measuring com-pressed natural gas for alternativefuel vehicles. Here they compete pri-marily with turbine flowmeters.

ThermalThermal flowmeters are used

almost exclusively to measure gas flow.Thermal flowmeters typically injectheat into the flow stream and thenmeasure how quickly it dissipates. Thisvalue is proportional to mass flow. Twomethods used are called constant cur-rent and constant temperature.

Thermal flowmeters grew out of theuse of hot-wire anemometers for

research applications. Early compa-nies to develop thermal flowmetersinclude Sierra Instruments (www.sier-rainstruments.com), Kurz Instruments(www.kurz-instruments.com), and FluidComponents International (www.fluid-components.com). Thermal flowmetersexcel at measuring gas at low flowrates. Measuring low flows is a diffi-cult proposition for some meters,including vortex, which makes ther-mal meters a hot commodity for gasmeasurement. Accuracy levels areimproving for thermal flowmeters aswell, as suppliers introduce productimprovements.

One application where thermalflowmeters are widely used is in themeasurement of stack flows. Gas flowhas to be measured in smoke stacks toconform to Environmental ProtectionAgency reporting requirements.Insertion thermal flowmeters are usedto measure the flow of sulfur dioxide(SO2), nitrogen oxide (NOx), andother industrial pollutants. Because ofthe large size of the stacks, insertion

By Bob Steinberg

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When installing an insertion-type thermal flowme-ter, establish the height of the mounting hardwareand depth to the point of average velocity as per theprobe insertion guidelines.

www.sierrainstruments.com

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thermal meters that use multiplemeasuring points are a good fit forthese applications. DP flowmeterswith averaging pitot tubes, and ultra-sonic flowmeters are also commonlyused for smoke stack applications.

Mass Flow ControllersMass flow controllers not only

measure flow; they also control it.They differ from thermal flowmetersin that most divert a small portion ofthe flow into a parallel channel, andthen measure the flow through thatparallel channel. The flowmeter thenperforms a calculation to determinemass flow through the entire flowme-ter. Most also contain an integratedvalve that is used to control flow. Asetpoint is determined, usually by theuser, and the valve is adjusted so thatflow reaches that setpoint.

Most mass flow controllers use ther-mal methods to determine flow rate inthe parallel flow path, though some use adifferential pressure principle. And some

mass flow controllers are sold withoutthe valve, meaning they are functioningas flowmeters rather than controllers.Mass flow controllers are widely used inthe semiconductor industry, but manyhave industrial applications. It is impor-tant for a mass flow controller to knowwhat gas is being measured in order toensure an accurate measurement. Massflow controllers can also be used to meas-ure liquid flow.

Other TypesVortex and variable area flowme-

ters are also used to measure gas flow.Vortex flowmeters are one of the fewtypes of meters, besides DP, that canaccurately measure the flow of liquid,steam, and gas. However, vortex metersespecially excel at measuring steam flow,since they can handle high temperatures.

Variable areas meters can measurethe flow of both liquid and gas, andthey also are used for a limited amountof steam flow measurement. Variable-area meters rely on a float that rises in

proportion to flow rate. They are pri-marily a low-cost alternative where avisual indication of flow is sufficient.While most still must be read visually,some variable-area meters have beenmanufactured with transmitters.

With eight types of flowmeters measur-ing gas flow, there is no shortage of tech-nology available. Some applications, suchas custody transfer of natural gas, CNGmeasurement, and stack flow measure-ment, have become the focal point of com-petition among the meter types.

Jesse Yoder, Ph.D., is a regu-lar contributor to Flow Control.The president of Flow ResearchInc., Dr. Yoder has been a leadinganalyst in the process controlindustry since 1986. He has writ-ten over 60 market studies onindustrial automation andprocess control and has published numerous journalarticles. Dr. Yoder’s latest study, The World Market for Gas Flow Measurement, covers eight of the most prominent flowmeter types in the gas measure-ment field. Dr. Yoder can be reached at jesse@flowresearch.com or 781 245-3200.

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