technical resource library from cole-parmer.pdf

Back

Technical Resources

Post Date:9/25/2009

Entry type:Articles and White PapersProduct Selection Guide

Topics:Energy, Fluid Handling,Food and Beverage,Industrial Process,Laboratory/Research,Natural Resources,Pharmaceutical, Waterand Wastewater,Chemical Process,Semiconductors andElectronics, Energy,Utilities, IndustrialManufacturing,Transportation, FluidHandling

Tags: Flowmeters,Flowmeters

Related/Recommended

Complete selection ofFlowmeters

flowmeter

Flowmeter Applications

Flowmeter FAQs

Gear flow meters

High ViscosityFlowmeters: Solution to aSticky Problem

How VolumetricFlowmeters Work

Installing YourPaddle-Wheel FlowSensor

Magnetic flowmeters

Mass flow controllers

Selecting the RightFlowmeter—Part 2

Turbine flowmeter

United Arab Emirates02-666 1331 or Live Chat

Welcome,log in or register

Search

Shop All Products Shop by | Service & Support | Technical Resources |

Related Searches:Scientific Instruments | Scientific Laboratory Equipment | Laboratory Supplies Medical Laboratory EquipmentSearch

Technical Resource Library from Cole-Parmer http://www.coleparmer.com/TechLibraryArticle/667

1 of 11 12/02/2013 9:57

Share

Selecting the Right Flowmeter—Part 1

By Corte SwearingenReprinted from the July 1999 edition of Chemical Engineering magazine

("Choosing the Best Flowmeter")

Table 1: A Comparison of Flowmeter Options Variable-Area Flowmeters Table 2: The Effect of Pressure Deviations on a Variable-Area Flowmeter

Mass Flowmeters Coriolis Flowmeters Differential-Pressure Meters Turbine Meters Oval-Gear Flowmeters References

With the many flowmeters available today, choosing the most appropriate one for a givenapplication can be difficult. This article discusses six popular flowmeter technologies, interms of the major advantages and disadvantages of each type, describes some uniquedesigns, and gives several application examples.

Dozens of flowmeter technologies are available. This article covers six flowmeter designs—variable-area, mass, Coriolis, differential-pressure, turbine, and oval-gear. Table 1compares the various technologies.

Table 1A Comparison of Flowmeter Options

Attribute Variable-area Coriolis Gasmass-flow

Differential-Pressure Turbine Oval Gear

Clean gases yes yes yes yes yes —Clean Liquids yes yes — yes yes yes

ViscousLiquids

yes (specialcalibration) yes — no

yes(special

calibration)

yes, >10centistokes

(cst)CorrosiveLiquids yes yes — no yes yes

Accuracy, ± 2-4% fullscale

0.05-0.15%of reading

1.5% fullscale

2-3%full-scale

0.25-1% ofreading

0.1-0.5%of reading

Repeatability,±

0.25% fullscale

0.05-0.10%of reading

0.5% fullscale

1%full-scale

0.1% ofreading

0.1% ofreading

Maxpressure, psi 200 and up 900 and up 500 and up 100 5,000 and

up4,000 and

upMax temp.,

°F 250 and up 250 and up 150 and up 122 300 and up 175 and up

Pressuredrop medium low low medium medium medium

Turndownratio 10:1 100:1 50:1 20:1 10:1 25:1

Averagecost* $200-600 $2,500-5,000$600-1,000 $500-800 $600-1,000$600-1,200

*Cost values can vary quite a bit depending on process temperature and pressures,accuracy required, and approvals needed.

Go to Top

Variable-Area Flowmeters

Design overview: The variable-area flowmeter (Figure 1) is one of the oldest

Ultrasonic flow meter

Variable area flowmeters

Velocity-ProfileDeviations InfluenceFlowmeter Performance

Volumetric Flow Rates vsMass Flow Rates

Related Searches:Scientific Instruments | Scientific Laboratory Equipment | Laboratory Supplies Medical Laboratory Equipment


2 of 11 12/02/2013 9:57

Figure 1The plasticor glass tubeof thevariable-areaflowmeterlets the uservisuallyinspect thefloat, whoseposition inthe taperedtub isproportionalto thevolumetricflowrate.

Figure 2This variable-area meter with aspring-loaded float can beinstalled at any angle. Thisaccommodation is not availablefor traditional variable-areaflowmeters, whose operationrelies on gravity.

tube (usually plastic or glass) and a metal or glass float. The volumetricflowrate through the tapered tube is proportional to the displacement ofthe float.

Fluid moving through the tube form bottom to top causes a pressure dropacross the float, which produces an upward force that causes the float tomove up the tube. As this happens, the cross-sectional area between thetube walls and the float (the annulus) increases (hence the term variable-area).

Because the variable-area flowmeter relies on gravity, it must be installedvertically (with the flowtube perpendicular to the floor). Somevariable-area meters overcome this slight inconvenience by spring loadingthe float withing the tube (Figure 2). Such a design can simplifyinstallation and add operator flexibility, especially when the meter mustbe installed in a tight physical space and a vertical installation is notpossible.

Two types of variable-area flowmeters are generally available: direct-reading and correlated. The direct-reading meter allows the user to readthe liquid or gas flowrate in engineering units (i.e., gal/min and L/min)printed directly on the tube, by aligning the top of the float with the tickmark on the flowtube.

The advantage of a direct-reading flowmeter is that the flowrate is literallyread directly off the flowtube. Correlated meters, on the other hand, havea unitless scale (typically tick marks from 0 to 65, or 0 to 150), and comewith a separate data sheet that correlates the scale reading on theflowtube to the flowrate in a particular engineering unit. The correlationsheets usually give 25 or so data points along the scale of the flowtube,allowing the user to determine the actual flowrate in gal/min, L/min, orwhatever engineering unit is needed.

The advantage of the correlated meter is that the same flowmeter can beused for various gases and liquids (whose flow is represented by differentunits) by selecting the appropriate correlation sheets, where additionaldirect-reading meters would be required for different fluid applications.Similarly, if pressure or temperature parameters change for a givenapplication, the user would simply use a different correlation sheet toreflect these new parameters. By comparison, for a direct-reading meter,a change in operating parameters will compromise the meter's accuracy, forcing it to bereturned to the factory for recalibration. In general, the average accuracy of avariable-area flowmeter is ±2-4% of fullscale flow.

Advantages: The major advantage of thevariable-area flowmeter is its relative low cost andease of installation. Because of its simplicity ofdesign, the variable-area meter is virtuallymaintenance-free and, hence, tends to have a longoperating life.

Another advantage is its flexibility in handling awide range of chemicals. Today, all-PTFE metersare available to resist corrosive damage byaggressive chemicals. The advantage of a PTFEflowmeter with a built-in valve is that you can notonly monitor the fluid flowrate, but you can controlit, as well, by opening and closing the valve. If theapplication requires an all-PTFE meter, chances arethe fluid is pretty corrosive, and many users wouldlike the option of controlling the flowrate by simplyturning a valve that is built into the flowmeteritself.

Disadvantages: One potential disadvantage of avariable-area flowmeter occurs when the fluidtemperature and pressure deviate from thecalibration temperature and pressure. Because temperature and pressure variations willcause a gas to expand and contract, thereby changing density and viscosity, thecalibration of a particular variable-area flowmeter will no longer be valid as theseconditions fluctuate. Manufacturers typically calibrate their gas flowmeters to a standardtemperature and pressure (usually 70°F with the flowmeter outlet open to theatmosphere, i.e., with no backpressure).

During operation, the flowmeter accuracy can quickly degrade once the temperatures andpressures start fluctuating from the standard calibration temperature and pressure. Metersused for water tend to show less variability, since water viscosity and density changesRelated Searches:Scientific Instruments | Scientific Laboratory Equipment | Laboratory Supplies Medical Laboratory Equipment


3 of 11 12/02/2013 9:57

More Details or OrderOnline:

Gilmont UnshieldedVariable Area Flowmeters

Gilmont Shielded VariableArea Flowmeters

Gilmont Shielded VariableArea Flowmeters without

Valve

very little with normal temperature and pressure fluctuations. While there is a way tocorrelate the flow from actual operating conditions back to the calibration conditions, theconventional formulas used are very simplified, and don't take into account the effect ofviscosity, which can cause large errors.

Table 2The Effect of Pressure Deviations on a Variable-Area Flowmeter

Maximum flowrate, L/min Fluid temperature, °F Outlet pressure, psiFluid type: Air

2.23 70 01.65 70 151.30 70 352.26 90 02.28 110 02.32 150 0

Fluid type: water4.82 70 04.82 70 154.82 70 354.86 90 04.89 110 04.95 150 0

As Table 2 shows, the effect of pressure deviations can be quite significant. This table wascreated using data from a variable-area flowmeter that was calibrated for air at 70°F andwith the outlet of the flowmeter vented to the open atmosphere (i.e. , 0 psi of outletpressure).

The flowmeter was calibrated to read a maximum of 2.23 L/min at this temperature andpressure. When the outlet pressure increases as all other parameters remain constant, theflowrate drops off. This pressure change affects the viscosity and density of the gas andwill cause the actual flowrate to deviate from the theoretical, calibrated flowrate. Thisrelationship is extremely important to be aware of, and underscores the difficulty inmeasuring gas flow. Also note that even though gas flowrate changes with a change in gastemperature (with all other parameters remaining constant), this effect is much lesssignificant with air than with other gases.

Table 2 shows this same variation with a meter calibrated for water at 9 psi ventingpressure and a temperature of 70°F. Here, one can assume water to be incompressible. Asshown, there is no direct effect on water flow with a change in back-pressure. Thetemp-erature change is not that significant either. But, for various fluids, a change intemperature could change the viscosity enough to degrade the accuracy below acceptablelimits.

The bottom line is that the user must be aware of anyvariation between calibration conditions and operatingconditions for gas flows, and must correct the readingaccording to the manufacturer's recommendations. Someusers have the manufacturer calibrate the meter to existingconditions, but this presumes that operating conditions willremain the same—which they rarely do.

The effect of viscosity changes is another potentialdisadvantage of the variable-area meter when measuringliquids. When a viscous liquid makes its way through avariable-area flowmeter, drag layers of fluid will build up onthe float. this will cause a slower-moving viscous liquid toyield the same buoyant force as a faster-moving fluid oflower viscosity. The larger the viscosity, the higher the error. The general rule of thumb isas follows—unless the meter has been specifically calibrated for a higher-viscosity liquid,only water-like liquids should be run through a variable-area flowmeter.

Sometimes, for liquids that are slightly thicker than water, a manufacturer-suppliedcorrection factor can be used without the need to recalibrate the whole meter. As always,check with the manufacturer if you plan on deviating from its calibration fluid andcalibration conditions. For a more-detailed discussion of the proper correction equations toapply to variable-area flowmeters in both water and gas service when they deviate fromstandard conditions, consult Refs. 9 and 10.

Applications:Variable-area flowmeters are well suited for a wide variety of liquid and gas applications,including the following:

Measuring water and gas flow in plants or labs



4 of 11 12/02/2013 9:57

Figure 3Because the massflowmeter measures massflow rather thanvolumetric flow, thispopular device is relativelyundaunted by fluctuationsin line pressures andtemperatures, especiallycompared with avariable-area flowmeter.The unit shown providesan integral digital display,as well as a built-in controlvalve.

Figure 4Inside a mass flowmeter, the gas issplit. Most goes through a bypass tube,while a fration goes through a sensortube containing two temperature coils.Heat flux is introduced at two sectionsof the sensor tube by means of twowound coils. As gas flows through thedevice, it carries heat from theupstream, to the downstream, coils. Thetemperature differential, generates aproportional change in the resistance ofthe sensor windings. Special circuitsmonitor the resistance change, which is

Purging instrument air lines (i.e., lines that use a valved meter) Monitoring filtration loading Monitoring flow in material-blending applications (i.e., lines that use a valved meter) Monitoring hydraulic oils (although this may require special calibration) Monitor makeup water for food & beverage plants

Go to Top

Mass Flowmeters

Design Overview:Mass flowmeters are one of the mostpopular gas-measurement technologies in use today(Figure 3). Most thermal mass flowmeters for gases arebased on the following design principles, which are shownin Figure 4. a gas stream moves into the flowmeterchamber and is immediately split into two distinct flowpaths. Most of the gas will go through a bypass tube, but afraction of it goes through a special capillary sensor tube,which contains two temperature coils.

Heat flux is introduced at two sections of the capillary tubeby means of these two wound coils. When gas flowsthrough the device, it carries heat from the coils upstreamto the coils downstream. The resulting temperaturediffererential creates a proportional resistance change inthe sensor windings.

Special circuits, known as Wheatstone bridges, are used tomonitor the instantaneous resistance of each of the sensorwindings. The resistance change, created by thetemperature differential, is amplified and calibrated to givea digital readout of the flow.

As shown in Figure 3, the mass flowmeter is available witha built-in valve for flow-control applications. This allows forexternal control and the programming of a setpoint for acritical flowpoint. Most mass flowmeters also have ananalog or digital output signal to record the flowrate. Theaverage mass flowmeter has an accuracy of ±1.5-2% offullscale flow.

Advantages: The main advantage of amass flowmeter for gas streams is itsability (within limitations) to "ignore"fluctuating and changing line temperaturesand pressures. As mentioned above forvariable-area flowmeters, fluctuatingtemperatures and pressures will cause gasdensity to change, yielding significant flowerrors. Because of the inherent design ofthe mass flowmeter, this problem is muchless significant than that found invariable-area flowmeters. Mass flowmetersmeasure the mass or molecular flow, asopposed to the volumetric flow. One canthink of the mass flowrate as thevolumetric flowrate normalized to a specifictemperature and pressure.

A more intuitive way to understand massversus volumetric measurement is toimagine a gas-filled ballon. Although thevolume of the balloon may be altered bysqueezing it (changing the gas pressure),or by taking the balloon into a hot or coldenvironment (changing the gastemperature), the mass of the gascontained inside the balloon remainsconstant. So it is with mass flow asopposed to volumetric flow.

A variable-area flowmeter measuresvolumetric flow. The flowrate on theflowtube reflects the volume of gas passingfrom the inlet to the outlet. This volume



5 of 11 12/02/2013 9:57

proportional to mass flow, and calibrateit to give a digital readout of the flow.


Aluminum Thermal GasMass Flowmeters

316SS Thermal Gas MassFlowmeters

Figure 5a (left). In a coriolis flowmeter, theCoriolis force F

Cor, pushes out toward the

z-axis as the fluid moves up through thetube. this force develops as the tube rotatesat a rate of W around the x-axis, and causes

pressures change. Because a massflowmeter is measuring the actual mass ofgas passing form inlet to outlet, there isvery little dependence on fluctuating temperatures and pressures. If you were piping anexpensive gas, you would certainly want to keep track of the amount of gas used based onmass, not volumetric, flow.

Makers of mass flowmeters measure their products' abilityto withstand changing pressures and temperatures bygiving coefficients that state the deviation of accuracy perdegree or psi change. For example, typical coefficient valuesare 0.10% error per degree C, and 0.02% error per psi.This means that each degree or psi change away from themeter's calibration conditions will degrade the accuracy bythese coefficient amounts. So, although there is adependence on pressure and temperature for a mass meter,its is very small, if not negligible. This is the biggestadvantage of a mass flowmeter. Another is that there are no moving parts to wear out.

Disadvantages: Aside from the fact that the gas going through the mass flowmetershould be dry and free from particulate matter, there are no major disadvantage to themass flow technology. Mass flowmeters must be calibrated for a given gas or gas blend.

Applications:Applications for mass flowmeters are diverse, but here are some typical uses:

Monitoring and controlling air flow during gas chromatography Monitoring CO

2 for food packaging

Gas delivery and control for fermenters and bioreactors Leak testing Hydrogen flow monitoring (e.g., in the utility industry) Control of methane or argon to gas burners Blending of air into dairy products Regulating CO

2 injected into bottles during beverage production

Nitrogen delivery and control for tank blanketing

Go to Top

Coriolis Flowmeters

Design Overview: The Coriolis flowmeter is named for the Coriolis effect, an inertial forcediscovered by 19th-century mathematician Gustave-Gaspard Coriolis. as a result of theCoriolis force, the acceleration of any body moving at a constant speed with respect to theEarth's surface will be deflected to the right (clockwise) in the northern hemisphere, andto the left (counter-clockwise) in the southern hemisphere.

The basic design of the Coriolis meter makes use of this Coriolis force by subjecting a setof curved measuring tubes to rotary oscillations about an axis. This oscillation is normallydriven by two electromagnetic coils, which also physically couple the two curvedmeasuring tubes. As a particular fluid flows through the tubes, it will move through pointsof high rotational velocity, to points of lower rotational velocity.

Upon approaching the tube plane inwhich the rotational axis is located,the rotational motion of the fluidelement is decelerated at a uniformrate, until it finally reaches zero in theplane of the rotational axis. As thefluid element flows away form therotational axis plane, toward pointswith higher rotational velocity, it isuniformly accelerated to increasinglyhigher rotational velocities. Thisproduces a force (the Coriolis force)that causes a twisting motion withingthe sensor tubes (Figure 5a).

If v is the velocity of the fluid in themeasuring tube, m/s, w theinstantaneous angular speed ofrotation, radians/s, and m the mass ofliquid in the tube section, kg, then thefollowing applies to the Coriolis force,kg(m/s) (Note that if the flow is low,



6 of 11 12/02/2013 9:57

the tube to distort out of the x-y planeFigure 5b (right). As an example of asingle-tube Coriolis flowmeter, this figureshows the fluid forces that generate thetwisting motion of the flow tube

represent smaller forces):

FCor

= -2m(w x v)

The design of the Coriolis flowmetertakes advantage of this force in thefollowing manner. First, the electromagnetic drivers initiate a vibration or oscillation in thesensor tube. This oscillation occurs even when there is no fluid moving in the meter.

The amplitude and frequency of this oscillation varies from manufacturer to manufacturer,but in general, the amplitude is about 3 millimeters, and the frequency is roughly 75-100cycles/s. As the fluid element passes through the sensor tubes, the Coriolis forces comeinto play. The Coriolis forces cause a twisting, or distortion, in the measuring tube, whichcauses a vibrational phase difference between the two tubes.

Some designs use only one sensor tube (figure 5b). In this case, the distortion caused bythe Coriolis force in the tube is compared to the tube at "no flow" conditions. In bothcases, however, a correlation to the mass flowrate is achieved, because the measuredphase difference or distortion is directly proportional to the mass flowrate of the fluid.Meanwhile, temperature-compensation techniques nullify the temperature dependence ofthe tube oscillations, creating a high-accuracy correlation to mass flow.

Advantages: The biggest advantage of the Coriolis design is that it measures mass flowinstead of volumetric flow. Because mass is unaffected by changes in pressure,temperature, viscosity and density, reasonable fluctuations of these parameters in thefluid line have no affect on the accuracy of the meter, which can approach 0.05% of massflow.

Coriolis meters can also determine fluid density by comparing the resonant frequency ofthe fluid being measured with that of water. Knowing density, the software can thenconvert mass to volume or percent solids.

Since there are no obstructions in the fluid path, Coriolis meters have inherently lowpressure drop for low-viscosity liquids. Turndown ratios (the ratio of maximum tominimum flow) of 100:1 are not uncommon. In addition, the lifetime and reliability of theCoriolis meter are high as the flow path is free of moving parts and seals. And, if installedproperly, vertically installed Coriolis meters are self draining, so they will not hold fluidwhen the line is down. A variety of wetted parts, communications outputs and connectionsare available.

Disadvantages: Because of their high accuracy and reliability, Corilois meters tend to berelatively expensive. This is not necessarily a disadvantage, however, if one looks at therelatively low cost of installation and ownership over time (Table 1). Because of theiraccuracy, Coriolis meters can help increase operating efficiency and save on productioncosts.

The main limitation of the Coriolis meter is that pressure drop can become large as fluidviscosity increases. For viscous products, check with the manufacturer to make sure thepressure drop at you max flowrate is acceptable and within your design parameters.

Applications:Coriolis flowmeters are suitable for:

General-purpose gas or liquid flow Custody transfer Monitoring concentration and solids content Blending ingredients and additives Conducting a primary check on secondary flowmeters Metering natural-gas consumption Monitoring such fluids as syrups, oils, suspensions and pharmaceuticals

Go to Top

Differential-Pressure Meters

Design overview: While many different types of differential-pressure flowmeters areavailable, this discussion will focus on one type. The technology discussed here involvesthe measurement of a pressure differential across a stack of laminar flow plates (Figure6). During operation, a pressuredrop is created as fluid enters through the meter's inlet.The fluid is forced to form thin laminar streams, which flow in parallel paths between theinternal plates separated by spacers.

The pressure differential created by the fluid drag is measured by a differential-pressuresensor connected to the top of the cavity plate. The differential pressure from one end ofthe laminar flow plates to the other end is linear and proportional to the flowrate of the



7 of 11 12/02/2013 9:57

Figure 6Using a differential-pressure flowmeter, apressure drop is created as fluid enters theinlet. The fluid is forced to form thinlaminar streams, which flow in alongparallel plates. The pressure differentialcreated by fluid drag from one end of thelaminar flow plates to the other is linearand proportional to the flowrate of theliquid or the gas.

What makes this technology unique isthe linear relationship betweendifferential pressure, viscosity and flow,which is given by the following equation

Q = K[P1-P

2)/n

2]

where (units vary per approach):Q = Volumetric flowrateP

1 = Static pressure at the inlet

P2 = Static pressure at the outlet

n = Viscosity of the fluidK = Constant factor determined by thegeometry of the restriction

This direct relationship betweenpressure, viscosity and flow allows themeter to switch easily among differentgases without recalibration. This isnormally accomplished by programmingin the various gas viscosities andallowing the user to dial in theappropriate gas, via a set of switches.

Variances in temperature and pressure,which often cause errors invariable-area flowmeters, can be easilyhandled by adding a pressure sensor(separate form the differential-pressuresensor in the basic design) and atemperature sensor to the design, toconstantly monitor fluctuations in stream pressure and temperature, and correct the flowreadings to standard pressure and temperature (77°F and 1 atm). This is critical for gasflowmeters, which are very sensitive to these parameters. Typical accuracy for the designis ±2-3% fullscale.

Advantages: As with mass flowmeters, the differential-pressure meter has no movingparts to wear out. And, unlike with mass flowmeters, users of differential-pressure meterscan measure different gases, such as air, hydrogen, ethane, methane, nitrous oxide,carbon dioxide, carbon monoxide, helium, oxygen, argon, propane and neon, by setting aswitch on the unit, without the need for recalibration.

For control applications, these meters are available with a built-in proportioning valve foronboard or remote control of the flowrate. With a wide variety of flow ranges and modelsfor both gases and liquids, the differential-pressure meter is one of the most versatiledesigns currently on the market.

Disadvantages: These meters are generally reserved for use with clean gases andliquids. particulates with diameters >20 to 30 micrometers could get caught between theplates.

Applications:Viable applications include the following:

Chemical applications (ratio, metering, and additive control) Pharmaceutical applications (liquid injection and batching) Research and development, and laboratory applications (gas blending, injection and aeration) Food and beverage applications (CO

2 measurements, air drying, and process control)

Go to Top

Turbine Meters

Design Overview: Many designs exist for turbine flowmeters, but most are a variation onthe same theme. As fluid flows through the meter, a turbine rotates at a speed that isproportional to the flowrate (Figure 7). Signal generators, usually located within the rotoritself, provide magnetic pulses that are electronically sensed through a pickup coil (theyellow pickup coil shown in Figure 7) and calibrated to read flow units. In some designs,an integral display may show both the flowrate and the total flow since power-up. Turbinemeters are available for both gas and liquid flow.

Because of the rotating blades in a turbine meter, the output signal will be a sine wavevoltage (V) of the form:



8 of 11 12/02/2013 9:57

Figure 7This cutaway viewof a turbineflowmeter showsthe turbines andsignal generatorsused to producevoltage pulses thatare proportional tothe flowrate.


Turbine Meters with4-20 mA Output

Turbine Meters withBattery-Powered Display

V=KwsinNwt

where:K = The amplitude of one sine wavew = The rotational velocity of the bladesN = The number of blades that pass the pickup in one full rotationt = Time

Because the output signal is proportional to the rotational velocityof the turbines—which, in turn, is proportional to the liquidflow—the signal is easily scaled and calibrated to read flowrateand flow totalization. Turbine flow sensors generally haveaccuracies in the range of ±0.25-1% fullscale.

Advantages: The main advantages of the turbine meter are itshigh accuracy (±0.25% accuracy or better is not unusual) andrepeatability, fast response rate (down to a few milliseconds),high pressure and temperature capabilities (i.e., up to 5,000 psi and 800°F withhigh-temperature pick coils), and compact rugged construction. Some manufacturer'shave taken turbine meter design to the next level by incorporating advanced electronicsthat perform temperature compensation, signal conditioning and linearization, all within afew milliseconds. This advanced technology will allow the meter to automaticallycompensate for viscosity and density effects.

Disadvantage: The disadvantage of the turbine meter isthat is relatively expensive and has rotating parts that couldclog from larger suspended solids in the liquid stream. And,most turbine meters need a straight section of pipeupstream from the flowmeter in order to reduce turbulentflow. This may make installation a challenge in small areas.However, some newer turbine meters reduce or eliminatethe amount of straight pipe required upstream, byincorporating flow straighteners into the body of the unit.

Another disadvantage in some designs is a loss of linearity at the low-flow end.Low-velocity performance and calibration can be affected by the natural change in bearingfriction over time. However, today's self-lubricated retainers, low-drag fluid bearings, andjeweled-pivot bearings all help to reduce the friction points, thereby allowing for greateraccuracy and repeatability in lower-flow applications.

Applications:Turbine flowmeters can be found in a wide variety of industries and applications:

Rotometer replacement Pilot plants Research and development facilities Cooling water monitoringInventory controlTest stands Water consumption Makeup water

Go to Top

Oval-Gear Flowmeters

Design Overview: The design of the oval-gear flowmeter is relatively simple:oval-shaped, gear-toothed rotors rotate within a chamber of specified geometry (Figure8). As these rotors turn, they sweep out and trap a very precise volume of fluid betweenthe outer oval shape of the gears and the inner chamber walls, with none of the fluidactually passing trough the gear teeth. Normally, magnets are embedded in the rotors,which then can actuate a reed switch or provide a pulse output via a specialized,designated sensor (such as a Hall Effect sensor). Each pulse or switch closure thenrepresents a precise increment of liquid volume that passes through the meter. The resultis a high accuracy (usually ±0.5 percent of reading) and resolution, and almost negligibleeffects for varying fluid viscosity, density and temperature.

When sizing an oval-gear flowmeter, keep in mind that thehigher the fluid viscosity, the more pressure will berequired to "push" the fluid into the flowmeter and aroundthe gears. Essentially, the pressure drop is the onlylimiting factor when the application requires the meteringof highly viscous liquids.



9 of 11 12/02/2013 9:57

Figure 8During operarion, eachgear rotation in theoval-gear meter traps apocket of fluid betweenthe gear and the outerchamber walls. Adesignated sensor countsthe pockets of fluidspassing from inlet tooutlet, and correlates thisvalue to a flowrate.


Oval Gear Flowmeterswith Integral Display

as long as there is enough system pressure, the oval-gearmeter will be able to measure the flow. In applicationswhere the lowest possible pressure drop is required, somemanufacturers can replace the standard rotors withspecially cut, high-viscosity rotors. The manufacturer willbe able to provide a graph of flowrate versus pressuredrop for various viscosities.

The oval-gear flowmeter works best when there is a littlebackpressure in the line; a throttling valve on the meteroutlet usually works just fine. The oval-gear meter is notsuitable for gases, including steam and multi-phase fluids.

Advantages: The advantage of the oval-gear flowmeter isthe it is, withing certain limits, largely independent of thefluid viscosity (users should just remain aware that higherpressures will be required to push higher-viscosity fluidsthrough the meter). This opens up a whole range ofapplications, including the metering of oils, syrups andfuels.

Ease of installation is another advantage of th oval design. Because no straight pipe runsor flow conditioning is required, these meters can be installed in tight areas, allowing formore flexibility in application design.

Disadvantage: Oval-gear meters are generally not recommended for water or water-likefluids, because the increased risk of fluid slippage between the gears and chamber walls.Fluid slippage will cause a slight degradation in accuracy, with low-viscosity fluids beingmore prone to degradation. As viscosity increases, the wall slippage quickly becomesminimal, and the best accuracy is realized. Since the oval-gear meter is really designed forhigher-viscosity fluids, it can be argued that running water through them is not a viableapplication anyway.

Applications:Oval meters are best suited for the following applications:

Measurement of net fuel use in boilers and engines

Verification of proper bearing-lubricant delivery in hydraulic applications Monitoring of paper-finishing chemicals Monitoring the flow of wax finishes Monitoring syrup injection in main beverage lines Monitoring and batching volumes of thick candy coating Monitoring and automating the dispensing of cooking oils

The specifications for the six flowmeter designs discussed above will vary widely frommanufacturer to manufacturer, and the performance values provided represent anaverage. When selecting a flowmeter for a given attribute, the engineer should consideradditional attributes—including velocity-profile deviations, the effect of non-homogeneousor pulsating flow, and cavitation, all of which will affect flowmeter choice, installation andoperation. While beyond the scope of this article, a thorough discussion of theseparameters can be found in Ref. 5.

Go to Top

References

Cole-Parmer, 1999-2000 catalog, Vernon Hills, IL, 19991.Hammond, Michael, "Is a Turbine Flowmeter Right for Your Application?," FlowControl, Vol. IV, No. 4, 1998, Witter Publishing Corp., N.J.

2.

Patrick, D., and Fardo, S., "Industrial Process Control Systems," Delmar Publishers,N.Y., 1997

3.

Parr, E. A., "Industrial Control Handbook," 2nd ed., Butterworth-Heinemann,England, 1995

4.

Miller, R. W., "Flow Measurement Engineering Handbook," 2nd ed., McGraw-Hill,N.Y., 1983

5.

Reif, David, "Matching the Flowmeter to the Job," Flow Control, Vol. III, No. 5, 1997,Witter Publishing Corp., N.J.

6.

Swearingen, C., "New Differential Pressure Flow Controllers Offer Exciting Benefits,"1997, European Process Engineer, Volume 7, No. 1, Setform Ltd, England.

7.

Swearingen, C., "High Viscosity Flowmeters: Solution to a Sticky Problem," FlowControl, Vol. IV, No. 5, 1998, Witter Publishing Corp., N.J.

8.

Gilmont, R., and Roccanova, B. "Low-flow rotameter coefficient," Instruments andControl systems, Vol. 39, p. 89, 1966.

9.



10 of 11 12/02/2013 9:57

February 1992, p. 124.

Customer ServiceTechnical SupportReturnsRepairsCalibrationsShipping PolicyTerms of SaleSite SupportForgot PasswordFAQs

Contact UsHow to Reach UsFeedbackAuthorized DealersFree Catalog Request

Look Up My...OrderQuoteInvoice

Company InfoAbout UsCareersPress ReleasesA "Greener" BusinessEvent CalendareNewsletter Archives

Subscribe Today

Stay Informedof the latest news, products,special offers and more

Follow Us

Privacy Policy | Site Map | © 2012 Cole-Parmer All Rights Reserved.

Technical Scientific Enterprises (TECHNI)P.O. Box 5100 Office No. 801, Al Sawari Tower C, Emirates Airlines Building, West Corniche

Road WAl Khalidiya Abu Dhabi United Arab EmiratesPhone: 02-666 1331 Fax: 02-666 4114 E-mail: [email protected]

View products by: Title | Brand | Category | Price | Popularity | Best SellersView product categories by: Popularity | Our Choices | All-Around Favorites | Title | Product

Category | RecommendedView popular items by: Popularity | Favorites | Title | Price | Part Number | Category

View Popular Categories by: Popularity | Our Choices | All-Around Favorites | Product Category |Recommended | Title



11 of 11 12/02/2013 9:57

technical resource library from cole-parmer.pdf

Documents