2741666 Flow Measurement Technology[1]

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Complete Selection ofFlowmeters

New-Technology

FlowTraditional Tech Fluid Flows Flow Research Just for Fun

FlowCoriolis FlowDP GasFlows Duonyms LevisBlueJeans

FlowMags FlowPD LiquidFlows FlowHandbook JesseYoder

FlowUltrasonic FlowTurbine MassFlows FlowLab Duonyms

FlowVortex FlowOpenChannel OilFlows FlowTimes Shades

FlowThermal FlowVA PumpFlows TempFlows SensorsTheory

FlowSonar FlowPlate SteamFlows Worldflow FlowMath

FlowOptical FlowEverything ValveFlow CustodyTransfer FlowCalendar

FlowMFC WorldPressure FlowReport NewTechFlow FlowCD

Complete Selection ofFlowmeters

Flow metersGas mass flow meterGear flowmetersMagnetic flowmetersTurbine flowmetersUltrasonic flowmetersVariable area flow metersFlowmeter ApplicationsHow Volumetric Flowmeters WorkInstalling Your Paddle-Wheel Flow Sensor

Selecting the Right FlowmeterPart 1Selecting the Right FlowmeterPart 2

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Ultr asonic F low Measur ementTechnolog y

Transit-time Ultrasonic Flow Meter

A typical transit-time flow measurement system utilizes two ultrasonictransducers that function as both ultrasonic transmitter and receiver. The flowmeter operates by alternately transmitting and receiving a burst of soundenergy between the two transducers and measuring the transit time that ittakes for sound to travel between the two transducers. The difference in thetransit time measured is directly and exactly related to the velocity of theliquid in the pipe.

To be more precise, let's assume that Tdown is the transit-time (or time-of-flight) of a sound pulse traveling from the upstream transducer A to the

downstream transducer B, and Tup is the transit-time from the oppositedirection, B to A. The following equations hold:

Tdown = ( D / sin) / ( c + V*cos), (1)

Tup = ( D / sin) / ( c - V*cos), (2)

where c is the sound speed in the liquid, D is the pipe diameter and V is theflow velocity averaged over the sound path. Solving the above equationsleads to

V = ( D / sin2) * T / (Tup * Tdown), (3)

where T = Tup - Tdown. Therefore, by accurately measuring the upstreamand downstream transit-time Tup amd Tdown, we are able to obtain the flowvelocity V. Subsequently, the flow rate is calculated as following,

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Q = K *A* V, (4)

where A is the inner cross-section area of the pipe and K is the instrumentcoefficient. Usually, K is determined through calibration.

From equations (3) and (4), we see that the measurement results, V andQ, are independent of fluid properties, pressure, temperature, pipematerials, etc. The sound speed term does not appear in the finalequations. These characteristics, plus large turn-down ratio, no

pressure drop, no moving parts, nodisturbance to the flow and manyother features, make ultrasonic transit-time flowmeter extremely attractive.

The transducers come with two types,

one is clamp-on type, the other is wettedtype. The wetted type can be furthercategorized into insertion type and flowcell (or spool piece) type. A brief

comparison among those types can be found here.

The transducers can be mounted in three ways: Z-method, V-method and W-method. With Z-method, the two transducers are mounted on opposite sidesof the pipe (see the figure on the top) and the sound pulse crosses the pipeflow once. This method is usually used for large pipe size, say above 12".

With V-method, the two transducers are mounted on the same side of thepipe and the sound pulse crosses the pipe flow twice. This is the mostcommonly used installation method, which could apply to pipe size from 1" upto 12".

With W-method (refer to the following drawing), the two transducers are stillmounted on the same side of the pipe. However, the spacing between thetwo transducers is doubled comparing with V-method. The sound pulse isbounced twice from the other side of the pipe, thus it intercepts the flow fourtimes. This method is used for small pipe, usually less than 1 1/2", for betteraccuracy.

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It should be mentioned that the actual implementation of the above principleis much more complex than what it looks like. The challenges include:

how to accurately measure the transit-time,

how to reduce the discrepancy between the upstream and the downstream signalpaths,

how to provide stable results when the signal quality is degraded due to old pipe

material, low sound conductivity fluid, the presence of minor particles or air bubbles,and etc.

how to treat the short-circuit wave (or pipe-wall born wave),

how to reduce installation-induced errors, how to make the installation easy and

reliable,

for high temperature application, how to design a high temperature transducer

how to provide a user-friendly operation interface, how to provide more

functionalities,

and, of course, how to reduce the cost.

Different manufacturers have different answers to the above questions. As a result, there aremany brands of ultrasonic flowmeters, some of them may work well in a wide range ofapplications, some may not. Some may be expensive, and some may be less expensive.Some may be easy to use, some may be difficult.A low-cost, high accuracy, reliable andeasy to use ultrasonic flowmeter is always the pursuing goal of all ultrasonicflowmeter manufacturers. Shenitech believes itself to be the top performer along this line.We offer guaranteed high quality, guaranteed lowest price for all our ultrasonic flow meters.

Ultrasonic Flowmeter Basics

Doppler and transit-time flowmeters are gaining ground in liquid, and in some instancesgas, flow measurement applications. Understanding how they work will help guaranteeoptimum performance.

Users and designers of flow metering systems can profit by keeping abreast of newdevelopments. The ultrasonic flowmeter, a recent arrival on the scene, has profited fromtechnological advances, especially those in electronic circuitry. For example, fast Fourier

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transform (FFT) signal processing is being used in one transit-time flowmeter design.And a supplier of Doppler flowmeters credits proprietary software and superiorelectronics design with opening up new application areas for this well-known technique.

Both types of ultrasonic flowmeters feature clamp-on designs with transducer assemblies

that detect flow rate from the outside. Installation entails neither breaks in the line norinterruption of flow. One recommendation is that where practical, the new userexperiment with a clamp-on meter to investigate the feasibility of a permanentinstallation, perhaps with wetted transducers and the requisite changes in piping.

Why Check Out Ultrasonic Types?

Figure 1 is a typical example of the ultrasonicflowmeters offered by at least 30 suppliers in theU.S. and Canada. Following are some of thecapabilities of this particular model.

The meter can measure pure water, washwater, sewage, process liquids, oils, andother light homogeneous liquids. Thebasic requirement is that the fluid be capable of ultrasonic wave propagation and

have a reasonably axis-symmetrical flow. Clamp-on types measure flow through the pipe without any wetted parts, ensuring

that corrosion and other effects from the fluid will not deteriorate the sensors. A corollary to the above is that clamp-on types simplify and speed up meter

installation and minimize maintenance. This design and others are portable, a feature particularly advantageous for

backing up an already installed flowmeter or checking out existing meters in anumber of locations.

Depending on the model, the flowmeters can operate on pipe diameters from 0.5in. (13 mm) to 20 ft (6 m); fluid temperatures from 40F (40C) to 392F (200C);and flow rates from 1.0 ft/s (0.3 m/s) to 106 ft/s (32 m/s).

Measurement accuracy can be in the range of 1% of flow rate, and speed ofresponse can be as fast as 1 s. The handheld, microprocessor-based converter provides a local graphics display

and has a keypad for calling up page menus for flow data, trend displays, settingup site parameters, and other requirements.

The converter can log data for as many as 20 sites and 40,000 data points. It canalso provide a PC interface via RS-232 serial communication, and an output of 4-20 mA DC for operating a digital controller, DCS, PLC, or recorder.

Figure 1. A clamp-on design with rail-mounted transducers makes thistypical transit-time flowmeter easy toposition. The microprocessor-basedconverter is also shown.

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As is true of most such meters, operation is linear and bidirectional. The flowmeter has a built-in, rechargeable battery and can operate continuously

for five hours. Advanced digital signal processing improves its performance where the flowing

fluid contains air or gas bubbles.

Some suppliers offer ultrasonic measurements of both level and flow velocity to calculateflow quantities in open channels with weirs or flumes. Others carry ultrasonic metersespecially adapted to measure the flow rate of gases. This class of meter is attractivecompared to conventional flow metering methods because, in addition to the points listedabove, the meters inherently provide linear calibration; have wide rangeability; induce nopressure drop or disturbance in the flow stream; and may offer the most economical costof ownership.

Basic Operating PrinciplesTo detect flow through a pipe, ultrasonic flowmeters use acoustic waves or vibrations of a

frequency >20 kHz. Depending on the design, they use either wetted or nonwettedtransducers on the pipe perimeter to couple ultrasonic energy with the fluid flowing in thepipe.

Doppler Flowmeters. Doppler flowmeters arenamed for the Austrian physicist andmathematician Christian Johann Doppler (1803-1853), who in 1842 predicted that the frequenciesof received sound waves depended on the motionof the source or observer relative to thepropagating medium. To use the Doppler effect to

measure flow in a pipe, one transducer transmitsan ultrasonic beam of ~0.5 MHz into the flowstream (see Figure 2). Liquid flowing through thepipe must contain sonically reflective materialssuch as solid particles or entrained air bubbles.The movement of these materials alters thefrequency of the beam reflected onto a second,receiving transducer. The frequency shift islinearly proportional to the rate of flow of

materials in the pipe and therefore can be used to develop an analog or digital signalproportional to flow rate.

The basic equations defining the Doppler flowmeter are:

(1)

and by Snell's law:

(2)

Figure 2. Doppler ultrasonicflowmeters operate on the Dopplereffect, whereby the transmittedfrequency is altered linearly by beingreflected from particles and bubbles inthe fluid. The net result is a frequencyshift between transmitter and receiverfrequencies that can be directly relatedto the flow rate.

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Thus, from Equations (1) and (2), we have:

(3)

where:

Equation (3) clearly shows that flow velocity is a linear function of the Doppler

frequency shift. Now, because the inside diameter of the pipe, D, is known, volumetricflow rate (e.g., in gallons per minute) can be measured using the following expression:

(4)

where:

One Doppler meter design mounts both the transmitting and the receiving transducers inthe same case, attached to one side of the pipe. Reflectors in the flowing liquid return thetransmitter signals to the receiver, with a frequency shift proportional to the flow velocity,as is the case when the two transducers aremounted separately on opposite sides of thepipe.

A portable, clamp-on Doppler meter capable ofoperating on AC power or from a rechargeablepower pack has recently been developed. A setof 4-20 mA DC output terminals permits the unitto be connected to a strip chart recorder or otherremote device for readout and/or control.

Transit-Time Flowmeters. Transit-time meters,as the name implies, measure the difference intravel time between pulses transmitted in thedirection of, and against, the flow. This type ofmeter is also called time of flight and time oftravel.

Figure 3. Transit-time flowmetersmeasure the difference in travel timebetween pulses transmitted in a singlepath along and against the flow. Twotransducers are used, one upstream ofthe other. Each acts as both atransmitter and receiver for theultrasonic beam.

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In the example shown in Figure 3, the sonic beam is at a 45 angle, with one transducerlocated upstream of the other. Each transducer alternately transmits and receives bursts ofultrasonic energy; the difference in the transit times in the upstream vs. the downstreamdirections (TU - TD) measured over the same path can be used to calculate the flowthrough the pipe:

(5)

where:

This equation shows that the liquid flow velocity is directly proportional to the meas-ureddifference between upstream and downstreamtransit times. Because the cross-sectional areaof the pipe is known, the product of that areaand the flow velocity will provide a measure ofvolumetric flow. Such calculations are easilyperformed by the microprocessor-basedconverter. With this type of meter, particles orair bubbles in the flow stream are undesirablebecause their reflecting qualities interfere withthe transmission and receipt of the appliedultrasonic pulses. The liquid, however, must bea reasonable conductor of sonic energy.

Figure 4 shows three placements that can beused for the two transducers. All are identifiedas single measuring path because the sonicbeam follows a single path, and in all three thetwo transducers are connected by cable to aconverter that can output a 4-20 mA DC signal.The selection of one configuration over anotheris dictated by several factors associated with theinstallation, including pipe size, space available

Figure 4. For single-pathmeasurements with the transit-timeflowmeter, there are three methods ofmounting the two transducers, Z, V, andW. The choice is dictated by installationfactors such as size and condition ofthe pipe-line.

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for mounting the transducers, condition of the inside pipe walls, type of lining, and natureof the flowing liquid.

The Z configuration places the transducers on opposite sides of the pipe, one downstreamof the other. Generally, the distance downstream is ~D/2, where D = pipe diameter. The

converter uses specific data on piping parameters to compute the optimum distance. TheZ method is recommended for use only in adverse conditions such as where space islimited, the fluid has high turbidity (e.g., sewage), there is a mortar lining, and when thepipe is old and a thick scale has built up on the inside wall that tends to weaken thereceived signals. It is not recommended for smaller pipes, where its measuring accuracytends to degrade.

In most installations, the V method is recommended, with the two transducers on thesame side of the pipe about a pipe diameter apart. The rail attachment that can beclamped on the pipe facilitates sliding the transducers horizontally along the pipe andpositioning them the calculated distance apart.

The W method should be considered on pipe 1 in. down to in. dia. Its main limitationis a possible deterioration in accuracy due to buildup of scale or deposits on the pipewall-note that the sonic signal must bounce off the wall three times. Turbidity of theliquid also could be harmful since the signal has a longer distance to travel.

Open-Channel Flowmetering. Ultrasonic flowmeters have been used successfully forcertain open-channel flow measurements, in conjunction with weirs or flumesdownstream. The transducer is installed above the channel, beaming pulses down on thesurface of liquid in the channel. The pulses are reflected back to the transducer and thetravel time can be related to the height of the liquid in the channel. Essentially, this is an

application of an ultrasonic level detector. By relating the channel level with the flowvelocity at the weir or flume, the metering system can provide a volumetric meas-ure offlow.

Application NotesIt is essential to carefully follow the manufacturer's operating instructions. Earlyproblems with ultrasonic flowmeters were perhaps due, at least in part, to the users' notunderstanding the importance of certain fundamentals such as proper mounting of thetransducers on the pipe. The acoustic coupling to the pipe and the relative alignment ofthe transducers must be retained despite events such as a large change in pipe temperatureor unusual vibration.

For both Doppler and transit-time flowmeters to indicate true volumetric flow rate, thepipe must always be full. A Doppler meter on a partially full pipe, however, will continueto indicate flow velocity as long as the transducers are both mounted below the liquidlevel in the pipe.

Most manufacturers specify the minimum distance that the meter must be from valves,tees, elbows, pumps, and the like, both upstream and downstream. This is usually

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expressed in pipe diameters and typically should be 1020 diameters upstream and 5diameters downstream.

Transit-time meters rely on an ultrasonic signal's completely traversing the pipe, so thepath must be relatively free of solids and air or gas bubbles. Bubbles in particular tend to

attenuate the acoustic signals, a problem that has been addressed in the Fuji Portaflow Xshown in Figure 1. The unit's electronic circuitry uses a proprietary Fourier transformtechnique to provide what is termed an advanced antibubble measurement.

Doppler meters, on the other hand, rely on reflectors in the flowing liquid. To obtainreliable measurements, therefore, attention must be given to the lower limits forconcentrations and sizes of solids or bubbles. The flow must also be rapid enough to keepthese materials in suspension. One manufacturer gives as typical the values of 6 ft/s (1.8m/s) for solids and 2.5 ft/s (0.75 m/s) for small bubbles.

Over the past few years, some suppliers of Doppler meters have introduced models that

operate at frequencies >1 MHz. The claim for such units is that they will operate onvirtually clean liquids because reflections will occur off the swirls and eddies of theflowing liquid. A cautionary note has been sounded, however, advising prospective usersto limit the technique to low concentrations of bubbles and particles.

Because in the operation of ultrasonic flowmeters the energy for measurement passesthrough only part of the measured liquid, Reynolds number, which can be thought of asthe ratio between the inertial forces and the viscous forces in a flowing stream, affects theperformance of the meter. For example, to perform within their stated specifications,some Doppler meters and a type of transit-time meter require minimum Reynoldsnumbers of 4000 and 10,000, respectively. Here again, for such limitations the

manufacturer's instruction should guide the user.Clamp-on meters typically require that the thickness of the pipe wall be relatively smallin relation to the distance the ultrasonic energy must pass through the measured liquid. Asa general rule, the ratio of pipe diameter to wall thickness should be >10:1; i.e., a 10 in.pipe should not have a wall thickness >1 in.

When it comes to the stated accuracies of ultrasonic flowmeters there are still not a lot ofindependent test data to confirm or refute the claims made by various manufacturers. Asthe use of these meters becomes more widespread, one can hope that the availability ofsupporting data will equal that on orifice meters, supported by a wealth of test data and

standards.Both types of ultrasonic meters are finding new applications. One market researchorganization has determined that transit-time meter applications are increasing at a fasterrate than are Dopplers. At present, the installations are split about 60/40 in favor of transittypes. Developments in technology, however, can greatly affect this picture and only timewill tell.

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Transit-time meters are not limited to measuring flow velocity.

Controlotron's (Hauppauge, NY) SonicMass flowmeter combines high-

resolution sonic velocity and temperature compensation to convert

volumetric flow to mass flow rate. The flowmeter, which is available in a

variety of mounting styles and pipe sizes (3/8- to 48-in. dia.), is said to bethe first commercially available liquid mass meter based on transit-time

technology. Performance accuracy exceeds 0.25% for mass flow of

selective fluids and 0.15% for volumetric readings over a specified range of

Reynolds numbers.

Doppler meters work differently than transit-time devices, most using

continuous transmission of a single sound frequency rather than pulses.

The beam is transmitted into the media at some angle to the direction of

flow. Bubbles, entrained solids, or eddies in the flow then reflect or scatter

the sound back to a receiver. Motion in these inclusions will cause a

Doppler (frequency) shift of the returned signal. In short, Doppler-based

flowmeters pass the signals between a transducer and inclusions in the

flow steam and back, rather than between two transducers.

Each of the inclusions, which have random physical distribution and

velocities, reflect sound while in the sonic stream. Hence, their reflected

composite signal is a random distribution of frequencies that add up to

what appears to the receiver as a single waveform. The difference

between the scattered and received frequencies is proportional to themotion of the flow inclusions or the flow velocity.

Other variations of ultrasonic flowmeters are available. There is a hybrid of

the two basic technologies intended for use in process (closed-pipe)

applications. Ultrasonic flowmeters can be used for determining flow ratein open channels and rivers. The technology, which is also available to

measure flow rate in partially filled pipes, determines flow by measuring

level in the pipe.

Application dependenceThe types of media suitable for measurement ultrasonically are quite

extensive for transit-time and Doppler meters alike. Either type requires

some prerequisites for successful operation. Applicable media must

support the passage of sound, be in a full conduit, and contain no material

that will deposit on the inside pipe wall. Flow must be continuous and non-

pulsing for either type to function accurately.

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There are additional requirements for using Doppler meters. In these

cases, the media must provide enough inclusions (suspended bubbles,

small solids/particulate, etc.) so that sound energy has something from

which to reflect. However, the media must not have so many of these

"scatterers" that sound cannot penetrate the flow.

According to Doug Weerstra, instrumentation chemist at Mesa

Laboratories Inc., NuSonics Div. (Lakeland CO), there is an overlap in the

amount of suspended solids by volume that both flowmeter types can

handle. Transit-time meters work well even with 0-2% by volume of solids

in small pipes. Doppler meters work in most flow situations with 0.1-10%

solids by volume. As the amount of solids increases, however,

instrumentation functionality can suffer.

Doppler meters can also function if eddies exist as the inclusions in theflow. "However, eddies can be tough to pick up. And since they are created

by downstream conditions, they cannot be counted on," says Mr. Weerstra.

Locating those sound beams

For both transit-time and Doppler flowmeters, the number of sound beams

that pass through a pipe can be increased or modified electronically to

raise the accuracy of the average velocity reading. Often a single acoustic

beam passed between 3 and 9 o'clock (recommended) in a horizontal pipe

run provides sufficient accuracy. However, for any ultrasonic flowmeter to

provide accurate readings, it must be located in a straight section of pipe at

least 10 pipe diameters upstream and three pipe diameters downstream

from the nearest flow disturbance (pump, elbow, tee, reducer, etc.).

If a sufficient straight run cannot be "found" or built into the process piping

then multi-beams can be used to cancel out the effects of the disturbed

flow. An additional beam(s) can be placed at "other angles of the clock"

and their signals combined to provide more accurate representation of

average flow velocity. Keep in mind, sensor configurations can be quite

different depending on the size of pipe and flow conditions encountered. Inshort, there just are no typical installations or application rules.

Attaching sensors

Sensor mounting styles are varied and often depend on where and when

the device is placed in service. Mounting types include direct-mounting or

non-intrusive. Direct-mounting devices include spool piece metersso

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named because the short flanged pipe section that contains the

transducers resembles a thread spoolbolted in place and weld-in style

devices. Most of these sensors styles are mounted in the early stages of a

process piping installation to avoid "taking the process down" later in a

retrofit situation. Weld-in transducers can be hot-tap mounted, however,allowing some flexibility as to when they can be added into a system.

According to Randy Brekke, vp sales at J-Tec Associates Inc. (Cedar

Rapids, IA) the greatest advantage of using ultrasonic flowmeters, whether

transit-time or Doppler, is that no pipe cutting is required. Ability to use

externally mounted transducers provides the control engineer with greatly

increased installation flexibility both in when and where the flowmeter is

mounted.

Externally mounted transducers can be used on both transit-time andDoppler meters. There are two basic types. Clamp-on types mount on the

outside of a pipe where flow velocity is needed. For clamp-on transducers

to work, the pipe wall to which it is attached must be capable of passing

sound and be clean and smooth. The inside of the pipe must be free of

sound-absorbing material, such as dirty grease or scale. Use of an

acoustic coupling material between the transducer and pipe (oil, grease, or

epoxy) is recommended.

Where clamp-on transducers can not be adapted, wetted flush-mount

sensorssome designs resemble spark plugsmust be used to provide a

good interface for passing sound energy into the media. In the case of

these transducers, no pipe need be cut, but mounting holes must be drilled

and tapped into the pipe, requiring process shutdown and/or pipe draining

during the process.

For new construction and where process interruptions (scheduled

downtime for periodic maintenance, general cleaning and sanitation, etc.)

are not a problem, installation of spool-piece devices is simple and

straightforward. Siemens Energy & Automation (Grand Prairie, TX) offersthe Sitrans F US, an ultrasonic flowmeter intended for use in liquids. This

device is offered as spool-piece mounting, available in 1-, 2-, 3-, and 4-in.

nominal diameters, and four standard DIN sizes with suitable flange

designs. Because it is available in a limited number of pipe sizes, mounting

flexibility as compared to the "one size fits most" clamp-on type is limited.

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However, a dedicated metering tube for each size allows the ultrasonic

path to be more precisely controlled. In the case of Sitrans F US, its

dedicated flow tube design uses built-in reflection points such that the flow

velocity along the measuring path corresponds to the average flow velocity

for all flow profiles. The patented helical sound path in this unit is said toresult in high accuracy for a wide flow range.

Whether a flowmeter is dedicated or portable is more a function of the

electronics than the type of sensor mounting. Dedicated devices are

specified for a given location and, as such, are obtained from the factory

calibrated to a given pipe size and flow range.

Truly portable ultrasonic flowmeters are microprocessor-based devices

that can be reranged and recalibrated in the field, allowing them to be

moved from one location to another with relative ease. Often used fortesting and verification purposes, devices such as J-Tec's Compu-Flow

Model JC5 feature an onboard keypad and LCD, clamp-on transducers,and a carrying case. Unlike dedicated units, the portable electronics are

not meant to be permanently mounted.

Adapted to gas

Clamp-on transit-time ultrasonic flowmeters are most often applied to

liquids, however, advancements in the transducer and signal processing

technology have extended their use to gas applications as well. In the case

of the GE Panametrics (Waltham, MA), Model GC868 clamp-on gas

flowmeter extends the technology to gas applications for pipes 3 in. or

greater in diameter and to pressures over 90 psig.

According to GE Panametrics' application engineer Daryl Belock, transit-

time technology was always adaptable to gas flow if its density was high

and delivery pressures were in the several-thousand psig range, not a

common real-world situation. Getting the instrument to read flow accurately

at smaller pipe sizes and lower pressures was the breakthrough, one that

was electronics based. Initial investigation of the technology was done inan actual application to prove product feasibility.

Ultrasonic measurement techniques have made steady progress overthe years as a viable flowmeter technology. Wide adaptability andease of installation, two of its most important features, have beengreatly enhanced through advancements in electronics and

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transducer design. And although no flow instrument is universallyadaptable, ultrasonic flowmeters make a good run at it. Ultrasonicflowmeter handles wide range of variables

The Esso Petroleum Co. Ltd. refinery at Fawley on Southampton Water is the largest refinery in

the U.K., and one of the largest refineries in Europe. Its 330,000 barrel/day capacity supplies over15% of all oil products used in Britain, with 85% of its products delivered by pipeline to seven

major airports including Heathrow and Gatwick. The Fawley site also incorporates an integrated

chemical plant operated by Exxon Mobil Chemical Ltd.

Each type of crude oil has its own composition. To maintain flexible operations, the Fawley

refinery must process up to 30 different types of crude oil from all over the world. As part of a

crude tank farm optimization project, Esso developed a method to manage its crude oil inventory

using flowmeters as part of an on-line blending process to maximize efficiency and cut costs.

Tackling the unattainableKrohne Inc. (Peabody, MA) supplied six 12-in. and six 20-in. dia. dual-beam UFM

500 ultrasonic flowmeters. The flowmeters are arranged in pairs, with the first

flowmeter configured to protect the pump against low discharge flow, and the

second for regulating the flow via a control valve to enable the blending of correct

crude ratios. Ultrasonic flowmeters were chosen to meet the accuracy stipulated

by the project for the entire range of flows and viscosities required for the

blending process. Additionally, the off-site piping networkextends over an area of

three square miles, thus line size flowmeters helped to minimize inherent

pressure drop within the overall system.

Esso's engineering team at the Fawley site provided exact and extremely

demanding flowmeter specs needed for the project. Because of the low velocities

and high viscosities, coupled with the overall range of velocities and viscosities

taking the flow through laminar, transitional, and turbulent regions, engineering

felt it could be difficult to meet the required accuracy and performance standards.

This process requirement, together with physical restrictions resulting in multiple

out-of-plane pipe bends upstream of the flowmeters, has taken the flowmeters

outside their normal performance ranges. Velocities in the range of 3.4-12.5 ft/s

were required for the 12-in. flowmeters, and 1.3-4.5 ft/s for the 20-in. flowmeters.Viscosities ranged from 3 to 1,800 centistokes.

Even with the wide range of process variables, UFM Series flowmeters met

performance criteria. With pairs of the flowmeters arranged in series, consistent

flow outputs have been achieved despite the potential for a distorted flow profile

from the upstream pipe bends. The blending system's control software conducts

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a self check of the flowmeters against tank movement via the radar tank gauging

system installed on the crude tanks.

Ultrasonics to the rescue

At times, convenient to install clamp-on ultrasonic flowmeters are the instrument

of choice for retrofit situations. A case in point is the Independent Gas Producers

(IGP, Gillette, WY) adaptation of a Dynasonics (Racine, WI) Series TFXL Small-

Pipe ultrasonic flowmeter to replace damaged mechanical flowmeters at IGP's

coal-bed methane wells located on nearby Bureau of Land Management (BLM)

land.

IGP uses flowmeters to measure the amount of water pumped to the surface

during well operation. Because the work is on BLM land, IGP is required to report

the amount of water brought to the surface. IGP also uses flowmeters to optimize

production of its wells and to prevent the pumps, which can be located as far as1,500 ft under ground, from burning out should a pipe get plugged. All sensor

signals are fed to small PCs with telemetry where they are monitored and

recorded. TXFL flowmeters are offered for pipe sizes 1/2-2 in., and operate

linearly over a 50:1 measuring range. Unlike the turbine meters they have

replaced, bidirectional TXFLs do not impede flow and plug in the presence of

coal and rock fragments often found in these wells. They eliminate the need for

bypass lines, do not falsely record gas flow as water flow, and automatically

compute volumetric compensation for gas bubble content in the water, leading to

more accurate flow measurement.

A short history of ultrasonic flowmeters

Ultrasonic flowmeters got their start in 1963 when Tokyo Keiki (now Tokimec) first

introduced them to industrial markets in Japan. In 1972, Controlotron Corp.

(Hauppauge, NY) brought clamp-on ultrasonic flowmeters to the U.S. market.

Others joined the market later in the 1970s and 1980s.

When ultrasonic flowmeters were first introduced, correct application conditions

for the two basic types, transit-time and Doppler, were not well understood. Early

on, some users misapplied these meters, which led to inaccurate measurements.These experiences gave some users a negative impression of ultrasonic

technology. Fortunately, the market has recovered from these events.

Use of ultrasonic flowmeters for gas flow measurement got its start in the early

1980s, when both Ultraflux (Poissy, France) and Panametrics (now GE

Panametrics) ran tests on ultrasonic flowmeters for gas applications. However,

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the biggest events in gas flow measurement didn't occur until 1995, when the

Groupe Europeen de Recherche GaziSres (GERG) published the Technical

Monograph 8, which laid out criteria for using ultrasonic flowmeters for custody

transfer of natural gas. At this point, ultrasonic flowmeters became a serious

alternative to differential pressure and turbine meters for custody transferapplications, especially in Europe.

In June 1998, the American Gas Association published AGA-9, a report that did

for the U.S. market what the GERG report had done for the European market.

Both reports specified use of multi-path ultrasonic flowmeters, meaning that more

than one ultrasonic signal is used to calculate flow rate. Publication of these

reports gave a major boost to ultrasonic flowmeter sales.

Use of ultrasonic flowmeters for liquid applications has also been growing.

However, most of the growth has been on the transit-time side. By using

advanced electronics, transit-time meters have become more adept at measuring

the flow of liquids containing some impurities, giving them a wider application

base than earlier models.

A number of suppliers have brought new products onto the market in the past five

years. These include multi-path devices for custody transfer of natural gas and

clamp-on flowmeters for general gas applications. The ultrasonic flowmeter

market has been one of the most active in terms of new product releases. This

trend is likely to continue.

There are two main types of ultrasonic flowmeters: transit time and Doppler. A transittime ultrasonic flowmeters has both a sender and a receiver. It sends two ultrasonicsignals across a pipe at an angle: one with the flow, and one against the flow. The meterthen measures the transit time of each signal. When the ultrasonic signal travels withthe flow, it travels faster than when it travels against the flow. The difference between thetwo transit times is proportional to flowrate.

Doppler flowmeters also send an ultrasonic signal across a pipe. Instead of tracking the

time the signal takes to cross to the other side, a Doppler flowmeter relies on having thesignal deflected by particles in the flowstream. These particles are traveling at the samespeed as the flow. As the signal passes through the stream, its frequency shifts inproportion to the mean velocity of the fluid. A receiver detects the reflected signal andmeasures its frequency. The meter calculates flow by comparing the generated anddetected frequencies. Doppler ultrasonic flowmeters are used with dirty liquids orslurries. They are not used to measure gas flow.

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------------------------------------------------------------------------------------

Mass Flow

While the majority of flowmeters measure volumetric flow, three types of flowmetersmeasure mass flow. These three types are Coriolis, thermal, and multivariableflowmeters. This article discusses the reasons for measuring mass flow, and then looks atthe advantages and disadvantages of Coriolis, thermal, and multivariable flowmeters.

Volumetric flow is measured in a number of different ways. Ultrasonic, magnetic, vortex,and turbine meters use various methods for determining average speed or velocity of theflow at some point in the flowstream. They then multiply this velocity value by thecross-sectional area of the pipe to yield volumetric flowrate.

Positive displacement flowmeters measure volume directly by separating portions of the

flow into small containers of known volume, and counting how many times this is done.This is a highly accurate method of flow measurement, and positive displacementflowmeters are widely used for custody transfer applications.

Why Mass Flow is Measured

One reason to measure mass flow is to achieve greater accuracy. Because the quantity ofa fluid varies with temperature and pressure, fluid flow can vary with changingtemperatures and pressures. This is most notable for gases. Pressure and temperaturevariations have minimal effects on liquids, so these effects are often disregarded when

measure liquid flows. However, temperature and pressure have a much more pronouncedeffect on gases, so much mass flow measurement is measurement of gases.

In the process industries, it is sometimes desirable to measure mass flow for greateraccuracy and to accommodate measurement standards. Chemical reactions often refer tomass rather than volume, so mass flow is often measured in the chemical industry. Someproducts are sold by weight rather than volume, and in these cases it is necessary tomeasure mass flow. Gas flow is widely measured in the process industries.

There is a close relation between volumetric flow and mass flow measurement. If thevolumetric flow of a fluid is known, multiplying this value by the density of the fluid

yields mass flow. Some flowmeters, such as multivariable flowmeters, computevolumetric flow and then determine mass flow by using a calculated density value.

What percent of the total flow measurements are volumetric as opposed to mass flow? Ina recent worldwide survey of conducted by Flow Research and Ducker Worldwide, 75%of flow measurements were volumetric and 25 percent were of mass flow. It is clear,then, that mass flow accounts for a significant percentage of total flow measurements.

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Coriolis Flowmeters

Coriolis flowmeters use fluid momentum to measure mass flow directly. The fluid entersthe meter and passes through one or more vibrating tubes, and accelerates as it reachesthe point of maximum vibration. As the fluid leaves this point, it decelerates. Thiscauses a twisting motion in the tubes. The Coriolis meter measures this twisting motion,and mass flow is directly proportional to the amount of twist.

While it is natural to think that users choose Coriolis meters because of their ability tomeasure mass flow, user surveys show differently. In the previously mentioned usersurvey, respondents were asked why they are using Coriolis meters. The leading answergiven was accuracy, which was mentioned by 63 percent of respondents worldwide.Reliability was the second leading reason, and was mentioned by 14 percent ofrespondents. Only a small percentage measure ability to measure mass flow.

Coriolis flowmeters are among the most accurate meters. Their main limitations are linesize and cost. Over 90 percent of Coriolis flowmeters are used on line sizes of two inchesand less. Coriolis meters become very large and unwieldy, especially in sizes from fourto six inches. Cost also increases with size. Even smaller size meters are generally moreexpensive than other comparable new-technology flowmeters. Users who areconsidering Coriolis flowmeters need to balance their need for accuracy and reliabilityagainst purchase price. Some users select Coriolis meters despite their higher initial cost,because low maintenance requirements reduces their cost over the life of the meter.

Thermal Flowmeters

While thermal flowmeters also measure mass flow, they do so very differently fromCoriolis meters. Instead of using fluid momentum, thermal flowmeters make use of thethermal or heat conducting properties of fluids to determine mass flow. While themajority of thermal flowmeters are used to measure gas flow, they are also used tomeasure the flow of liquids.

The origins of thermal flowmeters lie in hot wire anemometers. These consist of aheated, thin wire element, and are very small and fragile. Hot wire anemometers wereused in velocity profile and turbulence research. Because they are susceptible tobreakage and to dirt, they are not suited to industrial environments.

There are several different thermal flowmeter technologies. Some measure the speedwith which heat that is added to the flowstream disperses. Others measure thetemperature difference between a heated sensor and the ambient flowstream. Thermalflowmeters typically require one or more temperature sensors to measure the fluidtemperature at specific points.

Thermal flowmeters have several main advantages. One is a relatively low purchaseprice. Secondly, thermal flowmeters can measure the flow of some low-pressure gases

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that are not dense enough for Coriolis meters to measure. Both of these advantages givethermal flowmeters their own unique niche in flow measurement.

The main disadvantage of thermal flowmeters is low accuracy. While some thermalflowmeters may achieve accuracy levels of one percent, other thermal flowmeters have

accuracies in the three to five percent range. It is the accuracy level of thermalflowmeters that is the main barrier to classifying them as new-technology flowmetersrather than traditional technology meters. Users who are considering thermal flowmetersneed to balance their accuracy needs with their cost requirements.

Multivariable Flowmeters

Multivariable flowmeters measure mass flow by combining volumetric flowmeasurement with density measurement. Density is usually measured either byconsulting a table, or by dynamically measuring pressure and temperature. This is calledan inferred method, because a formula is used to compute mass flow. The main types ofmultivariable flowmeters are differential pressure (DP), vortex, ultrasonic, and magnetic.

One main advantage of multivariable DP flowmeters is that only one process penetrationis required to get three process readings: flow, temperature, and pressure. This reducesthe chance of fugitive emissions, and also the number of leak points. Another advantageof multivariable DP meters is that users who are already measuring volumetric flow witha DP flowmeter can upgrade to a multivariable DP meter with a minimum of changes.

One disadvantage of multivariable flowmeters is that accuracy levels are not as high asaccuracy levels of Coriolis meters. This is due to the number of variables involved, andto the fact that it is an inferred method of computing mass flow. On the other hand, thepurchase price of multivariable flowmeters is substantially below that of most Coriolismeters.

Summary

With at least three main ways to measure mass flow, users are advised to determine theiraccuracy requirements and their budgetary constraints before making a decision aboutwhich type of flowmeter to select. When considering cost, it is also advisable to considerthe lifetime costs of a flowmeter, rather than just the purchase price. There are many highquality products available in all three categories.

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Positive displacement (PD) flowmeters

Positive displacement (PD) flowmeters operate by repeatedly filling and emptyingcompartments of known volume with the liquid or gas from the flowstream. Flowrate is

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calculated based on the number of times these compartments are filled and emptied. Themain types of PD flowmeters used for gas applications are diaphragm and rotary. Thesetypes of meters differ according to the way they trap the liquid into compartments withdifferent shapes.

Diaphragm meters have several diaphragms that capture the fluid as it passes throughthe meter. Differential pressure across the meter causes one diaphragm to expand andone to contract. A rotating crank mechanism helps produce a smooth flow of gas throughthe meter. This mechanism is connected via gearing to the index, which registers theamount of fluid that passes through the meter. Diaphragm meters are used for gasapplications.

Rotary flowmeters have one or more rotors that are used to trap the fluid. With eachrotation of the rotors, a specific amount of fluid is captured. Flowrate is proportional tothe rotational velocity of the rotors. Rotary meters are used for gas applications.

Thomas Glover of England invented the first diaphragm meter in 1843. Glovers meterwas made in response to difficulties with liquid sealed drum meters, which were createdin the early 1800s. This meter had diaphragms of sheepskin and with sheet metalenclosures. Today diaphragm meters are made from cast aluminum and have diaphragmsof synthetic rubber-on-cloth.

Large Installed Base

One major growth factor for positive displacement flowmeters is the large installed baseof positive displacement flowmeters worldwide. Because they were introduced morethan 100 years before new-technology meters, positive displacement flowmeters have

had much more time to penetrate the markets in Europe, North America, and Asia.Installed base is a relevant growth factor because often when ordering flowmeters,especially for replacement purposes, users replace like with like. The investment in aflowmeter technology is more than just the cost of the meter itself. It also includes thetime and money invested in training people how to install and use the meter. In addition,some companies stock spare parts or even spare meters for replacement purposes. As aresult, when companies consider switching from one flowmeter technology to another,there is more than just the purchase price to consider. The large installed base of positivedisplacement flowmeters worldwide will continue to be a source of orders for new andreplacement meters in the future.

High Accuracy a Major Factor

Accuracy and reliability continue to be the strongest driving forces behind the flowmetermarket. Positive displacement meters are highly accurate because they actually separatethe fluid into compartments and count the number of times this is done. There is no needfor the inferential method that occurs with meters that correlate flow with velocity, or usethe differential pressure method to measure flow. PD meters are widely used for billing

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applications because they are both accurate and reliable. Both the degree of accuracy andreliability vary with the manufacturer and the type of PD meter.

Utility Applications Dominate

While new-technology flowmeters are making inroads into traditional meters in manyareas and applications, this is less true for positive displacement flowmeters for gasapplications. Close to 80 percent of revenues from PD meters for gas flow measurementderive from utility applications, where PD meters are highly entrenched (see Figure 1).This includes PD meters for commercial and industrial applications, where utilitycompanies use them to measure the amount of gas consumed by restaurants, officebuildings, and other establishments. While there has been a shift from diaphragm torotary meters PD for these applications, these applications have seen no strong shift awayfrom PD meters. Turbine meters are used for high speed flow utility applications,however.

The use of PD diaphragm and rotary meters for gas applications is somewhat like the useof nutating disc and piston PD meters for utility and billing applications in the waterindustry. PD meters, along with single-jet and multi-jet turbines are still the dominantmeter for utility measurement of water flows, especially in residential and smallercommercial applications. However, some new-technology flowmeters such as magneticand Coriolis are beginning to gain approvals from industry associations for use in utilitymeasurement of water. It is likely, then, that new-technology meters will eventually beused on a more widespread basis for utility gas measurement.

GAS FLOW

Flow measurement is primarily concerned with measuring the flow of liquids and gases.Steam, which is water vapor, can be considered as a form of gas. Steam is also animportant flow measurement.

Gas takes many forms. Types of gas include natural gas, fuel gas, atmospheric gas,compressed natural gas, and many others. Individual gases that are especially importantinclude hydrogen, oxygen, nitrogen, and carbon dioxide. Air is a gas, although it is a

mixture of gases that include nitrogen, oxygen, argon, and many other gases. Just aswater is an essential element of life, so life as we know it would not be possible withoutthe air we breathe.

Natural Gas

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Natural gas plays an especially important role in the flowmeter industry. Natural gas isan important source of fuel and energy. Like coal and petroleum, it is a fossil fuel. Likeair, natural gas is a mixture of gases. These include methane, ethane, propane, butane,and other alkanes. Natural gas is found in the ground, along with petroleum. It isextracted and refined into fuels that supply a significant portion of the worlds energy

supply.Compressed natural gas (CNG) has taken on importance as a source of fuel for alternativeenergy vehicles. Vehicles powered by natural gas are highly efficient, and emitsignificantly lower amounts of carbon monoxide, nitrogen oxide, and pollutants harmfulto the ozone layer than gasoline powered vehicles. CNG powered vehicles are fueled atfilling stations that are very much like gasoline stations, and several companies havecreated flowmeters specifically designed for these CNG filling stations.

Gas flow measurement can also be divided into three broad categories: industrial,commercial, and residential. Industrial gas flow measurement includes flow measurement

that occurs in manufacturing and process plants, including chemical plants and refineries.Commercial gas flow measurement occurs at businesses and commercial buildings suchas restaurants, office buildings, and apartment complexes. This is a form of utilitymeasurement, since these flowmeters typically measure the amount of natural gas usedby the business or in the commercial building. Residential gas flow measurement refersto flowmeters that measure the amount of gas used at individual homes and apartments.

Utility vs. Industrial. Meters used to measure gas or water entering a building or plantfor the purpose of billing the plant for their use of gas or water are considered to be utilitymeters. These meters are typically sold to a gas or water utility company and installed atthe building or plant by the utility company. Meters used within a building or plant for

internal allocation purposes of gas, water, or other liquids, are considered to be industrialmeters. They are typically sold to the owners of the plant or building itself, as opposed toa utility company.

Positive Displacement Flowmeters

Positive displacement (PD) flowmeters are widely used for utility measurements of gasflow. One main type is the diaphragm meter. Diaphragm meters have severaldiaphragms that capture the fluid as it passes through the meter. Differential pressureacross the meter causes one diaphragm to expand and one to contract. A rotating crankmechanism helps produce a smooth flow of gas through the meter. This mechanism isconnected via gearing to the index, which registers the amount of fluid that passesthrough the meter.

Another type of PD meter for gas flow measurement is the rotary meter. Rotaryflowmeters have one or more rotors that are used to trap the fluid. With each rotation ofthe rotors, a specific amount of fluid is captured. Flowrate is proportional to therotational velocity of the rotors. Rotary meters are used for both liquid and gasapplications. Rotary meters are used for industrial applications.

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Turbine Flowmeters

Turbine flowmeters have a rotor that spins in proportion to flowrate. There are manytypes of turbine meters, but many of those used for gas flow are called axial meters.Axial turbine meters have a rotor that revolves around the axis of flow. Most flowmeters

for oil measurement and for measuring industrial liquids and gases are axial flowmeters.Axial meters differ according to the number of blades and the shape of the rotors. Axialmeters for liquids have a different design from axial meters for gas applications. Like PDmeters, turbine meters are used as a billing meter to measure the amount of gas used atcommercial buildings and industrial plants.

Ultrasonic Flowmeters

The use of ultrasonic flowmeters to measure natural gas flow gained momentumfollowing the publication of AGA-9 in June 1998. This report from the American GasAssociation gives criteria for using ultrasonic flowmeters for custody transfer of natural

gas. The AGA had previously issued reports on differential pressure (DP) flowmeters(AGA-3) and turbine flowmeters (AGA-7). Since the publication of AGA-9, the AGAhas also issued a report on the use of Coriolis flowmeters (AGA-11).

The use of ultrasonic flowmeters is continuing to grow, both for custody transfer andprocess gas measurement. Unlike PD and turbine meters, ultrasonic flowmeters do nothave moving parts. And pressure drop is much reduced with an ultrasonic meter whencompared to PD, turbine, and DP meters. Installation of ultrasonic meters is relativelystraightforward, and maintenance requirements are low.

Ultrasonic flowmeters come as both inline and clamp-on configurations. Some have one

ultrasonic beam, while those with higher accuracy use multiple beams. These are knownas multipath ultrasonic flowmeters. Meters used for custody transfer purposes are inlinemultipath meters. Most of these custody transfer meters use four, five, or six paths,depending on manufacturer, to make a highly accurate measurement. Manufacturersinclude Instromet, Emerson Daniel, and FMC Measurement Solutions.

Differential Pressure Flowmeters

DP flowmeters consist of a differential pressure transmitter, together with a primaryelement. The primary element places a constriction in the flowstream, and the DPtransmitter measures the difference in pressure upstream and downstream of theconstriction. The transmitter or a flow computer then computes flow, using Bernoullistheorem.

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 arestill the most widely used type of primary elements. Their disadvantages are the amountof pressure drop caused, and the fact that they can be knocked out of position byimpurities in the flowstream. Orifice plates are also subject to wear over time.

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DP flowmeters are used to measure the flow of liquid, gas, and steam. Like ultrasonicand turbine meters, they are used for the custody transfer of natural gas. In many cases,end-users buy their pressure transmitters and primary elements from different suppliers.However, several vendors have integrated the pressure transmitter with the primaryelement to form a complete flowmeter. These have the advantage that they can be

calibrated with the primary element and DP transmitter already in place.

Coriolis Flowmeters

Coriolis flowmeters are the most highly accurate meter. However, liquid flowmeasurement still predominates for Coriolis meters because gases are less dense thanliquids, and the measurement is somewhat more difficult. A number of suppliers havebrought out Coriolis meters for gas flow measurement, however, and this is a growingarea for Coriolis. One application that Coriolis has come to excel in is in measuringcompressed natural gas (CNG) for alternative fuel vehicles. Here they compete primarilywith turbine flowmeters.

Thermal

Thermal flowmeters are used almost exclusively to measure gas flow. Thermalflowmeters typically inject heat into the flowstream and then measure how quickly itdissipates. This value is proportional to mass flow. Two methods used are calledconstant current and constant temperature.

Thermal flowmeters grew out of the use of hot-wire anemometers for researchapplications. Early companies to develop thermal flowmeters include Sierra Instruments,

Kurz Instruments, and Fluid Components International (FCI). Thermal flowmeters excelat measuring gas at low flowrates. Measuring low flows is a difficulty for some meters,including vortex, but this is where thermal flowmeters shine. Accuracy levels areimproving for thermal flowmeters, as suppliers introduce product improvements.

One application where thermal flowmeters are widely used is in the measurement ofstack flows. Gas flow has to be measured in smoke stacks to conform to EnvironmentalProtection Agency (EPA) reporting requirements. Insertion thermal flowmeters are usedto measure the flow of sulfur dioxide (SO2), nitrogen oxide (NOx), and other industrialpollutants. Because of the large size of the stacks, insertion thermal meters that usemultiple measuring points are used for these applications. Other flowmeters used for

smoke stack applications are DP meters with averaging pitot tubes, and ultrasonicflowmeters.

Mass Flow Controllers

Mass flow controllers not only measure flow; they also control it. They differ fromthermal flowmeters in that most divert a small portion of the flow into a parallel channel,

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and then measure the flow through that parallel channel. The flowmeter then performs acalculation to determine mass flow through the entire flowmeter. Most also contain anintegrated valve that is used to control flow. A setpoint is determined, usually by the user,and the valve is adjusted so that flow reaches that setpoint.

Most mass flow controllers use thermal methods to determine flowrate in the parallelflow path, though some use a differential pressure principle. And some mass flowcontrollers are sold without the valve, meaning they are functioning as flowmeters ratherthan controllers. Mass flow controllers are widely used in the semiconductor industry,but many have industrial applications. It is important for a mass flow controller to knowwhat gas is being measured, in order to insure an accurate measurement. Mass flowcontrollers can also be used for to measure liquid flow.

Other Types

Vortex and variable area flowmeters are also used to measure gas flow. Vortexflowmeters are one of the few types of meters, besides DP, that can accurately measurethe flow of liquid, steam, and gas. However, vortex meters especially excel at measuringsteam flow, since they can handle the high temperatures involved.

Variable areas meters can measure the flow of both liquid and gas, and they also are usedfor a limited amount of steam flow measurement. Variable area meters rely on a float thatrises in proportion to flowrate. They are primarily a low-cost alternative where a visualindication of flow is sufficient. While most still must be read visually, some variable areameters have been manufactured with transmitters.

Ultrasonic Flow Meter and Doppler Flowmeters

Cole Parmer UKs range of ultrasonic flow meters and doppler

flowmeters offers process contamination free measurement of liquidsand gas flow. A typical ultrasonic flow meter will consist of a

transducer that is mounted on to the exterior of the pipe and an

indicator or totalizer to display the output. This can easily be installedwithout disturbing existing pipe work. Most ultrasonic flow meters will

work with pipe sizes between and 200 diameter. There are two

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main types of ultrasonic flow meters, doppler flow meters and timetransit ultrasonic flow meters.

A doppler flow meter requires particulates or bubbles in the media.

The minimum diameter size of the particulate is typically 30 microns

and requires a minimum concentration levels of 25 ppm. Since somemeters may require slightly larger concentrations of particulate, it is agood idea to check the specifications table. Doppler flow meters are

mainly used for liquid applications (roughly 88%) while the rest are

used for gas (11%) and steam (1%) applications.

A time transit ultrasonic flow meter requires a clean liquid without

particulates or bubbles and can be used for both liquid and gasapplications. It has better accuracy than that of a doppler flow meter

and will typically offer accuracies of 2% full scale.

To find the right flow meter for your application use Cole-Parmers on-line flow meter quick search. Alternatively contact our Application

Specialists for free technical support and let us bring over 50 years of

flow control experience to help solve your application needs.

Key Points for ultrasonic flow meters

The doppler flow meter and the time transit ultrasonic flow meter

measure the frequency shift of an ultrasonic signal sent throughthe media. In the case of a doppler flow meter it utilises particles

or bubbles in a fluid as a reflective mechanism to gauge thevelocity of the media. For a time transit ultrasonic flow meter it

relies on a frequency difference in forward and reverse signalssent through a clean liquid to gauge the velocity of the media.

A ultrasonic flow meter has an accuracy of typically 2% full

scale

A ultrasonic flow meter normally has a turn down ratio in the

region of 20:1

Ultrasonic flow meters are non intrusive

Ultrasonic flow meters can be used with a wide variety of pipe

materials, but some will not allow the signal to pass through.

Although pipe material recommendations will vary depending onthe sensor design, you should not expect to have any problems

with carbon steel, stainless steel, PVC, and copper. However,

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pipes made of concrete, fibreglass, iron, and plastic pipes withliners can pose transmission problems.

Advantages for ultrasonic flow meters

Process contamination free measurement of flow No leak potential

Allows for easy installation without disturbing existing pipe work

Suitable for aggressive chemicals as no contact with the media

Doppler flow meters are suitable for slurries

Disadvantages for ultrasonic flow meters

Requires sufficient knowledge of the flow and media to give

some confidence in readings Higher initial setup costs than other flow technologies

Pipe material must be compatible with ultrasonic sensor

Typical Application for ultrasonic flow meters

Influent and effluent water flow (doppler flow meters only)

Clarifier monitoring

Digester feed control (doppler flow meters only)

Waste water (doppler flow meters only) Cooling water

Makeup water

Pure and ultra-pure fluids in semiconductor, pharmaceutical, and

the food & beverage industries (time transit ultrasonic flow

meters only)

Acids and liquefied gases in the chemical industry

Light to medium crude oils in the petroleum refining industry

(time transit meters only)

Water distribution systems used in agriculture and irrigation Cryogenic liquids (time transit ultrasonic flow meters only)

Gas-stack flow measurement in power plant scrubbers

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Selecting the Right FlowmeterPart 1

Selecting the Right FlowmeterPart 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 FlowmetersTable 2: The Effect of Pressure Deviations on a Variable-Area Flowmeter

Mass Flowmeters Coriolis FlowmetersDifferential-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 unique

designs, and gives several application examples.

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

Table 1

A Comparison of Flowmeter Options

AttributeVariable-

areaCoriolis

Gasmass-

flow

Differential-Pressure

Turbine Oval Gear

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

Viscous

Liquids

yes (specialcalibration)

yes noyes (specialcalibration)

yes, >10centistokes

(cst)Corrosive

Liquidsyes yes no yes yes

Accuracy, 2-4% full

scale

0.05-0.15% of

reading

1.5%full

scale

2-3% full-scale

0.25-1% ofreading

0.1-0.5% ofreading

Repeatability,

0.25% fullscale

0.05-0.10% ofreading

0.5%full

scale

1% full-scale

0.1% ofreading

0.1% ofreading

Max pressure,

psi200 and up

900 andup

500and up

100 5,000 and up4,000 and up

Max temp., F 250 and up250 and

up150

and up122 300 and up 175 and up

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Pressure drop medium low low medium medium mediumTurndown

ratio10:1 100:1 50:1 20:1 10:1

25:1

Average cost* $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 andpressures, accuracy required, and approvals needed.

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Variable-Area Flowmeters

Design overview: The variable-area flowmeter (Figure 1) is one of theoldest technologies available and arguably the most well-known. It isconstructed of a tapered tube (usually plastic or glass) and a metal orglass float. The volumetric flowrate through the tapered tube isproportional to the displacement of the 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). Some variable-area meters overcome this slight inconvenience by spring loading thefloat withing the tube (Figure 2). Such a design can simplify installation

and add operator flexibility, especially when the meter must be installedin a tight physical space and a vertical installation is not possible.

Two types of variable-area flowmeters are generally available: direct-reading andcorrelated. The direct-reading meter allows the user to read the liquid or gas flowrate inengineering units (i.e., gal/min and L/min) printed directly on the tube, by aligning thetop of the float with the tick mark on the flowtube.

The advantage of a direct-reading flowmeter is that the flowrate is literally read directlyoff the flowtube. Correlated meters, on the other hand, have a unitless scale (typically tickmarks from 0 to 65, or 0 to 150), and come with a separate data sheet that correlates the

scale reading on the flowtube to the flowrate in a particular engineering unit. Thecorrelation sheets 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, or whateverengineering unit is needed.

The advantage of the correlated meter is that the same flowmeter can be used for variousgases and liquids (whose flow is represented by different units) by selecting theappropriate correlation sheets, where additional direct-reading meters would be required

Figure 1The plastic or

glass tube ofthe variable-areaflowmeterlets the uservisuallyinspect thefloat, whoseposition inthe taperedtub isproportionalto thevolumetric

flowrate.

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for different fluid applications. Similarly, ifpressure or temperature parameters change for agiven application, the user would simply use adifferent correlation sheet to reflect these newparameters. By comparison, for a direct-reading

meter, a change in operating parameters willcompromise the meter's accuracy, forcing it tobe returned to the factory for recalibration. Ingeneral, the average accuracy of a variable-areaflowmeter is 2-4% of fullscale flow.

Advantages: The major advantage of thevariable-area flowmeter is its relative low costand ease of installation. Because of its simplicityof design, the variable-area meter is virtuallymaintenance-free and, hence, tends to have a

long operating life.Another advantage is its flexibility in handling a wide range of chemicals. Today, all-PTFE meters are available to resist corrosive damage by aggressive chemicals. Theadvantage of a PTFE flowmeter with a built-in valve is that you can not only monitor thefluid flowrate, but you can control it, as well, by opening and closing the valve. If theapplication requires an all-PTFE meter, chances are the fluid is pretty corrosive, andmany users would like the option of controlling the flowrate by simply turning a valvethat is built into the flowmeter itself.

Disadvantages: One potential disadvantage of a variable-area flowmeter occurs when the

fluid temperature and pressure deviate from the calibration temperature and pressure.Because temperature and pressure variations will cause a gas to expand and contract,thereby changing density and viscosity, the calibration of a particular variable-areaflowmeter will no longer be valid as these conditions fluctuate. Manufacturers typicallycalibrate their gas flowmeters to a standard temperature and pressure (usually 70F withthe flowmeter outlet open to the atmosphere, 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 changesvery 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 2

The Effect of Pressure Deviations on a Variable-Area Flowmeter

Maximum flowrate, L/min Fluid temperature, F Outlet pressure, psi

Figure 2This variable-area meter with aspring-loaded float can be installed atany angle. This accommodation is notavailable for traditional variable-areaflowmeters, whose operation relies

on gravity.

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Fluid type: Air

2.23 70 01.65 70 151.30 70 35

2.26 90 02.28 110 02.32 150 0

Fluid type: water

4.82 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 tablewas created using data from a variable-area flowmeter that was calibrated for air at 70Fand with 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 ingas temperature (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 70F. Here, one can assume water to be incompressible. Asshown, there is no direct effect on water flow with a change in back-pressure. The temp-erature change is not that significant either. But, for various fluids, a change intemperature could change the viscosity enough to degradethe accuracy below acceptable limits.

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 toexisting conditions, but this presumes that operatingconditions will remain the samewhich they rarely do.

More Details or Order Online:

Gilmont Unshielded VariableArea Flowmeters

Gilmont Shielded VariableArea Flowmeters

Gilmont Shielded VariableArea Flowmeters without

Valve

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The effect of viscosity changes is another potential disadvantage of the variable-areameter when measuring liquids. When a viscous liquid makes its way through a variable-area flowmeter, drag layers of fluid will build up on the float. this will cause a slower-moving viscous liquid to yield the same buoyant force as a faster-moving fluid of lowerviscosity. The larger the viscosity, the higher the error. The general rule of thumb is as

followsunless 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 equationsto apply to variable-area flowmeters in both water and gas service when they deviatefrom standard 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 Monitoring chemical lines 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 & beverageplants

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Mass Flowmeters

Design Overview:Mass flowme

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2741666 Flow Measurement Technology[1]