mag flow ref

8/8/2019 Mag Flow Ref

1/5

H-3

MAGNETIC FLOWMETERSFlow Reference Section

INTRODUCTIONMagnetic flowmeters are lowpressure drop, volumetric, liquidflow measuring devices. The lowmaintenance designwith no

moving parts, high accuracy, linearanalog outputs, insensitivity tospecific gravity, viscosity, pressureand temperature, and the ability tomeasure a wide range of difficult-to-meter fluids (such as corrosives,slurries and sludges)differentiatesthis type of metering system fromother flowmeters. Two basic stylesof magnetic flowmeter are currentlyavailable from OMEGA Engineering:1) Wafer-style, where highestaccuracy (up to +0.5% of reading)measurements are required; and 2)Insertion-style, for greater economy

and particularly for larger pipe sizes.All OMEGA magnetic flowmetersemploy the state-of-the-art dcpulsed magnetic field system. Thefollowing discussion details theprinciple of operation, as well as theadvantages, of dc pulsed typemagnetic flowmeters.

PRINCIPLE OF OPERATION

Faradays Law

The operation of a magnetic

flowmeter is based upon FaradaysLaw, which states that the voltageinduced across any conductor as itmoves at right angles through amagnetic field is proportional to thevelocity of that conductor.

Faradays Formula:

E is proportional to V x B x D

where:

E = The voltage generatedin a conductor

V = The velocity of the conductor

B = The magnetic field strength

D = The length of the conductorTo apply this principle to flowmeasurement with a magneticflowmeter, it is necessary first tostate that the fluid being measuredmust be electrically conductive forthe Faraday principle to apply.

As applied to the design of magneticflowmeters, Faradays Law indicatesthat signal voltage (E) is dependenton the average liquid velocity (V)

the magnetic field strength (B) andthe length of the conductor (D)(which in this instance is thedistance between the electrodes).

In the case of wafer-style magneticflowmeters, a magnetic field isestablished throughout the entirecross-section of the flow tube(Figure 1). If this magnetic field is

considered as the measuringelement of the magnetic flowmeter,it can be seen that the measuringelement is exposed to the hydraulicconditions throughout the entirecross-section of the flowmeter. Withinsertion-style flowmeters, themagnetic field radiates outwardfrom the inserted probe (Figure 2).

Figure 2: Insertion-type flowmeter operating principle

Figure 1: In-line magnetic flowmeter operating principle


2/5

H-4

MAGMETER SELECTIONThe characteristics of the fluid to bemetered, the liquid flow parameters,and the environment of the meter

are the determining factors in theselection of a particular type offlowmeter.

Conductivity

Electrical conductivity is simply away of expressing the ability of aliquid to conduct electricity. Just ascopper wire is a better conductorthan tin, some liquids are betterconductors than others. However,of even greater importance is thefact that some liquids have little orno electrical conductivity (such ashydrocarbons and many non-

aqueous solutions, which lacksufficient conductivity for use withmagmeters). Conversely, mostaqueous solutions are well suitedfor use with a magmeter. Dependingon the individual flowmeter, theliquid conductivity must be abovethe minimum requirementsspecified. The conductivity of theliquid can change throughoutprocess operations withoutadversely affecting meterperformance, as long as it ishomogeneous and does not dropbelow the minimum conductivity

threshold. Several factors should betaken into consideration concerningliquids to be metered usingmagnetic flowmeters. Some ofthese are:

1. All water does not have the sameconductivity. Water varies greatlyin conductivity due to various ionspresent. The conductivity of tapwater in Maine might be verydifferent from that of tap waterin Chicago.

2. Chemical and pharmaceuticalcompanies often use deionized or

distilled water, or other solutionswhich are not conductive enoughfor use with magnetic flowmeters.

3. Electrical conductivity is afunction of temperature.However, conductivity does notvary in any set pattern for allliquids as temperature changes.Therefore, the temperature of theliquid being considered shouldalways be known.

4. Electrical conductivity is afunction of concentration.Therefore, the concentration ofthe solution should always beprovided. However, avoid what

normally is a logical assumption,such as: That electricalconductivity increases asconcentration increases. Thisis true up to a point in somesolutions, but then reverses.For example, the electricalconductivity of aqueous solutionsof acetic acid increases asconcentration rises up to 20%,but then shows a decrease withincreased concentration to theextent that, at someconcentration above 99%, it fallsbelow the minimum requirement.

Acid/Caustics

The chemical composition of theliquid slurry to be metered will be adetermining factor in selecting theflowmeter with the proper designand construction.

Operating experience is the bestguide to selection of liner andelectrode materials, especially inindustrial applications, because, inmany cases, a process liquid orslurry will be called by a genericname, even though it may contain

other substances which affect itscorrosion characteristics. Commonlyavailable corrosion guides may alsoprove helpful in selecting the propermaterials of construction.

Velocity

The maximum (full scale) liquidvelocity must be within the specifiedflow range of the meter for properoperation. The velocity through theflowhead can be controlled byproperly sizing the meter. It isntnecessary that the flowhead be the

same line size, as long as suchsizing does not conflict with othersystem design parameters.Although the meter will increasehydraulic head loss when sizedsmaller than the line size (becausethe meter is both obstructionlessand of short lay length), the amountof increase in head loss is negligiblein most applications. The amount ofhead loss increase can be furtherlimited by using concentric reducers

and expanders at the pipe sizetransitions. As a rule of thumb,meters should be sized no smallerthan one-half of the line size.

Because of the wide rangeability ofmagnetic flowmeters, it is almostnever necessary to oversize a meterto handle future flow requirements.When future flow requirements areknown to be significantly higher thanstart-up flow rates, it is imperativethat the initial flows be sufficientlyhigh and that the pipeline remain fullunder normal flow conditions.

Abrasive Slurries

Mildly abrasive slurries can behandled by magnetic flowmeters

without problems, providedconsideration is given to theabrasiveness of the solids and theconcentration of the solids in theslurry. The abrasiveness of a slurrywill affect the selection of theconstruction materials and the useof protective orifices. Abrasiveslurries should be metered at6 ft/sec or less in order to minimizeflowmeter abrasion damage.Velocities should not be allowed tofall much below 4 ft/sec, since anysolids will tend to settle out. An idealslurry installation would have the

meter in a vertical position. Thiswould assure uniform distribution ofthe solids and avoid having solidssettle in the flow tube duringno-flow periods.

Consideration should also be givento use of a protective orifice on theupstream end of a wafer-stylemagnetic flowmeter to preventexcessive erosion of the liner. Thisis especially true since Tefzel linerhave excellent chemical resistance,but poor resistance to abrasion. Inlined or non-conductive piping

systems, the upstream protectiveorifice can also serveas a grounding ring.

Sludges andGrease-Bearing Liquids

Sludges and grease-bearing liquidsshould be operated at highervelocities, about 6 ft/sec minimum,in order to reduce the coatingtendencies of the material.


3/5

H-5

Viscosity

Viscosity does not directly affectthe operation of magneticflowmeters, but, in highly viscous

fluids, the size should be kept aslarge as possible to avoid excessivepressure drop across the meter.

Temperature

The liquids temperature is generallynot a problem, providing it remainswithin the mechanisms operatinglimits. The only other temperatureconsiderations would be in the caseof liquids with low conductivities(below around 3 micromhos percentimeter) which are subjectto wide temperature excursions.Since most liquids exhibit apositive temperature coefficientof conductivity, the liquids minimumconductivity must be determinedat the lower temperature extreme.

Advantages of theDC Pulse Style

From the principles of operation,it can be seen that a magneticflowmeter relies on the voltagegenerated by the flow of aconductive liquid through itsmagnetic field for a direct indicationof the velocity of the liquid or slurry

being metered. The integrity of thislow-level voltage signal (typicallymeasured in hundreds ofmicrovolts) must be preserved so asto maintain the high accuracyspecification of magnetic flowmetersin industrial environments. Thesuperiority of the dc pulse over thetraditional ac magnetic meters inpreserving signal integrity can bedemonstrated as follows:

Quadrature

Some magnetic flowmeters employalternating current to excite the

magnetic field coils which generatethe magnetic field of the flowmeter(ac magnetic flowmeters). As aresult, the direction of the magneticfield alternates at line frequency,i.e., 50 to 60 times per second.If a loop of conductive wire islocated in a magnetic field, avoltage will be generated in thatloop of wire. From physics, we candetermine that this voltage is 90out of phase with respect tothe primary magnetic field.

The magnitude of this error signal isa function of the number of turnsin the loop, and the changein magnetic flux per unit time. In a

magnetic flowmeter, the electrodewires and the path through theconductive liquid between theelectrodes represent a single turnloop. The flow-dependent voltage isin phase with the changingmagnetic field; however, flow-independent voltage is alsogenerated, which is 90out of phasewith the changing magnetic field.

The flow-independent voltage istherefore an error voltage whichis 90 out of phase with the desiredsignal. This error voltage is oftenreferred to as quadrature. In order

to minimize the amount ofquadrature generated, the electrodewires must be arranged so that theyare parallel with the lines of flux ofthe magnetic field.

In ac field magmeters, becausethe magnetic field alternatescontinuously at line frequency,quadrature is significant. It isnecessary to employ phase-sensitive circuitry to detect andreject quadrature.

It is this circuitry which makesthe ac magnetic meter highlysensitive to coating on theelectrodes. Since coatingscause a phase shift in thevoltage signal, phase-sensitive circuitry leads torejection of the true voltageflow signal, thus leadingto error.

Since dc pulse magmetersare not sensitive to phaseshift and require no phase-sensitive circuitry, coatingson the electrodes havea very limited effect onflowmeter performance.

Wiring

In ac magnetic flowmeters,the signal generated by flowthrough the meter is at linefrequency. This makes thesemeters susceptible to noisepickup from any ac lines.Therefore, complicated wiringsystems are typically requiredto isolate the ac flowmetersignal lines from both its ownand from any other nearby

power lines, in order to preservesignal integrity.

In comparison, dc pulse magmetershave a pulse frequency much lower

(typically 5 to 10% of ac linefrequency) than ac meters. Thislower frequency eliminates noisepickup from nearby ac lines,allowing power and signal lines tobe run in the same conduit and thussimplifying wiring. Wiring is furthersimplified by the use of integralsignal conditioners to providevoltage and current outputs. Noseparate wiring to the signalconditioners is required.

Power

By design, ac magnetic flowmeters

typically have high powerrequirements, owing to the factthat the magnetic field is constantlybeing powered. Because of thepulsed nature of the dc pulsemagmeter, power is suppliedintermittently to the magnetic fieldcoil. This greatly reduces bothpower requirements and heatingof the electronic circuitry, extendingthe life of the instrument.

MAGNETIC FLOWMETERSFlow Reference Section

Figure 3: Vertical installation of inline meter


4/5

H-6

Auto-Zero

In traditional ac magneticflowmeters, it is necessary afterinstallation of the meter to null

or zero the unit. This isaccomplished by manualadjustment which requires that theflowmeter be filled with processliquid in a no-flow condition.Any signal present under full pipe,no-flow conditions is consideredto be an error signal. The ac fieldmagmeter is therefore nulledto eliminate the impact of theseerror signals.

Many OMEGA Magmeters featureautomatic zeroing circuitry toeliminate the need for manualzeroing. When the magnetic field

strength is zero between pulses,the voltage output from theelectrodes is measured. If anyvoltage is measured during thisperiod, it is considered extraneousnoise in the system and issubtracted from the signal voltagegenerated when the magnetic fieldis on. This feature insures highaccuracy, even in electrically noisyindustrial environments.

Installation

OMEGA magnetic flowmeters aredesigned for easy installation.

FMG600 Series Magmeters areideal substitutes for the flangedspool type meter, which are heavierand significantly more expensive.The thin wafer style of the FMG600Series allows them to be slippedbetween standard flanges, withoutthe need to cut away pipe to makeroom for the meter. Furthermore,the low weight of the meter meansthat, in many cases, no additionalpipe supports are required aftermeter installation. Recommendedpiping configurations includethe installation of by-pass piping,

cleanout tees and isolation valvesaround the flowmeter(Figures 3 and 4).

With insertion-style magmeters,even greater reductions in weightand cost have been achieved.Installation is accomplished bythreading the piping system into

the tee fitting supplied with themeter, or by drilling a tap into theline to accept the fitting that comeswith the meter.

Prior to installation of the meter, thefollowing recommendations anditems of general information shouldbe considered.

First, before installing a magmeter,it is important to consider location.

Stray electromagnetic orelectrostatic fields of high intensitymay cause disturbances in normaloperation. For this reason, it isdesirable to locate the meter awayfrom large electric motors,transformers, communicationsequipment, etc., whenever possible.

Second, for proper and accurateoperation, it is necessary that theflowmeter be installed so that thepipe will be full of the process liquidunder all operating conditions.When the meter is only partiallyfilled, even though the electrodesare covered, an inaccuratemeasurement will result.

Third, for magnetic flowmeters,grounding is required to eliminatestray current and voltage which maybe transmitted through the pipingsystem, through the process liquid,or can arise by induction from

electromagnetic fields in the samearea as the magmeter. Grounding isachieved by connecting the pipingsystem and the flowmeter to aproper earth ground system.Unfortunately, this is not alwaysdone properly, resulting inunsatisfactory meter performance.In conductive piping systems, athird wire safety ground to the

power supply and a conductive pathbetween the meter and the pipingflanges are typically all that isrequired. In non-conductive or linedpiping systems, a protectivegrounding orifice must be suppliedto provide access to the potential ofthe liquid being metered. Dedicatedor sophisticated grounding systemsare not normally required. Detailedinformation concerning properflowmeter grounding is providedwith the owners manual that comeswith each flowmeter.

Finally, the position of the flowtube

in relation to other devices in thesystem is also important in assuringsystem accuracy. Tees, elbows,valves, etc., should be placed atleast 10 upstream and 5downstream pipe diameters awayfrom the meter to minimize anyobstructions or flow disturbances.

Figure 4: Horizontal installation of in-line meter


5/5

One Omega Drive | Stamford, CT 06907 | 1-888-TC-OMEGA (1-888-826-6342) | [email protected]

www.omega.com

More than 100,000 Products Available!

CANADAwww.omega.ca

Laval(Quebec)1-800-TC-OMEGA

UNITED KINGDOMwww. omega.co.uk

Manchester, England0800-488-488

GERMANYwww.omega.de

Deckenpfronn, Germany0800-8266342

FRANCEwww.omega.fr

Guyancourt, France

088-466-342

BENELUXwww.omega.nl

Amstelveen, NL

0800-099-33-44

UNITED STATESwww.omega.com

1-800-TC-OMEGAStamford, CT.

CZECH REPUBLICwww.omegaeng.cz

Karvin, Czech Republic596-311-899

TemperatureCalibrators, Connectors, General Test and Measurement

Instruments, Glass Bulb Thermometers, Handheld Instrumentsfor Temperature Measurement, Ice Point References,

Indicating Labels, Crayons, Cements and Lacquers, Infrared

Temperature Measurement Instruments, Recorders Relative

Humidity Measurement Instruments, RTD Probes, Elements

and Assemblies, Temperature & Process Meters, Timers and

Counters, Temperature and Process Controllers and Power

Switching Devices, Thermistor Elements, Probes and

Assemblies,Thermocouples Thermowells and Head and Well

Assemblies, Transmitters, Wire

Pressure, Strain and ForceDisplacement Transducers, Dynamic Measurement

Force Sensors, Instrumentation for Pressure and Strain

Measurements, Load Cells, Pressure Gauges, Pressure

Reference Section, Pressure Switches, Pressure Transducers

Proximity Transducers, Regulators,

Strain Gages, Torque Transducers, Valves

pH and ConductivityConductivity Instrumentation, Dissolved Oxygen

Instrumentation, Environmental Instrumentation, pH

Electrodes and Instruments, Water and Soil Analysis

Instrumentation

HeatersBand Heaters, Cartridge Heaters, Circulation Heaters,Comfort Heaters, Controllers, Meters and SwitchingDevices, Flexible Heaters, General Test and MeasuremenInstruments, Heater Hook-up Wire, Heating CableSystems, Immersion Heaters, Process Air and Duct,Heaters, Radiant Heaters, Strip Heaters, Tubular Heaters

Flow and Level

Air Velocity Indicators, Doppler Flowmeters, LevelMeasurement, Magnetic Flowmeters, Mass Flowmeters,

Pitot Tubes, Pumps, Rotameters, Turbine and Paddle Wheel

Flowmeters, Ultrasonic Flowmeters, Valves, Variable Area

Flowmeters, Vortex Shedding Flowmeters

Data AcquisitionAuto-Dialers and Alarm Monitoring Systems,

Communication Products and Converters, DataAcquisition and Analysis Software, Data LoggersPlug-in Cards, Signal Conditioners, USB, RS232, RS485and Parallel Port Data Acquisition Systems, WirelessTransmitters and Receivers

click here to go to the omega.com home page
http://www.omega.com/http://www.omega.ca/http://www.omega.co.uk/http://www.omega.co.uk/http://www.omega.de/http://www.omega.de/http://www.omega.de/http://www.omega.fr/http://www.omega.fr/http://www.omega.nl/http://www.omega.nl/http://www.omega.com/http://www.omega.com/http://www.omegaeng.cz/http://www.omegaeng.cz/http://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/http://www.omega.com/http://www.omega.com/http://www.omega.com/heaters/hsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/green/gsc.htmlhttp://www.omega.com/pressure/psc.htmlhttp://www.omega.com/das/dsc.htmlhttp://www.omega.com/temperature/tsc.htmlhttp://www.omega.nl/http://www.omegaeng.cz/http://www.omega.fr/http://www.omega.co.uk/http://www.omega.de/http://www.omega.ca/http://www.omega.com/http://www.omega.com/

mag flow ref

Documents