Types of Flow Meters

7/31/2019 Types of Flow Meters

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TYPESOF FLOWMETERS

YASEMN YILDIZ


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FLOWMETERS IN GENERAL

Flowmeters are devices that measure the amountof liquid, gas or vapor that passes through them.Some flowmeters measure flow as the amount offluid passing through the flowmeter during a time

period .Other flowmeters measure the totalizedamount of fluid that has passed through theflowmeter .

Flowmeters consist of a primary device, transducerand transmitter. The transducer senses the fluid

that passes through the primary device. Thetransmitter produces a usable flow signal from theraw transducer signal. These components are oftencombined, so the actual flowmeter may be one ormore physical devices.


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Flow measurement can be described by

Q = A v, which means that the volume of fluid

passing through a flowmeter is equal to the cross-sectional area of the pipe (A) times the averagevelocity of the fluid (v); and

W = r Q, which means that the mass flow of fluidpassing through a flowmeter (A) is equal to the fluiddensity (r) times the volume of the fluid (Q).


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UNITSOFMEASUREMENT

Both gas and liquid flow can be measured involumetric or mass flow rates, such as liters persecond or kilograms per second. Thesemeasurements can be converted between one

another if the material's density is known. Thedensity for a liquid is almost independent of theliquid conditions; however, this is not the case forgas, the density of which depends greatly upon

pressure, temperature and to a lesser extent, thegas composition.


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GAS

Gases are compressible and change volume whenplaced under pressure, are heated or are cooled. Avolume of gas under one set of pressure andtemperature conditions is not equivalent to the

same gas under different conditions.

Gas mass flow rate can be directly measured,independent of pressure and temperature effects,with thermal mass flow meters, Coriolis mass flow

meters, or mass flow controllers.


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LIQUID

For liquids, various units are used depending uponthe application and industry, but might includegallons (U.S. liquid or imperial) per minute, litersper second, bushels per minute or, when describing

river flows, cumecs (cubic metres per second).common unit to measure volume transport (volumeof water transported by a current for example) is asverdrup(Sv) equivalent to 106 m3 / s.


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REYNOLDSNUMBERS

The performance of flowmeters is also influenced by a dimensionlessunit called the Reynolds Number. It is defined as the ratio of theliquid's inertial forces to its drag forces.

The equation is:

R = 3160 x Q x GtD x h

where:

R = Reynolds number

Q = liquid's flow rate, gpm

Gt = liquid's specific gravity

D = inside pipe diameter, in.

h = liquid's viscosity, cp


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The flow rate and the specific gravity are inertiaforces, and the pipe diameter and viscosity are dragforces. The pipe diameter and the specific gravityremain constant for most liquid applications. At very

low velocities or high viscosities, R is low, and theliquid flows in smooth layers with the highestvelocity at the center of the pipe and low velocitiesat the pipe wall where the viscous forces restrain it.

This type of flow is called laminar flow. R values arebelow approximately 2000


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FLOWMETER TYPES

Numerous types of flowmeters are available forclosed-piping systems. In general, the equipmentcan be classified as differential pressure, positivedisplacement, velocity, and mass meters.


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FLOWMETER TYPES

Differential

Pressure

Positive

Displacement

Velocity Mass Open-

Channel

Orifice PlateVenturi TubeFlow Tube

Flow NozzlePitot TubeElbow Tap

TargetVariable-

Area(Rotameter)

ReciprocatingPiston

Oval Gear

Nutating DiskRotary Vane

TurbineVortex

Shedding

SwirlConada Effect &

MomentumExchange

ElectromagneticUltrasonic,

DopplerUltrasonic,

Transit-Time

CoriolisThermal

WeirFlume
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Differential pressuredevices (also known ashead meters) includeorifices, venturi tubes,

flow tubes, flownozzles, pitot tubes,elbow-tap meters,target meters, and

variable-area meters,Fig.


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DIFFERENTIAL PRESSURE METERS

In a differential pressure (or differential head) flowmeter,a restriction is used to create a pressuredrop. Measuring the pressure drop across therestriction can be used to determine the flowrate.

The basic operating principle of differential pressure

flowmeters is based on the premise that the pressuredrop across the meter is proportional to the square ofthe flow rate. The flow rate is obtained by measuring thepressure differential and extracting the square root

Differential pressure flowmeters, like most flowmeters,have a primary and secondary element. The primary

element causes a change in kinetic energy, whichcreates the differential pressure in the pipe. Thesecondary element measures the differential pressureand provides the signal or read-out that is converted tothe actual flow value


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Orifices are the most popular liquid flowmeters inuse today. An orifice is simply a flat piece of metalwith a specific-sized hole bored in it. Most orificesare of the concentric type, but eccentric, conical(quadrant), and segmental designs are alsoavailable.

In practice, the orifice plate is installed in the pipebetween two flanges. Acting as the primary device,the orifice constricts the flow of liquid to produce a

differential pressure across the plate. Major advantages of orifices are that they have no

moving parts and their cost does not increasesignificantly with pipe size.


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Venturi tubes have the advantage of being able tohandle large flow volumes at low pressure drops. Aventuri tube is essentially a section of pipe with atapered entrance and a straight throat. As liquid

passes through the throat, its velocity increases,causing a pressure differential between the inletand outlet regions.

The flowmeters have no moving parts. They can be

installed in large diameter pipes using flanged,welded or threaded-end fittings. Four or morepressure taps are usually installed with the unit toaverage the measured pressure.


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Flow tubes are somewhat similar to venturi tubesexcept that they do not have the entrance cone.They have a tapered throat, but the exit iselongated and smooth. The distance between the

front face and the tip is approximately one-half thepipe diameter. Pressure taps are located aboutone-half pipe diameter downstream and one pipediameter upstream


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Flow Nozzles, at high velocities, can handleapproximately 60 percent greater liquid flow thanorifice plates having the same pressure drop.Liquids with suspended solids can also be metered.

However, use of the units is not recommended forhighly viscous liquids or those containing largeamounts of sticky solids


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Pitot tubes sense two pressures simultaneously,impact and static. The impact unit consists of a tubewith one end bent at right angles toward the flowdirection. The static tube's end is closed, but a

small slot is located in the side of the unit. Thetubes can be mounted separately in a pipe orcombined in a single casing.

Pitot tubes are generally installed by welding a

coupling on a pipe and inserting the probe throughthe coupling.


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Elbow tap meters operate on the principle thatwhen liquid travels in a circular path, centrifugalforce is exerted along the outer edges. Thus, whenliquid flows through a pipe elbow, the force on the

elbow's interior surface is proportional to thedensity of the liquid times the square of its velocity.In addition, the force is inversely proportional to theelbow's radius.


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Target meters sense andmeasure forces caused byliquid impacting on a target ordrag-disk suspended in theliquid stream. A directindication of the liquid flow

rate is achieved bymeasuring the force exertedon the target. In its simplestform, the meter consists onlyof a hinged, swinging platethat moves outward, alongwith the liquid stream. In such

cases, the device serves as aflow indicator.


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Variable-area meters, often called rotameters,consist essentially of a tapered tube and a float.Although classified as differential pressure units,they are, in reality, constant differential pressure

devices


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POSITIVE-DISPLACEMENT METERS

Operation of these units consists of separatingliquids into accurately measured increments andmoving them on. Each segment is counted by aconnecting register. Because every increment

represents a discrete volume, positive-displacement units are popular for automaticbatching and accounting applications. Positive-displacement meters are good candidates for

measuring the flows of viscous liquids or for usewhere a simple mechanical meter system isneeded.


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Reciprocating pistonmeters are of the singleand multiple-piston types.The specific choicedepends on the range of

flow rates required in theparticular application. Pistonmeters can be used tohandle a wide variety ofliquids. A magneticallydriven, oscillating piston

meter is shown in Fig. .Liquid never comes incontact with gears or otherparts that might clog orcorrode.


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Oval-gear meters have two rotating, oval-shapedgears with synchronized, close fitting teeth. A fixedquantity of liquid passes through the meter for eachrevolution. Shaft rotation can be monitored to obtain

specific flow rates.


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Nutating-disk meters have a moveable diskmounted on a concentric sphere located in aspherical side-walled chamber. The pressure of theliquid passing through the measuring chamber

causes the disk to rock in a circulating path withoutrotating about its own axis. It is the only moving partin the measuring chamber.


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Rotary-vane meters are available in severaldesigns, but they all operate on the same principle.The basic unit consists of an equally divided,rotating impeller (containing two or more

compartments) mounted inside the meter's housing.The impeller is in continuous contact with thecasing. A fixed volume of liquid is swept to themeter's outlet from each compartment as the

impeller rotates. The revolutions of the impeller arecounted and registered in volumetric units.


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VELOCITYMETERS

These instruments operate linearly with respect tothe volume flow rate. Because there is no square-root relationship (as with differential pressuredevices). Velocity meters have minimum sensitivity

to viscosity changes when used at Reynoldsnumbers above 10,000.


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Turbine meters have foundwidespread use for accurate liquidmeasurement applications. Theunit consists of a multiple-bladedrotor mounted with a pipe,perpendicular to the liquid flow. Therotor spins as the liquid passesthrough the blades. The rotational

speed is a direct function of flowrate and can be sensed bymagnetic pick-up, photoelectriccell, or gears. Electrical pulses canbe counted and totalized, Fig..

The number of electrical pulsescounted for a given period of timeis directly proportional to flowvolume. A tachometer can beadded to measure the turbine'srotational speed and to determinethe liquid flow rate. Turbine meters,when properly specified andinstalled, have good accuracy,particularly with low-viscosityliquids.


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Vortex meters makeuse of a naturalphenomenon thatoccurs when a liquid

flows around a bluffobject. The frequencyof the vortexshedding is directlyproportional to thevelocity of the liquidflowing through themeter, Fig.


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The three majorcomponents of theflowmeter are a bluffbody strut-mountedacross the flowmeter

bore, a sensor to detectthe presence of thevortex and to generate anelectrical impulse, and asignal amplification and

conditioning transmitterwhose output isproportional to the flowrate, Fig.


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Electromagnetic meters canhandle most liquids andslurries, providing that thematerial being metered iselectrically conductive. Majorcomponents are the flow tube(primary element), Fig.. Theflow tube mounts directly inthe pipe. Pressure dropacross the meter is the sameas it is through an equivalentlength of pipe because thereare no moving parts or

obstructions to the flow. Thevoltmeter can be attacheddirectly to the flow tube orcan be mounted remotelyand connected to it by ashielded cable.


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Electromagnetic flowmetersoperate on Faraday's law ofelectromagnetic inductionthat states that a voltage willbe induced when a conductormoves through a magneticfield. The liquid serves as theconductor; the magnetic fieldis created by energized coilsoutside the flow tube, Fig.The amount of voltageproduced is directlyproportional to the flow rate.

Two electrodes mounted inthe pipe wall detect thevoltage, which is measuredby the secondary element.


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Ultrasonic flowmeters can bedivided into Doppler meters andtime-of-travel (or transit) meters.Doppler meters measure thefrequency shifts caused by liquidflow. Two transducers aremounted in a case attached to

one side of the pipe. A signal ofknown frequency is sent into theliquid to be measured. Solids,bubbles, or any discontinuity inthe liquid, cause the pulse to bereflected to the receiver element,Fig. Because the liquid causingthe reflection is moving, the

frequency of the returned pulseis shifted. The frequency shift isproportional to the liquid'svelocity.


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Time-of-travel meters have transducers mountedon each side of the pipe. The configuration is suchthat the sound waves traveling between the devicesare at a 45 deg. angle to the direction of liquid flow.

The speed of the signal traveling between thetransducers increases or decreases with thedirection of transmission and the velocity of theliquid being measured.


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MASS FLOWMETERS

The continuing need for more accurate flowmeasurements in mass-related processes(chemical reactions, heat transfer, etc.) has resultedin the development of mass flowmeters. Various

designs are available, but the one most commonlyused for liquid flow applications is the Coriolismeter. Its operation is based on the naturalphenomenon called the Coriolis force, hence the

name.


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Coriolis meters are true mass meters that measure themass rate of flow directly as opposed to volumetric flow.Because mass does not change, the meter is linearwithout having to be adjusted for variations in liquidproperties. It also eliminates the need to compensate for

changing temperature and pressure conditions. Themeter is especially useful for measuring liquids whoseviscosity varies with velocity at given temperatures andpressures.

Coriolis meters are also available in various designs. A

popular unit consists of a U-shaped flow tube enclosedin a sensor housing connected to an electronics unit.The sensing unit can be installed directly into anyprocess. The electronics unit can be located up to 500feet from the sensor.


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Inside the sensorhousing, the U-shapedflow tube is vibrated atits natural frequency by

a magnetic devicelocated at the bend ofthe tube.


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Thermal-type mass flowmetershave traditionally been used forgas measurements, but designsfor liquid flow measurements areavailable. These mass metersalso operate independent ofdensity, pressure, and viscosity.Thermal meters use a heatedsensing element isolated fromthe fluid flow path. The flowstream conducts heat from thesensing element. The conductedheat is directly proportional to themass flow rate. The sensor nevercomes into direct contact with

the liquid, Fig.


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OPEN CHANNEL METERS

The "open channel" refers to any conduit in whichliquid flows with a free surface. Included aretunnels, nonpressurized sewers, partially filledpipes, canals, streams, and rivers. Of the many

techniques available for monitoring open-channelflows, depth-related methods are the mostcommon. These techniques presume that theinstantaneous flow rate may be determined from a

measurement of the water depth, or head. Weirsand flumes are the oldest and most widely usedprimary devices for measuring open-channel flows


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Weirs operate on the principle that an obstructionin a channel will cause water to back up, creating ahigh level (head) behind the barrier. The head is afunction of flow velocity, and, therefore, the flow rate

through the device. Weirs consist of vertical plateswith sharp crests. The top of the plate can bestraight or notched. Weirs are classified inaccordance with the shape of the notch. The basic

types are V-notch, rectangular, and trapezoidal.


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Flumes are generally used when head loss mustbe kept to a minimum, or if the flowing liquidcontains large amounts of suspended solids.Flumes are to open channels what venturi tubes

are to closed pipes. Popular flumes are the Parshalland Palmer-Bowlus designs.


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Types of Flow Meters

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