B-5

• Up to 1⁄4% of Reading Repeatability

• Simple to Install and Maintain

• No Up- or Downstream Piping Limitations

• Broad Rangeability

• Glass, Plastic and Metal Tube Designs

• Especially Suited for Low Flowrate Metering

• Piston and Vane Units Can Be Mounted in Any Position

ROTAMETERSThe rotameter is an industrialflowmeter used to measure the flowrate of liquids and gases. Itsoperation is based on the variablearea principle: fluid flow raises afloat in a tapered tube, increasingthe area for passage of the fluid.The greater the flow, the higher thefloat is raised. The height of the floatis directly proportional to theflowrate. With liquids, the float israised by a combination of thebuoyancy of the liquid and the

velocity head of the fluid. Withgases, buoyancy is negligible, and the float responds to thevelocity head alone.

The float moves up or down in thetube in proportion to the fluidflowrate and the annular areabetween the float and the tube wall.

The float reaches a stable positionin the tube when the upward forceexerted by the flowing fluid equalsthe downward gravitational forceexerted by the weight of the float. A change in flowrate upsets thisbalance of forces. The float thenmoves up or down, changing theannular area until it again reaches aposition where the forces are inequilibrium. To satisfy the forceequation, the rotameter floatassumes a distinct position for everyconstant flowrate. However, it isimportant to note that because thefloat position is gravity dependent,rotameters must be verticallyoriented and mounted.

The rotameter is popular because it has a linear scale, a relatively long measurement range, and low

INTRODUCTIONVariable area flowmeters are very simple yet versatile flowmeasurement devices for use on alltypes of liquids, gases and steam.

They operate on the variable areaprinciple, whereby a flowing fluidchanges the position of a float,piston, or vane to open a larger area for the passage of the fluid.The position of the float, piston, orvane is used to give a direct visualindication of the flowrate.

The variable area family offlowmeters includes the followingfeatures and capabilities:

• Flowrate Meters and Controllers

• Measurement of Liquids and Gases

• Direct Visual Indication

• Low Pressure Drop

• 6 to 76 mm (1⁄4 to 3") Typical Size

• Visual, Transmitting andAlarm Models

• Up to 2% of Reading Accuracy

VARIABLE AREA FLOWMETERSFlow Reference Section

FL-8115A, $735,see page B-58.

FL6715A, $211,see page B-65.

FL-9204, $94,see page B-49.

FL-1601A, $1100,see page B-44.

FL-75K, $607,see page B-39.

FL4212-V, withoptional valve,$65, see pageB-17.

B-6

B

but the operating pressure for alarge 51 mm (2") tube may be aslow as 100 psig. The practicaltemperature limit for glassrotameters is 204°C (400°F),although operation at such hightemperatures substantially reducesthe operating pressure of the meter.In general, there is a linearrelationship between the operatingtemperature and pressure.

METAL TUBEROTAMETERSFor higher pressures andtemperatures beyond the practicalrange of glass tubes, metal tubesare used. These are usuallymanufactured of stainless steel, with stainless steel floats. Theposition of the float is determined by magnetic or mechanical followersthat can be read from the outside ofthe stainless steel metering tube. As with glass tube rotameters, the tube-and-float combinationdetermines the flowrate, and thefittings and materials of constructionmust be chosen so as to satisfy the demands of the applications.

These meters are used for serviceswhere high operating pressure ortemperature, water hammer, orother forces would damage glassmetering tubes. Like the generalpurpose type, armored rotameterscan be used for most fluids,including corrosive liquids andgases. They are particularly wellsuited for steam applications, whereglass tubes are unacceptable.

PLASTIC TUBEROTAMETERSPlastic tubes are also used in somerotameter designs due to their lowercost and high impact strength. Theyare typically constructed ofpolycarbonate, with either metal or plastic end fittings. With plasticend fittings, care must be taken ininstallation, not to distort thethreads. Rotameters with all plasticconstruction are available forapplications where metal wettedparts cannot be tolerated, such aswith deionized water or corrosives.

SIZING ROTAMETERSThe reading of a rotameter isdependent upon the nature of thefluid being metered. Rotameters aretypically supplied with either a directreading scale for air or water, orcalibration data for air and water. It is therefore necessary to performmathematical calculations whenusing rotameters with other fluids.

The meter’s ability to repeat orreproduce this flowrate is of primary importance. Rotameters are repeatable up to ±1⁄4% of theinstantaneous flowrate. This featureenables the operator to reset oradjust the flow with confidence.

GLASS TUBEROTAMETERSThe basic rotameter is the glasstube indicating-type. The tube isprecision formed of borosilicateglass, and the float is preciselymachined from metal, glass orplastic. The metal float is usuallymade of stainless steel to providecorrosion resistance. The float has a sharp metering edge where thereading is observed by means of ascale mounted alongside the tube.

End fittings and connections ofvarious materials and styles areavailable. The important elementsare the tube and float, often calledthe tube-and-float combination,because it is this portion of therotameter which provides themeasurement. In fact, similar glass tube and stainless steel float combinations are generallyavailable, regardless of the type ofcase or end fittings the applicationcan demand, so as best to meetcustomer requirements. The scaleof the rotameter can be calibratedfor direct reading of air or water, or it may have a scale to read apercent of range or an arbitraryscale to be used with conversionequations or charts. Safety-shieldedglass tube rotameters are in general use throughout industry for measuring both liquids andgases. They provide flow capacitiesto about 60 GPM, and aremanufactured with end fittings of metal or plastic to meet thechemical characteristics of the fluid being metered.

The only fluids for which thesemeters are not suited are thosewhich attack glass metering tubes,such as water over 90°C (194°F),with its high pH which softens glass;wet steam, which has the sameeffect; caustic soda, which dissolvesglass; and hydrofluoric acid, whichetches glass.

The primary limitations of generalpurpose rotameters are thepressure and temperature limits ofthe glass metering tube. Small,6 mm (1⁄4") tubes are suitable forworking pressures up to 500 psig,

pressure drop. It is simple to install and maintain. It can bemanufactured in a variety ofconstruction materials and designed to cover a wide range ofpressures and temperatures. Therotameter can easily be sized orconverted from one kind of serviceto another. In general, it owes itswide use to its versatility ofconstruction and applications.

Because of its functionaladvantages the rotameter is an exceptionally practical flowmeasurement device. The pressuredrop across the float is low andremains essentially constant as theflowrate changes. Float response to flowrate changes is linear, and a 10-to-1 flow range or turndown isstandard. In the case of OMEGA®

laboratory rotameters, far greaterrangeability is possible through theuse of correlation equations.Rotameters can be installed directlyadjacent to pipe fittings withoutadverse effects on their meteringaccuracy, and the meters areinherently self-cleaning. Flowingbetween the tube wall and the float,the fluid provides a scouring actionwhich discourages the build-up offoreign matter.

Variable area flowmeters are used primarily to set flowrates. The operator observes the meter,and adjusts the valve to bring theprocess flow to the proper flowrate.

Variable area flowmeter, also called arotameter, has a float that moves up ordown in a tapered tube. The distance itmoves is proportional to the liquid flowrateand the annular area between the floatand the tube well.

B-7

Flowmeters Operating Principle ofFL-O Series

Flowmeters Operating Principle of FL-X Series

Piston-type VariableArea Flowmeter

General Purpose RotametersFor Liquids:GPM water equivalent = GPM metered liquid flow x

where: SGL= specific gravity of meteredliquid at operating conditions andSGF= specific gravity of rotameterfloat.

This equation converts the reading ofa rotameter calibrated for water to areading appropriate for another fluidof different density. Most rotametersare very sensitive to liquid viscosity,and this equation does not takeviscosity into account. Therefore, itmust be considered to provide only arough approximations.Specific gravity of Glass = 2.53Specific gravity of 316 SS = 8.04

For Gases:SCFM air flow from rotameterreading = SCFM true gas flow rate x

where:SG = specific gravity of metered gas (air = 1.0) at STP T0=temperature at operatingconditions, Rankine (F + 460) P0 = pressure at operatingconditions in psia = (psig + 14.7).

This formula converts the meteredflow, making allowance for theoperating temperature and pressure,to an equivalent flow of air in SCFMat 21°C (70°F) and 14.7 psia.Capacity tables are in SCFM atstandard temperature and pressureof 14.7 psia and 21°C (70°F).

End Cap

RetainingRing

Body FlowIndicator

DustGuard

ScaleAssembly

SealBumper

SpringMagnet/PistonAssembly

MeteringConeSpider

Plate

EndFitting

Flow

(SGF-1) x SGL(SGF - SGL)

(SG) (T0) (14.7)(1.0) (530) (P0)

VARIABLE AREA FLOWMETERSFlow Reference Section

B-8

B

When used with purge rotameters(meters with +10% full scaleaccuracy) these equations aresatisfactory, although not precise.When used with ±2% full scaleaccuracy meters, the correlationholds as long as the viscosity of thefluid does not exceed 6 centistokes.Above this viscosity, field calibrationis required.

General PurposeRotametersFL-1800 SeriesGeneral purpose glass tuberotameters with ball floats cannot becorrelated with precision, due totheir strong sensitivity to variationsin viscosity. OMEGA FL-1800Series (page B-29) rotameters aresupplied with correlation charts fornumerous gases at varioustemperatures and pressures. For highest precision, fieldcalibration is recommended.

Piston and Vane Type VariableArea FlowmetersPiston-type flowmeters use anannular orifice formed by a pistonand a tapered cone. The piston isheld in place at the base of the cone(in the “no flow position”) by acalibrated spring. Flow through

Correlation of Spring and Piston FlowmetersThese units are calibrated toindicate standard cubic feet perminute (SCFM) of air at 100 psigand 21°C (70°F). When used atother conditions, the followingcorrections apply:

SCFM (true flowrate) = SCFM(indicated)/ (f1 x f2 x f3) where:

f1 = � 114.7/(14.7 + operating psig)

f2 = � (460 + operating °F)/530

f3 = � molecular weight of gas

29

that moves the piston against thespring. Piston movement and orificearea are proportional to flowrate. Incase of the vaned-type units, thefluid flow forces the vane to rotateagainst a spring, increasing theorifice area for flow. The position ofthe piston or vane is then read on ascale to give the flowrate. Since theforce of a spring opposes the flow(in comparison to rotameters, whichuse gravity), these units may bemounted in any position. Scalesare based on specific gravities of0.84 for oil meters, and 1.0 for watermeters. Their simplicity of designand the ease with which they can be equipped to transmit electricalsignals has made them aneconomical alternative to rotametersfor flowrate indication and control.

Correlation of Spring and Piston Flowmeters

Although normally calibrated for oilor water, these units can be usedfor other fluids as well. The readingon the flowmeter must be multipliedby the following correction factors toaccount for fluid density:

�0.84/spec. grav. (for oil meters) or

�1.0/spec. grav. (for water meters)

These units are only slightlyaffected by viscosity, and nocorrection is normally required.

FL6715A, $211, see page B-65.

FL-8115A, $735,see page B-58.

FL-9204, $94,see page B-49.

FL-1601A, $1100,see page B-44.

FL-75K, $607,see page B-39.

FL4212-V, withoptional valve, $65, see page B-17.

Many fittings of various materials and styles are available. See pages T-35 to T-66.

All shown smaller than actual size.

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