Variable area Flowmeters – What Is Piping: All about Piping Engineering

A flow meter is a device that measures the flow rate of a fluid. Flow meters are flow measuring instruments used to measure the linear, nonlinear, mass, or volumetric flow rate of a gas or a liquid. Flow meters are also known as Flow Gauges. This article will provide the following information in short: Types of Flowmeters/ Flowmeter typesOperating Principle of Each type of FlowmetersDesign Features of each flowmeter.Advantages and Disadvantages of each type of FlowmetersSelection and Application of those Flowmeters Types of Flowmeters Broadly two types of flow meters are widely used in industries: Volumetric Flow meters and Mass Flow meters Volumetric Flowmeters Volumetric flow meters got its name because these flow meters measure the fluid volume passing through a specific location in a set period of time. Volumetric flow meters provide an instantaneous analog, digital, or pulse output of the volumetric flow rate of the liquid or gas. Various types of Volumetric Flowmeters are available as listed below Differential Head type Orifice platesVenturi metersAnnubar Differential Area type (Rotameters)Electromagnetic flowmetersUltrasonic flowmetersTurbine flowmetersVortex flowmetersPositive Displacement Meters Mass Flowmeters Mass flow meters measure the fluid mass flow rate that travels through a tube per unit time. There are two types of mass flowmeters as mentioned below Coriolis Mass flowmeter andThermal Mass flowmeters Differential Head Type Flowmeters The difference in pressure exists between the upstream & downstream sides of a restriction in a confined fluid stream, which related to the square of fluid velocity. Q α √ ▲P Where Q = Volume flow rate and ▲P = Differential pressure between taps Fig. 1:Differential head type flowmeters Types of Orifice plates (Fig. 1) Concentric orifice plate: Most commonly usedSegmental & Eccentric orifice plate: Used for fluids containing suspended solids. Tappings for the Orifice plates: Corner taps ( 16 inch) Features of Orifice Plates Design Pressure: No limitation. Limited by DP transmitterDesign Temperature: No limitation. Limited by DP transmitterSizes: Maximum size is the pipe sizeFlow range: limited only by pipe size.Fluids/ Applications: Cryogenic / clean gases & liquids/ Steam (saturated/superheated)MOC: No limitation (Steel/ monel/nickel/ haste alloy)Accuracy : It varies from ±0.25% to ±0. 5% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.Rangeability is 3:1 to 5:1.Upstream length/ Downstream straight length is 20 / 5 Advantages of Orifice Plates Easily installed between flanges.Fabrication simple and inexpensive.No limitations on the materials of construction, line size and flow rateCost relatively independent of pipe diameter since the cost of DPT is fixed.No process interruption for the exchange of DP transmitter. Disadvantages of Orifice Plates High permanent pressure loss & hence high energy consumption to overcome pressure loss.Impractical for systems with low static pressure.Measuring range to about 3:1 to 5:1.Accuracies decrease with Beta ratios above approximately 0.7.Subject to damage by water hammer and foreign objects. Venturi Meters A venturi tube (Fig. 2) also measures flow rates by constricting fluids and measuring a differential pressure drop.In the upstream cone of the Venturi meter, velocity is increased, the pressure is decreased.Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument Fig. 2: Figure showing Venturi meter and Annubar Flowmeter Features of Venturimeters Design Pressure: No limitation. Limited by DP transmitter/ pipe press.ratings.Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratingsSizes: 25 mm to 3000 mmFluids/ Applications: Clean Liquids/ clean gasesLimited applications: Dirty /corrosive/viscous Liquids & Dirty gasesFlow range: limited only by pipe size and beta ratio.MOC: No limitation (cast iron/ carbon steel/ SS/Monel, Titanium, Teflon, Hastelloy, Naval Bronze/haste alloy)Accuracy : It varies from ±0.25% to ±0. 75% of actual flow. The accuracy of DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.Rangeability is 3:1 to 5:1.Upstream length/ Downstream straight length is 20 / 5 Advantages of Venturimeters Lower head losses than orifice plates reducing the capital expenditure on pumping eqpt. / save pump energy costsNo process interruption for the exchange of DP transmitter.Can be used for temperature extremesCryogenics or High Temperatures Disadvantages of Venturimeters Highly expensiveLarger and heavier to handle. Annubar Flowmeter The probe is installed in the media line as a pressure sensor.With the flow, the probe records both the static and the dynamic pressure via the probe openings.In the minus chamber of the annubar, lying on the opposite side, only the static pressure has any effectThe differential pressure corresponds to the dynamic pressure in the pipeline & the flow can is calculated directly. Features of Annubar Flowmeters Design Pressure: Upto 97 bars (38 Deg.C) / 55 bars (370 Deg.C)Design Temperature: Upto 400 deg.CSizes: 50 mm to 3000 mmFluids : Clean Liquids, gases and steamMOC: Brass / steel/ stainless steel/ HastelloyAccuracy : It varies from ±1% to ±2% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.Rangeability is 3:1 to 5:1.Upstream length/ Downstream straight length is 20 / 5 Advantages of Annubar flowmeters The integral manifold head allows direct mounting of DP transmittersHot tapping: Insertion/ installation without system shutdownVery low-pressure drop Disadvantages of Annubar Not suitable for viscous and slurry applicationsCan be used for only for clean fluids. Variable Area Flowmeters/ Rotameters A free moving float is balanced inside a vertical tapered tubeAs the fluid flows upward the float remains steady when the dynamic forces acting on it are zero.The flow rate indicated by the position of the float relative to a calibrated scale. Fig. 3: Variable area flowmeters Design Features of Rotameters Design Pressure: Upto 350 PSIG (GLASS TUBE) / 720 PSIG (METAL TUBE).Design Temperature: Upto 400 deg.C (GLASS TUBE) / 538 Deg.C (METAL TUBE) .Sizes: upto 75 mmFluids/ Applications : Clean liquids, gases and vapoursFlow range: upto 920 cub.m/hr for liquids & 2210 cub.m/hr for gasesMOC: Borosilicate glass/ brass / steel/ stainless steel/ HastelloyAccuracy : It varies from ±1% to ±2% of actual flow.Rangeability is 10:1Upstream length/ Downstream straight length is 10 / 5 Advantages of Rotameters Simple, robust and linear outputIt does not require external impulse or lead lines.The pressure drop is minimal and fairly constant. Disadvantages of Rotameters Vertical installation only.Glass tubes limit pressure & temperature and subject to breakage from hydraulic & thermal shockGlass tubes eroded by undissolved solids & unsuitable for metering alkaline solutionsMetal tube meters more expensive.Foreign particles can accumulate around the float & block the flow Magnetic Flowmeters Operate on Faraday’s Law of magnetic induction.When a conductive fluid moves in a magnetic field, a voltage is generated between two electrodes at right angles to the fluid velocity and field orientation.The flow tube has a fixed area & field intensity so the developed voltage is linearly proportional to the volumetric flow rate. Fig. 4: Figure showing Magnetic Flowmeters Design features of Magnetic Flowmeters Design Pressure: 20 BARS to 172 BARSDesign Temperature: Upto 120 deg.C with teflon liners / 180 Deg.C with ceramic linersSizes: 2.5 mm to 3000 mmFluids : Liquids (clean/ corrosive/dirty/viscous/ slurry)Velocity range: 0.1 to 10 m/sMOC: Liners: ceramic/ teflon/rubber : Electrodes: Platinum/ hastelloy/SSAccuracy: It varies from ±0.5% to ±1% of actual flow.Rangeability is 10:1Upstream length/ Downstream straight length is 10 / 5 Advantages of magnetic Flowmeters Flow rate unaffected by fluid density, consistency, viscosity, turbulence, or piping configuration.Highly accurate due to the absence of moving parts/ external sensing linesCorrosion-resistant using Teflon liner and platinum electrodesWide flow measuring ranges & no pressure drop Disadvantages of Magnetic Flowmeters Costly, relative to other flowmeter types.Temperature of the fluids being metered limited by the liner material rating.Cannot be used for gas flow measurements Vortex Flowmeters An obstruction is placed across the pipe bore at right angle to fluid flow.As fluid flows, vortices are shed from alternating sides of the body & this shedding frequency is directly proportional to fluid velocity.Detection of the vortices by means of pressure changes in the vortex stream.Rate of creation of vortices directly proportional to the flow rate. Design Features of Vortex Flowmeters Design Pressure: 138 barsDesign Temperature: -200 Deg. C to 400 Deg.CSizes: 15 mm to 300 mmFluids : Gases (clean/ dirty) and clean liquidsVelocity range: 0.3 to 10 m/s (liquids) and 6 to 80 m/s (gases)MOC: mostly in stainless steel, some in plasticAccuracy : It varies from ±0.5% to ±1% of actual flow for liquids and ±1% to ±1.5% for gasesRangeability is 20: 1Upstream length/ Downstream straight length is 20 / 5 Advantages of Vortex Flowmeters Minimal maintenance, no moving parts.Calibration using fluid flow not required & unaffected by viscosity, density, pressure, and temperature within operating specification.Digital or analog output. Disadvantages of Vortex Flowmeters At low flows, pulses are not generated and the flowmeter can read low or even zero.Reynolds number should be greater than 10000.Vibration can cause errors in accuracy.Correct installation is critical as a protruding gasket or weld beads can cause vortices to form, leading to inaccuracy.Long, clear lengths of upstream pipework must be provided, as for orifice plate flowmeters. Fig. 5: Figure showing Vortex Flowmeter, Ultrasonic flowmeter and Turbine Flowmeter Ultrasonic Flowmeters A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on the pipe wall, one (set) on the upstream and the other (set) on the downstream.The time for acoustic waves to travel from the upstream transducer to the downstream transducer (td) is shorter than the time it requires for the same waves to travel from the downstream to the upstream (tu).The larger the difference, the higher the flow velocity. Design Features of Ultrasonic Flowmeters Design Pressure: 207 bars (insertion type)/ unlimited (clamp on type)Design Temperature: -180 Deg. C to 260 Deg.CSizes: 3 mm to 3000 mmFluids : clean gases, clean/corrosive liquids (with little/no solids/ bubbles)Velocity range: 0.3 to 15 m/sMOC: mostly in stainless steel/ alloyicAccuracy is: +0.5% of flowrate for insertion type/+1% to +3% of flowrate for clamp on typeRange ability is 10 : 1 to 300 : 1Upstream length/ Downstream straight length is 10 / 5Bidirectional flow measurementFor insertion type, hot tapping in pressurised pipelines possible Advantages of Ultrasonic Flowmeters No obstruction/ moving parts in the flow pathNo pressure dropLow maintenance costMulti-path models have higher accuracy for wider ranges of Reynolds numberCan be used in corrosive fluid flowPortable models available for field analysis and diagnosis Disadvantages of Ultrasonic Flowmeters Only clean liquids and gases can be measuredHigher initial set up cost Turbine Flowmeters Consists of a multi-bladed rotor mounted at right angles to the flow & suspended in the fluid stream on a free-running bearing.The diameter of the rotor is slightly less than the inside diameter of the flow metering chamber.Speed of rotation of rotor proportional to the volumetric flow rate. Features of Turbine Flowmeters Design Pressure: 1500 PSIGDesign Temperature: 150 Deg. CSizes: 5 mm to 600 mm (Full bore type)/ > 75 mm for insertion typeFluids : Clean liquids/ gases and vapoursVelocity range: 0.3 to 15 m/sMOC: mostly in stainless steel/ hastelloyAccuracy is: +0.25% to + 0.5% of flowrate for full bore type/+1% to +3% of flowrate for insertion typeRange ability is 10 : 1Upstream length/ Downstream straight length is 15/ 5Bidirectional flow measurementFor insertion type, hot tapping in pressurized pipelines possible Advantages of Turbine Flowmeter Very accurate. Commonly used to prove other meters.Digital output provides for direct totalizing, batching, or digital blending without reducing accuracy.There is less tendency to read high in pulsating flow than in head or variable-area type meters. Disadvantages of Turbine Flowmeters Not usable in dirty streams or with corrosive materials.Subject to fouling by foreign materials -fibers, tars, etc.Bearings subject to wear or damage. Shift in calibration if bearings replacedIt can be damaged by overspeeding (over 150 percent) or by hydraulic shock.Pressure loss at rated flow varies & can be high. Positive Displacement Flowmeters This meter repeatedly entraps the fluid into a known quantity and then passes it out.The quantity of the fluid that has passed is based on the number of entrapments.The volume flow rate can be calculated from the revolution rate of the mechanical device. Features of Positive Displacement (PD) Flowmeters Design Pressure: 1500 PSIG (liquids)/ : 100 psig (gases)Design Temperature : 293 Deg. C (liquids)/ : -34 to 60 Deg. C (gases)Sizes: 6 mm to 400 mmFluids : Clean Liquids/ gasesFlow range: 0 - 20000 GPM (liquids)/ : 0 - 3000 cub.m/hr (gases)MOC: mostly in aluminum, stainless steel, plastics, hastelloyAccuracy is + 0.5% to + 1% of flowrateRange ability is 15 : 1 Fig. 6: Positive Displacement Flowmeters and Thermal mass flowmeters Advantages of PD Flowmeters Good accuracy and high range abilityCan be used in viscous liquid flowLow to medium initial set up costRequire no power supply and available in wide variety of read out devices Disadvantages of PD Flowmeters Maintenance required at frequent intervals because of the `moving parts.High pressure drop due to obstructionNot suitable for low flow rateNot suitable for fluids with suspended solidsGas (bubbles) in liquid could significantly decrease the accuracy Thermal Mass Flowmeter Operates by monitoring the cooling effect of a gas stream as it passes over a heated transducer.Gas flow passes over two PT100 RTD transducers.The temperature transducer monitors the actual gas process temperature, whilst the self-heated transducer is maintained at a constant differential temperature by varying the current through it.The greater the mass flow passing over the heated transducer, the greater the current required to keep a constant differential temperature.The measured heater current is, therefore, a measure of the gas mass flow rate. Design Features of Thermal Mass Flowmeters Design Pressure: 1200 PSIGDesign Temperature: 176 Deg. CSizes: 15 mm to 1000 mmFluids : Clean gasesFlow range: 0 – 2500 SCFMMOC: mostly in stainless steel/ glass, teflon, monelAccuracy is +1% to + 2% of flowrateRange ability is 10 : 1 to 100:1Upstream length/ Downstream straight length is 5/ 3 Advantages Of Thermal Mass Flowmeter No temperature or pressure compensation requiredLinear output (as temperature differential is proportional to mass flow)Can be used on corrosive process streams if proper materials are specifiedDC voltage or 4 to 20 mA dc outputs available Disadvantages of Thermal Mass Flowmeters Practical for gas flow onlySubject to blockage by foreign particles or precipitated deposits due to small openings in flowmeterPower requirements excessive in larger pipe sizesHas to taken out of process line for servicingAccurate field calibration is difficult Coriolis Mass Flowmeter When a moving mass is subjected to an oscillation perpendicular to its direction of movement, Coriolis forces occur depending on the mass flow.When the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube.On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist.This twisting movement is sensed by a pick-up and is directly related to the mass flow rate Coriolis Mass Flowmeter Characteristics Design Pressure: 345 barDesign Temperature: 200 to 426 Deg. CSizes: 1.5 mm to 150 mmFluids/ Applications : Liquids (clean/ dirty/viscous/ slurries) clean /liquified gasesFlow range: 0 – 25000 lb/mMOC: mostly in stainless steel, hastelloy/titaniumAccuracy is + 0.15% to + 0.5% of flowrateRange ability is 20 : 1Bidirectional flow measurement Fig. 7: Coriolis Mass Flowmeter Advantages of Coriolis Mass Flowmeters Capable of measuring difficult handling fluidsIndependent of density changes, flow profile and flow turbulence. Hence straight lengths are not required.No routine maintenance required since no moving partsHigh accuracy Disadvantages of Coriolis Mass Flowmeters Not available for large pipes (upto 150 mm only)High flow velocities required for detection resulting in high pressure dropExpensive compared to other flowmetersDifficulty in measuring low pressure gases. Application of Flowmeters / Selection of Flowmeters Clean liquids/gases OrificesVenturiAnnubarVariable AreaMagnetic (only liquids)UltrasonicVortexCoriolis Mass FlowmetersThermal mass flowmeter (only gases)PD meters Dirty Liquids Most suited: Magnetic/Coriolis Mass FlowmetersLimited applications: Venturi meters Dirty Gases Most suited: Vortex metersLimited applications: Venturi meters/Thermal mass flowmeter/Variable area flowmeter CORROSIVE LIQUIDS: Magneticflowmeters/Ultrasonic flowmeters NON-NEWTONIAN LIQUIDS: Coriolis Mass Flowmeters VISCUOUS LIQUIDS: Coriolis/Magnetic/Positive Displacement Meters ABRASIVE SLURRIES: Magnetic flowmeters/Coriolis Mass Flowmeters FIBROUS SLURRIES: Magnetic flowmeters/Coriolis Mass Flowmeters (limited applications) Saturated Steam Most suited: Orifice DP meters/ Vortex flowmetersLimited applications: Venturi meters/Variable area meters Super-heated Steam Most suited: Orifice DP metersLimited applications: Venturi meters Cryogenic Applications: Venturi-meters/Orifice plates