industrial flow measurement - isa interchange · chapter 2 fluid flow fundamentals 3 ... able...

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Industrial FlowMeasurement

3rd Edition

David W. Spitzer

vii

Contents

Preface Why Measure Flow? xiii

About the Author xv

About the Book xvii

Acknowledgments xix

Chapter 1 INTRODUCTION 1Objectives, 1Prerequisites and Audience, 2Learning Objectives, 2

Chapter 2 FLUID FLOW FUNDAMENTALS 3Introduction, 3Temperature, 3Pressure, 4Expansion of Liquids, 8Expansion of Solids, 8Expansion of Gases, 10Specific Gravity, 16Flow, 17Inside Pipe Diameter, 19Kinematic Viscosity, 27Dynamic (Absolute) Viscosity, 27Velocity Profile and Reynolds Number, 32Newtonian and Non-Newtonian Liquids, 36Friction Losses, 37Miscellaneous Hydraulic Phenomena, 44

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viii Industrial Flow Measurement

Chapter 3 PERFORMANCE MEASURES 49Introduction, 49Performance Statements, 49Repeatability and Hysteresis, 53Linearity, 54Accuracy, 56Composite Accuracy, 59Turndown, 61Rangeability, 61Long Term Stability, 61

Chapter 4 LINEARIZATION AND COMPENSATION 63Introduction, 63Linear and Nonlinear Flowmeters, 63Gas Flow Pressure and Temperature Compensation, 65Liquid Temperature Compensation, 67Pressure and Temperature Tap Location, 70Flow Computers, 70Multivariable Flowmeters, 71

Chapter 5 TOTALIZATION 73Introduction, 73Analog and Digital Flowmeters, 73Implementation, 75

Chapter 6 FLOWMETER CALIBRATION 79Introduction, 79Calibration Techniques, 79Dry Calibration, 81Verification of Operation, 85

Chapter 7 MEASUREMENT OF FLOWMETER PERFORMANCE 87Introduction, 87Applicable Range, 87Flowmeter Composite Accuracy, 88Transmitter Accuracy, 88Linearization Accuracy, 89Digital Conversion Accuracy, 90Indicator Accuracy, 91Totalization Accuracy, 91Overall Flowmeter System Accuracy, 91

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Table of Contents ix

Chapter 8 MISCELLANEOUS CONSIDERATIONS 97Introduction, 97Materials of Construction, 97Piping Considerations, 100Safety, 107Wiring, 108

Chapter 9 INTRODUCTION TO FLOWMETERS 111Introduction, 111Flowmeter Classes, 111Flowmeter Types, 113Introduction to Flowmeter Technology Sections, 115

Chapter 10 DIFFERENTIAL PRESSURE FLOWMETERS 117Introduction, 117Orifice Plate Flowmeters, 117Other Technologies, 151

Chapter 11 MAGNETIC FLOWMETERS 161Introduction, 161Principle of Operation, 161Construction, 168Operating Constraints, 172Performance, 173Applications, 174Sizing, 174Installation, 175Maintenance, 181

Chapter 12 MASS FLOWMETERS 185Introduction, 185Coriolis Mass Flowmeters, 185Hydraulic Wheatstone Bridge, 198

Chapter 13 OPEN CHANNEL FLOWMETERS 201Introduction, 201Weirs, 201Parshall Flumes, 205

Chapter 14 OSCILLATORY FLOWMETERS 211Introduction, 211Fluidic Flowmeters, 211Vortex Precession Flowmeters, 216Maintenance, 217Vortex Shedding Flowmeters, 218

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x Industrial Flow Measurement

Chapter 15 POSITIVE DISPLACEMENT FLOWMETERS 245Introduction, 245Helical Gear Positive Displacement Flowmeter, 247Nutating Disc Positive Displacement Flowmeter, 253Oscillating Piston Positive Displacement Flowmeter, 256Oval Gear Positive Displacement Flowmeter, 261Piston Positive Displacement Flowmeter, 270Rotary Positive Displacement Flowmeter, 277

Chapter 16 TARGET FLOWMETERS 283Introduction, 283Principle of Operation, 283Construction, 284Performance, 287Applications, 287Sizing, 288Installation, 289Maintenance, 291

Chapter 17 THERMAL FLOWMETERS 293Introduction, 293Principles of Operation, 293Construction, 296Operating Constraints, 298Performance, 298Applications, 299Sizing, 299Installation, 300Maintenance, 300

Chapter 18 TURBINE FLOWMETERS 303Introduction, 303Axial Turbine Flowmeters, 303Other Turbine Flowmeter Designs, 315

Chapter 19 ULTRASONIC FLOWMETERS 319Introduction, 319Principle of Operation, 319Construction, 324Operating Constraints, 325Performance, 326Applications, 327Sizing, 327Installation, 328Maintenance, 329

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Table of Contents xi

Chapter 20 VARIABLE AREA FLOWMETERS 331Introduction, 331Principle of Operation, 331Construction, 332Operating Constraints, 337Performance, 338Applications, 339Sizing, 339Installation, 341Maintenance, 342

Chapter 21 CORRELATION FLOWMETERS 345Principle of Operation, 345

Chapter 22 INSERTION FLOWMETERS 351Introduction, 351Principle of Operation, 351Available Technologies, 357Operating Constraints, 361Performance, 363Applications, 363Sizing, 363Installation, 364Maintenance, 366

Chapter 23 BYPASS FLOWMETERS 369Introduction, 369Principle of Operation, 369Types of Bypass Flowmeters, 370

Chapter 24 FACTORS IN FLOWMETER SELECTION 373Introduction, 373Flowmeter Categories, 373Flowmeter Types, 374Performance, 375End Use, 376Power Requirement, 377Safety, 378Rangeability, 378Materials of Construction, 378Maintainability, 378Ease of Application, 379Ease of Installation, 379Installed Cost, 379Operating Cost, 380Maintenance Cost, 380

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xii Industrial Flow Measurement

Chapter 25 DATA REQUIRED FOR FLOWMETER SELECTION 387Introduction, 387Performance, 388Fluid Properties, 389Installation, 391Operation, 392Future Considerations, 393Risk, 393Flowmeter Information Sheet, 393

Chapter 26 FLOWMETER SELECTION PROCEDURE 395Introduction, 395Flowmeter Selection Procedure, 395Applications, 398

Appendix A REFERENCES 425

Appendix B ANSWERS TO EXERCISES 427

INDEX 439

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319

19Ultrasonic

Flowmeters

IntroductionA relative newcomer to the field of flow measurement, ultrasonic shows consider-able promise as a viable flowmeter technology for liquid applications and somegas applications. Some designs allow measurements to be made external to thepipe and utilize no wetted parts, while other designs require that the sensor be incontact with the flowstream. As a result, in some designs the sensor is clampedonto the flowstream pipe, while other designs a section of pipe is supplied by themanufacturer with the sensors already mounted for insertion into the flowstream.

Principle of OperationUltrasonic flowmeters use acoustic waves or vibrations to detect the flow travel-ing through a pipe. Ultrasonic energy is typically coupled to the fluid in the pipeusing transducers that may be wetted or non-wetted, depending upon the design ofthe flowmeter. Time of flight and Doppler measurement techniques are available.

DopplerThe Doppler effect can be illustrated by the change in frequency that occurs whena vehicle approaches a bystander with its horn on. As the vehicle approaches, thehorn is perceived by the bystander to be higher pitched since the velocity of thevehicle causes the sound waves to be more closely spaced than if the vehicle werestanding still. Likewise, the horn is perceived to be lower pitched as the vehiclemoves away from the bystander; the sound waves tend to become farther apart,resulting in a lower frequency. The Doppler shift is proportional to the relativevelocity along the path between the source and the observer.

Doppler ultrasonic flowmeters utilize the Doppler effect to detect and mea-sure flow in a pipe. A transducer transmits continuous or pulsed (modulated)

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320 Industrial Flow Measurement

acoustic energy into the flowstream to a receiver (see Figure 19-1). Under no flowconditions, the frequency received is identical to the frequency at the transmitter;however, when there is flow, the frequency reflected from particles or bubbles inthe fluid is altered linearly with the amount of flow through the pipe due to theDoppler effect. The net result is a frequency shift between the transmitter and thereceiver that is linearly proportional to flow. The two signals are then “beat”together to generate a frequency signal at the difference between the transmittedand received frequencies, which is then converted to an analog signal proportionalto flow.

Figure 19-1. Doppler effect.


Chapter 19 – Ultrasonic Flowmeters 321

Most designs have two transducers, one each for transmitting and receiving,while some designs utilize a common transducer to achieve both functions.

Transit TimeTransit time ultrasonic flowmeters measure the difference in travel time betweenpulses transmitted along and against the fluid flow and beamed at an angle in thepipe. One transducer is located upstream of the other and the times of transit of theultrasonic beam in the upstream and downstream directions are measured over thesame path and used to calculate the flow through the pipe, as illustrated in Figure19-2.

Clamp-on transducers that utilize the time of flight principle are usually capa-ble of retransmitting sooner and operating faster as the sonic echo is away fromthe receiver and is not caught up in an “echo chamber” as is the case with insertedtransducers that face each other. However, inserted transducers typically makebetter sonic contact with the fluid. Any variations in sonic velocity due to fluidproperty changes will affect the performance of the flowmeter.

Clamp-on transducers can be designed to generate shear or axial beams in thepipe wall (see Figure 19-4). Each type has its advantages and limitations; forexample, shear mode ultrasonic energy is transmitted into the fluid such that the

Figure 19-2. Transit time flowmeter operation.

TuL

Co VF θcos–-------------------------------=

TDL

C0 VF θcos+--------------------------------=

VFk Tu TD–( )⋅

Tu TD⋅--------------------------------=

Tu = Upstream transit timeTD = Downstream transit timeVF = Liquid flow velocityCo = Velocity of sound in fluid



beam signal can shift in time and position as the flow varies. As a result, if thesonic properties of the liquid vary significantly, the beam could conceivably missthe receiver and not be sensed. Axial beam injection avoids this potential problemby transmitting the ultrasonic energy axially along the pipe. As a result, the place-ment of the receiver is not as critical, and the flowmeter is less sensitive tochanges in liquid sonic velocity. In addition, at least one manufacturer uses theshear wave traveling in the pipe to determine the integrity of the flow measure-ment.

Note that some time of flight ultrasonic flowmeters intentionally bounce theultrasonic beam off the pipe walls. These reflex designs allow longer paths thatresult in longer transit times that can be measured more accurately. Some manu-facturers offer ultrasonic flowmeters with over 5 bounces. Reflex mode is com-

Figure 19-3. Pulse repetition flowmeter operation.

L



monly used in small pipes where short paths result in short transit times that canadversely affect performance.

Figure 19-4. Wave propagation.




Pulse RepetitionPulse repetition ultrasonic flowmeters are time of flight devices in which thetransducer is positioned so that the ultrasonic energy is beamed at an angle in thepipe. One transducer is located upstream of the other, and the frequencies of theultrasonic beam in the upstream and downstream directions are detected and usedto calculate the flow through the pipe, as shown in Figure 19-3. The frequencyshift is linearly proportional to the velocity of the fluid and independent of thevelocity of sound in the fluid.

ConstructionConstruction of ultrasonic flowmeters can be classified by the mounting of thetransducers as either clamp-on or wetted. Clamp-on transducers offer convenienceand, in some cases, rather good accuracy. Wetted transducers are usually requiredfor more accurate liquid measurement, especially when multiple ultrasonic pathsare needed. A spool piece with wetted sensors can provide even better perfor-mance.

Clamp-on TransducerClamp-on transducers are attached to the pipe externally, typically with a pipeclamp on a small pipe. As there are no wetted parts, fluid compatibility is not aconsideration. Clamp-on designs typically employ one or two transducers,depending upon manufacturer (see Figure 19-5).

Figure 19-5. Typical clamp-on Doppler installation.



Wetted TransducerThe flowmeter body (spool piece) houses transducers in their proper orientationand permits direct contact with the fluid, usually resulting in a superior signal-to-noise ratio and more precise positioning. Design issues, such as correction forrefraction of the ultrasonic beam as it enters the pipe, can be avoided with wettedtransducers. Typically, two or more transducers are required for this design. Asthe spool piece and the transducers are wetted, attention must be paid to the mate-rials of construction. Spool pieces are typically stainless steel in the smaller sizesand carbon steel in the larger sizes. Other materials are also available. Somedesigns allow removal of transducers from the body while the liquid is flowingthrough the pipe, while others require that the flow be interrupted and the pipe bedrained for transducer removal.

Wetted PartsClamp-on transducer designs have not wetted parts, other than those exposed tothe surrounding atmosphere, while the wetted transducer design requires that thetransducer, any required seals, and the flowmeter body be wetted. Metal parts aretypically of stainless steel, although other materials of construction are available.

Transducer LocationSome manufacturers of wetted spool pieces locate their transducers off-center(chordal) to reduce the velocity profile and Reynolds number effects on perfor-mance.

Multi-Path TransducersFor better performance, some manufacturers offer spool-piece construction withmultiple ultrasonic paths. Typically, 3 or 5-paths are offered, but one manufac-turer offers an 18-path ultrasonic flowmeter.

Operating ConstraintsUltrasonic flowmeters are available for measurement in sonically conductivepipes greater than 1/8 inch in size. They can measure flows greater than approxi-mately 0.1 gpm at temperatures of up to approximately 400°C. Pressure for wettedtransducers is limited by the flowmeter flange rating and sensor design. Whilemost ultrasonic flowmeters measure liquid flow only, time of flight designs thatmeasure gas flows are available.

As the ultrasonic energy passes through only part of the liquid being mea-sured, Reynolds number affects the performance of the flowmeter. Some Dopplerand differential time flowmeters require minimum Reynolds numbers of 4000 and10,000, respectively, in order to perform within their stated specifications. Thedifferential frequency design can operate in the laminar flow regime at Reynolds



numbers of less than 2000, and in the turbulent flow regime at Reynolds numbersof over 4000.

Most Doppler flowmeters require that some entrained gas or particles bepresent in the fluid to reflect ultrasonic energy to indicate the velocity of the flow-stream to the receiving transducer. The maximum allowable entrained gas or sol-ids varies with design of the flowmeter and is often not specified. Should thepercentage be above the maximum, the ultrasonic energy will not sufficiently pen-etrate the flowstream, resulting in a loss of accuracy.

The velocity of the fluid must be above the threshold velocity of the flowme-ter, which is typically between 0.1 foot per second for the time of flight and differ-ential frequency designs, and 0.5 foot per second for the Doppler design.

Clamp-on ultrasonic flowmeters typically require that the thickness of thepipe wall be small in relation to the distance that the ultrasonic energy passesthrough the fluid. As a rule of thumb, the ratio of the pipe diameter to the wallthickness should be greater than 10:1.

PerformanceUltrasonic flowmeter accuracy is typically in the ranges of ±0.15 percent rate to 5percent FS. It should be noted that manufacturers often state flowmeter perfor-mance in terms of percent without stating whether this is percentage of rate, fullscale, or meter capacity. As some flowmeters are specified as a percent of metercapacity, the manufacturer should be consulted when there is any doubt as towhich specification is intended. A flowmeter that is specified a as a percent ofmeter capacity will exhibit significant errors at velocities encounted in typicalapplications, as meter capacity typically represents a velocity of 40 feet per sec-ond. It should also be noted that some specifications may reflect operation of theflowmeter under simulated conditions as opposed to operating conditions, whichdoes not accurately define the expected performance of the flowmeter.

There is little independent flow test data for ultrasonic flowmeters to confirmor deny manufacturers’ accuracy claims. Nevertheless, the differential frequencyand time of flight technologies generally achieve better performance than flowme-ters using Doppler technology.

The time of flight technologies transmit signals that usually travel through theentire flowstream between transducers on opposite sides of the pipe, while theDoppler technology relies on reflections of ultrasonic energy form particles orentrapped gas in the flowstream. Doppler technology sometimes has the addeduncertainty of the depth of penetration of the ultrasonic energy; the velocity pro-file, fluid properties, or fluid composition change can result in errors of greaterthan 30 percent under process conditions. In other words, there is uncertainty as towhether a flowmeter using Doppler technology is measuring the average velocityin the pipe or some other velocity, since the depth to which the ultrasonic energypenetrates the flowstream is not well defined, especially as the amount of particlesor entrapped gas varies. Slurries are particularly susceptible to large shifts in accu-racy; the particles can cause the slurry to be opaque to ultrasonic energy, causing



lack of penetration into the flowstream and hence a considerable loss of accuracyor loss of signal.

ApplicationsDoppler flowmeters can be applied to fluids that have some amount of

entrained gas or particles to reflect ultrasonic energy. Differential frequency andtime of flight technologies can measure flows of clean liquids as well as liquidsthat contain solids, depending upon manufacture. Clamp-on sensor designsrequire that the pipe be sonically conductive, as the ultrasonic energy must be effi-ciently transmitted to and received from the liquid being measured.

Ultrasonic flowmeters can be applied to pipes of all sizes. Since the flowmeterelement is virtually the same above certain sizes, this technology has economicadvantages over other flowmeter technologies in applications in large pipe.

SizingIn general, ultrasonic flowmeters are the same size as the pipe size to take advan-tage of the obstructionless design of the flowmeter, unless the flow is such that theReynolds number and velocity constraints are not satisfied. In such a case theflowmeter size may be altered as necessary. Compensation for pipe size is usuallyperformed electronically in the transmitter, so field modification of the transmitterto another size pipe is usually possible. Wetted transducers in spool-piece designsrequire that the flow primary be changed when the pipe size is changed. Clamp-onsensors may require replacement if more ultrasonic energy is required to penetratea different size pipe.

EXAMPLE 19-1

Problem: Size a Doppler flowmeter for a 100 gpm full scale flow of a liquid with a specific gravity of 1.0 and a viscosity of 1.0 cP

Solution: Assuming that Doppler flowmeters operate in a velocity range of 0.5 to 40 feet per second and typical liquid velocities are 6 to 8 feet per second, a 2-inch flowmeter could be applied and would operate at a velocity of 9.56 feet per second at full scale. Reynolds number can be calculated as follows:

RD = (3160 × 100 gpm × 1.0) / (1.0 cP × 2.067 in.) = 152,879

which is sufficiently high to ensure that the flow operates in the turbulent flow regime for all applicable flows.




InstallationProper installation of ultrasonic flowmeters is important to proper operation.

As the performance of most ultrasonic flowmeters cannot be verified in typicalapplications involving large pipe sizes, manufacturer recommendations should befollowed as closely as possible to achieve the best performance possible.

Of extreme importance is the use and data entry of proper and accurate fluidand pipe data. Failure to do so can drastically degrade the performance of theseflowmeters. For example, entering the wrong pipe size can cause significant error.

Hydraulic RequirementsUltrasonic flowmeters, which are sensitive to the velocity profile entering theflowmeter require 10 to 30D/5 to 10D upstream and downstream straight run,depending upon manufacture and technology. In general, increasing the straightrun of the flowmeter will decrease the possibility of shifts in measurement due toan improperly developed velocity profile at the inlet of the flowmeter.

Piping OrientationAs gas or solids collecting at or flowing on a transducer can affect the transmis-sion of ultrasonic energy in the flow, thereby affecting the accuracy of the mea-

EXAMPLE 19-2

Problem: Size a Doppler flowmeter for a 60 gpm full scale flow of a liquid with a specific gravity of 1.2 and a viscosity of 40 cP.

Solution: Typical liquid design velocity is 6 to 8 feet per second, so a 2-inch flowmeter could be applied and would operate at 5.74 feet per second at full scale flow. Reynolds number is calculated by:

RD = (3160 × 60 gpm × 1.2) / (40 cP × 2.067 in.) = 2752

and is found to be in the transition regime, which is unsatisfactory.

If the size of the flowmeter were decreased, the velocity as well as Reynolds number will increase. The increase in Reynolds number will not be sufficient to ensure that part of the desired flow measurement range will not be in the transition flow regime, so another alternative should be pursued.

Increasing the size of the flowmeter to 3 inches reduces Reynolds number as well as the velocity, so that the flowmeter can be operated totally in the laminar flow regime with a maximum velocity of 2.60 feet per second. As the differential frequency technology can be applied to laminar flow and can measure velocities as low as 0.1 foot per second, a 3-inch flowmeter would be applicable:

RD = (3160 × 60 gpm × 1.2) / (40 cP × 3.068 in.) = 1854



surement, ultrasonic transducers should be orientated in a manner to eliminate thispossibility. This can be accomplished by locating the transducers in the horizontalplane.

Piping VibrationThe frequencies at which ultrasonic flowmeters operate are usually selected to beoutside the realm of frequencies at which pipes will vibrate. Nevertheless, it maybe possible for the receiving transducer to respond to shock or high intensityvibration. As many transducers are temperature- and moisture-sensitive, careshould be exercised to avoid attributing all “unidentifiable responses” to vibra-tion.

Sensor MountingClamp-on transducers typically require that a coupling material be applied to thepipe and/or transducer before installation so as to provide satisfactory acousticcontact.

CablingMost ultrasonic flowmeters are 4-wire devices that have maximum distance limi-tations between the transmitter and the transducers. Special cable is usuallyrequired between the transmitter and the transducers to minimize attenuation ofsignals.

MaintenanceUltrasonic flowmeters require no routine maintenance other than routine calibra-tion checks. Problems such as transducer failure, lack of sufficient contactbetween the transducer and the pipe wall, and electronic failures can occur.

Transducer SpacingThe transit time of a clamp-on time of flight ultrasonic flowmeter is affected bythe spacing of the transducers. Should the spacing change, such as due to mechan-ical vibration or maintenance procedures, the performance of the flowmeter willbe degraded.

Transducer FailureDifficulty of transducer replacement is dependent upon transducer design.Replacement may require interrupting flow and opening the pipe, such as in thecase of wetted transducers that have no valving arrangement with which to isolatethe transducer from the pipe. Clamp-on transducers, which are mounted externallyto the pipe, can be replaced without interrupting flow.



Loss of Contact between Transducer and Pipe WallMaterials used to improve acoustic coupling between a clamp-on transducer andthe pipe can become ineffective over a period of time due to dehydration or mate-rial loss. The lack of proper coupling reduces the ultrasonic energy by reflection,often to the point of causing the flowmeter to cease to operate. This condition canbe corrected by removing the transducers and replacing the conducting materialper manufacturer specifications.

Electronic FailureElectronic failures can occur and are usually remedied by board replacement. Itshould be noted that process data and calibration information may need to beentered into a replacement board.

Spare PartsSpare parts inventory varies with flowmeter design, but the transducer and anyassociated mounting hardware such as gaskets should be stocked. Identical trans-ducers are usually used for many pipe sizes, while the transmitter for a particulardesign is typically identical for all pipe sizes, both of which minimize spare partsrequirements.

CalibrationCalibration of ultrasonic flowmeters is performed by electronically simulating thesignals that would be present under flow conditions and making the necessaryadjustments to the transmitter. A better calibration could be obtained if the flow-meter were calibrated at the manufacturer’s flow facility.

EXERCISES19.1 Size an ultrasonic flowmeter for a flow of 0 to 1400 gpm of a liquid with

a specific gravity of 0.98 and a viscosity of 3.3 cP. Can Dopplertechnology be applied? Why or why not?

19.2 Size an ultrasonic flowmeter for a flow of 0 to 600 gpm of a liquid with aspecific gravity of 1.13 and a viscosity of 150 cP. Can Dopplertechnology be applied? Why or why not?

19.3 Why must Doppler technology be applied to fluids with particles orbubbles?



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