Measuring process fluid flow: an exacting task

To perform properly, instruments for measuring process variables must be specified correctly—a demanding job at best. And when it comes to measuring process fluid flow, many control engineers will tell you that specifying the right flowmeter is the toughest job of all. Newer technologies are gaining on more traditional sensing methods, recent research suggests.

By Dick Johnson Control Engineering June 1, 2005

Types of flowmeters used has not changed significantly in four years. The top five remain orifice, magnetic, turbine, mass flow, and Coriolis.

To perform properly, instruments for measuring process variables must be specified correctly—a demanding job at best. And when it comes to measuring process fluid flow, many control engineers will tell you that specifying the right flowmeter is the toughest job of all. Newer technologies are gaining on more traditional sensing methods, recent research suggests.

Flowmeters measure gross movement of liquids or gases. In custody transfer and batching applications, this task must be done with extreme accuracy. Available devices range from simple head meters to a host of new technology instruments. With such variety of applications and devices, matching meter to applications requires a lot of science and even a little art.

In February 2005, users of flowmeters shared their thoughts and views about the specification and application of these devices in a flowmeter survey conducted by Control Engineering magazine and the Reed Research Group. Results were based on 182 qualified responses from Control Engineering subscribers answering a Web-based, email survey; all specify, recommend, and/or buy flowmeters. Of those responding, 70.3% specified these devices for in-plant requirements and 17.6% selected them for OEM (resale) requirements . The remaining 12.1% chose flowmeters for both in-plant and OEM requirements . Of those using flowmeters for in-plant operations, 46.8% did so in continuous processing applications, 25.3% used them in batch processing , and 19% used them in utility services . A wide variety of other applications accounted for 8.9%. These included test and measurement duty, pipeline monitoring, and environmental remediation service.

Application of flowmeters remained highest in full flow service. Respondents indicated full flowmeters were used at a rate slightly more than three times that of open-channel or partial flow pipe applications. Although partial flow and open-channel meters are classified as traditional technology, newer technologies have been adapted to these sensors. Both magnetic and ultrasonic technologies can now measure partially filled pipe flow, which should come as good news to industries that make use of partial-flow devices.

Many and varied

In the lineup of flowmeter types in use, top five remain orifice, magnetic, turbine, mass , and Coriolis (see accompanying diagram), compared to the survey four years ago. The only mentionable changes occurred farther down the list, where increased use of positive displacement and ultrasonic flowmeters can be noted at 13.1% and 12.3% respectively.

According to Daryl Belock, commercial programs manager at GE Infrastructure Sensing ( ), the company has seen significant growth in its ultrasonic flowmeter product line sales during this time. One key growth area has been clamp-on ultrasonic technology for gas applications. Reasons include innovation in ultrasonic transducers and gas-specific signal processing. Improved technology has also impacted liquids, with devices now capable of handling more difficult applications. Ultrasonics has made headway in other areas as well, notes Belock: ‘In an area beyond clamp-on gas, we’ve seen very good growth in flare-gas ultrasonic flowmeter usage and combined that with service offerings to accommodate TCEQ regulations in Texas and AQMD regulations in California.’ (Acronyms are Texas Commission on Environmental Quality and Air Quality Management District.)

Oil and gas and chemical/petrochemical industries now use ultrasonic flowmeters for high-temperature, high-pressure applications, leveraging their very high turndown with no pressure drop capability. However, these industries are looking to ultrasonics with an eye on accuracy. Says Belock, ‘We are seeing increased interest by customers to use multi-path, wetted ultrasonic flowmeters with higher accuracy performance in applications found in oil/gas and chemical/petrochemical industries. These industries are seeing the advantages of no pressure drop, no moving parts, and wide rangeability as very desirable with the combination of multi-path, robust, and high accuracy ultrasonic flow measurements.’

Crowded at the top

Orifice plate flowmeters still top the list of types in use. According to Jesse Yoder, president of Flow Research Inc. ( ), this is not surprising, especially in process plants. ‘We see orifice plate flowmeters as remaining a strong force in the flowmeter market. The main reason for this is their very large installed base. Users are reluctant to change technologies unless they have a good reason to. For this reason, in many cases, users will select another orifice plate flowmeter to replace an existing one, if replacement is needed. If they have a problem with their orifice plate flowmeter, however, they may look at another technology, such as magnetic, ultrasonic, or Coriolis.’

Full-flow service meters have felt the pressure of newer technologies. On the list of flowmeters used, magnetic, ultrasonic, and Coriolis devices are clustered with turbine and positive displacement devices, all within striking distance of the top. The enhanced reliability and accuracy of new technology flowmeters will undoubtedly keep this trend going. (For more on new technology flowmeters, see the February 2005 issue of Control Engineering .)

‘At the same time, suppliers of dp flowmeters are giving their customers reasons to think twice before switching technologies,’ says Yoder. ‘High-tier differential pressure transmitters offer enhanced accuracy and stability. Manufacturers are now integrating primary elements with pressure transmitters to form a complete dp flowmeter, and multivariable pressure transmitters offer mass flow measurement.’

Amy Johnson, director of pressure marketing at Emerson Process Management ( ) expects differential pressure (dp) type devices to continue to be the most popular flow measurement methodology. ‘This technology provides a combination of features and capabilities not available in other technologies, she says. ‘And today’s dp flowmeters continue to improve. This technology can now provide 0.7% mass accuracy over a flow turndown of 10:1 or better. Orifice technology is an old technology only because of how long it has been in existence. It is the workhorse of flow measurement techniques and forms the heart of dp flow measurement. Differential pressure flow and orifice technology have gone through some significant changes over the past few years, resulting in improvements in both performance and capabilities.’

Improvements that have earned orifice plate flowmeters the top spot include multivariable measurement capabilities that improve the accuracy and ease with which compensated flow measurement is obtained—an especially important factor in gas and steam measurement—and a tenfold improvement, from 0.25% to 0.025% of span, in transmitter performance standards. These flowmeters can provide complete point solutions that arrive assembled, calibrated, leak-tested, and ready-to-install. Primary elements have also improved, with some offering built-in conditioning.

No failure to communicate

Like most instruments, flowmeters must communicate with the overall control system. For decades, 4-20 mA and, to a lesser extent, 0-10 V methods have led the communications competition. Highway Addressable Remote Transducer (HART) protocol has gained ground in the past 10 years because its digital-over-analog signal configuration enables it to exchange more data with the control system. Features such as remote calibration, range adjustments, self-checking characteristics, and log maintenance are making it among the fastest growing communication protocols for process sensing systems. However, survey respondents indicate a shift is coming. Of those expecting to have a choice of protocol, 82.7% picked 4-20 mA , 48% picked HART , 44.6% picked Ethernet , and 37.1% picked 0-10 V . This lineup shows a decided increase in Ethernet and decrease in 0-10 V. (See this article online for a breakdown of communication protocols in use.)

Jonathan Fiedler, product specialist at Krohne Inc. ( ) sees a similar trend. ‘We rarely receive requests for 0-10 V outputs, and would almost say it is a non-requirement for flow devices at this time,’ observes Fiedler. ‘By far, the number one request for output type is 4-20 mA, probably around 95% of the time, with approximately 20% of these requests looking for HART capability. We do not get requests for Ethernet outputs, although other protocol requests are increasing, including FOUNDATION Fieldbus and Profibus. These still account for less than 5% of the inquiries for flow devices. We do not see the 4-20 mA standard being displaced from the top spot for the foreseeable future.’

Which ones and why?

When asked why they chose a particular flowmeter supplier, 33.6% of survey respondents said the ability to take noninvasive measurements was their number-one concern. Digital communication ability (32%) and relaxed installation requirements/reduced process penetrations (16.1%) ranked second and third, respectively. Two other factors generated significant numbers: on-line configuration/troubleshooting (13.6%) and low cost of ownership (6.3%). Other factors mentioned included ease of calibration (1.6%), improved accuracy (1.5%), and improved reliability (0.8%). According to these responses, ‘not messing with the process setup’ clearly seems of greatest interest to control engineers when they decide which flowmeters to specify. The move to digital communication also is a clear priority.

When queried about the survey results, Fluid Components International’s (FCI’s, ) director of engineering, Eric Wible, observed that noninvasive measurements/reduced process penetrations should be very important to flowmeter users. ‘Experienced control engineers,’ said Wible, ‘know to include pipe invasiveness factors in their total installed cost evaluation and equation because installation labor costs escalate proportionally with the number of pipe taps or cuts. Larger flow elements can cause significant pressure drop, can clog, are more susceptible to wear and damage by the fluid, and disrupt the flow profile. This leads to greater energy costs to boost line pressure as well as significantly higher labor cost to service and maintain the flowmeter.’

Wible also sees increased customer interest in digital communications at FCI, especially in HART and Profibus, as standard product options. ‘Customers are increasingly looking beyond the single process variable analog output to digital bus communications that they can simply plug-in to their computer or DCS to provide multiple process variables and preventive diagnostics.’

Flowmeter products

For more manufacturers, visit . For more information on the products described here, visit the Web sites listed.

Simplifying vortex technology

Model 8800CR Reducer Vortex product line is said to reduce the cost associated with applying vortex flowmeters by eliminating the need for field assembly of reducer piping. Gasket-free design builds reducers into the flowmeter, resulting in a meter that can measure lower flows without the expense and complexity of field assembled pipe reductions. Product also matches the face-to-face dimension of standard Rosemount 8800C design to allow the user to change meter selection without the risk of impacting piping layout or drawings. Rosemount Div., Emerson Process Management

Small size Coriolis meter

F-Series product line now includes a 3-in. Coriolis meter with standard mass flow accuracy up to 0.15% in a compact package. F-Series flowmeters feature an increased temperature rating of 356nd beverage applications. Micro Motion

Transmitter for hard-to-handle materials

CFT50 Coriolis mass flow transmitter features patented technology that corrects for two-phase (gas/liquid) flow and allows accurate measurement of traditionally difficult-to-handle materials. Device incorporates two advanced signal processing techniques to provide accurate measurements of both mass flow and density. The first includes an advanced control and measurement system with high-speed digital signal processing that responds to changing flow conditions many times faster than standard Coriolis flowmeters. The second relates to detecting and compensating for two-phase flow conditions and generating a validated mass flow measurement. Foxboro M&I Div., Invensys Process Systems

Flow control for gas-fired processes

ST75 Series mass flowmeter measures gas and provides three outputs: mass flow rate, totalized flow, and gas temperature. It combines a thermal sensing element with microprocessor-based electronics and is said to deliver fast, direct mass flow measurement without routine maintenance. Product comes in nine line size configurations, 0.25 to 2 in. dia. with T-fittings. Flow range is 0.008 to 839 scfm depending on line size, making it well suited for low or high flow applications in industrial furnaces, ovens, and heat-treating systems. Accuracy is Fluid Components International

Flowmeter for petroleum industry

Proline Promass F 10-in. Coriolis mass flowmeter is specifically designed for petroleum industry applications. Device provides accurate and repeatable measurement (possibility of leaks and promote process safety. Endress+Hauser

Sensing fluids containing particulate

Signet 2551 Magmeter insertion style electro-magnetic flow sensor is intended for conductive fluid applications that contain particles, solids, and fibers. Device has no moving parts to wear or foul. Typical applications include flow monitoring for water and waste treatment, municipal drinking water, food and beverage operations, and power plants. Unit is said to provide the benefits of magnetic flow sensors without the cost of full-line units. All units have stainless steel electrodes and can be quickly mounted into pipe sizes from 0.5-0.8 in. using standard Signet installation fittings. Dynamic flow range is 0.15 to 33 ft/sec with repeatability of George Fischer Inc.

Conductive fluids

FSM4000 electromagnetic flowmeter is designed to measure a wide range of flow applications in the pulp and paper and metals and mining industries. Device measures conductive liquids with quick response and a noise-free output, and features improved ac magnetic field excitation, digital signal processing, andflanged, tri-clamp, wafer design, and for sanitary applications. Fluid temperature limits are -40 to 266 ºF; a high-temperature design is also available. ABB Instrumentation

Flowmeters for difficult applications

Rotamass Coriolis mass flowmeters have multi-measurement and multi-parameter capabilities and a robust, field-proven sensor and converter. Features include remote configuration via HART, multiple languages, advanced diagnostics, infrared programming, and a four-line display for easy setup and operation. Self-draining design meets global standards for sanitary applications. Device measures materials with a temperature range from -328 to 662 Yokogawa Corp. of America

Ultrasonic flowmeter

DigitalFlow CTF878 clamp-on gas flowmeter claims to be the first device to use patented ultrasonic flow pattern recognition or correlation tag technology for gas flow measurement. Device makes clamp-on flow measurement techniques applicable to metal pipes at atmospheric pressure. Correlation tag technology lets it operate at lower gas density than common transit-time meters and with no pipe size or gas composition restrictions. It measures gas flow noninvasively in pipes from 6-30 in. dia. over a range of 3.5 to 150 ft/sec. GE Infrastructure Sensing

Flowmeters feature diagnostics

Optiflux family of electromagnetic flowmeters consists of nine models for applications in water and wastewater, chemical, pulp and paper, and pharmaceutical industries. All feature three kinds of proprietary diagnostics: 100% application, 100% accuracy, and 100% instrument to provide information on the state of the flowmeter, measurement quality, and fault notification. ‘All in one’ IFC 300 converter allows any model to be adapted to virtually any application. Modular devices offer high repeatability and accuracy to 0.15% of measured value. Krohne Inc.

Battery operated magnetic flowmeter

Mag 8000 battery-operated magnetic flowmeter is intended for irrigation applications, including measuring water transport to farm outlets. Meters range in size from 2-12 in. dia. and are said to be a competitively priced alternative to propeller-style devices. They feature accuracies to 1% of actual flow and can be installed directly on the water well discharge. No propellers or bearings need to be replaced. Device has no moving parts and is said to operate maintenance free for six years in a typical application. Tube design improves low-flow performance with minimal pressure drop. Each unit comes with an IP68/NEMA 6P enclosure, integrated data logger, and open communications platform. Siemens Energy & Automation Inc.

Real-time metering, control

FLR2000 Series meters and FLV-2000 Series controllers are intended for use in real-time metering and control of chemicals, which are dosed, injected, pumped, or dispensed. Flow ranges of 0.1 to 400 gal/hr at pressures from 20-300 psi are said to meet the general range of use in water, wastewater, industrial, CPI, food and beverage, and pharmaceutical industries. Features include 1% full-scale accuracy, a 4-line x 20-character digital display, and RS-232 connectivity. The device’s PVC body features an advanced material coating that provides resistance to aggressive flow media. The ‘wet’ side of the flow system is isolated from the electronics yet close coupled for easy installation, operation, and maintenance. Omega Engineering Inc.

Low viscosity fluid meters

Coolpoint vortex-shedding flowmeter measures and monitors flows of water and low viscosity fluids, such as water-based machine coolant and water/glycol mixtures. Standard scaled pulse output increases meter’s ability to measure, monitor, and control the total amount of liquid used in a process. Meter accuracy is Universal Flow Monitors Inc.

Measuring gas flow

Thermatel TA2 mass flow transmitter provides reliable measurement in air and gas flow applications. Easy-to-use electronics are contained in a compact, explosion-proof enclosure with a 2-line, 16-channel display. Device can be mounted integrally on the probe or at a remote location and its 4-20 mA output set for active or passive operation. It has high turndown ratios, withstands process temperatures up to 400 Magnetrol

Applying the Hall Effect

SFI-800 flow sensor uses a patented Hall Effect sensor to detect flow rates in 1/2 in. and 3/4 in. pipe sizes. Incorporated paddle wheel rotates when fluid flows through the sensor. It includes a disc with a small bar of ferromagnetic metal in it. A sensor package attached to the rear of the device near the disc on the rotor contains both a magnet and a Hall Effect sensor. As flow spins the paddle wheel proportionately to the flow rate, the ferromagnetic bar rotates passing the magnet in the sensor with each rotation and generating a magnetic field that excites the Hall Effect sensor. The action produces a pulsed output; the frequency of the pulses is proportional to the flow rate. Dwyer Instruments Inc.

Measuring liquid, gas, or steam flow

V-Cone flowmeter is a differential pressure-type device with built-in flow conditioning for accuracy todia. Device features no moving parts, long life, and low operating costs, and needs no recalibration. McCrometer

Online Extra

High-pressure piston-style flowmeter Model 213 piston-style flowmeter now comes in a 7,500 psi-rated version. Small size, 0.9 cc/rev displacement, and high pressure rating make it suitable for leak testing on hydraulic test stands. With reduced blow-by piston design, device offersg, and analog output, based on the Model 213 meter. Max Machinery Inc.

Electromagnetic flowmeter M-4000 Series electromagnetic flowmeter comes in pipe size configurations 1/4-54 in. dia. (DN6-DN1400) for use in municipal water/wastewater and a variety of industrial applications. Housings are available in meter-mount and remote-mount versions that incorporate a rotatable display for easy viewing regardless of meter position. FM-approved Class I, Div. 1 models are available 1/4-12 in. dia (DN6—DN300). Class I, Div. 2 versions are available 1/4-24 in. dia. (DN6—DN600). Badger Meter

Flow control in critical applications Quantim Coriolis family of flowmeter products is intended to provide precise measurement and control of liquids and gases at lower and higher flow rates through improved tube sensitivity in high-value laboratory, research, and pilot plant applications. A modular hardware platform using improved sensing tube geometries and optical sensing techniques enable the devices to handle liquids and gases, and measure and control flows as low as 1 g/hr and as high as 28,000 g/hr. Coriolis mass flow controller features a new valve with a larger orifice to tolerate variations in line pressures and reduce the chance of clogging. Brooks Instrument

Strain gage target flowmeter Strain Gage Target flowmeter is said to be the only one of its kind on the market, accurately measuring the flow of liquid, gases, and superheated and saturated steam from -320 to 500 Aaliant

Measuring abrasive flows COIN segmented wedge type meter measures most flows, including abrasives. Accuracy ofo handle various flow ranges. Device is said to maintain the necessary square root relationship between flow rate and differential pressure for almost any type flow including clean liquids, high viscosity fluids, steam, slurries, corrosive processes, and gas/air. It has no moving parts, comes in sizes 1/2-48 in. dia., and performs bi-directional flow measurement. Preso, a division of Racine Federated Inc.

Transmitter or on/off control Type 8041 electromagnetic flow transmitter operates as a transmitter or for on/off control. It functions at high temperatures and pressures and is compatible with pipe diameters from 1/2 to 16 in. dia. Applications include water/wastewater treatment, drinking water, industrial/commercial laundries, food processing cleaning cycle monitoring, and irrigation. It accommodates liquids with conductivities greater than 20 Burkert