Ultrasonic Flowmeters: A ‘Sound’ Technology
Ultrasonic flow measurement is not new technology. However, it has recently become something of a hot commodity among other well-represented technologies in the industrial flowmeter market. According to the Ultrasonic Flowmeter Worldwide Outlook, a market analysis forecast through 2005 published by the ARC Advisory Group (Dedham, MA), ultrasonic flowmeters are one of the few field devices for which double-digit growth is projected. The report goes on to say that although this is highly unusual for a mature technology, improvements in the last few years have lead to this significant resurgence. This is the result of experience, combined with recent technology advancement, which form the next generation of flowmeters that are essentially brand new.
The two types of ultrasonic flowmeters applied in closed-pipe flow measurements most often encountered in the process industries use either transit-time or Doppler technology.
Transit-time flowmeters use the difference in time for a sonic pulse to travel a fixed distance in the media, first with the direction of flow and then against it. These flowmeters can have one of two operating modes, time domain and frequency domain. Although they work similarly, transmitting pulses from a transducer to a receiver and back again through the flowing media, time domain meters use the difference in time between the two trips to provide information on the fluid’s motion.
Frequency domain transit-time flowmeters use the same sensors as time domain meters, however the signals are read differently. Time is in not read directly. Instead frequency-domain units convert the time information into a frequency. As soon as a sonic pulse is received it is immediately retransmitted to form a pulse rate (frequency) proportional to the transit time. If two such paths, one in each flow direction are used, two frequencies are generated. The difference in frequency is proportional to flow velocity.
Transit-time meters are not limited to measuring flow velocity. Controlotron’s (Hauppauge, NY) SonicMass flowmeter combines high-resolution sonic velocity and temperature compensation to convert volumetric flow to mass flow rate. The flowmeter, which is available in a variety of mounting styles and pipe sizes (3/8- to 48-in. dia.), is said to be the first commercially available liquid mass meter based on transit-time technology. Performance accuracy exceeds 0.25% for mass flow of selective fluids and 0.15% for volumetric readings over a specified range of Reynolds numbers.
Doppler meters work differently than transit-time devices, most using continuous transmission of a single sound frequency rather than pulses. The beam is transmitted into the media at some angle to the direction of flow. Bubbles, entrained solids, or eddies in the flow then reflect or scatter the sound back to a receiver. Motion in these inclusions will cause a Doppler (frequency) shift of the returned signal. In short, Doppler-based flowmeters pass the signals between a transducer and inclusions in the flow steam and back, rather than between two transducers.
Each of the inclusions, which have random physical distribution and velocities, reflect sound while in the sonic stream. Hence, their reflected composite signal is a random distribution of frequencies that add up to what appears to the receiver as a single waveform. The difference between the scattered and received frequencies is proportional to the motion of the flow inclusions or the flow velocity.
Other variations of ultrasonic flowmeters are available. There is a hybrid of the two basic technologies intended for use in process (closed-pipe) applications. Ultrasonic flowmeters can be used for determining flow rate in open channels and rivers. The technology, which is also available to measure flow rate in partially filled pipes, determines flow by measuring level in the pipe.
The types of media suitable for measurement ultrasonically are quite extensive for transit-time and Doppler meters alike. Either type requires some prerequisites for successful operation. Applicable media must support the passage of sound, be in a full conduit, and contain no material that will deposit on the inside pipe wall. Flow must be continuous and non-pulsing for either type to function accurately.
There are additional requirements for using Doppler meters. In these cases, the media must provide enough inclusions (suspended bubbles, small solids/particulate, etc.) so that sound energy has something from which to reflect. However, the media must not have so many of these ‘scatterers’ that sound cannot penetrate the flow.
According to Doug Weerstra, instrumentation chemist at Mesa Laboratories Inc., NuSonics Div. (Lakeland CO), there is an overlap in the amount of suspended solids by volume that both flowmeter types can handle. Transit-time meters work well even with 0-2% by volume of solids in small pipes. Doppler meters work in most flow situations with 0.1-10% solids by volume. As the amount of solids increases, however, instrumentation functionality can suffer.
Doppler meters can also function if eddies exist as the inclusions in the flow. ‘However, eddies can be tough to pick up. And since they are created by downstream conditions, they cannot be counted on,’ says Mr. Weerstra.
Locating those sound beams
For both transit-time and Doppler flowmeters, the number of sound beams that pass through a pipe can be increased or modified electronically to raise the accuracy of the average velocity reading. Often a single acoustic beam passed between 3 and 9 o’clock (recommended) in a horizontal pipe run provides sufficient accuracy. However, for any ultrasonic flowmeter to provide accurate readings, it must be located in a straight section of pipe at least 10 pipe diameters upstream and three pipe diameters downstream from the nearest flow disturbance (pump, elbow, tee, reducer, etc.).
If a sufficient straight run cannot be ‘found’ or built into the process piping then multi-beams can be used to cancel out the effects of the disturbed flow. An additional beam(s) can be placed at ‘other angles of the clock’ and their signals combined to provide more accurate representation of average flow velocity. Keep in mind, sensor configurations can be quite different depending on the size of pipe and flow conditions encountered. In short, there just are no typical installations or application rules.
Sensor mounting styles are varied and often depend on where and when the device is placed in service. Mounting types include direct-mounting or non-intrusive. Direct-mounting devices include spool piece meters-so named because the short flanged pipe section that contains the transducers resembles a thread spool-bolted in place and weld-in style devices. Most of these sensors styles are mounted in the early stages of a process piping installation to avoid ‘taking the process down’ later in a retrofit situation. Weld-in transducers can be hot-tap mounted, however, allowing some flexibility as to when they can be added into a system.
According to Randy Brekke, vp sales at J-Tec Associates Inc. (Cedar Rapids, IA) the greatest advantage of using ultrasonic flowmeters, whether transit-time or Doppler, is that no pipe cutting is required. Ability to use externally mounted transducers provides the control engineer with greatly increased installation flexibility both in when and where the flowmeter is mounted.
Externally mounted transducers can be used on both transit-time and Doppler meters. There are two basic types. Clamp-on types mount on the outside of a pipe where flow velocity is needed. For clamp-on transducers to work, the pipe wall to which it is attached must be capable of passing sound and be clean and smooth. The inside of the pipe must be free of sound-absorbing material, such as dirty grease or scale. Use of an acoustic coupling material between the transducer and pipe (oil, grease, or epoxy) is recommended.
Where clamp-on transducers can not be adapted, wetted flush-mount sensors-some designs resemble spark plugs-must be used to provide a good interface for passing sound energy into the media. In the case of these transducers, no pipe need be cut, but mounting holes must be drilled and tapped into the pipe, requiring process shutdown and/or pipe draining during the process.
For new construction and where process interruptions (scheduled downtime for periodic maintenance, general cleaning and sanitation, etc.) are not a problem, installation of spool-piece devices is simple and straightforward. Siemens Energy & Automation (Grand Prairie, TX) offers the Sitrans F US, an ultrasonic flowmeter intended for use in liquids. This device is offered as spool-piece mounting, available in 1-, 2-, 3-, and 4-in. nominal diameters, and four standard DIN sizes with suitable flange designs. Because it is available in a limited number of pipe sizes, mounting flexibility as compared to the ‘one size fits most’ clamp-on type is limited.
However, a dedicated metering tube for each size allows the ultrasonic path to be more precisely controlled. In the case of Sitrans F US, its dedicated flow tube design uses built-in reflection points such that the flow velocity along the measuring path corresponds to the average flow velocity for all flow profiles. The patented helical sound path in this unit is said to result in high accuracy for a wide flow range.
Whether a flowmeter is dedicated or portable is more a function of the electronics than the type of sensor mounting. Dedicated devices are specified for a given location and, as such, are obtained from the factory calibrated to a given pipe size and flow range.
Truly portable ultrasonic flowmeters are microprocessor-based devices that can be reranged and recalibrated in the field, allowing them to be moved from one location to another with relative ease. Often used for testing and verification purposes, devices such as J-Tec’s Compu-Flow Model JC5 feature an onboard keypad and LCD, clamp-on transducers, and a carrying case. Unlike dedicated units, the portable electronics are not meant to be permanently mounted.
Adapted to gas
Clamp-on transit-time ultrasonic flowmeters are most often applied to liquids, however, advancements in the transducer and signal processing technology have extended their use to gas applications as well. In the case of the GE Panametrics (Waltham, MA), Model GC868 clamp-on gas flowmeter extends the technology to gas applications for pipes 3 in. or greater in diameter and to pressures over 90 psig.
According to GE Panametrics’ application engineer Daryl Belock, transit-time technology was always adaptable to gas flow if its density was high and delivery pressures were in the several-thousand psig range, not a common real-world situation. Getting the instrument to read flow accurately at smaller pipe sizes and lower pressures was the breakthrough, one that was electronics based. Initial investigation of the technology was done in an actual application to prove product feasibility.
Ultrasonic measurement techniques have made steady progress over the years as a viable flowmeter technology. Wide adaptability and ease of installation, two of its most important features, have been greatly enhanced through advancements in electronics and transducer design. And although no flow instrument is universally adaptable, ultrasonic flowmeters make a good run at it.
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Ultrasonic flowmeter handles wide range of variables
The Esso Petroleum Co. Ltd. refinery at Fawley on Southampton Water is the largest refinery in the U.K., and one of the largest refineries in Europe. Its 330,000 barrel/day capacity supplies over 15% of all oil products used in Britain, with 85% of its products delivered by pipeline to seven major airports including Heathrow and Gatwick. The Fawley site also incorporates an integrated chemical plant operated by Exxon Mobil Chemical Ltd.
Each type of crude oil has its own composition. To maintain flexible operations, the Fawley refinery must process up to 30 different types of crude oil from all over the world. As part of a crude tank farm optimization project, Esso developed a method to manage its crude oil inventory using flowmeters as part of an on-line blending process to maximize efficiency and cut costs.
Tackling the unattainable
Krohne Inc. (Peabody, MA) supplied six 12-in. and six 20-in. dia. dual-beam UFM 500 ultrasonic flowmeters. The flowmeters are arranged in pairs, with the first flowmeter configured to protect the pump against low discharge flow, and the second for regulating the flow via a control valve to enable the blending of correct crude ratios. Ultrasonic flowmeters were chosen to meet the accuracy stipulated by the project for the entire range of flows and viscosities required for the blending process. Additionally, the off-site piping network extends over an area of three square miles, thus line size flowmeters helped to minimize inherent pressure drop within the overall system.
Esso’s engineering team at the Fawley site provided exact and extremely demanding flowmeter specs needed for the project. Because of the low velocities and high viscosities, coupled with the overall range of velocities and viscosities taking the flow through laminar, transitional, and turbulent regions, engineering felt it could be difficult to meet the required accuracy and performance standards. This process requirement, together with physical restrictions resulting in multiple out-of-plane pipe bends upstream of the flowmeters, has taken the flowmeters outside their normal performance ranges. Velocities in the range of 3.4-12.5 ft/s were required for the 12-in. flowmeters, and 1.3-4.5 ft/s for the 20-in. flowmeters. Viscosities ranged from 3 to 1,800 centistokes.
Even with the wide range of process variables, UFM Series flowmeters met performance criteria. With pairs of the flowmeters arranged in series, consistent flow outputs have been achieved despite the potential for a distorted flow profile from the upstream pipe bends. The blending system’s control software conducts a self check of the flowmeters against tank movement via the radar tank gauging system installed on the crude tanks.
Ultrasonics to the rescue
At times, convenient to install clamp-on ultrasonic flowmeters are the instrument of choice for retrofit situations. A case in point is the Independent Gas Producers (IGP, Gillette, WY) adaptation of a Dynasonics (Racine, WI) Series TFXL Small-Pipe ultrasonic flowmeter to replace damaged mechanical flowmeters at IGP’s coal-bed methane wells located on nearby Bureau of Land Management (BLM) land.
IGP uses flowmeters to measure the amount of water pumped to the surface during well operation. Because the work is on BLM land, IGP is required to report the amount of water brought to the surface. IGP also uses flowmeters to optimize production of its wells and to prevent the pumps, which can be located as far as 1,500 ft under ground, from burning out should a pipe get plugged. All sensor signals are fed to small PCs with telemetry where they are monitored and recorded. TXFL flowmeters are offered for pipe sizes 1/2-2 in., and operate linearly over a 50:1 measuring range. Unlike the turbine meters they have replaced, bidirectional TXFLs do not impede flow and plug in the presence of coal and rock fragments often found in these wells. They eliminate the need for bypass lines, do not falsely record gas flow as water flow, and automatically compute volumetric compensation for gas bubble content in the water, leading to more accurate flow measurement.
A short history of ultrasonic flowmeters
Ultrasonic flowmeters got their start in 1963 when Tokyo Keiki (now Tokimec) first introduced them to industrial markets in Japan. In 1972, Controlotron Corp. (Hauppauge, NY) brought clamp-on ultrasonic flowmeters to the U.S. market. Others joined the market later in the 1970s and 1980s.
When ultrasonic flowmeters were first introduced, correct application conditions for the two basic types, transit-time and Doppler, were not well understood. Early on, some users misapplied these meters, which led to inaccurate measurements. These experiences gave some users a negative impression of ultrasonic technology. Fortunately, the market has recovered from these events.
Use of ultrasonic flowmeters for gas flow measurement got its start in the early 1980s, when both Ultraflux (Poissy, France) and Panametrics (now GE Panametrics) ran tests on ultrasonic flowmeters for gas applications. However, the biggest events in gas flow measurement didn’t occur until 1995, when the Groupe Europeen de Recherche GaziSres (GERG) published the Technical Monograph 8, which laid out criteria for using ultrasonic flowmeters for custody transfer of natural gas. At this point, ultrasonic flowmeters became a serious alternative to differential pressure and turbine meters for custody transfer applications, especially in Europe.
In June 1998, the American Gas Association published AGA-9, a report that did for the U.S. market what the GERG report had done for the European market. Both reports specified use of multi-path ultrasonic flowmeters, meaning that more than one ultrasonic signal is used to calculate flow rate. Publication of these reports gave a major boost to ultrasonic flowmeter sales.
Use of ultrasonic flowmeters for liquid applications has also been growing. However, most of the growth has been on the transit-time side. By using advanced electronics, transit-time meters have become more adept at measuring the flow of liquids containing some impurities, giving them a wider application base than earlier models.
A number of suppliers have brought new products onto the market in the past five years. These include multi-path devices for custody transfer of natural gas and clamp-on flowmeters for general gas applications. The ultrasonic flowmeter market has been one of the most active in terms of new product releases. This trend is likely to continue.
Jesse Yoder is the president of Flow Research Inc. (Wakefield, MA) and the author of the monthly report, Worldflow Monitoring Service. Contact him at email@example.com .