Of notable importance to the oil & gas industry is the phenomenal growth in unconventional gas production along-side that of conventional resources, and, further, the critically important require-ment for metering that spans the entire gas value chain. Flowmetering is essen-tial to quantify the bulk movement of gas from wellhead to the point of flame.

Companies in the natural gas industry have critical operational demands. They require accurate flowmetering technol-ogy to measure and analyze the quality and volume of gas throughout its value chain. The industry categorizes its me-

ters as being: (1) Traditional; or (2) New Technologies. This is a relative categori-zation based on a historical timeline that extends to a number of technological centuries. Frequent use of this terminol-ogy is evident within published material, such as The World Market for Ultrasonic Flowmeters, 3rd Edition, a marketing report dedicated to ultrasonic flowmeter-ing.3 However, traditional flow measure-ment devices can be, within certain ap-plications, unreliable and costly to install and maintain, which is why newer tech-nologies have been developed.

One such new technology is ultra-

sound-based flow measurement. For natural gas processes with dry, wet, or corrosive and abrasive gases, or requir-ing bi-directional measurement with minimal or no pressure drop, ultrasonic measurement devices generally offer better performance, greater reliability, and lower capital and ownership costs than mechanical-type meters.

Ultrasound is employed in elec-tronic, navigational, industrial, and security applications. It is also used in medicine to view internal organs of the body. Examples of audible sound sur-round us in everyday life, and man has been pursuing a deeper knowledge of the influence of sound for centuries. The speed-of-sound (sometimes called velocity-of-sound) is known to be the speed at which sound travels in a given medium under specified conditions.

The first known attempt to measure the speed-of-sound (SoS) in air (a gas) was by Pierre Gassendi, who, in 1635, timed the delay between the powder flash of a distant cannon and the ex-plosion that sounded later. By the early 1700s, estimates for the speed-of-sound using cannon fire were within 0.5 m/s (1.64 ft/s) of modern-day values. This was substantiated by the first analyti-cal determination of SoS given by Isaac Newton (1642 to 1727) in Proposition 49 of Book II of the Principia. For sea-level air at a typical ambient temperature, he computed a value of 298 m/s (979 ft/s), which is too low by about 15 per-cent, the true value being closer to 340

NATURAL GAS MeasurementHow ultrasonic flowmeters have evolved to support the efficient measurement of gas flow

By Martin Bragg

77%Industrial & Electrical Power sectors project increases

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–

65%Natural Gas continues to be favored as an Environmentally attractive fuel

820Quadrillion Btu of Gas

World increase in supply

16.7+Trillion Cubic Feet of SHALE GAS representing 50% of total USA production

1 BtuEnergy release from burning a Wooden match

22 March 2015 Flow Control Magazine

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The natural gas industry is a key segment of many global economies. It is stated, within the public domain1,2, that

natural gas energy consumption is set to reach 23 percent of world energy demand by 2040. It is the fastest-growing fuel, rising at approximately 1.7 percent per year between 2010 and 2014. This represents a predicted consumption in the order of 820 quadrillion Btu of gas.

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m/s (1116 ft/s). Nevertheless, the basic method conceived by Newton was essentially correct. As early as the 1800s, experi-ments to determine the SoS for water were recorded by Col-ladon and Sturm in their reported experiment on Lake Geneva.

Metering, in one form or another, has existed for many centuries, although techniques based on ultrasound have only been more recently utilized. Records dating back to the 1930s indicate the use of Doppler techniques and, circa 1970, the transit-time method. It is important to note that all metering techniques are valid when considering a specific end-use or application; however, the exclusive use of transit-time for fiscal gas meters has developed over the last 40 or more years. Sig-nificant developments are recorded with reference to meters that employ more than one measurement point (known as a “path”) within any single, dedicated metering device.

Over the past 30 years, gas ultrasonic multi-path meters have transitioned from the engineering laboratory to wide commercial usage as the primary device-of-choice to mea-sure gas volume for fiscal accounting. The metering path ar-rangements are notably different (by physical configuration/location) between various designers/manufacturers, howev-

er the basic underlying principles remain the same.The development of published standards4, 5, 6, 7 often in-

creases end-user confidence of a particular metering tech-nology, of which the most notable ultrasound example is that of the American Gas Association’s Report No. 9.4 There is no doubt that as meter technologies develop, the requirement for impartial standards grows exponentially and is often an indication of the uptake rate of that technology.

Standards are updated from time to time to align with both the technology and its market developments that are in most parts based on experiential learning from a vast number of sources. Sources that extend beyond the manufacturer to the specific technology end-users, calibration facilities, indepen-dent research facilities, type-testing and approvals agencies, independent consultants, other technologies representatives, along with any other party that has a technology touch point.

The acceptance of multi-path ultrasonic flowmeters by gas pipeline companies has occurred due to the following features and benefits:

Reliability Repeatability

www.flowcontrolnetwork.com March 2015 23

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24 March 2015 Flow Control Magazine

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Capacity Rangeability Low maintenance Adoption of industry standards

for fiscal measurement applications Condition Based Monitoring (on

line, real-time).

If one is to consider the specific ap-plication of ultrasonic meters for cus-tody transfer or allocation metering, there are vast sources of published data, in direct response to published standards, in which it can be seen that an optimized path layout would be one with a three-dimensional element to its design. Such a layout provides for a complex, yet accurate and repeat-able measurement during both ideal and severely disturbed flow conditions. One such path layout was initially de-veloped in the 1990s; although there are other examples of multi-path me-ters that may be seen within the stan-dards documents. This unique six-path arrangement is optimized for three-

dimensional accuracy and provides performance benefits compared to the traditional two-dimensional paths. The three-dimensional element of the de-sign is shown as follows.

Virtually all ultrasonic flowmeters used for fiscal measurement are flow calibrated at metrological traceable test laboratories. Flow tests are conducted at multiple points over the flowmeter’s operating range to characterize its per-formance curve. Meter factor(s) are then calculated and applied to correct the output to the laboratory’s traceable and certified reference standards.

An advantage of modern ultrason-ic meters is that once a meter is flow calibrated, diagnostic assessments can describe performance (i.e., meter fac-tor shifts due to a fault in the device’s operating elements, such as transduc-ers and/or processing electronics) so that frequent (typically defined as once per calendar year) re-calibration gener-ally isn’t required, although some regula-tory authorities mandate re-certification

at set intervals, these mandates vary by local jurisdiction. This diagnostic assess-ment is growing in its complexity and is being driven by the ultimate requirement to validate the device online and real-time to ensure its optimum performance.

All commercially viable gas ultra-sonic meters offer diagnostic outputs that indicate meter operating condition, up to and including the ability to judge whether or not measured bulk flow rate output is accurate. These diagnostics have developed over several decades and must continue to do so in the in-terest of achieving the best metered performance. The nature of the meter’s operating principle helps define these outputs and also their interpretation for which the more widely used terminology is Condition Based Monitoring (CBM). CBM is a collection of an individual me-ter’s discrete diagnostics brought into a piece of dedicated software that as-sists the end-users to interpret the vari-ous datasets. Combinational datasets are combined to create more intuitive graphical representations supported, quite often, by the quick reference col-or-coding of key diagnostic data.

Transit-time ultrasonic meters depend on transmission and subsequent recog-nition of sonic pulses using precise tim-ing measurements and known geometry (i.e., path length and angle) to accurately measure gas velocity. Manufacturers have incorporated signal (pulse) recog-nition and processing algorithms as well as highly accurate clocks to make timing measurements. Therefore, signal gen-eration, signal reception, signal strength, signal-to-noise ratio and clock accuracy are fundamental to precise and reliable meter performance. Diagnostics, such as a meter’s signal strength have been noted for decades where the strength is known to be the inverse of the electronics amplification applied to the measurement

Operator dashboard used for online validation of an ultrasonic flowmeter

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www.flowcontrolnetwork.com March 2015 25

system. Prior to the implementation of dedicated software, and certainly prior to digital electronics, such a determination was achieved by the manual use of an oscilloscope.

One of the most powerful, and proven, CBM results is that of a path ratios analysis in considering multi-path meters. The analysis of the path ratios can provide a great deal of informa-tion relating the meter’s response to the actual fluid hydrau-lics within its location. Under disturbed and non-ideal fluid flow conditions, defined as the flow profile not being fully de-veloped and classic, there is valuable information contained within such a path’s analysis. However, it should be noted that there are many situations where there is no one single analysis of a single diagnostic but a combination of several. This is where the technology will continue to develop. Many end-users are keen to continue to explore the topic of online, in-situ validation for which CBM is central to success.

Online and in-situ validation is key to the continued growth of ultrasonic flowmeters within the custody-transfer and allocation-metering arena. In many installations the cost in removing and recalibrating such a meter is one of the largest contributors to operational expenditure, while an

end-user also wishes to maintain traceability within a prede-termined audit trail. The instantaneous presentation of CBM via a dedicated software package must continue to evolve to become ever more intuitive, however it must also develop into data historian analytics. Vast data historical archives

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It is important to note that all meter-ing techniques are valid when considering a specific end-use or application; however, the exclusive use of transit-time for fiscal gas meters has developed over the last 40 or more years. Significant devel-opments are recorded with refer-ence to meters that employ more than one measurement point (known as a ‘path’) within any single, dedi-cated metering device.

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Flowmeter Calibration | Flowmeter Verification | Natural Gas | Oil & Gas | Ultrasonic Flowmeter

Find related content @ flowcontrolnetwork.com…Search on: FLOWSTREAM

exist within many end-user businesses that include flowrate data. Creating and subsequently analyzing the much bigger CBM data is a future imperative for the industry in realizing experiential learning from data archives. As the volume of in-dividual datasets increases, and as the total CBM packaged data is collected within an historical archive, there must be information contained therein that is not known because there is no subsequent analysis. This subject resides within the auditing arena for which there is a great deal of work to be done within the future application of ultrasonic flowme-ters for custody transfer and allocation metering.

Going forward, gas—both conventional and unconven-tional—figures to continue to be in great demand within the global economy, and, as such, so too shall the requirement for metering. As such, existing and new standards are likely to evolve and emerge. And as most of the industrial suppli-ers to the gas metering industry are operating on a global basis, it would be helpful if some element of the various regional standards were globally aligned where practicable.

It is considered essential to the growth of ultrasonic flow-meters, in terms of installed base, that Condition Based Monitoring continue to be ever more intuitive from an end-users’ perspective, and that it may be used as the basis to decide on the recalibration frequency of an installed meter. Of key importance, especially for auditing, is the continued requirement to analyze historical datasets for individual me-ters giving rise to new experiential learning.

One very challenging area for consideration in looking into the future, is that of process historical data archives giving rise to the subject of “virtual metering,” a topic outside this article, which can significantly decrease the uncertainty in any valida-tion activity. However, it may be considered that such an ap-proach could lead to the ultimate goal of in-situ calibrations of ultrasonic flowmeters that is, in its own right, a vast topic for discussion within the industry. It is a discussion that has been ongoing for over two decades, but perhaps there are clues with-in this article as to how this may be achieved in the future. FC

References1. The United States Energy Information Administration, Independent Statistics and Analysis; International Energy Outlook 2013.2. The Centre for Strategic and International Studies Energy and National Security Program hosted the release of

the U.S. Energy Information Administration’s International Energy Outlook 2013 (IEO2013), July 25; Adam Sieminski, Administrator of the EIA, presented key findings from the report. Supporting information may be located at: • U.S. Energy Information Administration home page, www.eia.gov • Short-Term Energy Outlook, www.eia.gov/steo • Annual Energy Outlook, www.eia.gov/aeo • International Energy Outlook, www.eia.gov/ieo

3. Flow Research Inc., The World Market for Ultrasonic Flowmeters, 3rd Edition, Book 1 and 2, Wakefield, MA, USA.

4. AGA Report No. 9, Measurement of Gas by Multipath Ultrasonic Meters, June 1998, American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209.

5. ISO 17089-1:2010 Measurement of fluid flow in closed conduits; Ultrasonic meters for gas — Part 1: Meters for custody transfer and allocation measurement.

6. BS 7965:2013 Guide to the selection, installation, oper-ation and calibration of diagonal path transit-time ultrasonic flowmeters for industrial gas applications.

7. International Organisation of Legal Metrology Amendment 2014 to OIML R 137-1 & 2 Edition 2012 (E), Part 1: Metrological and technical requirements; Part 2: Metrological controls and performance tests.

www.honeywellprocess.com

Martin Bragg is Chief Technology Officer for Honeywell Process Solutions. Mr. Bragg is responsible for the technol-ogy portfolio of Honeywell’s Engineered Field Solutions (“EFS”) business, which is active in the market for gas and oil transmission, distribution and storage, including for individual measurands and terminal automation. He is a member of

the BSI, ISO, and AGA technical committees for flow and was sponsored through his MSc by The Royal Academy of Engineering, London, which administered the Panasonic Trust. He holds an MBA with a focus on Strategy within the Private Sector. Mr. Bragg can be reached at martin.bragg@honeywell.com or +44 8701 149880.

Martin Bragg

26 March 2015 Flow Control Magazine