Ultrasonic flowmeters positioned to lead in renewable energy measurement

The ultrasonic flowmeter is still a relatively new technology. In a few decades, ultrasonic meters have gone from being mistrusted to one of the fastest growing flowmeter technologies.

In 1993, when the American Gas Association (AGA) began holding meetings to discuss ultrasonic metering, ultrasonic meters were analog devices with wetted transducers. There were few suppliers, and ultrasonic meters were used mainly for large-diameter, high-pressure, stable-flow natural gas transmission applications, with unidirectional flow between the Gulf Coast and Western Canada, regulated by the Federal Energy Regulatory Commission (FERC). There were no large ultrasonic flow calibration facilities in North America. Flow conditioners barely existed, and software was aimed at developers and technicians.

Today, ultrasonic flowmeters are found across the entire supply chain – from the wellhead to the residence – for production, gathering, transmission and distribution. They are used for wet gas, cryogenic gas (LNG), biomethane and hydrogen blends. The meters themselves are highly integrated digital devices incorporating comprehensive software, advanced transducers, computational fluid dynamics, flow diagnostics and flow conditioning. Global standards are well established, and calibration is required and competitive.

Ultrasonic suppliers are now moving into new frontiers, including in energy transition with renewable natural gas (RNG) and hydrogen blends. They are trying to solve problems resulting from wet gas, fouling and contamination.

Classifying ultrasonic flowmeters

It used to be standard practice to divide the ultrasonic flowmeter market up according to whether the meters are transit-time, Doppler or hybrid. Traditional use of transit-time meters was for clean liquids, while Doppler meters handle fluids with impurities. Hybrid meters are a combination of transit-time and Doppler, and use one technology or the other, depending on the fluid. In the past 10 years, transit-time suppliers made great progress getting transit-time meters to measure fluid flows with some impurities. As a result, Doppler and hybrid meters are less important since transit-time meters are now used for applications previously reserved for Doppler meters. Another reason for growth in transit-time meters is their use in energy industries, mainly oil and gas, within which Doppler flowmeters play no major role.

Mounting type is now the more useful way to classify ultrasonic flowmeters, rather than transit time vs. Doppler. Three main mounting types for ultrasonic flowmeters include:

• Inline
• Clamp-on
• Insertion

Inline ultrasonic flowmeters are mounted with a meter body in the pipe. Inline meters achieve the highest accuracy of any ultrasonic meters, and multipath ultrasonic meters are inline meters. Multipath meters have three or more ultrasonic signals or “paths” to determine flow velocity. This gives them greater accuracy than single- and dual-path meters. The most common number of paths is four, five and six, but some multipath meters have eight, 12 or even 18 paths. Inline meters are used for custody transfer applications.

Clamp-on meter disadvantages limit their usefulness in certain situations. The ultrasonic signal can be attenuated by the pipe wall. Knowing pipe-wall thickness and composition can be important. In addition, build-up on the inside of the pipe wall can affect the internal diameter of the pipe. Knowing the internal pipe diameter is important to getting a correct flowmeter reading.

Insertion meters are sometimes used in large pipes when a spoolpiece would be expensive. They have a cost-advantage over inline meters since there is no meter body. Insertion meters go into a hole drilled in the pipe wall. They are widely used in stack-gas and exhaust-flow monitoring. Here they compete with differential pressure (DP) flowmeters using averaging Pitot tubes and with thermal flowmeters.

Advantages of ultrasonic flowmeters

Inline ultrasonic flowmeters have a host of benefits that have made the technology a popular choice for a wide range of flow measurement applications in gas, liquid and steam environments. These advantages include high flow measurement accuracy, high reliability, high turndown ratios, competitive pricing, no moving parts, low maintenance, valuable diagnostics, bi-directional flow measurement, redundancy capabilities and long service life.

Ultrasonic flowmeters have distinct advantages over other new-technology meters. Unlike Coriolis meters, ultrasonic flowmeters do very well in large pipe sizes. Over half of the Coriolis meters sold today are for pipe sizes two inches or less. While some Coriolis meters have successfully been used in four-inch and six-inch lines, they begin to become unwieldy and quite more expensive in sizes over two inches. Size can be an advantage for inline ultrasonic flowmeters, since larger pipes provide more distance for the ultrasonic signal to cross. For pipes six inches and larger, ultrasonic flowmeters are in most cases a better choice than Coriolis meters.

The main competitors to ultrasonic meters for natural gas pipeline applications in the larger line size range are turbine and differential pressure flowmeters. Here, ultrasonic flowmeters have the advantage of being highly accurate, non-intrusive and highly reliable over time, with no moving parts to wear. For a long time, DP flowmeters also had the advantage of approvals from the American Petroleum Institute (API) and the AGA that helped them establish a large installed base in the oil and gas industry. This advantage has diminished as ultrasonic flowmeters have gained their own sets of approvals from international bodies.

Ultrasonic flowmeters have an advantage over magnetic flowmeters in that they can be used to measure the flow of nonconductive liquids, gases and steam. Magnetic flowmeters have very limited use in oil and gas production, transportation and refining sectors because petroleum-based liquids are nonconductive. Ultrasonic flowmeters have an advantage over vortex flowmeters in that they can measure low and zero flows. Vortex meters have a difficult time with low flows because if the flow is too low, it may not generate sufficient vortices for measurement purposes.

Frontiers of research

Suppliers are devoting development and marketing to significant enhancements to ultrasonic flowmeters. The frontiers of research include:

• Innovative enhancements increase accuracy, reduce profile uncertainty and enhance self-diagnosis
• Wet gas in upstream measurement
• Hydrogen and renewable natural gas

Innovative enhancements increase accuracy, reduce profile uncertainty and enhance self-diagnosis

Suppliers are introducing ultrasonic meters with innovative path arrangements and other features to provide greater accuracy as well as a clearer and more timely view of what is happening with meters in the field. This is important as ultrasonic meter users and regulators demand continuous assurance of the accuracy of their measurements while they are in service and need better means of self-diagnosis. Suppliers are also realizing that flow profiles are not constant, and that pulsation, vortices, harmonics and other effects can result from a variety of causes – pumps and compressors, sharp edges in a pipe, sharp changes in flow direction, liquids in a gas flow, blockages from dirt and other build-up, and even foreign objects. Consequently, they are incorporating self-verification and enhanced diagnostics, including remotely, and reducing the effects from flow disturbances, even without flow conditioners.

Wet gas in upstream measurement

The complex upstream market is a frontier of interest for many ultrasonic flowmeter manufacturers, who see untapped opportunities for measurement as well as possibilities for making inroads into applications currently using differential pressure and turbine meters. Issues in the upstream market include unplanned downtime and greenhouse gas (GHG) and emissions monitoring. One of the main frontiers for ultrasonic meters in this market is the presence of liquids and other elements at gas measuring points.

Raw natural gas at gas production wells and pipelines or associated gas at oil fields can contain natural gas liquids (NGLs) – hydrocarbons such as ethane, propane, butane, isobutane and pentane; water, oil or glycol, as well as dirt, dust and mechanical particles. Shale play areas often have very high BTU gas that consequently may contain NGLs. Some gas producers find it is easier to allow the NGLs to flow with the gas until they are separated at a compressor station, measured (often with a Coriolis meter), and then re-injected into the downstream pipeline after compression. This process continues until the gas and the liquids arrive at a gas processing plant that removes the NGLs and delivers pipeline quality gas.

Hydrogen and renewable natural gas

Some ultrasonic flowmeter manufacturers now offer meters that measure hydrogen and are continuing to explore how to best measure hydrogen and hydrogen blends. Hydrogen can be produced from multiple sources, including natural gas, nuclear power, biomass and from renewable power sources such as solar and wind. Hydrogen is an attractive fuel option for electricity generation and transportation applications. It can be used in houses, in cars, for portable power, and for many other fuel-related applications. 

Hydrogen’s physical properties are quite different from other gases, including natural gas, and flowmeter suppliers have had to adapt to those differences, as well as figure out ways for ultrasonic meters to accurately measure hydrogen blends.

Most hydrogen today is extracted from fossil fuel natural gas (sometimes called “grey hydrogen”). It is widely used today in industrial processes, including, for example, in manufacturing high-grade steel and as a reactive agent to refine petroleum products by breaking down heavy molecules and removing impurities. In a production process for fatty alcohols, hydrogen is used as a circulating gas. Within the process, hydrogen is used both as a raw material and the primary carrier medium.

Now two types of carbon-neutral hydrogen, plus a mixed type, are on the rise:

• Blue hydrogen, extracted from natural gas, captures the CO2 generated during the process and stores it permanently underground for a carbon-neutral footprint.
• Green hydrogen (H2) is produced by splitting water into hydrogen and oxygen in an electrolyzer powered by renewable energy. It is a versatile energy carrier that can be used directly instead of grey hydrogen or combined with other elements to create synthetic electrofuels (e-fuels) like e-ammonia and e-methanol.
• Mixed hydrogen combines blue or green hydrogen in natural gas pipelines to reduce carbon emissions.

A look ahead

Expect ultrasonic flowmeters to continue to build on their advantages, and to maintain their fast-paced growth. Suppliers are investing heavily in these meters to add more sophisticated software, advanced diagnostics, more communication protocols and enhanced reliability. Look for ultrasonic meters to remain competitive in the market for custody transfer of natural gas. And as hydrogen and renewable gases become more important in the energy markets, ultrasonic meters are well-positioned to take the lead in their measurement.

For more information on the ultrasonic flowmeter market, visit www.flowultrasonic.com. To learn about the entire flowmeter market, visit www.flowvolumex.com.