Ultrasonic Flow Meters Set a New Standard for Natural Gas Custody Transfer

Watching the changes in oil and gas supply chains over the last few years has been exciting, to say the least. With changes in sources and political upheaval in global energy markets, many traditional relationships have been upset. The result has been that many new connections between buyers and sellers have taken shape, which makes understanding how custody transfer works all the more important.

Rosemount Ultrasonic Flow MeterThis coincides with growing sophistication of Emerson’s Rosemount Ultrasonic Flow Meter technologies, making them especially desirable for this type of service. I unpack how this all works in my article in the Q2 2023 issue of Process Instrumentation magazine, titled Improved Precision and Reliability of Flow Measurement Ensures Accurate Custody Transfers.

Accuracy is paramount, given the volumes and values involved. For example, an error as small as 0.25% equates to financial risk of nearly $500,000 per year on 3 million standard cubic meters per day at $5 per million BTU. Measurement errors lead to financial exposure and disagreements, costly in both money and time. For most industrial applications, a flow meter with accuracy of ±0.25% is exceptionally precise, but the higher the annual economic trading value, the more accuracy required from the flow meter and corresponding meter assembly.

The kind of accuracy required, combined with the scale and complexities of the application, poses challenges for flow metering. International metrology standards for custody transfer call for a maximum allowable shift in accuracy of ±0.167% compared to an international reference standard with disturbed flow conditions present. Now, apply that to a 30 inch, or even larger, pipeline with variable flows and all sorts of disturbances caused by valves, fittings, and so forth. It’s certainly one of the most challenging applications imaginable.

But, it’s well within the capabilities of our most sophisticated model, Rosemount 3418 Eight-Path Gas Ultrasonic Flow Meter, and line size is not a limitation. This is the best approach, as the article argues, because it provides this level of accuracy across a wide range of operating conditions.

Some of the newest ultrasonic flow meters use eight transducer/receiver pairs, each placed to read velocity at four positions, with the first set of four set in the mirror image of the second within the body, with flow in either direction. Using this eight-path approach, it is possible to characterize the flow profile to create the most accurate picture of gas movement and volume for exceptional measuring precision. Each position provides two chords across the pipe diameter, so there are eight measurements taken over 32 milliseconds.

The article goes into more detail, but this approach is easy to install, largely tolerant of piping system issues, and creates no pressure drop. It is also well suited to more products than just pipeline gas. If you’re involved in creating or upgrading a custody transfer site, this should be top of the list to consider.

Visit the Custody Transfer Measurement System pages at Emerson.com. You can also connect and interact with other engineers in the Oil & Gas Groups at the Emerson Exchange 365 community.

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