Flow measurement is critical in fiscal measurement applications from custody transfer to allocations between parties. The choice of technologies is broad and dependent on the fluid being measured and properties of the application.In an Applied Automation article, Selecting flowmeters for natural gas, Emerson’s Dan Hackett describes the need for different flow measurement technologies across different parts of the natural gas delivery supply chain—from the wellhead to delivery or sale location.
Dan explains that all forms of fiscal measurements:
…can be affected by product quality, fluid properties and composition, operating parameters, maintenance practices, and technology type.
Accuracy is essential given the value of natural gas:
At current gas prices, an error of 1% in measuring 300 million ft3 of gas per day can lead to a difference of about $2 million per year.
Dan highlights three flow measurement technologies: differential pressure (DP), ultrasonic, and Coriolis. The first two measure volumetric flow and Coriolis directly measures mass flow. What is actually paid for in a fiscal measurement is the energy delivered.
For this reason, accurate and repeatable measurement of natural gas flow requires simultaneous measurement of several other variables, including pressure, temperature, density, and gas composition.
Dan describes DP measurement technology.
DP meters measure only differential head. To measure either mass or volumetric flow, they must be corrected for density (mass) or temperature, pressure, and gas composition to obtain a standard reading.
DP meters have a low turndown ratio— “…the range of flow that a flow meter is able to measure with acceptable accuracy”. Changing the size of the orifice plate changes the flow measurement range. Fouling can also occur. Other drawbacks are that DP meters are sensitive to the profile of the fluid flow and can cause a medium-to-large pressure drop. Use in custody transfer applications:
…is governed by API 14.3/AGA3 in North America and ISO 5167 globally.
Since Coriolis meters measure mass and density, they:
…do not need pressure and temperature compensation for fluid properties. They feature wide turndown ratios and do not experience performance or calibration drift.
Dan shares some advantages including applications with unstable densities, no flow conditioning requirement, and self-diagnostics for fouling detection. They are also good at detecting when two-phase flow begins to occur. Disadvantages is their physical size, relative cost to other technologies and pressure drop. They:
…generally are not available in sizes larger than 16 in. (and, in fact, are often one or two pipe sizes smaller than the lines in which they are installed, which contributes to pressure drop). For this reason, they are generally restricted to applications in which pressure drop is of limited concern.
AGA-11 governs their use in custody transfer applications.
Ultrasonic meters have several advantages including:
…standard volume flow accuracy of 0.35% to 0.5%, with 0.25% available, as well as negligible pressure drop and high turndown capability. The high turndown makes them useable in applications subject to wide variations in flow, which means that a single ultrasonic meter can replace multiple other meters…
Also embedded diagnostics can detect a range of issues including two-phase flow and blockages. Disadvantages include:
…the need for sufficient straight-run upstream or a flow conditioner. Because the accuracy of an ultrasonic meter depends on the accuracy with which the flow profile inside the instrument is known, ultrasonic meters for custody transfer applications generally use a minimum of four paths.
Read the article for more on the role of flow computers, the differences in natural gas between conventional and shale production, and managing the challenges across the supply chain. The Flow Solutions Technology Advisor site is also great for helping you find the right flow measurement technology for your application.
You can also connect and interact with other flow measurement experts in the Flow group in the Emerson Exchange 365 community.