Multipath Ultrasonic Meters For Natural Gas Custody Transfer
Figure 1: Schematic of a transducer pair’s geometry that puts the vector sums into context.
Over the past 15 years, natural gas ultrasonic meters have transitioned from the engineering lab to wide commercial use as the primary device of choice to measure gas volume for fiscal accounting. Wide acceptance and use by gas pipeline companies has occurred during this time due to the device’s
- Capacity (rangeability),
- Commercial availability that translates into product support, and
- Adoption of industry standards for fiscal measurement applications.
Briefly, the historical development of fluid velocity measurement in closed conduits with sonic pulses was first considered in the 1920s with discovery that transmission and reception of repetitive sound bursts could be used to describe the location and speed of moving objects; this principle was soon used to build sonar and radar arrays. Attempts were made over the years to apply the principle to measurement of fluid velocity in conduits, but it wasn’t until the development of economical high-speed electronics and digital signal processing in the late 1970s that a repeatable instrument with sufficient resolution for gas applications was devised.
Over the following decade and a half, the practical challenges of making the technology commercially viable as a flow measurement device were described and addressed through innovation and development that resulted in production of a gas ultrasonic meter that utilizes:
- Robust transducers that generate repeatable pulses (amplitude and frequency),
- Multiple paths to average axial velocity components over the cross-section of a closed conduit, (i.e. pipe),
- High-speed electronics complete with an accurate clock, to detect, resolve and time transmission and reception of sonic pulses with sufficient time-domain resolution.
- Combining transducer and electronics to permit high pulse transmission rates, and their transit time measurement, to allow rapid integration of fluid flow velocity so that accurate measured values can be reported once per second.
Virtually all ultrasonic meters used by pipeline companies for fiscal measurement are flow-calibrated at meteorologically traceable test labs. Flow tests are conducted at multiple points over the meter’s operating range to characterize its proof curve. Meter factor(s) are then calculated and applied to correct the meter’s output to the lab’s reference standards.
An advantage of modern ultrasonic meters is that once flow-calibrated, diagnostic assessments can describe whether proof, i.e. meter factor, shifts due to fault in the meter’s operating elements (i.e. transducers and/or processing electronics), so that re-calibration generally isn’t required (although some regulatory authorities mandate re-certification at set intervals: these mandates vary by jurisdiction).
Knowledge regarding the measuring principle of ultrasonic meters lays a foundation for optimal field application as well as providing the basis for understanding whether the meter continues to accurately and reliably measure gas volume. Multipath ultrasonic meters that are typically used for gas custody transfer calculate gas flow rate from velocity measurements made over a pipe’s cross-section, using the following process:
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- Contractor, pipeline
- Contractor, river crossing/ directional drilling
- Directional drilling rigs, large
- Fittings, valves: plastic
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