An evolving philosophy: Reducing operational uncertainty in large volume gas transfers
Importance of managing fiscal risk in large volume gas custody transfer.
Industry Best Practice for Setting Gas Measurement Uncertainty Budgets
Apart from the annual trading value, these recommended uncertainty budgets are the result of the desire to weigh the cost of more robust measurement solutions against the cost of uncertainty and account for the current state of the art measurement technology capabilities, prices and uncertainties.
Three Prevailing Large Volume Gas Measurement Technologies
For larger volume gas custody transfer, there are only three prevailing technologies and ultrasonic is most often the first choice.
Differential Pressure (DP), dating back to the 1930s can be offered in a wide range of sizes and are very good in harsh environments. The orifice fitting, one type of DP measurement, is the most common type of flow meter type used today and was the preferred gas measurement technology until the 1980s when turbine meters became accepted for custody transfer measurement and started to displace orifice fittings, particularly in cleaner downstream environments like gas pipelines and utilities. They are highly accurate, repeatable and have a wider turndown ratio, or the range of velocities a meter can measure accurately. This provides for more flexibility for operational changes such as growth or decline and/or seasonal fluctuations of flow rates.
The Rise and Evolution of Ultrasonic Flow Meters
Early ultrasonic flow meters were often misapplied because of poor designs of Doppler type ultrasonic meters and a poor understanding of the new technology in general. They were perceived initially as “black box” devices with performance issues and received a bad reputation in the industry. Advances in path configurations, micro processing speed, digital signal processing (DSP) and software led to significant improvements in their performance, diagnostics and usability.
It is also why global metrology approvals that govern their use and application have been so important to the acceptance of ultrasonic among end users. Ultrasonic meters have continued to evolve exponentially over the last ten years offering new designs with more paths (including dedicated diagnostic and verification paths), more advanced diagnostics and more robust designs to name a few improvements.
The meter sensors have also become more robust with designs that protect them from direct contact with the process, making harsh environments suitable. Applications for ultrasonic meters have continued to expand as a result. These improvements, decreasing price and fundamental advantages (Figure 2) have accelerated the evolution from older technologies to ultrasonic as the pros and cons are weighed. This and changing needs have prompted the industry to reevaluate older standards.
Minimizing the Effect of Process Disturbances on Operational Uncertainty
However, in 2013 when the European Union started changing the pipeline infrastructure to ensure security of supply, the option for two different types of ultrasonic meters started to become accepted for bidirectional installations as well as for unidirectional installations in many cases.
The Problem of Common Mode Error
One way this can be addressed is by combining different path types and path locations and then by comparing speed of sound (SOS) and/or flow velocity. Each path configuration has different strengths and weaknesses on their own (Figure 3), but combined, they can make very powerful check measurements and diagnostics. Flow profile changes in the process as a result of (partial) blockage or roughness can be detected quickly in a very early stage comparing a meter with more robust direct path meter to a check meter with a single reflective verification path.
Furthermore, multi-path meters already do sophisticated diagnostic checks on SOS agreement per path. If meters with both reflective and direct path types are compared, greater deviations in the SOS differences between paths will be observed. Reflective paths that are sensitive to pipe wall effects will show a clear SOS shift compared to negligible SOS shifts on direct paths.
That is how liquid and buildup contamination can be diagnosed. If the reflective measurement path is positioned to come into direct contact with the pipe wall buildup or liquid, it will create a significant shift in SOS per path and average SOS. The graph below shows the SOS spread that resulted from just 1-1.5 mm of anti-seize sprayed across different sections of the bottom of the meter in the lab.
Both meters will overstate the volumetric flow rate and have the same directional bias in this case, but the risk can still be mitigated and detected. Figure 4 illustrates the sensitivity of the reflective path to buildup. The speed of sound shift of the single path is calculated by comparing measured speed of sound to calculated SOS, using the AGA 10 method. This method uses gas composition, pressure and temperature to determine what the speed of sound should be. This is one of the most fundamental diagnostic checks for ultrasonic meters.
In Test 5C where the anti-seize is only on the very bottom of the pipe, the reflective path does not reflect off this small area of buildup and the SOS shifts marginally. In Test 6C the reflective path comes directly into contact with the test condition and SOS shifts over 0.12%, orders of magnitude over the 0.2% shift allowed by AGA, signaling a problem.
Reflective paths are critical to detection, but reflective path location is also important. Test 5C could be detected using SOS if a vertical reflective path were present. SOS comparisons between path types and between meters is how the operator can reduce the impact of common mode error in the case where the flow rate responds with the same directional bias.
New Test Qualification for Limiting Operational Uncertainty
With exception to liquid and buildup, reflective paths typically respond with the opposite flow error bias to that of direct paths, which drives greater separation between two meters using opposite path types for faster detection.
DNV GL created a new test qualification for custody meters that was designed to account for uncertainty that occurs due to process conditions in the field. Other internationally renowned metrology standards organizations (OIML R-137, AGA 9, ISO 17089) focus on calibration, design and installation effects but fail to account for real-world operating conditions in the field.
DNV GL based the test criteria for their new meter qualification on their independent research on the most common process conditions that affect operational uncertainty. These conditions were found at real sites all throughout the world. Below are the results of the test qualification on a meter design with both direct and reflective path types.
On all blockage tests and on the temperature test, the direct path meter and the reflective path meter had the opposite flow error bias, preventing common mode effect and providing an early warning of process changes. For these process conditions velocity comparisons are enough for verification. For liquids, buildup or roughness effects, SOS comparisons between meters and path types are needed to reduce common mode error.
The meter under test is the Daniel 3416 (Figure 5), not an 8-path meter but a so called 4 + 2, which combines two independent meters into the same body: the first uses 4 direct paths in the British Gas layout and the second uses two reflective paths in different locations.
The reflective path which is 30° off vertical is a check measurement path and the vertical reflective path is a diagnostic path used for its sensitivity to pipe bottom contamination. Emerson has a 20-year legacy with 1-path and 2-path reflective type meters used for allocation measurement applications and a 34-year legacy with the robust 4-path British Gas layout designed for custody transfer.
In 2015, in recognition of the potential to improve inline meter verification using dissimilar path types and the potential to exploit the weaknesses of the reflective paths for process condition detection, these meter types were combined into a single meter body.
This allowed comparisons between these path types to be made in the meter or online with new comparisons and diagnostics. Several ultrasonic meter manufacturers now offer two-in-one solutions but with different combinations and results. Emerson is the first and only manufacturer who combines the robustness of the direct paths (chordal) with sensitivity of the reflective paths for checking, verification and diagnostics enabling early warnings.
The key to limiting the operational uncertainty due to these types of process issues is by monitoring the comparison of the two measurements. Most ultrasonic meters due to the robustness of the diagnostic data have Ethernet connectivity, and some can do their own internal inter-meter comparisons that can be easily extracted discretely without external flow computer or SCADA programming.
An Ideal Combination
Several manufacturers of ultrasonic meters have also released new models with more paths; many now offer an 8-path gas ultrasonic meter. The availability of these higher accuracy meters negates many of the previous restrictions of using ultrasonic flow meters, it allows operators to reduce upstream piping lengths and eliminates flow conditioners as they are less sensitive to flow profile changes.
8-paths provide a more robust measurement that is not as affected by process disturbances upstream piping, tees, piping intrusions, blockages, etc. and they improve measurement availability as they can maintain their high accuracy even without all paths, making maintenance less critical.
Additionally, this makes putting two ultrasonic meters in series even more economical while increasing measurement assurance. Emerson’s recent OIML R-137 Accuracy Class 0.5 classification results demonstrate that the 8-path meter can maintain the highest accuracy class in both mild and severe disturbances without conventional upstream piping lengths and without flow conditioning (Figure 6).
Figure collection 6.
Traditional process diagnostics have low correlations to measurement error even with very high degrees of swirl. Even if the diagnostics in the field differ from the calibration, this meter will maintain its calibrated accuracy level.
Combining this highly robust measurement with one that can mitigate common error provides the solution with the lowest operational uncertainty possible with today’s technology. Emerson’s ideal combination is the Daniel 3418 combined with the Daniel 3416 for maximum accuracy, repeatability, reliability and process intelligence (Figure 7).
With a Rapidly Changing Technological Environment, what is Next?
As the technology has rapidly developed over the last ten years, ultrasonic has been the fastest growing flow metering technology. They were traditionally applied in gas transmission and widely accepted in clean, dry gas environments. Now, they are now accepted at the well head, in wet gas, in corrosive gas environments.
Ultrasonic meters are also supplied in biogas, coal bed methane, high C02 and many liquid applications including LNG.
There are fewer restrictions for use and a much wider operating envelope than ever before. Additionally, ultrasonic manufacturers have developed condition-based monitoring systems with machine learning and are exploring use of artificial intelligence to improve online verification tools and to eliminate diagnostic interpretation.
More operators are investing in IOT infrastructure and with more secured networks are open to the possibility of moving diagnostic data to the cloud.
Users recognize the potential to do more with the data to manage processes, system uncertainty and advance predictive maintenance for additional operation savings. With all the new improvements and changes in the oil and gas industry, we can confidently state today's technology has opened the door to moving away from calendar-based maintenance or re-calibration to condition or risk-based maintenance or re-calibration without a high risk for common mode effects.
Question remains whether operators will believe we have arrived at a mature solution for managing operational uncertainty will a new horizon emerge for mitigating or even quantifying risk?
Either way, ultrasonic metering solutions are progressing such that they are unparalleled in robustness and intelligence and may also soon exceed the long-term repeatability of turbine meters.
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