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Flow, level & control


National Standard


Calibration House


Company ‘Master’


Produced Product


The pyramid of traceability


conditions (i.e. at line pressure and temperature). Meanwhile, the volumetric flow rate of water, oil, and gas needs to be estimated at standard conditions because of the business requirement that any fluid must be sold at stabilised conditions, which is referred to as ‘standard conditions’. To accurately convert meter readings from


in-situ to report flow rates at standard conditions requires knowledge of fluid properties, or pressure-volume-temperature (PVT) following pressure and temperature change, and this is irrespective of the type of meter or technology used. MFMs are particularly affected by the use of


the PVT information for two main reasons. They can be working at extremely high pressure and temperature, which requires a significant correction to convert the measured flow rates to standard conditions. For example, a flow meter may be calibrated with fluid at 20°C, and atmospheric conditions, but the fluid temperature in-situ could actually be as great as 100°C and > 50 bar(a). This will have a significant effect on flow measurement accuracy if not properly accounted for. Also, because no separation is made between the different flow phases, a large amount of gas can be dissolved inside the oil, or some of the condensates could be in gas phases at meter conditions when liquid at standard conditions. An uncertainty budget for the flow meter


when in use must therefore be constructed, taking account of additional uncertainties arising from interpolation and extrapolation


Instrumentation Monthly April 2021


from calibration conditions. Developing realistic uncertainty budgets for complex measurement processes is not trivial, requiring much more than just the calibration certificates for the individual components. Calculations for an uncertainty budget must therefore capture all the sources of uncertainty. Traceability is the process of proving


the performance of equipment for each stage of calibration. To provide the highest quality of measurement and therefore, the lowest uncertainty budget, flow meters should be traceable to primary standards, which are calibrated against National Measurement Standards. It is important to have proper mapping of


the performance/uncertainty of an MFM and this is based on the expected profile of oil and gas production. However, end-users face a substantial challenge when trying to select and review accurate meter performance because MFM manufacturers use the set of output parameters to state the performance of their product with the most attractive uncertainty or very generic information. TÜV SÜD National Engineering Laboratory


research has shown that stated meter performance levels are in general, overly optimistic. It was also shown that usually, manufacturers do not provide the end-user with the expected output specification of oil, water and gas flow rates. Instead they provide a combination of different parameters at line conditions because they do not have the expertise in fluid properties to translate to


Continued on page 34... 33


Decreasing Uncertainty Increasing Cost


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