Oil & gas of oil and gas flow meters
In this article, Craig Marshall, flow measurement consultant at TUV SUD NEL, discusses calibration of flow meters and factors that must be taken into account to ensure a correct and transferable calibration is performed
are 95 per cent confident that the true value lies within the uncertainty range stated around the measured value. Or, that for 20 different measurements, at least 19 will fall with the uncertainty range. For any reporting of uncertainty, and therefore any reported calibration results, there must be a stated uncertainty and ‘coverage factor’ on the calibration report or certificate.
measurement, the standard is a system comprising of a measure of fluid quantity over a specified time period as well as measurements of other important factors like density, viscosity, temperature and pressure of the fluid. It is important to note that all measurements are part of the standard, not just the measured quantity. Another feature of the standard is that there
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must be confidence that the measurement taken by the standard is accurate. To achieve this, all the measurements in the system have to show traceability to higher level measurements and ultimately to national and international standards. There are written standards and guidelines documents that provide prescriptive direction on application of measurement systems. To correctly express the performance of any
measurement system its uncertainty must be quantified and presented. The total measurement uncertainty of any device is the summation of individual sources of uncertainty combined in a well-defined manner e.g. ‘Guide to the Expression of Uncertainty in Measurement’. Uncertainty is expressed as the measured value alongside the absolute or relative uncertainty in that value where there is a given confidence that the true value lies within the uncertainty range. For flow measurement, the confidence in the
result lying within the uncertainty is commonly quoted with a ‘coverage factor’ of k=2, which is approximately 95 per cent confidence i.e. we
42 March 2019 Instrumentation Monthly
alibration is a comparison between the reading of a measurement device and that of a standard. For flow
ThE IMporTaNCE of CaLIbraTIoN fLUID aND CoNDITIoNS The conditions of calibration are vitally important for the transferability of any meter performance. All flow meters are dependent on a dynamic non-dimensional parameter called Reynolds number. Reynolds number is function of the physical properties of the fluid, the geometry of the pipe and the flowrate of the fluid itself. The calibration factor (e.g. K Factor, discharge coefficient, meter factor etc) can change significantly with Reynolds number and it is therefore important to ensure the operational Reynolds number is covered during any
calibration. For instance, calibrating a flow meter in water (viscosity = 0.9 cP at 25 °C) and using it in a hydrocarbon with a viscosity of 100 cP, will likely result in a significant measurement bias introduced through an inappropriate calibration factor. In addition to physical properties, careful
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attention must be given to the flow profile within a pipe. Flowmeters are designed to operate in what is referred to as ‘fully developed’ flow, i.e. where there is minimal distortion to the symmetrical distribution of velocity over the cross section of a pipe. The presence of intrusive process equipment or pipe geometry changes can alter the profile drastically as bends, double bends, valves, etc. all introduce asymmetry or swirl to the flow profile. These disturbances in flow profile are called installation effects and typically cause errors in the measured values from flow meters. The magnitude and direction of these errors depend on the disturbance significance, measurement technology and proximity. In an ideal world, the meter should be calibrated in situ using the exact pipework and
Calibration
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