PC-APR23-PG09.1_Layout 1 11/04/2023 10:11 Page 9


CALIBRATION RESEARCH RESULTS IN RECOMMENDATIONS


Dr Chris Mills, senior consultant engineer at TÜV SÜD National Engineering Laboratory, discusses calibrating oil and gas flowmeters for real-world process effects


T


he oil and gas industry appear to be favouring a move towards using “newer” and more “advanced” flow measurement technologies, such as ultrasonic and Coriolis devices, as an alternative to turbine and positive displacement meters. The advantages of Coriolis flow meters, such as high accuracy, claimed insensitivity to installation and direct measurement of mass flow, have led to wide scale adoption across the process industries, including the food and pharmaceutical sectors. Though the adoption of Coriolis flow meters is a logical move for the oil and gas industry, the effect of elevated conditions on their measurement uncertainty is generally not well understood by end users. Several factors affect the performance of Coriolis devices, including temperature, pressure, fluid viscosity and the Reynolds number. However, meter manufacturers incorporate corrections in an attempt to compensate for these effects. Whilst calibrating “in situ” at service conditions can eliminate these effects, industry appears to be moving away from proving meters onsite. Partly due to a lack of space, maintenance and cost, provers are becoming scarce in offshore oil and gas applications. The more favoured design appears to consist of Coriolis master and duty flow meters. The Coriolis duty meter remains in- situ and the master meter is periodically sent to an accredited laboratory for a flow calibration to minimise oil and gas production downtime. The performance of the duty meter is then compared with the calibrated master meter. However, the temperature, pressure and fluid


properties of produced oil and gas from a reservoir can differ considerably from conditions at the calibration laboratory. Our research has explored the performance of Coriolis flow meters that have been calibrated in our Elevated Pressure and Temperature (EPAT) oil flow facility and the UK National Standards oil flow facility in Glasgow. We analysed the calibration results in terms of fluid viscosity, Reynolds number, temperature, pressure and flow rate to identify trends and to ascertain whether manufacturers’ performance claims were valid. The experimental results used were from a combination of Department for Science, Innovation & Technology (DSIT) funded research, Joint Industry Projects, internal TÜV SÜD National Engineering Laboratory research and commercial calibrations.


Overall, the research results reinforce the concept that Coriolis flow meters cannot simply be utilised at service conditions without suitable consideration, characterisation, and calibration. In summary, Coriolis water calibration does not replicate service conditions, and it is vital that end users remember that pressure corrections published by manufacturers are not fully traceable at present.


From our research, we would make these recommendations for calibrating Coriolis flow meters when onsite proving is not available: Temperature - Temperature is a significant effect for Coriolis flow meters. However, Coriolis flow meters have an onboard Resistance


Temperature Detectors (RTD) and incorporate algorithms to correct for temperature effects on the flow tube material. This means that the temperature effect is dynamically corrected. The temperature compensation coefficient cannot be easily modified by the end user. Instead, a more practical approach would be to calibrate the device as close to the service temperature as possible. This would allow the end user to ascertain whether temperature effects are significant, and a correction allowed for via an adjustment to the Coriolis mass factor. Pressure - As the pressure effect has shown to be linear, it can be corrected either via an adjustment to the meter mass factor, a static fixed pressure correction or a dynamic “live” correction via a pressure transmitter. If the process conditions are stable, then a static fixed pressure correction could potentially be applied. This involves the device being adjusted for the effects of pressure via an adjustment to the device mass factor or to the flow computer. However, it should be noted that if the pressure effect is significant (e.g. –0.020 % per bar) then even a 5 bar variance could produce a meter offset of –0.10 % using a static fixed correction. This means that a traceable dynamic “live” pressure correction via a pressure transmitter should be used where possible.


Viscosity / Reynolds Number - If operating in high viscosity conditions, a Coriolis flow meter should be characterised against the Reynolds number with a suitable fluid to ascertain the effects. However, correcting for the adverse effects of viscosity/Reynolds number can be challenging. It should also be noted that installation has a significant effect on the Reynolds number at which the laminar-turbulent transition occurs. Hence, the robustness of any Reynolds number correction might require further investigation at alternative entry lengths. It is also important for the end user to remember that the performance of Coriolis meters from one manufacturer are not necessarily similar to meters from other manufacturers. There are many variables such as meter design, flow tube dimensions, patented corrections and the quantity and quality of any internal R&D. It is also hoped that ISO 10790, which provides guidance on the selection, installation and use of Coriolis meters, will be updated in the near future with the latest available traceable data on temperature, pressure, and viscosity/ Reynolds number effects


TÜV SÜD National Engineering Laboratory www.tuvsud.com/en-gb/nel


APRIL 2023 | PROCESS & CONTROL 9


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70