Towards the Validation of Ultrasonic Flowmeters Operating in Hydrogen-enriched Natural Gas Mixtures though Speed of Sound Measurements Obtained by a Clamp-on Meter

New flow metering challenges are presented by the energy transition program since the available and new infrastructures might be used to transport energy using energy vectors, such as hydrogen-enriched natural gas mixtures, including blends never adopted before in current distribution lines. In this framework, it is necessary to have the possibility to verify the performance of flowmeters, which are currently calibrated using natural gas and nitrogen as reference fluids, even when operating with fluids that are not yet in use. For this reason, a commercial clamp-on ultrasonic flowmeter was used to measure the speed of sound in a mixture of hydrogen and iso-butane after being calibrated using helium as reference fluid. Helium is actually much more expensive than nitrogen, but in our case it is advantageous because, in the temperature and pressure ranges considered in this work, the speeds of sound of helium are more comparable with those of the binary mixture of hydrogen and isobutane than the speeds of sound of nitrogen under the same thermodynamic conditions. A specifically developed control apparatus was designed to adjust the temperature and the pressure of the gas filling a DN50-PN100 spool where the ultrasonic meter was mounted on. The instrument was calibrated for temperatures between (270 and 320) K and for pressures up to 3 MPa by using the prediction of the reference equation of state for helium of Ortiz-Vega et al. The measurements of the speed of sound were obtained in a binary mixture containing mainly hydrogen with a small content of iso-butane since, for these compounds, new results are necessary to validate and improve the predictions of thermodynamic models installed in flowmeters and in flow computers. The expanded relative uncertainty was evaluated to be of 0.09% ( = 2) that was estimated by considering the contributions of the main influence quantities, repeatability and reproducibility of the measurements. The obtained results were compared with the AGA-8-92DC and GERG-2008 equations of state and found to be consistent with the values predicted by both models, demonstrating the feasibility of using a clamp-on ultrasonic flowmeter to determine the speed of sound and possibility to verify the performance of flowmeter installed on the gas networks using the speed of sound as transfer quantity.