Effect of flow-rate measurement accuracy on unaccounted for gas in transmission networks

Unaccounted-for-Gas (UAG) in modern natural gas transmission networks is a very debated topic among researchers and network managers, since UAG significantly affects the physical and commercial balancing of the network itself. On the technical hand, UAG is caused by the unavoidable errors in measuring and estimating gas quantities in the network and it is either positive or negative. However significant and tendentially positive/negative UAG values often occur in modern networks and this can be ascribed to the rise of systematic errors. Furthermore, cyclic trends of UAG are often observed and the correlation with the accuracy of flow-rate measurements has not been adequately investigated, despite this latter is widely recognized as a crucial issue. In this paper, the rangeability faults of flow-meters installed at the interconnections with regional and city networks have been investigated, together with the effect of the drift of the instrument due to the lack of subsequent calibrations. From the analysis carried out, it was found that about 12% of the average daily flow-rates measured in a whole year at the exit points of the Italian network are below the minimum flow-rate of the flow-meter and that a significant correlation between monthly UAG and the number of flow-meter rangeability faults exists.