One of the main issues in the management and control of Natural Gas (NG) transmission networks is represented by Unaccounted-for-Gas (UAG), that is the quantity to be necessarily considered in the network balancing equation to take into account the unavoidable measuring errors. Even though this aspect has been continuously investigated in the scientific literature, few studies are available concerning UAG sources and related uncertainty estimation by considering pipelines typology, operative and environmental conditions, metrological performance of the installed measuring systems, procedures used to perform the network balancing and to estimate each term of the balancing equation including the losses and line-packs terms. In this paper the authors investigate the UAG sources and evaluate the criticalities related to UAG estimationeprediction, besides the possible actions aimed at reducing UAG quantity in different NG transmission networks. To this purpose, the authors investigate the UAG in some relevant NG trans- mission networks, statistically analyzing annual and monthly trends of UAG and evidence UAG sources, with particular reference to the influence of the uncertainty of the measuring plants. In fact, inaccurate measurements can be responsible of large UAG, with potentially significant economic losses for all players in the system. Finally, the authors point out that the main UAG sources are related to some systematic measurement errors associated to climatic conditions and propose a simplified model to predict UAG value.
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