In new European and national legislations, there is increased focus on the reporting of the emmissions related to greenhouse gases from process plants. This includes reporting and documentation of uncertainty in the reported emmissions, in addition to specific uncertainty limits depending on the type of emission and type of measurement regime. In a process plant like the Statoil Mongstad oil raffinery, there may be a huge number of measurement points for mass flow. These measured mass flow rates have to be added in order to obtain the total emission for a given source. Typically, orifice plates are used at many of the measurement points. These orifice plate meters are ususally not equipped with individual densitometers. In stead, they are pressure and temperature corrected from a common upstream densitometer. This will give correllations between the individual flow meters. In the present paper, the flow meter set-up for Statoil Mongstad will be briefly addressed, thereafter, an uncertainty model suitable for the CO_2 emission from the Statoil Mongstad oil raffinery will be presented, included a practically method for handling the partial correllation between the uncertainty of the various flow meters. This model will comply with the ISO 5168 for measurement uncertainty. Various uncertainty contributions will be reviewed, in order to work out an uncertainty budget for the specific emission sources. From these results and the use of a specially developed calculation tool, improvements on the instrumentation have been suggested in order to reduce the uncertainty, both in a short and a long time range. In particular, it is necessary to plan the flow metering instrumentation so that the uncertainty is maintained also for the new gas flow regime that will appear when the new gas power plant at Statoil Mongstad is ready.
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