Pressure Influence in LPG Measurements by Uncertainty Evaluation

In Brazil, National Metrology Institute, INMETRO, elaborated an N° 64 Ordinance, on April 11^th 2003 [1], based on OIML [2]. This document approves the Metrological Technical Regulation, establishing the metrological and technical requirements applicable to measurement systems supplied with flow meters used in the measurement of oil, its derivatives liquid, alcohol anhydrous and hydrous alcohol fuel. For custody transfer purposes, liquefied petroleum gas (LPG) volumes are generally stated at a fixed base temperature and saturation pressure. As most volume transfers occur at temperatures and pressures other than standard conditions, these volumes are adjusted to standard conditions through the use of correction factors. The measurement uncertainty of these custody transfer systems must be controlled. Based in norms ASTM, IP, ISO and API – MPMS, the mathematical model for the calculation of the volume, it basically depends on quantities as: K factor, density, pulse number, turbine meter factor, temperature and pressure corrections. This work develops a methodology to calculate the LPG mass uncertainty that is completely different form the other liquid oil derivatives. This work details this calculation showing the complex temperature and pressure algorithms, including the equilibrium bubble point pressure. Based on this methodology, it is noticed that the pressure uncertainty contribution is not less than 43% overall the process and the expanded uncertainty is less than 1%, as required by INMETRO.