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Simplified calibration methodology of chromatographs used in custody transfer measurements of natural gas

Oliveira E. C.

Metrology and Measurement Systems

2012

Vol. 19, nr 2

405-416

The properties, superior calorific value (SCV) and the compressibility factor (z), of 77 natural gas (NG) samples are calculated from two different calibration approaches of gas chromatography, based on ISO 6974-2. The method A uses an analytical curve with seven points that the best adjust is confirmed by Analysis of Variance (ANOVA); it is required when the composition of the natural gas varies. The method B uses a single point calibration, with an allowed tolerance between the calibration gas mixture and sample mole fraction, so it is used to analyze constant natural gas streams. From natural gas composition analyzed by both methods, exceeding the method B allowed tolerance; SCV, z and its uncertainties are calculated and compared. The results show that all samples that comply with Brazilian legislation can be analyzed by method B, because there are no metrological differences in terms of SCV and z, even though the allowed tolerance has been exceeded. This simplified methodology minimizes operator exposure, besides saving about US$ 50,000.00 per chromatograph.