Impact of the Gas Mass Flow Rate on Orifice Measurements Accuracy

The present article analyses the causes of systematic errors in measurements of the quantity of natural gas that result from the changes in the differential pressure at the orifice meter with flange tapping. The analysed orifices had the internal diameter of 412.45 mm and the thicknesses of 12.2 mm or 13 mm. Numerical calculations of the orifice deformation indicated that at the differential pressure of 60 kPa the force affecting the orifice exceeds 15 kN. As a result of the force effect, the central axis of the orifice 12.2 mm thick was shifted in 2.012 mm in the axial direction and the internal diameter of the bore decreased in 0.115 mm. At the differential pressure of 45 kPa, a negative deviation of 12,973 m(3).day(-1) occurs in the measurement of the mass flow rate (at 20 degrees C and 101.325 kPa). This error is approximately 5.6-times higher than at the differential pressure of 8 kPa. The gas flow rate should be thus measured on a larger amount of parallel lines, with a smaller pressure drop across the orifice.