Sensitivity of orifice meter gas flow computations

The effect of fluctuations in temperature and gas composition on the isentropic exponent and viscosity of metered natural gas are often overlooked in custody meters flow calculation software. They are usually entered manually at constant nominal values. Data pertaining to an existing installation are used to verify quantitatively the effect of this approximation on the accuracy of flow calculations. An analysis of the sensitivity of the coefficient of discharge and expansion factor with respect to both the isentropic exponent and viscosity is presented. It is found that the sensitivity of the measured gas flow rate to fluctuations in either temperature or gas composition is smaller than the sensitivities of the expansion factor and coefficient of discharge to such fluctuations.

► Use of exact expansion factor formula improves accuracy by about 0.1%.
► Derivation of expansion factor sensitivity to isentropic exponent.
► Derivation of orifice coefficient of discharge sensitivity to viscosity.
► Isentropic exponent and viscosity assumed linearly related to calorific value.
► Orifice coefficient and expansion factor sensitivities exceed that of flow rate.