Technology Advancements Enhance Measurement Accuracy
Modern-day computers have altered volumetric flowrate measurement procedures in use by the natural gas industry. They have allowed implementation of an improved and highly accurate equation of state for the prediction of the supercompressibility factor of natural gases, including pipeline quality, high CO2, high N2, wet and/or sour natural gases. This correlation, developed in research sponsored by the Gas Research Institute, also allows accurate prediction of derived properties such as sonic velocity needed in sonic nozzle calculations.
The industry-standard report released by the American Gas Association in 1992, A.G.A. Transmission Measurement Committee Report No. 8, Compressibility Factors of Natural Gas and Other Related Hydrocarbon Gases (K.E. Starling and J.L. Savidge, authors), calculates compressibility and supercompressibility factors based on known natural gas composition or characterization methods including either natural gas gravity, heating value and mole percentage of carbon dioxide or natural gas gravity and mole percentages of carbon dioxide and nitrogen.
While tables of compressibility factors based on the work of R.H. Zimmerman (PAR Report NX-19) were used in the past, the increased accuracy gained over a wide range of gases in the computation-intensive A.G.A. Report No. 8 have caused gas volume and other pertinent calculations in the office to be made using computers. In the field, electronic flow meters and portable microcomputers are increasingly being used for calculations. The programmed calculator and computer with the appropriate software or firmware are replacing charts and tables as the working tool in those situations, such as field testing, where tabulations have traditionally been used.
A.G.A. Report NX-19 was developed for calculations of supercompressibility factors of natural gases containing up to 15 mole percent N2 and up to 15 mole percent CO2 with a specific gravity limit of 1.0. In practice, however, these composition limits have been exceeded as a matter of necessity. With the adoption of A.G.A. Report No. 8 for most natural gases at typical pipeline conditions, the uncertainties in computer supercompressibility and compressibility factors will be less than 0.05% and 0.1% on the average, respectively (the relative uncertainty of the supercompressibility factor is one-half that of the compressibility factor). For natural gases which have high gravities, due either to carbon dioxide or ethane plus heavier hydrocarbons, uncertainties using A.G.A. Report NX-19 will be much higher than these.
Electronic flow measurement (EFM) software must be efficient to allow for frequent flow calculations. The algorithms in the 1992 A.G.A. Report No. 8 were designed to minimize redundant calculations with the anticipation of EFM applications. The natural gas properties depend on three quantities, (1) gas composition, (2) temperature and (3) pressure. Quantities which depend on composition but not on temperature or pressure are calculated in one computation block (a group of subroutines). Quantities which depend on temperature but not pressure are calculated in a second computation block. Quantities which depend on pressure are calculated in a third computation block. If between two successive EFM calculations the flowing pressure changes but the temperature and composition remain unchanged, then computations of composition dependent and temperature dependent quantities are not performed as these calculations would be redundant.
Additionally, the capability for calculations and/or recalculations must be incorporated in the accounting system used. Software implementing the 1992 A.G.A. Report No. 8 calculations has been installed in numerous U.S. accounting systems, including mainframe, minicomputer and PC based systems. Few quantitative problems are encountered when care is taken to properly pass the data and calculation results across computation modules, whether the modules are subroutines, libraries of subroutines or dynamic link libraries (DLLs). Checks, of course, are needed for proper variable ranges and unreasonable variable values such as negative absolute pressure.
The editing of flow measurement data, especially for final accounting and monthly billings, is a necessary activity regardless of whether the flow data is from charts or electronic flow meters. However, because of the large quantity of data entries associated with EFM data, it is not feasible to edit each individual entry from a computer keyboard. It is more efficient to upload the EFM data to a computer with resident EFM editing software, enter the edit information (e.g., change of orifice diameter from first to second part of month) and allow the many recalculations of flow for the edited period. This edit can be performed while retaining the original data for archival purposes and updating the event log for the meter, so that a complete audit trail is maintained. EFM editing software using dynamic link libraries (DLLs) for the new orifice discharge coefficient and gas density calculations have been developed for PC applications, making it feasible for EFM editing as part of field or office operations. Installations also have been implemented to send the EFM data from the meter to the EFM editing in a real-time mode using dynamic data exchange (DDE).
