The integrated pulse velocity method applied to the calibration of flowmeters in high-pressure natural gas systems

The radioisotope Integrated-pulse-velocity Method has been previously demonstrated to measure air flowrates to uncertainties of less than +- 0.2 per cent (95 per cent confidence levels) by comparisons in a laboratory with a primary standard of gas flow measurement. This method of flow measurement, when applied with simultaneous measurement of gas density, can be used to measure mass flowrates of gas and is particularly attractive for the on-site calibration of gas flowmeters, where the effects of installation conditions and velocity distributions upstream of the meter are included in the calibration. On-site calibration can be readily effected by measuring the transit time of the gas by means of radiation detectors positioned on the outside of the gas pipe with provision required only for access of an injection probe to the flow. The volume of pipe between detectors is determined from external measurements of outside diameter and wall thickness of the gas pipe and the distance between the detector positions. The gas density is measured by sampling the flow through a calibrated vibrating cylinder density cell. In order to assess the accuracy of the method when applied to the on-site calibration of meters measuring the flow of natural gas, measurements of gas flowrate have been made in the high-pressure, natural gas test facility at Low Thornley. These experiments were carried out in both long and confined lengths of pipe and the measurements by the Integrated-pulse-velocity Method compared with those derived from a venturi meter previously calibrated in water flow. Tests were also carried out to establish the accuracy of determining the internal volume of a pipe by external measurements. The results of these tests indicate that accuracies of gas flow measurement of within +- 0.5 per cent can be achieved in a well-designed high-pressure gas system by the use of this method. (author)