Analysis and Elimination of Disturbing Flowrates Deviations Between a 24' ANSI 600 Ultrasonic Meter and Vortex Meter in a Natural Gas Meter Station

摘要

During the startup phase of a newly erected meter station for natural gas, considerable synchronism deviations were discovered between the ultrasonic and vortex meters arranged in series. In order to eliminate the difference, the inlet zone of the meters was extended by 40 D, among other things, and a second high pressure test was carried out for both meters. The renewed high pressure test showed for the ultrasonic meter a shift of 0,5% for which there is no explanation up to the present day, and this leads to a corresponding reduction of the synchronism difference. The extension of the inlet failed to produce any improvement. The synchronism quality of the measuring devices continued to be unacceptable. With the additional installation of a turbine meter, the vortex meter was positively identified as being the primary source of the shift. At the same time, a dependence of the synchronism on the position of the control ball valve located approx. 50 D downstream was determined. As verified by a detailed pulsation measurement, there is a systematic correlation between the differential pressure across the control ball valve, the stochastic pressure and velocity fluctuations within the measuring section as well as the synchronism deviation between the ultrasonic and vortex meters. Evidently, an increase of the flow turbulence at the vortex meter at constant volume flow led to a reduced indication. The vortex shedding frequencies (Strouhal number) decrease at constant Reynold's number with increasing turbulence (intensity and coherence length, respectively). As a remedial action, the control ball valve was equipped with two multi-hole discs so that the natural gas for expansion no longer has to flow through one cross-section but rather through a multitude of boreholes. In this case, the number of boreholes as well as the borehole diameters increase in amount and size with the increasing opening phase of the control ball valve. With this remedial action, the stochastic fluctuations in the natural gas flow were reduced significantly. At the same time, the synchronism between the vortex meter and the ultrasonic meter was improved significantly and was subsequently accepted by all participants.