TRANSIENT DIFFRACTION EFFECTS IN ULTRASONIC METERS FOR VOLUMETRIC, MASS AND ENERGY FLOW MEASUREMENT OF NATURAL GAS

摘要

The influences of transducer diffraction effects on measured USM transit times are investigated, in relation to “dry calibration” methods and consequences for the USM measurement accuracy in flow calibration and field operation. Diffraction effects depend on a number of operational factors. The magnitude of such systematic effects is evaluated using a plane piston model and finite element modelling of the vibration and radiation of typical USM piezoelectric transducers. Consequences for volumetric, mass and energy flow measurement using USMs are evaluated. In a cooperation with industry partners, data from flow calibration of a USM are used to illustrate the significance and potentials of improved control with diffraction effects. It is shown that especially for small-size meters (4”-12”) treatment of diffraction effects is highly recommended if high accuracy of the volumetric flow rate measurement is needed, and if the USM is used as a mass or energy flow meter. For the VOS measurement used in mass and energy flow metering, flow calibration does not reduce the error due to diffraction, whereas for the volumetric flow measurement, this error can be reduced by flow calibration. Methods to achieve improved control with diffraction effects are described.