Sound-Speed Sensor for Gas Pipeline Applications

A sensor, based on a small cylindrical acoustic resonator that may be suitable for measuring the speed of sound in natural and other process gases under pipeline conditions is described. The resonator is physically robust and requires minimal calibration. The speed of sound is obtained from the resonance frequency of a single longitudinal mode of oscillation that is isolated in the frequency spectrum of the cavity and can therefore be located and measured automatically. The design and acoustic model of the sensor are discussed. The performance of a prototype device was validated by means of measurements on three pure gases: argon, nitrogen and methane. The results of these measurements agree with the predictions of the most accurate equations of state with an absolute average deviation of about 0.02% and a maximum absolute deviation of 0.06% at temperatures between 293.15 K and 333.15 K and at pressures between 0.1 MPa and 10 MPa. Additional design features have been tested that may facilitate the deployment of the sensor in a pipeline system by: (a) preventing problems with condensate when operating near or below the dew temperature and (b) ensuring that the sensor is filled with a representative sample of the pipeline gas. Finally, we discuss how the design methodology may be applied to optimize the sensor dimensions for different operating conditions, such as high or low gas pressure.