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Measurement and prediction of speed of sound, with application to gas flow metering in Australian natural gases

Fawcett, Derek (1995) Measurement and prediction of speed of sound, with application to gas flow metering in Australian natural gases. PhD thesis, Murdoch University.

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Natural gas custody transfer between buyer and seller is now commonly based upon energy content. Measurement of energy transferred requires simultaneous measurement of volume flow, density and composition of natural gas in high pressure pipelines. Densitometers are used to determine the density of the pipeline gas. Densitometers, however, are subject to systematic errors, and the minimisation of this error requires an accurate measurement of the speed of sound within the gas.

This research investigation has developed an apparatus which is believed to be W1ique. It consists of four interconnected spherical pressure vessels, forming a Differential Burnett apparatus. Sequential isothermal expansions of a gas sample between the vessels allows the accurate determination of a (P,Z) isotherm, either directly, or by comparison to a reference gas. Each of the spheres is equipped with an acoustical source and receiver, and can thus act as an independent spherical acoustic resonator allowing the precise measurement of the speed of sound in the gas. The ability to conduct the volumetric measurements simultaneously with the acoustic measurements is the unique feature of this apparatus.

Measurements have been conducted in this apparatus with the pure gases He, N2 and CH4, for which there is a wealth of high quality PVT and speed of sound data. These measurements were conducted in order to calibrate the apparatus volumes and to determine the magnitude of a number of small correction factors required in the modelling of the acoustic response of the spheres. Comparison between the literature data and our measurements on these pure gases also provided a stringent check on the performance of our apparatus and the measurement accuracies attainable.

Following this detailed characterisation of our apparatus, acoustic and volumetric measurements were made on natural gas samples on a number of isotherms over a wide pressure range. These measurements have provided extensive data on various natural gas thermophysical properties - speed of sound, compressibility factor, isentropic exponent and virial coefficients.

The resulting thermophysical property information has been used by the industry partner in this research {AlintaGas} to correctly calibrate pipeline densitometers and in other areas of high pressure gas flow metering.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Physical Sciences, Engineering and Technology
Supervisor(s): Edwards, T.J. and Pack, D.
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