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Title: Density And Calorific Value Measurement In Natural Gas Using Ultrasonic Flow Meters
Author: Kjell-Eivind Frysa, Per Lunde
Source: 2005 North Sea Flow Measurement Workshop
Year Published: 2005
Abstract: Multipath ultrasonic transit time flow meters (USMs) are today extensively used by industry for volumetric flow metering of natural gas, for fiscal measurement, check metering, etc. As natural gas is typically sold on basis of mass or energy, the density and/or calorific value (GCV) of the gas is measured in addition. In current fiscal metering stations this is typically made using additional instrumentation like e.g. densitometers, calorimeter or gas chromatographs. In addition to the flow velocity and the volumetric flow rate, USMs give measurement of the velocity of sound (VOS) in the gas. The VOS is a quality parameter which contains valuable information about the gas. For example, under certain conditions the density and GCV of the gas can be derived from the VOS. This provides a potential for mass and energy flow rate measurements by the USM itself. Various approaches in this respect have been presented over the recent years, by various research groups. The present paper describes a new method for calculation of density and GCV of natural gas, from measurements of the pressure, temperature and the VOS only. That is, in the present method, no instrumentation is needed in addition to the USM itself and the pressure and temperature sensors. The method can thus be used on existing USM metering stations with only a software upgrade. Such a feature may be of interest for fiscal metering stations (e.g. for backup and redundancy) as well as simpler metering station (where density and GCV are not measured today, but where such information may be of interest e.g. for monitoring). Results for different real natural gas compositions are presented, and contributions to the measurement uncertainty discussed. The paper is intended to provide insight into the potentials and limitations of methods for calculating gas density and GCV from VOS also on a more general basis.

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