Total Energy Measuremen
Total Energy Measurement with Packaged Redundant Gas Ultrasonic Metering Skids

Total energy management of natural gas is possible using an integrated measurement system comprising of standard measurement devices, explains Dam Hackett

Introduction
Natural gas has most often been sold on a standard volume basis, however, the most useful basis of transfer is on an energy basis as the gas will ultimately be burned either to generate electricity or directly by consumers for industrial or domestic use. The ability to combine traditional volumetric measurement with analysis equipment to determine energy value is widespread and accepted; particularly in light of the fact that direct energy flow measurement devices are not currently available. Several measurement standards are therefore combined to yield a final energy measurement number. These include, but are not necessarily limited to:

AGA 9 (AGA 7) Ultrasonic Measurement

BS 7965 Ultrasonic Measurement

AGA 8 Supercompressibility Calculation

AGA 10 Velocity of Sound Calculation

ISO 6976 Natural Gas Physical Properties

GPA 2145 Natural Gas Physical Properties

API 21.1 Electronic Gas Flow Measurement

ISO 5168 System Uncertainty Calculations Ultrasonic Flow Measurement

Gas ultrasonic meters are a relatively new technology being applied to custody transfer measurement of natural gas. While the basic technology has been available since the mid 1980’s, two milestones have propelled the technology to widespread acceptance as custody transfer measurement. The first occurred in the early 1990’s with the introduction of digital signal processing electronics, replacing slower and less repeatable analog signal processing. The second milestone was the issuance of the American Gas Association Transmission Measurement Committee’s Report No. 9 (AGA 9), which has covered measurement of natural gas by multipath ultrasonic meters. This performance-based report on ultrasonic metering opened the door to broad acceptance of the technology for custody transfer measurement. However, one issue continues to concern operators and regulatory authorities, how to validate the performance of the metering systems. Orifice based systems can be validated by calibration of the differential pressure transmitters and inspection of the orifice plate and meter tubes for conformance to AGA 3 / API 14.3 or ISO 5167 construction and installation standards. Gas turbine based systems generally require periodic spin testing to identify bearing wear and periodic recalibration of the meters due to wear of the mechanical components including the rotors and bearings. This is an accepted element of gas turbine meters. The ultrasonic meter, however, does not have any moving parts to wear. The dimensions of the ultrasonic meter are not as easy to physically verify as the dimensions of an orifice plate and meter tube. However, ultrasonic meters have extensive built-in diagnostic functions to check meter performance. For most regulatory authorities, however, the diagnostics often are not considered as sufficient validation of meter performance. In order to lengthen the recalibration interval of ultrasonic meters, many systems are designed to allow two ultrasonic meters to be periodically placed in series to compare actual volume flow between the meters to validate meter performance. Hence, a typical metering system design utilizing gas ultrasonic meters will consist of 2 x 100% capacity parallel ultrasonic flow meters with provision for running the meters in series. This article will address some of the system design considerations being employed by operators of gas ultrasonic meters and the validation results that are attainable.

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