System
Design Basis for Packaged Redundant Gas Ultrasonic Metering Skids
Use of redundant metering
components allows continuous validation of each of the measurement devices
online, says Dan 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 direct 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 ISO 6976 Natural gas
Physical Properties AGA 8 Super-compressibility Calculation GPA 2145
Natural Gas Physical Properties AGA 10 Velocity of Sound Calculation
ISO 5168 System Uncertainty Calculations API 21.1 Electronic Gas Flow
Measurement 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
covered measurement of natural gas by multi-path ultrasonic meters.
This performance-based report on ultra-sonic metering opened the door
to broad acceptance of the technology for custody transfer measurement.
One of the concerns of using ultrasonic meters is the perceived expense
of utilising this technology. While the initial capital expense may
be higher for smaller size installations, larger meter stations are
actually much less expensive than similar capacity orifice installations.
Of more importance, is the reduced long term operating expense of ultrasonic
meter installations. One issue, that continues to concern operators
and regulatory authorities, is how to validate the performance of 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 and recalibration
cont....
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