Fiscal metering of a dry natural gas mixture

A national oil company operates several oil fields in the Middle East. To maximise the values of the hydrocarbons produced and to reduce gas flaring, the gas that is separated from the oil and produced water is further processed to achieve grid quality natural gas. In this way, large quantities of natural gas can be recovered, resulting in additional revenue.

The raw gas from the separator is basically dry natural gas (methane) with a negligible amount of water and traces of nitrogen, CO₂, propane, butane and other organic compounds, as well as a significant amount of hydrogen sulphur (H₂S). The high H₂S content makes the raw gas "sour" and requires amine treatment.

Process parameters:

The subsequent transport of the desulphurised gas to the gas processing plant requires fiscal metering. The oil company therefore needed a gas metering skid consisting of custody transfer (CT) flowmeters, pressure and temperature transmitters, as well as gas chromatographs, a water dew point analyzer and a fully equipped metering control system with certified flow computers, data acquisition system and PLCs.

With a long track record in oil and gas projects and an experienced team of local service engineers in the Middle East, KROHNE met all the customer’s requirements and won the EPC tender package. The dry gas metering solution comprised of a gas metering skid, an analyzer shelter with different gas analysing units, a metering control cabinet and various related services.

Dry gas metering skid

For fiscal metering KROHNE supplied two metering lines with ALTOSONIC V12 ultrasonic gas flowmeters (12", Cl 600) in a Z configuration, with one flowmeter as duty meter and the other one as check meter. The Z-crossover allows the duty meters to be periodically verified against the master meter. An additional ALTOSONIC V12 was provided as a spare.

As per AGA9, 10D/5D straight length meter spools were considered for each metering stream. The multipath flowmeters had been wet calibrated with natural gas at a third-party calibration lab prior to delivery. The metering skids are equipped with motor operated ball valves at the inlet, outlet and the cross-over of the streams as well as pressure transmitters, temperature sensors and all the necessary tubing, fittings and electrical equipment.

Analyzer house and shelter
The metering skid was accompanied by a fully assembled, configured and tested analyzer house and shelter for gas quality measurements. The galvanised steel analyzer unit was equipped with third-party equipment as per customer requirements, such as:

• a gas chromatograph for various components (in mol-%): methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, nitrogen, CO₂
• a dew point analyzer for moisture measurement
• HVAC equipment and gas detection for the Fire & Gas (F&G) system
• other equipment inside and outside the housing, e.g. a sampling conditioning cabinet

The analyzer solution was delivered integrated with the CalSys solution for analyzer management and data acquisition (AMADAS). This enables fully automated analyzer validation procedures to determine the performance, availability and maintainability of the various process analyzers. The project also included the integration and monitoring of the various existing online analyzers for oxygen, chlorine, total dissolved solids (TDS), pH, ORP and H₂S.

Metering control system
KROHNE also supplied a supervisory control system cabinet, which provides control of the metering skids and allows validation of the installed field instrumentation. It includes supervisory station computers, an AMADAS server, field signal and data transfer equipment, a PLC and two SUMMIT 8800 flow computers.

Based on the readings from the ALTOSONIC V12 and the signals from pressure and temperature transmitters, the SUMMIT 8800 calculates the standard volume flow rate of dry gas for billing purposes. To this end, the multi-stream flow computers are equipped with two I/O boards and one communication board. The dual pulse signal of the flowmeters is used as the primary measurement signal by the SUMMIT 8800. The flow computers receive backup measurement signals and diagnostic data via the serial RS-485 Modbus connection of the ALTOSONIC V12. The flowmeter features two serial RS-485 Modbus RTU ports which are connected to both flow computers. The Modbus connection of the flow computer is also used for speed of sound comparison calculated as per AGA10. The readings of the pressure and temperature measurements are transferred via 4…20 mA to the analogue input of the flow computer.

KROHNE’s range of solutions enables the gas producer to implement an AGA-compliant fiscal metering. Based on the ALTOSONIC V12 flowmeter, the metering skids generate accurate billing data for transparent transactions between the sour gas treatment facility and the gas processing plant. It also enables the management of several analyzers and chromatographs to monitor gas moisture and composition prior to dehydration and further processing.

Thanks to the CalSys analyzer management solution, the operator can effectively monitor the performance of its quality measurement instruments (QMI) to carry out targeted maintenance of their gas chromatographs, process analyzers and gas detectors. It also enables production management to keep track of asset KPIs to ensure a maximum uptime of the analytical equipment.

The customer benefited from the full scope of supply, including software and hardware components as well as the solution-related services conducted by KROHNE’s local service engineers. These ranged from the initial design drawings and calculations to engineering, manufacturing and welding of pipework to comprehensive quality assessments and non-destructive examination (NDE) to calibration and acceptance testing to installation, commissioning and training.