Gas Metering Validation (LNG Plant)
In year 2013, PowerQ performed validation of custody transfer gas metering system for a 6mpta LNG terminal. This is an onshore LNG import/export terminal comprising of facilities for LNG unloading/reloading, LNG storage, Boil-off Gas (BOG) Handling, LNG send-out, LNG re-gasification and natural gas metering and regulation.
LNG is imported from LNG tanker, offloaded through unloading arm and pumped at LP to storage of half million cubic meter capacity. From storage tanks, the LNG was pumped at HP to bulk consumers at pressure in excess of 500 psi.
The export system is equipped with specialized Gas Metering Systems that support the Gas metering and Gas Quality Monitoring. The metering stream configuration for each metering skid is configured with pay and check meters of 2 x 100% (i.e. one active and one standby).
This validation plan addresses the following sub-systems of the Gas metering system:
- Primary instrumentation
- Ultrasonic flow meters
- Secondary instrumentation
- Pressure and temperature transmitters
- Flow computer
- Functions and calculations, e.g. Totalization must be tested against independent off-line calculations to ensure they are correct. In addition, constants and system data such as pipe sizes and gas composition dependant factors entered into the flow computer were verified.
- Gas analyser
- Gas chromatographs were checked, tested and the validity of the data they produced was confirmed
PowerQ performed validation exercises in four phases as follows:
- Design Validation (DV) – PowerQ reviewed design document and issued the design and verification which were addressed by supplier of various instruments and loggers.
- Installation Validation (IV) – After DV, various site visits were conducted to inspect the skid for the installed instruments, sensors and transmitters. The tag number, make, model, serial number and installation of all the equipments related to gas metering system were inspected and recorded. This record was checked against the data sheets and FAT results. Installation of Gas Analysers, chromatographs and sampling systems were also inspected and their tag number, make and model were recorded and checked against data sheets and FAT results. Physical measurements were made for straight lengths of the metering skid and recorded.
- Operation Validation (OV) – OV was carried out during various stages of system operation. OV was first carried out on pipeline when first gas was charged in. During this OV, the metering skid involving pipeline-2 was validated for its operation. It involved checking the range of various measured parameters, values in pay and check systems, communication between GC, FC and their servers.
- Performance Validation (PV) – PV was carried nearly about a month after OV. The data collected over one month period after OV was checked for consistency. Indicated values in pay and check systems, communication between GC, FC and their servers, consistency of values over the one month period were verified and checked for detailed data recorded during PV.
The validation was performed under the available gas flow rate (approximately 60K cu.m/hr) and the condition of the gas. It was noted that the gas flow rate was within the design range but was not the expected design flow rate.
The gas metering system was validated based on hourly flow rate accuracy of ±1% as per specification and as per gas code. However, the metering accuracy varies according to flow rate.
PowerQ final analysis report recommended that calibration program of periodic calibration of critical instruments shall be implemented according to an approved schedule and approved calibration procedures. It is recommended for periodic test and calibration of meter installations as guided by codes.
In conclusion, the design, installation, operational and performance validation conducted by PowerQ validated that the pay and check metering system performed with reasonable accuracy. It is within tolerance limit of the overall inaccuracy of the system, and is acceptable and eligible for custody transfer.