At a regasification terminal, the gas sent out to the transmission grid is normally metered as gas, not as liquid. The custody transfer point is usually a natural gas metering station at the terminal outlet, downstream of the vaporizers, where the regasified product enters the high-pressure pipeline. Send-out metering is therefore a high-pressure, high-flow gas measurement task rather than a cryogenic one. What makes it specific to LNG is the system boundary, the variability of gas quality from cargo to cargo, and the role the station plays in the terminal's energy balance.
The general principles of custody transfer, such as uncertainty budgets, verification and audit trails, are covered on the custody transfer natural gas metering page. This page looks at how those principles are applied at the outlet of an LNG import terminal.
Where the meter sits in the terminal process
In a typical receiving terminal, LNG unloaded from ships or trucks is stored in tanks. High-pressure send-out pumps raise the liquid to roughly pipeline pressure, and vaporizers, for example seawater-heated open rack units or submerged combustion units, turn it back into gas. The send-out header pressure is then controlled, calorific value adjustment and odorization are applied where the project or grid requires them, and the gas passes through the metering station before entering the grid. The physical location of the station represents the delivery point defined in the contract.
Why measure gas instead of liquid?
The product delivered to the grid is natural gas, and title or capacity service usually changes hands at the grid entry point. Because pressure control, calorific value adjustment or odorization may sit between the vaporizers and the delivery point, the quantity that best represents what was delivered is the one measured after regasification. At that point, proven high-pressure gas techniques such as multipath ultrasonic meters and on-line gas chromatography can be applied.
Dynamic measurement of liquid LNG belongs to other duties: truck loading, small-scale transfers and bunkering. Ship cargoes are traditionally quantified by static tank measurement of liquid level, temperature and pressure on board. Liquid-side meters inside the terminal, for instance on vaporizer feed lines, may exist, but they mostly serve process control and allocation, not the commercial basis of grid delivery.
Defining the system boundary
Onshore LNG installation design treats LNG connections (ship manifold, truck and rail car connections) and natural gas pipeline inlets and outlets as separate boundary limits. For a send-out metering project, that boundary becomes concrete through a few decisions:
· Ownership and operation: Does the station belong to the terminal operator or to the transmission system operator, and who is responsible for verification and sealing?
· Position relative to other functions: Is calorific value adjustment or odorization upstream or downstream of the meters, and does the sample point represent the delivered mixture?
· Checks and comparison: How will duty and standby runs, a series check meter or comparison with the counterparty's measurement be arranged?
· Data interface: Which systems receive hourly and daily quantity and energy data, how often, and through what approval workflow?
Characteristics of send-out gas that matter for metering
Regasified LNG is generally clean and dry, since the gas is treated before liquefaction, but its composition depends on the source. Cargoes from different suppliers can carry different proportions of heavier hydrocarbons and nitrogen, so the heating value shifts from one cargo to the next. For energy-based delivery, this is why continuous composition analysis by gas chromatograph, and calculation of calorific value and compressibility from that composition, carry so much weight.
Flow range is the second driver. Send-out follows grid demand and tank inventory and can swing from very low rates to full capacity, so most projects use several parallel meter runs that are opened and closed as flow changes. Temperature is the third: vaporizer outlet temperature varies with operating conditions, and low-temperature monitoring and trip functions are usually designed so that abnormally cold gas does not reach the meter runs if a vaporizer underperforms.
Design inputs and how they shape the station
Parameter | Why it matters | Effect on selection |
Minimum and peak send-out rate | The terminal may operate across a wide range between low and peak demand | Number of parallel runs, meter size and run switching logic |
Grid entry pressure | Send-out pumps and pressure control are set around it | Pressure class, meter and transmitter ranges |
Vaporizer outlet temperature range | Abnormally cold gas is a risk to piping materials and to measurement | Temperature monitoring, low-temperature trip and material choice |
Cargo composition variability | Heating value and compressibility change with each cargo | On-line gas chromatograph and calculation method |
Location of heating value adjustment and odorization | The measured gas has to represent the delivered gas | Sample point and station position in the process chain |
Delivery contract and grid code | They define reference conditions, energy unit and verification frequency | Flow computer configuration, records and sealing |
Terminal energy balance requirements | Send-out is one line of the terminal's receipt and delivery reconciliation | Data transfer, time stamping and reporting design |
Role in the terminal energy balance
Send-out measurement is never judged in isolation. The energy received with each cargo, changes in tank inventory, truck loadings, gas used as terminal fuel, and boil-off gas (BOG) that is flared or recovered are all tracked together with the energy delivered to the grid. A systematic bias at the send-out station shows up as an unexplained gap in that balance. Station records should therefore share a common time base and comparable units with the other measurement points on site.
How TLY Enerji can support the project
TLY Enerji provides engineering support in selecting meter technology and run configuration from flow, pressure and composition data. Depending on the project scope, this can extend to supplying ultrasonic flow meters, pressure and temperature instruments and process gas chromatographs, system integration, installation, PLC and SCADA configuration, testing and commissioning. The split of responsibility between the terminal operator and the transmission system operator is agreed through the technical specification.
Information needed for an engineering review
· Send-out profile: minimum, normal and peak rates, plus expected future expansion
· Grid entry pressure and the temperature range at the vaporizer outlet
· Typical cargo compositions or the expected heating value range
· Where heating value adjustment and odorization units sit in the process chain
· Delivery contract or grid code requirements: energy unit, reference conditions, verification method
· Station ownership, operating responsibility and any counterparty metering
· Plot plan, hazardous area classification and control system interfaces
Related pages
· Custody transfer natural gas metering stations: Uncertainty, verification and record keeping for commercial gas measurement.
· Ultrasonic gas metering skids: The meter technology most often used on high-flow send-out runs.
· Natural gas quality measurement and analysis: On-line determination of a heating value that changes with each cargo.
· LNG custody transfer metering: Energy-based measurement of liquid LNG and how it differs from send-out.
· Boil-off gas (BOG) measurement: The other gas-phase line in the terminal energy balance.
Frequently asked questions
Can send-out be measured while the LNG is still liquid?
Liquid-side metering is technically possible and some terminals use it for process control. However, pressure control, heating value adjustment or odorization may take place after the vaporizers, so the point that best represents the quantity delivered to the grid is usually the gas metering station downstream of regasification. Which point serves as the commercial basis is set by the contract and the grid rules.
Why is a gas chromatograph important for send-out metering?
Cargoes arriving from different sources can differ in composition and therefore in heating value. In energy-based delivery, energy is calculated by multiplying the base volume or mass by a representative heating value, so composition has to be tracked continuously. The chromatograph data also feeds the compressibility calculation used for volume conversion.
How does send-out metering differ from LNG custody transfer metering?
Send-out metering measures regasified natural gas at high pressure as it enters the grid. LNG custody transfer covers liquid LNG changing hands during ship, truck or small-scale transfers, using tank gauging, weighbridges or cryogenic meters. Both are separate lines in the terminal's energy balance and are reconciled against each other.
What happens at the metering station if a vaporizer fails?
If a vaporizer cannot supply enough heat, the gas leaving it may be much colder than normal. Because this threatens both pipe materials and measurement accuracy, the send-out line usually has temperature monitoring and a function that stops flow below a set limit. The limit and the trip logic come from the project's risk assessment rather than from a fixed rule.