What changes hands in an LNG sale is usually energy, not liters or tonnes. The energy sold is the transferred quantity multiplied by the heating value of the product. For road tankers the quantity is normally established as mass, and the heating value is calculated from an analysis of a vaporized LNG sample. Gas returned to the other party and gas consumed as fuel during the transfer are then taken off. An LNG custody transfer metering system is therefore not a single meter but a chain: quantity measurement, sampling and analysis, and records that both parties can reconcile.
How custody transfer differs from process measurement and legal metrology in general is covered on the custody transfer natural gas metering stations page. This page looks at what those principles mean for a cryogenic liquid.
What goes into the energy figure
· Quantity: For road tankers, mostly the mass found by weighing before and after loading; with dynamic metering, the mass indicated by a cryogenic meter. For ship cargoes, liquid volume is measured and converted with density.
· Heating value: The gross calorific value calculated from LNG composition, on a mass basis when quantity is a mass, or on a volume basis together with density when quantity is a volume.
· Adjustments: Energy in vapor returned to the other party and LNG or boil-off gas (BOG) used as fuel during the transfer. The contract defines which items are deducted.
The international standard on LNG sampling notes that LNG custody transfer is commonly settled on calorific content, with total calorific content derived from liquid volume, liquid density and gross calorific value. On a road tanker, where mass is determined directly, density drops out as an intermediate step and energy can be treated as mass times mass-based heating value.
Scope: road tankers and small-scale transfer, not ship cargoes
Cargo measurement on LNG carriers is a separate discipline: quantity is determined statically from liquid volume, vapor volume, temperature and pressure in the ship's tanks, under its own international standard. This page covers road tanker loading and unloading, satellite plants and small-scale transfers instead. Terminal send-out is metered in the gas phase and has its own page.
Establishing quantity: weighbridge versus meter
The industry's LNG custody transfer handbook reports that delivered truck mass is usually determined by weighbridge, while dynamic metering with Coriolis and ultrasonic meters is gradually being introduced in small-scale fiscal applications. The two methods fall under different legal metrology frameworks and treat returned vapor and cool-down differently:
Aspect | Weighbridge | Dynamic cryogenic metering |
Framework | OIML R 76 for non-automatic weighing instruments; maximum permissible errors apply to gross and tare weighings, not to the calculated net value | OIML R 117-1:2019 for dynamic measuring systems for liquids other than water; LNG measuring systems sit in accuracy class 1.5 |
Indicated quantity | Gross and tare mass; net mass is the difference | R 117-1 requires LNG delivery systems to indicate quantity in mass |
Returned vapor | The mass of vapor leaving the tanker is automatically reflected in the difference between weighings | If a vapor connection exists, returned vapor must be measured, with an accuracy better than 20 percent, and deducted from the delivered quantity |
Cool-down | Weighing sequence and tanker pressure condition are set by procedure | A recirculation circuit downstream of the meter is allowed for cool-down; flow in that circuit does not count as delivery |
Site requirements | Periodic verification of the weighbridge and a defined weighing procedure | Product kept liquid in the meter; temperature measurement and confirmation that the line is cold |
R 117-1 also sets a tighter error limit for the meter than for the system: in class 1.5, 1.5 percent for the complete LNG measuring system and 1.0 percent for the meter. Data-sheet meter performance and system results are not the same thing; skid hardware is covered on the cryogenic metering skid page.
Heating value: sampling, vaporization and analysis
LNG composition is not constant. The sampling standard notes that minor-component concentrations vary with the source gas, pre-treatment and liquefaction process, and storage conditions. Heating value is therefore not taken from a catalog; it is determined from a sample representative of the product delivered. Liquid drawn through a sample probe on the transfer line is fully gasified in an LNG sample vaporizer and analyzed either by an online gas chromatograph or in a laboratory from a sample container. Heating value, and density where needed, are then calculated from the composition using standard methods.
Partial vaporization is the weak point of any LNG sample. If some liquid flashes in the sample line, the lighter components get ahead and the analyzed gas no longer represents the product, so sample point location, insulation between probe and vaporizer and keeping the liquid sufficiently subcooled are part of the design. For road tankers, the contract decides whether each loading is sampled or a periodic storage tank analysis is used, and the delivery ticket should state which.
Returned vapor and gas used during transfer
The handbook states that the energy balance of a transfer accounts for gas returned to the other party and for LNG or BOG used as fuel during the transfer. With a weighbridge, returned vapor mass is already in the weight difference, but the heating value applied to it still has to be defined, because returned vapor can differ in composition from the liquid. With meter-based loading, returned vapor is quantified by a separate gas measurement and deducted. How the return line affects pressure and the BOG system is covered on the LNG vapor return page.
Records, sealing and reconciliation
An energy-based delivery ticket should carry the weighings or meter totals, temperature and pressure data, the analysis result and heating value used, the returned-vapor correction and the calculation method. OIML R 117-1 requires the meter adjustment device to be sealable and prohibits adjustment by bypassing the meter. It also notes that, particularly for road tanker loading and by prior agreement with the supplier, a transaction need not be settled when the customer leaves the site; where the analysis result is finalized after loading, that process belongs in the contract.
The handbook itself is a reference, not a standard or specification. The basis for invoicing is set by the contract between the parties, national legal metrology rules and site regulations.
How TLY Enerji contributes
TLY Enerji provides system design, product supply, engineering, installation and commissioning for LNG tanker loading and unloading facilities. For custody transfer measurement this can cover choosing the measurement method, supplying instruments such as Coriolis meters and temperature and pressure transmitters, data collection and reporting through PLC and SCADA, and site commissioning. Scope of supply and legal metrology responsibilities are agreed per project in the technical specification.
Information to share for an assessment
· Type of transfer: tanker loading, unloading or delivery to a satellite plant
· Invoicing unit, mass or energy, and the heating value basis if energy
· Existing weighbridge, planned cryogenic meter, or both
· Whether a vapor return connection exists and how the contract treats it
· Sampling and analysis approach: per-loading sample, tank analysis or online analysis
· Applicable national legal metrology requirements and contracting parties
· Expected delivery ticket, reporting and data interfaces to business systems
Related pages
· Cryogenic metering skids: Meter selection, cool-down circuit and skid hardware.
· LNG truck loading systems: Weighing sequence and the stages of a loading operation.
· LNG vapor return systems: Return line design and its effect on the BOG system.
· LNG send-out metering: Gas-phase custody metering at the grid entry point.
· Natural gas quality measurement and analysis: Gas chromatographs and heating value from composition.
LNG custody transfer FAQ
Why is LNG invoiced on energy rather than mass?
The buyer is paying for the energy in the LNG, and two batches of equal mass can carry different energy depending on composition. The share of components other than methane varies with the source, the liquefaction process and time in storage, so energy-based invoicing protects both parties from that variability. Some small-scale sales are still invoiced by mass; the contract sets the unit.
Does every road tanker loading need its own sample?
There is no single mandatory approach. Each loading can be sampled from the transfer line, a periodic analysis of the storage tank can be used, or an online chromatograph result can be applied. The choice depends on how quickly tank contents change, how often tankers load and the uncertainty the parties accept. Either way, the delivery ticket should make clear which analysis applies to which loading.
If the weighbridge and the meter disagree, which one counts?
The contract names the billing measurement in advance; the other serves as a check. It should also define the acceptable difference between the two and what happens if it is exceeded. Typical causes of a discrepancy include returned vapor being treated differently by the two methods, product passing the meter during cool-down, weighing sequence, or product not staying single-phase liquid in the meter.
Are these systems used for LNG ship loading?
Ship cargoes are traditionally quantified statically from liquid and vapor volume, temperature and pressure in the ship's tanks, under their own international standard. The industry handbook notes that in-line LNG meters are in use at some terminals but are not yet conventional for ship-to-shore custody transfer. This page is limited to road tankers and small-scale transfers.