Liquefied natural gas (LNG) is natural gas composed predominantly of methane that has been cooled to approximately −160 °C and converted into liquid form, and it is stored in double-walled, vacuum-insulated tanks under conditions close to atmospheric pressure. This cryogenic (very low temperature) characteristic affects every part of the transfer and measurement system: lines and equipment must be cooled down before use, the product must remain liquid in the meter, boil-off gas continuously generated by external heat ingress must be managed, materials must retain their toughness at low temperatures, and safety layers must be established against possible cryogenic leakage.
What distinguishes LNG from LPG and NGL?
LPG and NGL are kept in liquid form under pressure at ambient temperature; LNG remains liquid not by pressure but by cooling. Therefore, the primary risk in an LNG system is not overpressure but heat ingress. Any heat entering the tank, line, or tanker vaporizes part of the product. Collecting, reusing, or safely disposing of the resulting boil-off gas (BOG) is part of the daily operation of an LNG terminal.
Requirements introduced by cryogenic transfer
Cool-down: LNG sent into a warm line rapidly vaporizes and creates two-phase flow. The metering line and transfer equipment are cooled down in advance; how the quantity passing through the meter during this process will be handled is defined.
Vapor return and BOG: As the tanker fills, the displaced vapor is returned to storage through the vapor return line; this flow is designed together with boil-off gas management on the storage side.
Material and insulation: Wetted parts are selected from materials that do not become brittle at low temperatures; insulation around piping, valves, and meters limits both heat ingress and icing on external surfaces.
Measurement: In LNG measurement systems, the product is expected to remain liquid in the meter and the quantity to be indicated as mass. For road tankers, the delivered mass is mostly determined by weighing; dynamic measurement with Coriolis and ultrasonic meters is becoming increasingly common in small-scale applications.
Safety: Cold impact of a cryogenic leak on structural steel, expansion of liquid trapped between closed valves, and low-temperature gas clouds are central considerations in ESD and relief system design.
Terminal scale and scope of this section
Design principles for onshore LNG facilities are addressed by different standards according to storage capacity; large facilities and satellite or small-scale facilities ranging from a few tonnes to a few hundred tonnes are covered under separate frameworks. At the system boundaries of large facilities, road tanker and rail loading/unloading connections with vapor return are defined alongside marine connections. This section does not cover ship cargo measurement; it focuses on road tanker loading and unloading, small-scale LNG applications, boil-off gas measurement, and measurement of gas send-out from the terminal to the grid.
Is liquid or gas measured?
Two different measurement worlds exist side by side in an LNG terminal. Cryogenic liquid is measured during tanker loading and unloading. In terminals where regasification is performed, the gas delivered to the grid is measured in the gas phase after the vaporizers and at the transmission line inlet; the configuration used here is a natural gas custody transfer metering station together with gas quality analysis. Boil-off gas is also measured in the gas phase using separate meters. The first step in selecting the correct meter technology is to determine the phase at the measurement point.
TLY Enerji’s role in LNG facilities
TLY Enerji provides system design, product supply, engineering, installation, commissioning, and maintenance services for LNG tanker loading and unloading facilities; the supply scope may include LNG metering systems, valves, and pumps. The project scope and supply boundaries are jointly defined according to the scale of the facility and the technical specification.
Solution pages for LNG terminals
LNG tanker loading systems: How cryogenic conditions change road tanker loading.
LNG tanker unloading systems: Unloading into the storage tank by pressure or pump.
Cryogenic metering skids: Differences in skid design for cryogenic liquid measurement.
LNG custody transfer metering systems: Calculation of the energy sold and the required measurements.
LNG loading and unloading skids: Scope of the preassembled cryogenic package.
Boil-off gas (BOG) metering systems: Why and how continuously generated boil-off gas is measured.
LNG vapor return systems: The effect of the vapor line on pressure and BOG during loading.
LNG terminal gas send-out metering: Where and in which phase the gas sent to the grid is measured.
Cryogenic ESD and safety systems: Safety layers required for cryogenic transfer.
Frequently asked questions
What is the difference between LNG and CNG?
Both are natural gas, but they are stored in different forms. LNG is cooled into liquid form and kept in insulated tanks under conditions close to atmospheric pressure. CNG, on the other hand, is stored in gaseous form by compressing it to high pressure. Therefore, LNG requires cryogenic liquid transfer and measurement, while CNG applications are designed around gas measurement and high-pressure equipment.
In LNG tanker loading, is the quantity determined by a meter or by weighing?
In road tanker loading, the delivered mass is mostly determined by weighing the tanker before and after loading. Dynamic measurement using Coriolis and ultrasonic meters is being used increasingly in small-scale applications. Which method will be the commercial basis depends on the agreement between the parties, legal metrology requirements, and how cool-down and vapor return are reflected in the measurement.
Why is the gas delivered from the terminal to the grid measured by a separate system?
The product delivered to the grid is no longer liquid; it is gas that has passed through the vaporizers. Measurement at this point is performed in the gas phase according to the delivery conditions of the transmission grid and is generally used as the basis for energy calculation together with gas quality analysis. The equipment used is more similar to natural gas custody transfer metering stations than to cryogenic liquid meters.