On paper, an LNG truck loading system has the same building blocks as any fuel loading point: a supply pump, a loading line, an arm or hose, and a way to measure what went into the trailer. In practice, cryogenic temperature changes how every one of those blocks behaves. Liquefied natural gas (LNG) boils at roughly −162 °C at atmospheric pressure, with the exact value depending on composition. That means nothing can be loaded until the line and equipment are cold, the liquid entering the trailer compresses a vapor space that has to go somewhere, and the quantity is usually determined as mass rather than volume.
The generic process flow of a single loading point is covered on our truck loading systems page. This page explains why that chain is arranged differently for LNG and what a typical loading sequence looks like.
What cryogenic service changes
· Heat ingress drives boil-off: LNG starts boiling as soon as it touches warm steel. A warm line cannot deliver controlled, measurable flow, so every loading starts with cool-down.
· Trailer pressure has to be managed: LNG road trailers are vacuum-insulated tanks with an inner pressure vessel and an outer jacket. Filling raises the pressure of the vapor space, which is handled through a vapor return line or another method suited to the trailer design.
· Quantity is usually mass: LNG density is highly sensitive to composition and temperature, so truck deliveries are normally quantified by direct mass measurement.
A typical loading sequence
The LNG custody transfer handbook published by GIIGNL, the international group of LNG importers, describes truck loading in distinct stages:
1. Decompression of the trailer, where a vapor return connection is used.
2. An initial low-flow period that cools the line, arm or hose (cool-down).
3. Ramp-up toward the nominal flow rate.
4. Stable loading at nominal flow.
5. Ramp-down and stop as the target quantity approaches.
Each stage is a step in the loading control sequence. Because flow can be two-phase during cool-down, facilities that meter dynamically need to define up front how that period is treated commercially. OIML R 117-1:2019, the international legal metrology recommendation for dynamic liquid measuring systems, allows a recirculation circuit downstream of the meter for cool-down, but requires that a delivery does not start, or is stopped, while product is flowing through that circuit.
Vapor return and trailer pressure
A vapor return hose or arm is normally connected to the trailer so that boil-off gas (BOG) generated during loading goes back to the facility. The abstract of EN 1473:2021, the European design standard for onshore LNG installations, likewise lists truck loading and unloading connections with vapor return among the plant boundary limits. Not every trailer design requires a vapor return connection, however, so the trailer types a bay will serve should be defined early in the project.
How returned vapor affects the commercial quantity depends on the measurement method. With a weighbridge, the before and after weights capture it automatically, provided the first weighing is taken before the trailer is decompressed. With in-line metering, returned vapor has to be measured separately and subtracted. Vapor line design and its interaction with the terminal BOG system are covered on the LNG vapor return page.
Weighbridge or flow meter?
Weighing the trailer before and after loading is still the most common way to determine the loaded LNG quantity. The alternative is dynamic metering with a Coriolis mass flow meter or an ultrasonic meter in the loading line, which GIIGNL notes is gradually being introduced for small-scale fiscal applications. Dynamic metering only works if the product in the meter stays liquid throughout the measurement, so the line must be verified as full and cold before measurement begins.
Many sites combine both: a meter controls the batch and stops the load at the target quantity, while the weighbridge ticket is used for the commercial document. Which measurement governs the invoice is set by the terminal rules and the agreement between buyer and seller. Cryogenic meter skid design and the energy calculation are covered on their own pages.
Fill limit and trailer acceptance
An LNG trailer is never filled to its full geometric volume. Heat keeps leaking in during transport and pressure rises, so the maximum load is established before each loading according to the trailer design and applicable transport rules, such as the ADR agreement on the carriage of dangerous goods by road in Europe. The loading controller receives that limit as a preset. Before its first load at a facility, the trailer owner is typically expected to provide the trailer technical data and a capacity certificate issued by an authorized body.
LNG-specific design inputs
Parameter | Why it matters in design | Effect on selection |
Trailer types and connection layout | Liquid and vapor connection position, size and type vary between trailers | Arm vs hose choice, couplings and bay layout |
Loading rate and trucks per day | Sets loading time and supply pump duty | Line size, number of pumps and meter size |
Supply tank pressure and pump type | Determines the margin that keeps liquid from boiling along the line | Pump selection, cool-down line and recirculation layout |
Vapor return requirement | Trailer pressure and returned BOG load the terminal BOG system | Vapor line size, BOG capacity and commercial calculation |
Quantity determination method | Weighbridge and meter need different infrastructure and verification | Weighbridge location or a cryogenic meter skid |
Cryogenic hose or loading arm | Length change and thermal cycling between cold and warm states create mechanical loads | Hose type, supports and emergency release arrangement |
EN 12434 sets design, manufacturing, testing and marking requirements for cryogenic flexible hoses; couplings are covered by separate standards. The emergency shutdown (ESD) chain, including loading arms and emergency release couplings (ERC), is detailed on the cryogenic ESD and safety systems page.
How TLY Enerji supports LNG loading projects
TLY Enerji provides system design, supply of instruments, valves and pumps, engineering, installation, commissioning and maintenance for LNG truck loading and unloading facilities. Depending on project scope, this can include a cryogenic metering system, temperature and pressure instrumentation, control valves, and PLC and SCADA based monitoring and reporting. The scope of supply is agreed on each project through the technical specification, taking into account interfaces with existing tanks, pumps and the BOG system.
What we need to review your loading bay
· Trailer types to be served, tank sizes, and liquid and vapor connection details
· Number of loads per day and target loading time
· Supply tank operating pressure, existing pumps and LNG composition range
· Terminal BOG handling and its capacity to accept returned vapor
· Preferred quantity method: weighbridge, meter or both
· Data required on the commercial document and the site control system architecture
Related pages
· Truck loading systems: The fluid-independent component chain and loading profile of a loading point.
· Cryogenic metering skids: Cryogenic meter skid design when dynamic metering is chosen.
· LNG vapor return systems: The vapor line, pressure balance and BOG impact during loading.
· LNG loading and unloading skids: Scope of supply when loading equipment is delivered as a pre-assembled skid.
· Cryogenic ESD and safety systems: ESD, emergency release and detection layers at the bay.
· LNG custody transfer metering: How the loaded mass becomes an invoiced energy quantity.
LNG truck loading FAQ
Why does an LNG loading line have to be cooled down first?
LNG flashes to vapor when it contacts a line, arm or trailer shell at ambient temperature. Starting at full flow on warm equipment would build up vapor pressure, create two-phase flow and make any quantity measurement unreliable. Loading therefore starts at a low rate and the flow is increased step by step as the system gets cold.
Does every LNG trailer need a vapor return connection?
No. Vapor return hoses or arms are common at loading facilities, but some trailer designs do not require one. The facility should define the trailer types it will serve at the start. When vapor return is used with in-line metering, the returned vapor must be measured and subtracted from the delivered quantity; with a weighbridge, the effect shows up in the weight difference.
Can a flow meter replace the weighbridge?
It can. Dynamic metering with Coriolis or ultrasonic meters is increasingly used in small-scale LNG. It requires single-phase liquid in the meter, separate measurement of returned vapor, and a measuring system that satisfies the applicable legal metrology requirements. Which method governs the invoice is a matter of agreement between the parties.
Why is an LNG trailer never filled completely?
The trailer keeps absorbing heat on the road; part of the liquid evaporates and tank pressure rises. Vapor space must be left to absorb that expansion. The maximum load is set before each loading based on the trailer design and transport regulations, and entered into the loading control system as a preset.
Loading arm or cryogenic hose?
The choice depends on trailer connection positions, loading rate, bay layout and operator ergonomics. Cryogenic hoses are flexible but need regular inspection because of length change and thermal cycling between cold and warm states. Loading arms offer a fixed geometry. Either way, an emergency release arrangement and ESD integration belong in the design.