There are two basic ways to move LNG from a road trailer into a storage tank. Either the trailer's own pressure build-up system raises the vapor pressure and pushes the liquid across, or a cryogenic pump transfers it. Many sites use both in a controlled combination. The right choice depends on how the trailer is equipped, the design and operating pressure of the receiving tank, how quickly the trailer has to be turned around, and what happens to the vapor left over at the end of the transfer.
The fluid-independent basics of unloading, including suction-side issues and quantity verification, are covered on our truck unloading systems page. This page focuses on the decisions that are specific to LNG.
Where LNG trailers are unloaded
LNG delivered by road usually serves sites without pipeline gas: satellite LNG storage at industrial plants, LNG fueling stations, and temporary or remote sites. Many of these installations are small. In Europe, onshore LNG installations with a storage capacity between 5 t and 200 t fall under a separate design standard, EN 13645, while larger plants are covered by EN 1473. Unloading capacity and automation level are typically sized for this smaller scale.
Unloading by pressure build-up
Some LNG road trailers carry pressure build-up (PBU) coils designed for pumpless offloading. A small stream of liquid from the bottom of the trailer is vaporized in the coil and returned to the top of the tank, which raises the vapor pressure and drives the liquid into the receiving tank on differential pressure alone. The main advantage is simplicity: no pump, no pump cool-down and no pump maintenance at the receiving site.
The limits come from the same physics. Flow lasts only as long as there is a pressure difference between trailer and tank; as the receiving tank fills and its pressure rises, the difference shrinks and the transfer slows. Liquid vaporized to pressurize the trailer also warms the remaining LNG, and the trailer ends the transfer at elevated pressure and full of vapor. A peer-reviewed modeling study that compared six ways of offloading LNG from a tanker truck to a fueling station found that transfer by pressure build-up alone could lead to high methane emissions in the cases studied.
Unloading by pump
With a pump, liquid is delivered to the receiving tank independent of the pressure difference. The cryogenic pump can sit at the receiving facility, on an unloading skid, or, as in some mobile storage units, on the receiving unit's frame. It needs cold liquid at the suction that is not close to boiling, and the pump and suction line must be cooled down before operation. The same study reported that combining a pump with a controlled pressure build-up gave the lowest risk of methane venting.
A pump makes unloading time largely independent of receiving tank pressure, at the cost of an additional rotating machine, a cool-down procedure and electrical infrastructure. The decision weighs deliveries per day, the acceptable unloading time and the site's approach to venting.
Comparing unloading methods
Aspect | Pressure build-up (PBU) | Pump, or pump with controlled PBU |
Site equipment | No pump needed at the receiving site | Cryogenic pump, cool-down line and power supply required |
Unloading time | Slows as receiving tank pressure rises | Less affected by receiving tank pressure |
Trailer after transfer | Left at elevated pressure, full of vapor | Trailer pressure can be kept lower |
Vapor and venting risk | High methane emissions possible in the modeling study | Lowest venting risk in the modeling study |
Operation | Simple; operator watches pressures | Pump cool-down and protective interlocks needed |
Receiving tank pressure and vapor handling
The limiting item is often the receiving tank rather than the trailer. Incoming liquid compresses the tank's vapor space and pressure rises; approaching the tank design pressure stops the transfer or can lift a relief valve. Some storage tanks have both top-fill and bottom-fill lines. Cold liquid sprayed in at the top tends to cool the vapor and lower the pressure, while bottom filling compresses the vapor. Operators switch between the two to manage tank pressure.
A balance line connecting the vapor spaces of trailer and tank can reduce the pressure difference, but it also reduces the driving force for pressure transfer and, where metering is used, affects the commercial calculation. What happens to vapor generated during and after unloading, whether it is used as fuel, compressed or flared, falls under BOG handling.
How is the received quantity established?
For LNG delivered by road, the commercial quantity is usually established at the loading terminal by weighbridge and arrives with the loading certificate. The receiving site then reconciles it operationally against storage tank level and pressure records. Liquid and vapor left in the trailer, and any gas vented during unloading, can create a difference between loaded and received quantities, and the supply contract should state how that difference is treated.
If a meter is installed at the unloading point, the LNG provisions of OIML R 117-1:2019 still provide the framework: the product in the meter must stay liquid throughout the measurement, the quantity is indicated in mass, and if the trailer and tank vapor spaces are connected, returned vapor must be measured and subtracted. Cryogenic meter skid design is covered on a separate page.
Connection, disconnection and a safe finish
Unloading hoses are selected to the standards that apply to cryogenic flexible hoses. At the end of the transfer, liquid in the hose and connecting line has to be drained safely and the line purged before disconnection. LNG trapped between two closed valves builds pressure as it warms, so draining and pressure relief of those sections must be designed in. Nitrogen purge and inerting interfaces, emergency shutdown (ESD) and emergency release are detailed on the related skid and safety pages.
How TLY Enerji supports unloading projects
TLY Enerji provides system design, supply of pumps, valves and instruments, engineering, installation, commissioning and maintenance for LNG truck unloading facilities. Depending on project scope, this can include monitoring of receiving tank pressure and level, temperature and pressure measurement on the unloading line, pump protection interlocks and PLC-based control. Interfaces with existing tanks, vaporizers and BOG equipment are defined at project start.
Information to share for an unloading system review
· Receiving tank volume, design pressure and normal operating pressure
· Pressure build-up equipment and connection layout of the incoming trailers
· Target unloading time and monthly delivery frequency
· Preference on pump use and available electrical supply
· Availability of top-fill and bottom-fill lines on the tank
· Site policy on venting and BOG use
· How the received quantity will be verified: loading certificate, tank gauging or meter
Related pages
· Truck unloading systems: Suction-side basics and verification of the received quantity.
· LNG loading and unloading skids: Pumps, valves and connections supplied as a pre-assembled skid.
· Boil-off gas (BOG) measurement systems: Measuring and routing vapor generated during and after unloading.
· LNG vapor return systems: Effects of a vapor balance line between trailer and tank.
· LNG truck loading systems: The loading-side process where the delivered quantity is determined.
LNG unloading FAQ
Is pressure transfer or pump unloading the better choice?
There is no single answer. Pressure build-up unloading is simple and needs no pump at the receiving site, but it slows down as tank pressure rises and leaves the trailer at elevated pressure. A pump shortens unloading time and eases pressure management, but adds equipment that needs cool-down and maintenance. Delivery frequency, tank design pressure and venting policy should be assessed together.
Why does receiving tank pressure rise during unloading?
Incoming liquid compresses the vapor space in the tank, and heat picked up in lines and connections vaporizes some LNG. If pressure approaches the tank design limit, unloading has to stop or vapor has to be handled in a controlled way. A top-fill line, where available, can help limit the rise by cooling the vapor space.
Does LNG from the trailer need to be measured again at unloading?
Not necessarily. The commercial quantity is usually set at the loading terminal by weighbridge and documented on the loading certificate, and the receiving site reconciles it against tank level records. If the contract calls for metering at unloading, the liquid must stay single-phase in the meter and any vapor returned through a balance connection must be subtracted.
Why can't the hose be disconnected immediately after unloading?
LNG left in the hose and connecting line vaporizes as it warms and builds pressure in any closed section. Before disconnection the liquid must be drained safely, the line purged where required and the pressure relieved. These steps are part of the unloading procedure for both personnel safety and avoiding gas release.