A pre-assembled LNG loading and unloading skid brings the liquid line, the vapor return connection, the cool-down circuit, the emergency shutdown valves and the instruments that supervise them onto a single structural frame between the storage tank and the road tanker. Its value lies less in the individual components than in what it settles in advance: which functions are shop-assembled on the frame, and exactly where the skid meets the rest of the plant. Scoping an LNG skid therefore starts with one question: what sits on the skid, and what stays on the facility side?
The generic loading chain of pump, meter, control valve and loading arm is covered on our Truck Loading Systems page. Here the focus is how that chain is packaged for a cryogenic liquid and where the skid meets the site; loading steps and unloading methods have their own LNG pages.
What makes an LNG skid different from an ordinary liquid transfer skid?
Liquefied natural gas (LNG) typically boils at about −162 °C at atmospheric pressure, depending on its composition, and starts to vaporize wherever it touches a warmer surface. That single property drives most skid design decisions. Piping and valves cycle between ambient and cryogenic temperature, so supports and flanges must accommodate thermal contraction, and insulation must limit both heat ingress and external icing. Any pocket where liquid can be trapped between closed valves becomes a pressure-rise hazard as it warms up.
Metering is affected too. Meters perform properly in single-phase flow, yet an LNG line carries a mix of liquid and vapor until it is fully cold. Pre-cooling the line and providing reliable thermal insulation along the metering section are the main ways to keep two-phase flow away from the measurement.
Functional groups typically found on the skid
Functional group | Role on the skid | Standard scope or option |
Cryogenic isolation and control valves | Route the flow, set the loading rate, isolate the line for maintenance | Standard |
Emergency shutdown (ESD) valves | Close the liquid and vapor lines on an emergency signal | Standard; required safety performance comes from the risk assessment |
Cool-down and recirculation circuit | Chills the line before transfer and keeps the initial vapor and warm flow out of the delivered quantity | Standard; tie-in point chosen to suit the metering rules if a meter is fitted |
Vapor return line | Carries vapor displaced from the tanker to storage or to the boil-off gas (BOG) system | Project-specific; depends on the loading method |
Mass flow meter | Measures the transferred quantity dynamically, in mass | Option; not needed where quantity is determined by weighbridge |
Pressure and temperature instruments | Monitor cool-down status, tanker pressure and permissive conditions | Standard |
Thermal relief valves | Limit pressure rise in liquid trapped between two valves | Standard; discharge destination agreed with the facility |
Nitrogen connections | Inerting and draining of the hose or arm before and after connection | Project-specific |
Hose or arm interface, breakaway coupling | Connection to the tanker and safe separation if the vehicle moves | Project-specific |
Local control panel | Cool-down and transfer sequence, permissives, alarms | Project-specific |
Cool-down circuit and protection of the delivered quantity
Before transfer starts, the skid piping is brought close to LNG temperature. The vapor generated during this phase, and the first slug of flow through warm pipe, can upset both the pump and the meter. OIML R 117-1:2019, the legal metrology recommendation for dynamic liquid measuring systems, requires LNG to remain liquid in the meter and the quantity to be indicated in mass; it permits a recirculation or cool-down circuit downstream of the meter provided that flow is indicated. On the skid this becomes a practical design question: where does the cool-down line branch off, and how is product used for cooling kept out of the delivered quantity? Meter selection and zeroing under cryogenic conditions are covered on the Cryogenic Metering Skids page.
Whether to fit a meter at all is a separate decision. An industry handbook on LNG custody transfer notes that the mass of LNG loaded into road tankers is usually determined on a weighbridge, full and empty, while dynamic metering with Coriolis and ultrasonic meters is gradually being introduced in small-scale fiscal applications. Dedicated cryogenic sensor options exist, but a datasheet accuracy describes the meter alone, not the uncertainty of the whole system.
Nitrogen, drains and relief valve outlets
Air must be removed from a cryogenic hose or arm before connection, and residual LNG and gas cleared before disconnection, so skids usually carry nitrogen supply and drain connections; the method follows the operator's procedure and the nitrogen available on site. Relief outlets and drains are piped to the vent, flare or BOG header the facility provides. LNG vapor at boiling temperature is initially heavier than air and rises only as it warms, so discharge locations deserve attention in the layout study.
Field interfaces and supply boundary
EN 1473, which covers the design of onshore LNG installations, lists road tanker and rail car loading and unloading connections, each with vapor return, among the boundary limits of an installation. Smaller plants with 5 t to 200 t of storage fall under EN 13645. At skid level the same logic means putting in writing who owns each interface. Typical field connections include:
· Liquid supply from the storage tank or the pump discharge
· Return line to the tank vapor space or to the BOG header
· Nitrogen supply and the vent or flare header
· Instrument air for pneumatic valves and electrical power
· Signals to and from the terminal ESD system and the gas and low-temperature detectors
· Grounding, tanker position and connection permissives
· Data exchange with the weighbridge, terminal automation or SCADA
Conformity is assessed for the whole skid
An approval for one meter, valve or transmitter does not demonstrate conformity of the complete skid. The European ATEX application guide explains that whoever combines equipment into a functional assembly becomes its manufacturer and must assess additional ignition hazards arising from the combination. Hazardous area classification, risk-based safety functions and pressure equipment requirements follow the project specification and applicable regulations. Marine documents do not apply directly to a road tanker skid: EN 1474-1 on marine transfer arms has been withdrawn and replaced by EN ISO 16904, which addresses the ship-to-shore interface.
Where TLY Enerji contributes
For LNG road tanker loading and unloading facilities, TLY Enerji provides system design, supply of instruments, valves and pumps, engineering, installation, commissioning and maintenance. On skid projects this can cover equipment selection from process data, implementation of cool-down and ESD sequences in the control system, and field installation and commissioning. Who fabricates the skid and where the supply boundary lies are agreed for each project through the technical specification.
What to define before requesting a skid proposal
· Skid duty: loading only, unloading only or both, and how many tankers connect at the same time
· Storage tank type, operating pressure and total LNG inventory
· Tanker types served, connection sizes, and hoses or loading arms
· Target transfer time and pump location
· Quantity determination by weighbridge, meter or both
· Tie-in point for returned vapor and the capacity of the existing BOG system
· Nitrogen, instrument air, power and ESD interfaces
· Hazardous area classification, plot constraints and transport limits
Related pages
· LNG truck loading systems: The loading process the skid serves and how cryogenic conditions change it.
· LNG truck unloading systems: How the choice between pressure transfer and pumping shapes the skid layout.
· Cryogenic ESD and safety systems: The safety layers that the skid's shutdown valves belong to.
· Cryogenic metering skids: Designing the metering section when a meter is added to the skid.
· Truck loading system fundamentals: The fluid-independent loading chain and its components.
LNG skid FAQ
Can one skid handle both loading and unloading?
Yes, if the valve arrangement is designed to reverse the flow path. However, whether unloading relies on building tanker pressure or on a pump changes the line arrangement, the direction of the vapor connections and the control sequence. If a meter is included, decide at the outset whether it must measure in both directions and how the cool-down circuit will be used in each operating mode.
Does an LNG skid need a flow meter?
Not necessarily. For road tankers, delivered LNG mass is commonly determined by weighing the vehicle full and empty. A meter is added when the quantity must be tracked during loading, when automatic batch termination is required, or when the contract calls for dynamic measurement. A fitted meter then requires single-phase liquid at the meter, cool-down flow kept apart and a rule for returned vapor.
Why is the cool-down circuit treated as a standard part of the skid?
LNG entering a warm line flashes rapidly, and liquid and vapor flow together until the line is cold. That can cause pump problems, erroneous meter readings and sudden pressure swings. A cool-down circuit brings the line to LNG temperature in a controlled way and routes the vapor generated to a suitable header. In cryogenic transfer skids, cool-down is therefore a core function rather than an optional extra.
Can EN 1474 be applied to road tanker skids?
The EN 1474 series addresses marine transfer systems. EN 1474-1, which covered transfer arms, has been withdrawn and replaced by EN ISO 16904, dealing with the design and testing of LNG marine transfer arms at conventional onshore terminals. EN 1474-2 on transfer hoses is also marine-oriented. Road tanker skids are designed around documents for onshore LNG installations, the project specification and applicable regulations.