Cryogenic Metering Skids

A cryogenic metering skid packages the meter, temperature and pressure measurement, valves and flow computer needed to measure liquids such as liquefied natural gas (LNG) that are held far below ambient temperature. What sets it apart from an ordinary liquid metering skid is that the fluid is always close to boiling. A little heat or a small pressure drop turns part of the liquid into vapor, and no meter measures two-phase flow reliably. Cryogenic skid design therefore starts with keeping the liquid liquid, before meter selection even comes up.

The general content of a custody transfer liquid metering skid is covered on our custody transfer truck loading skids page. This page explains why the same functions are solved differently in cryogenic service.

Rule one: single-phase liquid in the meter

OIML R 117-1:2019, the international legal metrology recommendation for dynamic liquid measuring systems, requires LNG measuring systems to be designed and operated so that the product in the meter stays liquid during measurement. The GIIGNL LNG custody transfer handbook makes the same point: LNG meters reach their full metrological performance only with single-phase flow and no vapor bubbles, operation beyond the bubble point must be avoided, and lines must be completely full and cold before measurement starts.

For liquefied gases under pressure such as LPG, this is mostly achieved by holding back pressure downstream of the meter. For LNG, the main tools are pre-cooling and reliable thermal insulation, because every pipe spool, valve body and instrument connection on the skid is a heat bridge. Pressure loss across meter and valves also pushes the liquid toward flashing, so meter sizing weighs pressure drop together with rangeability.

Cool-down circuit and line pack

The skid has to be brought down to operating temperature before measurement begins. OIML R 117-1:2019 allows a recirculation circuit downstream of the meter for this purpose, provided there is a means to indicate flow in that circuit and a delivery cannot start, or is stopped, while product is flowing through it. Product circulated for cool-down therefore never ends up in the delivered quantity.

If the piping or hose downstream of the meter does not stay full between deliveries, the same recommendation requires the quantity needed to fill it to be accounted for automatically. The calculator can start the transaction at a negative value, reset to zero once the hose is full, or hold the indication until the hose is charged. Which method applies should be fixed in the skid control logic from the outset.

Coriolis or ultrasonic?

LNG has a relative density of roughly 0.5 and a viscosity of about 0.1 cP. According to the GIIGNL handbook, it presents few metering challenges as long as the meter can physically handle cryogenic temperatures and no flashing occurs. Both Coriolis and ultrasonic meters have demonstrated custody-transfer-quality repeatability on LNG at cryogenic temperature, and the choice is driven by available pressure drop and required flow rate.

Two dynamic metering technologies in cryogenic service

Manufacturer datasheets often state different accuracy figures for cryogenic service than for room-temperature calibration, may declare the density specification not applicable below a certain temperature, and may recommend special cryogenic sensor versions. Ask for cryogenic-service figures explicitly when comparing offers. A datasheet value describes the meter only; it does not represent system uncertainty, which also includes insulation, temperature measurement and line pack effects.

Temperature measurement and insulation

LNG temperature is generally measured with three- or four-wire platinum resistance sensors; thermocouples are avoided because they are less sensitive at very low temperature. With volumetric metering, density is highly sensitive to temperature, so sensor location and installation feed straight into the quantity calculation. Vacuum-jacketed piping is common, but a modeling study found rigid foam insulation to be an economical alternative at LNG fueling stations. The choice depends on line length, duty cycle and the heat ingress the measuring system can tolerate.

Relief valves and vapor line metering

OIML R 117-1:2019 requires relief valves downstream of the meter in LNG systems to discharge to atmosphere or to the receiving tank, and prohibits piping a relief valve upstream of the meter to the downstream side, since that would bypass the meter. LNG trapped between two closed valves builds pressure as it warms, so every isolatable section of the skid needs a defined relief path.

If the vapor spaces of trailer and storage tank are connected, returned vapor has to be measured and subtracted from the delivered quantity. The recommendation asks for that vapor measurement to be better than 20 percent, and the GIIGNL handbook notes that errors in measuring returned vapor have a small effect on the net quantity, so differential pressure devices such as orifice plates can be used. A vapor line meter is therefore an optional but commercially important part of the skid.

Verification, calibration and conformity

Field proving, routine for liquid hydrocarbons, is not yet as established for LNG. The 6th edition of the GIIGNL handbook reports that no accepted practice for proving meters in LNG service against a traceable standard existed at the time of publication, and that an LNG flow calibration facility with gravimetric traceability had been built. The 7th edition published in 2026 may update this picture. Recalibration intervals depend on regional and national metrology requirements.

OIML R 117-1:2019 places LNG measuring systems in accuracy class 1.5 and sets a tighter error limit for the meter than for the complete system. A meter's type approval or hazardous-area certificate does not cover the whole skid on its own: meter, temperature measurement, calculator, cool-down circuit and vapor correction are assessed as one measuring system, and whoever assembles the components must assess any additional ignition hazards the combination introduces.

How TLY Enerji supports cryogenic metering projects

For LNG truck loading and unloading facilities, TLY Enerji offers customizable LNG metering systems, supply of instruments and valves, system design and engineering support. Depending on project scope, this can cover selection of Coriolis meters, temperature and pressure instruments and flow computers, installation and commissioning of flow meters, and PLC and SCADA integration. How the system will be treated under legal metrology and where the scope of supply ends are agreed through the project specification and applicable regulations.

Design data for a cryogenic skid

·         Purpose: custody transfer, check metering or process monitoring

·         Minimum, normal and maximum flow and batch size

·         Pressure and temperature range at the supply point and LNG composition range

·         Total pressure drop allowed across the skid

·         Whether a vapor return line exists and needs metering

·         Whether downstream hose or piping stays full between deliveries

·         Applicable metrology regulations and control system interfaces

Related pages

·         LNG custody transfer metering systems: Turning the metered mass into invoiced energy.

·         LNG truck loading systems: The loading operation and stages the skid serves.

·         LNG loading and unloading skids: Pre-assembled package scope including valves, nitrogen interfaces and site connections.

·         LPG metering skids: Comparison with a pressurized liquefied gas, where back pressure keeps the liquid phase.

·         Custody transfer truck loading skids: Fluid-independent custody transfer skid content and verification approach.

Cryogenic metering FAQ

Wouldn't a back-pressure valve be enough, as with LPG?

Back pressure helps prevent flashing by keeping the liquid above its vapor pressure, but in LNG service the main source of vapor is heat coming in from outside. In a line that has not been cooled down, or on a poorly insulated skid, vapor forms regardless of pressure. That is why LNG skids address pre-cooling, insulation and limiting pressure loss together.

Can a Coriolis meter also measure LNG density?

Coriolis meters measure mass, density and temperature together, but some datasheets do not state a density specification below a certain temperature. For mass-based deliveries density is not needed, so the quantity is unaffected. If density is required for other purposes, the skid design should define separately how that value will be obtained.

Is LNG circulated during cool-down counted as delivered?

It should not be. OIML R 117-1:2019 requires that a delivery does not start, or is stopped, while flow is detected in the recirculation circuit downstream of the meter. Product used to cool the line is therefore never counted as delivered to the customer. Measurement starts once the line has been confirmed full and cold.

How is a cryogenic meter verified in the field?

Proving with a prover, common for liquid hydrocarbons, is not yet as widespread for LNG. In practice, calibration at a flow facility, comparison in the field against an independent method such as weighbridge tickets, and monitoring of meter diagnostics are considered. Which approach is accepted depends on national metrology rules and the agreement between the parties.

How are meter electronics protected from the cold?

In cryogenic applications the transmitter is often mounted remotely from the sensor so it stays within its ambient temperature limits. The choice also matters for hazardous-area conformity: some manufacturers certify the remote-mount configuration, and limits for the integral version can be different.