Improving custody transfer accuracy in LNG truck loading through advanced flow measurement

As LNG continues to play a key role in the global energy mix, operators are placing increasing importance on the efficiency, accuracy, and reliability of loading and unloading operations. Whether LNG is handled at a terminal, a satellite facility, or a truck loading station, accurate measurement of the transferred mass flow is essential for both commercial transactions and operational control.

However, reliable measurement for commercial settlement in LNG applications is anything but simple. The cryogenic nature of the product presents a number of challenges that can compromise measurement performance, particularly during the initial phases of loading and unloading operations.

The challenge of measuring LNG

Natural gas liquefies when cooled to approximately -162°C, significantly reducing its volume and facilitating transport and storage. Keeping LNG in a liquid state requires strict temperature control throughout the supply chain.

One of the main challenges is the formation of boil-off gas (BOG). Even small amounts of heat from the environment can cause some of the LNG to vaporise. This effect is particularly noticeable during the start-up phase of loading operations, when transfer lines typically have a temperature significantly higher than that of the cryogenic liquid entering the system.

Before measurement can begin, the pipeline must first be cooled. During this process, some of the LNG vaporises by absorbing heat from the warmer pipelines, generating significant amounts of BOG. This results in a two-phase flow consisting of both liquid and gas. From an operational standpoint, this cooling phase is a necessary but costly step. It consumes product, extends loading times, and can complicate the transition to commercial metering.

Why conventional measurement approaches struggle

The presence of gas in a liquid flow has always posed challenges for many flow measurement technologies. In LNG applications, this issue becomes particularly evident during the start-up phase, when gas concentrations can fluctuate significantly.

For operators conducting metered transactions, it is essential to determine the exact moment when the pipeline has cooled sufficiently and the gas content has dropped to an acceptable level. Starting the measurement too early can compromise accuracy, while waiting unnecessarily too long extends loading times and results in product losses.

Conventional flowmeters often react to high gas content by triggering alarms, losing measurement stability, or entering an error state. In some cases, measurement must be restarted once process conditions have stabilised. These interruptions not only hinder operational efficiency but can also complicate the entire loading process.

Advances in LNG flow measurement

Recent advances in Coriolis mass flow measurement technology have made it possible to overcome many of these challenges. Modern flowmeters, specifically designed to operate at extremely low temperatures, can function at temperatures as low as -196°C while maintaining the accuracy required for legal metrology applications.

In addition to their ability to directly measure mass flow, the latest generation of Coriolis flowmeters can provide valuable diagnostic information on the state of the process fluid. This includes the detection of entrained gas and the identification of two-phase flow conditions that may occur during LNG handling.

Instead of interrupting the measurement in the presence of vapour, advanced signal processing techniques enable continuous operation throughout the cooling phase and in the event of fluctuating gas content. This allows operators to maintain a clear overview of the process, from start-up through to normal operation.

Using two-phase diagnostics to optimise loading operations

The ability to detect and quantify gas entrainment offers benefits that go beyond measurement continuity. Diagnostic results can provide operators with real-time information on the presence of vapour in the feed line, enabling them to determine the optimal time to start the graduated measurement.

This information can be directly integrated into filling procedures and automation systems. Instead of relying on conservative estimates or fixed waiting times, operators can base their operational decisions on actual process conditions.

This often reduces cooling times while ensuring that measurements for commercial transactions are taken under appropriate conditions. The result is a faster and more efficient filling process with fewer product losses.

Economic and operational benefits

For LNG terminal operators, shippers, and facility owners, even minor improvements in loading efficiency can translate into substantial long-term savings. Reducing unnecessary cooling phases helps minimise LNG losses due to vaporisation, while shorter loading cycles improve facility utilisation and throughput.

At the same time, continuous measurement under varying operating conditions enhances process transparency and operational safety. Operators gain a better understanding of what is happening within the transfer system, enabling them to make more informed decisions and reduce the risk of unplanned outages.

As LNG infrastructure continues to expand globally, the industry is increasingly moving away from simply measuring transferred volumes to focus on optimising the entire transfer process. Advanced flow measurement technologies play a key role in this transformation.

Looking ahead

The reliability of measurements for metered transactions remains one of the most critical requirements in LNG logistics. However, accuracy alone is no longer sufficient. Operators are also seeking technologies that improve operational efficiency, reduce product losses, and provide more detailed information on processing conditions.

The ability to ensure accurate measurements even in the presence of entrained gas and to detect the start and end of two-phase flow represents a significant advancement for LNG handling applications. By combining measurement accuracy with advanced diagnostics, modern flow measurement solutions help operators achieve both commercial accuracy and operational excellence.

As LNG continues to contribute to energy security and the transition to low-carbon energy systems, technologies that improve the efficiency of storage, transportation, and handling will become increasingly important throughout the value chain.

Read the article online at: https://www.lngindustry.com/special-reports/07072026/improving-custody-transfer-accuracy-in-lng-truck-loading-through-advanced-flow-measurement/