Measuring the LNG process with confidence

The first step in the liquefaction process chain, the pretreatment and dehydration stage, is critical for removing water and other contaminants from the feed gas. Excess moisture can cause ice formation and blockages in the cryogenic section of the plant. Moisture sensors are used to monitor the water content of the feed gas at the outlet of the desiccant beds and at the outlet header, informing operators that dryers are working properly. Sensors that can operate at these process pressures and that provide process dew points are ideal at this stage of the LNG process. Flow meters are also critical at this stage for measuring feed gas before and after the dryer processes.

The liquefaction stage is where the dehydrated feed gas is converted into LNG. This is done by cooling the gas to 111 Kelvin. The energy-intensive process requires accurate and reliable flow measurement to ensure that the LNG produced is fully liquefied and of the highest quality. Flow meters are used to monitor the flow rate of the feed gas and the LNG, so they must be able to operate in cryogenic conditions and compensate for thermal retraction. A critical factor at this stage is the monitoring of the boil-off gas (BOG) generated during the liquefaction process.

Fortunately, sensors using advanced technology enable operators to optimise the process and minimize safety and environmental risks.

During loading and offloading, it is necessary to transfer the LNG between storage tanks and transport vessels, often through pipelines. Accurate and reliable flow measurement is crucial to ensure proper custody transfer, optimise the transfer process, and minimise the risk of accidents and spills.

To make sure the LNG transfer is done safely and efficiently, operators have some essential considerations:

• Accurately accounting for every cubic metre of LNG and gas to ensure reliable energy balance and data reconciliation for custody transfer purposes.
• Managing changes in flow rates and conditions, which can make accurate flow measurement challenging.
• Optimising the transfer process to minimise the risk of accidents and spills.
• Ensuring accurate and reliable measurement to maintain safe transfer of the LNG from one location to another.
• Adhering to safety and environmental regulations to protect workers, vessels, and the environment.
• Accounting for the energy being transferred to accurately bill for LNG, including monitoring the gas density, calorific value, and deducting the energy returned through the BOG.

Liquefying natural gas is a convenient way to transport it over long distances since it represents 600 times less volume than gas. However, once the LNG arrives at its destination, it needs to be converted back into a gaseous form, making it suitable for distribution and consumption.

The complex process involves heating LNG above its boiling point of 111 Kelvin at atmospheric pressure. During this process, it is imperative to maintain strict control over temperature and pressure to ensure efficiency and safety in handling the volatile substance. Technologies, including flow meters and process analyzers, are employed to monitor and optimise the regasification procedure closely, further enhancing its reliability and effectiveness in meeting energy demands.

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Read the article online at: https://www.lngindustry.com/special-reports/02102023/measuring-the-lng-process-with-confidence/