Liquefied natural gas (LNG),
an inescapable development
Liquefied natural gas (LNG) is natural gas condensed to a liquid state: cooled to a temperature of -161°C, the gas (composed mainly of methane) takes the form of a clear, transparent and odourless liquid, and its original volume is reduced by about 1/600.
Reducing the volume of gas facilitates its transport and diversifies the sources of supply, without relying solely on onshore pipelines. This is why LNG is now playing an increasingly important role and is developing on a global scale.
The need for sampling of perfect liquefied natural gas
LNG is transported by gigantic vessels, the LNG tankers, which are loaded and unloaded in LNG terminals. When LNG arrives at an LNG terminal, it can be transported to land or marine fuelling stations, plants, or to be regasified to be injected into the natural gas system.
Density and calorific value calculations of the transferred LNG (e. g. during loading and unloading of LNG carriers) are carried out on the basis of the average composition of the LNG. This composition is obtained by LNG sampling and chromatographic analysis.
The value of an LNG shipment is generally in the order of $50 million. It is therefore essential to determine precisely the calorific value of the transferred LNG, because even a small mistake has a considerable financial impact. The very nature of LNG, its extreme temperature and the difficulty of keeping it in liquid form lead to very specific sampling and measurement issues.
LNG sampling, a delicate process
For the liquefied natural gas sample to be analyzed to be truly representative, it must change from liquid to gaseous state without partial vaporization and without loss of components. To do this, it must be kept in liquid phase until the precise moment it is sprayed. It must then be uniformly vaporized in a single-phase gas state.
As shown in the phase diagram below, the liquid sample must pass through a liquid/gas biphasic region before reaching its gaseous form. If the LNG sample begins to vaporize in the sampling lines, the nitrogen and methane will vaporize first producing pockets of gas in the liquid stream arriving at the vaporizer. As a result, the LNG sample reaching the vaporizer will be a non-representative liquid sample rich in heavy components with methane-rich bands.
The gas sample that will come out of the vaporizer will not be homogeneous, which will cause huge fluctuations in composition. This sample, which will feed the chromatograph and the sampling cylinders, will result in a non-representative analysis.
As mentioned in ISO 8943:2007, precautions should be taken to avoid partial vaporisation of the sample in the transfer line and the sampling line.
Pressure, temperature and flow rate in the transfer line and sampling system must be continuously monitored.
Regular inspections must be carried out, in particular to prevent any leakage and defects in the thermal insulation.
The construction materials of the sampling system must be strong enough to withstand pressure and temperature conditions to which they are exposed.
The pipe must be installed in a place where the LNG is at a temperature below the boiling point.
The heat exchange capacity of the vaporizer must be sufficient to vaporize all the sampled LNG. It must also be designed in such a way that the heaviest components do not remain inside.
A pressure regulator must be installed at the outlet of the vaporizer where the gasified LNG is transferred to the gasometer or at the compressor outlet where the gasified LNG is transferred.
The capacity of the regulator must be higher than the maximum flow rate of the vaporizer.
