LNG online analysis

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The last edit to this post was made by lijie2007521 on 2022-3-21 09:58. An LNG online analysis system obtains a small sample using certain methods in order to assess the quality of large quantities of material. Therefore, the collected samples must have representative rows, so that the properties of all materials can be understood through the quality of the samples. During transportation, LNG is in a state of high pressure and low temperature (around -162°C); therefore, it is necessary to completely vaporize its sample using the vaporizer in the sampling analysis system before taking a sample. The LNG analysis systems commonly used internationally today are of two types: continuous analysis and intermittent analysis, as specified in ISO8943-2007 \"Refrigerated light hydrocarbon fluids – Sampling of liquefied natural gas – Continuous and intermittent methods\". Continuous analysis has been widely used around the world as a conventional sampling system, whereas intermittent analysis is a sampling system that was introduced mainly by the European Union in recent years; as a result, this method is gradually being adopted in the EU region, while continuous sampling remains the preferred approach in other areas. At the LNG receiving stations that have been built in China, all but those in Shanghai use continuous analysis systems. 1 Continuous sampling analysis refers to the process in which, over a period of stable flow, the sample gas is vaporized and then flows at a constant rate through a constant-pressure sample container – typically a gas storage tank – into the sampling cylinder. A continuous sampling analysis system can be connected directly to the analysis unit; it also provides the sampling cylinder for analysis in the laboratory. The continuous sampling system mainly consists of a sampling probe, an LNG sample evaporator, a buffer tank, a gas storage tank, and auxiliary equipment. The gas storage tank can be of either non-sealed water-type or sealed water-type; the process flows for these two types are different (see the technical drawings for details). ◆ Principle of the continuous water-sealed analysis system: First, the sampling probe collects low-temperature liquid LNG from the LNG unloading pipeline and sends it into the sampling line, where it then enters the evaporator. Heating in the evaporator causes the liquid LNG to completely vaporize. The fully vaporized LNG sample enters a buffer tank, which is used primarily for safety reasons to prevent excessive or unstable system pressures. After passing through the buffer tank, the sample goes into a large-volume piston-type constant-pressure sample container (gas storage tank). Its main functions are to balance the sampling time and to ensure thorough mixing of the sample, thereby eliminating any unevenness that may occur after the sample passes through the evaporator. Nitrogen or other inert gases should be used as the sealing gas for the piston seals. Before sampling from a hermetically sealed water-type gas storage tank, it is necessary to bubble vaporized LNG through the water to dissolve and saturate the hydrocarbons in it. After the sample gas in the gas storage tank has been thoroughly mixed and stabilized for more than half an hour, a gas filling compressor is used to fill the sample gas into the sample container (sampling cylinder). The sample containers need to be sulfur-passivated; the special treatment or internal coating of the containers should ensure minimal reactivity with sulfides, while also being suitable for sample handling and facilitating purging and cleaning. The continuous sampling system is designed as a closed system; during the cleaning procedure prior to sampling, it can automatically verify whether the cleaning has achieved the required standard, thereby effectively preventing the possibility of sample contamination ； The sampling process is controlled by a flow controller, and the gas storage tank remains essentially at rest, which ensures the stability of the system ； The continuous sampling system achieves high accuracy because the sampling period is continuous and the gas is well mixed in the storage tank. At the same time, the continuous sampling system uses a large-volume gas storage tank, allowing for the collection of a relatively large number of samples. ◆Principle of the continuous anhydrous sealed analysis system: The samples collected by sampling probes on the LNG transmission pipeline are vaporized in a vaporizer. When the pressure of the natural gas after the vaporizer is high enough, it is transported from the outlet of the sampling vaporizer to the sampling NG container under its own pressure ； When the pressure is insufficient, the compressor is used to increase the pressure of the LNG after it passes through the carburetor. During this process, the gas pressure in the sampling line is controlled by a pressure regulator, while the valve at the inlet of the NG sampling container is used to maintain the flow rate of the gas entering it. The sample gas in the NG container is fed into the sampling bottle by a sampling compressor. 2 Intermittent sampling refers to the process of vaporizing the sample at predetermined time intervals or at predetermined flow rates, and then feeding the gas sample through 3 samplers (sampling pumps) into 3 constant-pressure gas cylinders respectively. The intermittent sampling system mainly consists of a sampling probe, an LNG sample evaporator, a buffer tank, a atmospheric pressure floating piston sample container, and auxiliary equipment. Compared to the continuous sampling system, the intermittent sampling system has a similar operating pressure to that of the continuous sampling system before the sample gas enters the gas storage tank; thereafter, the intermittent sampling type does not use a gas storage tank, but instead collects samples directly from 3 sample containers. The sample container in the intermittent sampling system separates the sample from the carrier gas using a movable piston. The pressures on both sides of the piston are in balance. The sample container, composed of high-pressure-resistant stainless steel cylinders, includes the following components: (1) Floating piston: to separate the sample gas from the carrier gas and maintain a constant pressure within the container ； (2) Piston position indicator: Similar to a magnetic follower or piston rod, it can be used to indicate the maximum percentage volume of sample gas that the container is allowed to hold and transport ； (3) Pressure gauge ； (4) Stainless steel needle valves at both ends ； (5) A safety pressure relief device at each end to prevent overcharging or overpressure caused by thermal expansion. Since the sample container in the intermittent sampling system uses a moving piston to separate the sample gas from the carrier gas, this moving piston may introduce the carrier gas that has passed through it into the sample during the sampling process; therefore, the choice of carrier gas is particularly important. In actual sampling, if argon is used as the carrier gas and a gas chromatograph is employed to analyze the sample, an oxygen peak will appear among the components. This occurs because the retention times of argon and oxygen are almost identical; such a situation arises when argon is introduced into the sample gas. Similar situations can cause significant difficulties in the settlement of LNG trades; therefore, when using an intermittent sampling system, it is essential to choose the most appropriate carrier gas based on the actual conditions. The intermittent sampling system improves upon continuous sampling by allowing high-volume gas flows to pass through the system; it uses direct sampling, which results in samples with good representativeness. However, the sampling container must be manually cleaned before sampling, and there is no automated mechanism in this system to verify whether the cleaning has achieved the required standard, thus posing a risk of contamination of the system ； At the same time, the sample container has a complex structure and is relatively fragile; solid particles present in the system may damage the cylinders, and even minor leaks of the carrier gas can cause difficulties in detection, increasing maintenance costs. Additionally, compared to continuous sampling systems, the amount of sample taken is less. If you have any technical or project needs, feel free to leave a message or send a text!