The Sampling of the
Conventional natural gas
Resulting from the natural transformation of organic matter, natural gas, odourless and colourless, is composed of:
- 95% methane,
- less than 4% ethane and nitrogen,
- less than 1% carbon dioxide and propane.
It is used as a raw material or fuel, for cooking and heating, in the chemical industry, petrochemicals and refining, for electricity production and finally as a fuel for vehicles.
Produced almost everywhere in the world, natural gas is transported by pipeline or by LNG carrier in its liquefied form. Before being transported to the consumer, natural gas passes through storage sites, in former depleted gas or oil fields, in aquifers or salt cavities. Storage is necessary to ensure the adjustment of gas consumption and resources at all times and to offer consumers permanently available energy.
Due to these diverse sources of supplythe composition of natural gas can vary significantly. For safety reasons and for economic reasons related to the transaction value of gas, This quality must be regularly checked, involving the taking of samples.
The ISO 10715 standard « Natural gas sampling The "Conventional Natural Gas Sampling Guidelines" provides concise guidelines for sampling conventional natural gas.
Fundamental principles of natural gas sampling
Natural gas sampling is the process by which the gas sample is collected, packaged and transported. This sample must be relevant, i.e. representative of the mass where it is collected. It should therefore not be altered during the acquisition processes. In order to have a representative sample, particular attention should be paid to the following points:
- Location of the sampling point:
The location must be chosen so that the gas sampled is relevant. The sampling point must be at a point where all the gases to be analysed pass and must not be at the end of the line, for example, and must not be affected by flow disturbing elements such as valves, bends, thermocouples, which can cause changes in the composition of the gas. It must also be accessible to allow easy maintenance. Finally, the sampling point must be in the upper first third of the pipe.
- Process conditions at the sampling point:
Conditions such as the temperature, the pressure or even the speed of the flow must be taken into account. In particular, a sampling tube must be designed to meet these process conditions. It must also be resistant to vibration.
- Wetted materials:
It is necessary to pay attention to wetted materials to avoid phenomena of sorption of gas, adsorption (retention of molecules by the surface) and desorption (release of molecules).
- Temperature :
The system temperature must be maintained above the dew point (temperature at which the gas condenses and forms water droplets) and must remain stable. The fact that natural gas condenses when its temperature drops makes the collection and preparation of the sample quite complicated. Indeed, if the natural gas enters the liquid/steam phase, the integrity of the sample will be questioned.
The sampling system should therefore be isolated. In addition, natural gas sampling often involves expanding the gas from 70-200 bar in the lines to the pressure required by the analyzer. The expansion of the gas causes the creation of cold and therefore condensation. This is the famous the Thomson-Joule effect that should be neutralized.
- Dead volume :
Volumes must be reduced to a minimum to avoid dilution effects.
- Cleanliness and maintenance of the sampling system:
To avoid contamination, the compounds must be carefully cleaned. Indeed, some contaminants have a strong sorption capacity.
The main equipment of a natural gas sampling system
Sampling probes
The design of the probe must take into account the possibility of vibration caused by the high speed of the flux. For lines free of liquids and with temperatures very higher than the dew point, any type of probe may be suitable. On the other hand, for lines where temperatures are close to the dew point, a specific probe is necessary to avoid problems related to condensation and liquids in the gas flow.
The most basic sampling tube is a straight sampling tube. The other type of rod frequently used in the gas industry is the sampling probe with pressure reduction. It is frequently used with continuous analysis systems. The pressure reduction takes place at the end of the rod through which the gas flow passes. The temperature drop is compensated by the thermal mass of the gas flow. It is recommended to install the pipe at 20 times the pipe diameter of the flow disturbing elements. The pipe should be equipped with a suitable valve system, which will allow the sampling line to be disconnected from the process line. It can be permanent or temporary.
Sampling and transfer lines
Sampling lines should be as short as possible and with the smallest diameter, but not less than 3 mm. The flow rate in the lines must be chosen so as to have a fast response time.
Separators
In general, separators are not recommended in sampling systems. However, they can be used to ensure that no liquid enters the analyzer or sample cylinder.
Pressure Reducers
In order to supply the analyzer with a gas with an appropriate pressure, a regulator is often required at the beginning or at the end of the line. The regulators are preferably made of stainless steel and PTFE. Due to the Thomson-Joule effect, the temperature drops to an average of 1°C every 2 bars, which potentially creates condensation. It is usual to prevent this phenomenon by heating to compensate for the temperature drop. The necessary heat depends on the composition of the gas, the pressure reduction, the pressure and the temperature, the flow rate ...
Heating appliances
Heating elements can be installed on the sampling probe or the sampling lines. They must be of the auto-limiting type and must respect the electrical standards of the areas where they are used.