The gas sample is taken under main line pressure of 30 to 60 bar directly without any pressure reducer or any other large volume instrumentation into a high safety gas cylinder three times. The cylinders are opened after the first and second sample load, but the gas is allowed to leave only with quite a low low speed. The sample cylinder finally contains 500 ml sample under main line pressure which shows no sample taking error larger than detectable by a analysis quality of s = + - 0.002 % absolute for methane. The sample temperature is adjusted to + 20 degr. C and delivers a constant micro gas flow through the inlet system of a micro process GC instrument. We never used any needle valve / gas pressure reducer system but a flat steel capillary flow resistor, allowing from a 10 ml/min flow from top gas pressure level only. Here a series of at least five consecutively repeated chromatograms are taken and the quantitative data for the inert gases and the hydrocarbons up to C6 integrated and stored in a process GC EXPORT data file. This file contains besides GC data also calculated physical gas quality values. The Micro GC uses two differing capillaries under isothermal conditions and uses two micro detectors. The substance values are automatically calibrated and electronically stored as mole-%. The physical data are calculated under user defined units based on standard pressure and standard temperature. 15 data lines are automatically written for each single run into a Process GC EXPORT file
(= PGC file). Its data structure differs necessarily from the chromatography standard data structure in normal chromatography EXPORT files because of the strict limitation for values from only 11 chemical substances (including one substance group called C6 plus) and 4 physical data. This PGC file structure is shown in Table 1 below.
Natural gas contains hundreds of further substances not taken by the micro process GC instrument. These are hydrocarbons above C6 and not ending at C40, but above C12...C14 in very low concentrations only. Their measurement is necessary only for very special reasons because of slow long range effects in wide ranging gas line systems. Besides hydrocarbons natural gas may contain alcohol traces (methanol, diols, triols) and water at trace concentration. The alcohol and the water concentrations are of technical importance thus their value is needed. The higher hydrocarbons in the range C6 to C12 identifies the gas source. It is considered as the individual fingerprint of a gas type.
There are further traces in natural gas including solid micro particles and higher poly aromatic hydrocarbons as complex as in raw mineral oils, but at so low concentrations, that their quantitation is outside the needs of a daily routine task. They can be measured by PLC. PLC is applicable to gas analysis
The alcohols and the higher hydrocarbons are quantitized by temperature programmed capillary gas chromatography using a flame ionization detector. In case the gas contains sulfur compounds, a further extra gas chromatographic analysis is needed using sulfur specific detection.
The water concentration is measured only in constantly flowing gas under line pressure because of otherwise serious data falsification. Water traces can be correctly quantitized in non polar gases using physical modes of analysis. The special instruments deliver water values in many differing units but can give results also in grams per m3. Knowing the gas analysis results the water data are transformed into mole-%.
The temperature programmed capillary GC data are automatically written into a standard EXPORT file, but in order to get qualified trace quantities for the higher hydrocarbons and correct alcohol values, the GC analysis needs high sample volumes. This falsifies the values for methane, even for ethane and propane and thus of course also all higher hydrocarbon trace values. This is a typical error caused by oversampling. The values for methane are necessarily far outside the quantitative working range.