Calculation of Moisture Content in Accordance with DIN EN ISO 18453
The calculation method developed by the European Gas Research Group (GERG) is the most reliable mathematical correlation between water content and dew point temperature. The advantage of this method of calculating the water content is that it exactly takes account of the gas composition.
The water dew point is an important parameter for determining and monitoring the gas quality. As water (vapor) reduces the calorific value and in the worst case can even cause system damages, its determination is required in many fields of gas production, gas processing, gas transport and even in applications of the end consumer.
As the application fields are constantly increasing, like for example the production and feed-in of biogas, and due to statutory provisions, there are more and more transfer points where natural gas changes its owner. At these points it is indispensable to determine the gas quality and, consequently, also to monitor the dew point as the gas quality is usually subject to stipulated limit values. These contractual stipulations usually do not take into account the gas pressures of the individual measuring stations and networks. The statement about the quality is often very general; this is why comparisons can only be made by means of calculation methods.
These calculation methods are necessary as the water dew point of the gas depends on the respective prevailing pressure. However, there have not been any internationally standardized calculation methods so far, a fact that often causes problems. The European Gas Research Group (GERG), a research and development organization, recognized the necessity of standardizing these methods and initiated empirical investigations of the water dew point behavior in different gas mixtures in order to establish a well-founded and practicable correlation .
DIN EN ISO 18453
The investigation results and their mathematical description (GERG water correlation) were integrated in the standard DIN EN ISO 18453 “Natural gas – correlation between water content and dew point“. This standard defines a method to calculate water contents from the dew point under frame conditions typical of natural gas. As the statement of the moisture content as absolute value already takes into account the gas pressure, it is a more reasonable way to express the prevailing moisture than the statement of the water dew point. This is why the moisture content, which is expressed in mg/m³, is getting more and more important.
The calculation tolerances stated in the standard apply to the pressure range from 0.5 MPa to 10 MPa and to a water dew point within the range from -15°C to +5°C. The so-called extended operating range covers pressures from 0.1 MPa to 30 MPa and can process water dew points from -50°C to +40°C .
Advantage of this method
The most significant advantage of this method of water content calculation is that it exactly takes into account the gas composition. Already when the gas composition is entered, Hygrophil F 5673 checks the concentration limits and the currently present mole percent in order to facilitate the handling. HYGROPHIL F 5673 works in connection with a combination sensor which determines the moisture content by means of fiber optics and determines the temperature within the medium via a PT100 sensor. In addition to the extremely robust construction of the sensor, it is above all the measurement method itself that offers several decisive advantages.
The plus factors of this patent-protected measurement principle are:
- no cross-sensitivity to impurities such as glycol, methanol or liquid hydrocarbons
- high measurement precision
- programmability of gas composition
- long-term stability of the sensor (no drift!)
- low maintenance
- simple installation and refitting
At present, the GERG calculation is applied in the biogas industry, which is a trend-setter in this field. Work sheet DVGW-G 262 (“Utilization of gases from regenerative resources“) refers to DIN EN ISO 18453 and prescribes the calculation according to the GERG method. Besides, it also prescribes limit values for the moisture content which should be adhered to when gas is fed into the line network.