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1. Scope
This Standard specifies the hygrometer for the determination of the water dew point of natural gas; the hygrometer hereof is determining the water dew point by testing the stability of the cooled surface vapor condensation or the surface condensation.
2. Application Area
The water dew point scope of the treated pipeline gas is generally -25℃~5℃; in the corresponding gas pressure, the water content range (volume fraction) is 50×10-6~200×10-6. However, in exceptional circumstances, the water dew point scope may be wider.[1]
During the system operation process, if the total test pressure of the sample is greater than or equal to the atmospheric pressure, the hygrometer described here in this standard may not be corrected, or may be used to determine the water vapor pressure, vapor partial pressure and the relationship between the measure dew points depends on the method and measurement level.
If the condensation temperature of the gas in the test environment is close to or higher than the water dew point, the water vapor condensation is hard to be measured.
3. Principle
3.1 Instrument Principle
Such kind of instrument is used to calculate the water content of the gas by determining the corresponding water dew point of the gas. The hygrometer for water dew point measurement usually carries with a mirror surface (metal mirror surface generally), when the gas sample flows over the mirror surface, the temperature may be reduced and may be measured accurately. When the mirror surface temperature is cooled until condensation appearing, the mirror surface may be observed to be condensing gradually.
If lower than this temperature (water dew point), the condensation will be increased gradually with time; if higher than this temperature, the condensation will reduce till it disappears, by then, the mirror surface temperature is the dew point of the measured gas passing through the instrument.
3.2 Determination of Water vapor Pressure
When the gas sampling pressure is identical to the gas pressure through the hygrometer, the saturated water vapor pressure correspondent by the measured dew point is the vapor partial pressure of the gas sample.
Check the manual, the relationship between the saturated water vapor pressure and temperature is available.
However, it has to be noted that: if the measured sample gas contains methanol, the dew point measured by this method is that of the methanol and water mixture; nevertheless, if the methanol content is given, it shall be used as the known information, and Appendix A presents the correction factor required for the practical water dew point calculation.
3.3 Announcements
When determining the water dew point, one fundamental point is: the sampling pipeline shall be short as far as possible, and the dimension shall neglect the pressure drop happen in the determination process; except mirror surface, the temperature of other parts of the instrument and the sampling pipeline must be higher than the water dew point.
[1]In order to meet the actual situation of China's natural gas production, the service scope shall be extended; this Standard adds content of "under particular environment, the water dew point scope may also be enlarged". |
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