Measuring the Moisture Content in Natural Gas

Natural Gas is mostly Methane; however contaminants are almost always present in one form or another. The contaminants which are most widely encountered are Glycol (TEG), Ethanol, Natural Gas Liquids (NGL), H2S, a variety of particulates as well as other contaminants depending on the gas source.

The gas can be at pressures of 700 psig to 1,400 psig. These operating conditions are taxing on the typical moisture analyzer, in most cases requiring complex cleaning procedures such as an elaborate rinse/wash procedures, detested by maintenance crews but needed to keep the typical sensor running reliably.

H2S vapor combined with water becomes a corrosive acid; unlikely at low moisture levels. Natural Gas is typically dried to 7 pounds of water per million standard cubic feet (LBS/MMSCF), this water vapor density corresponds to approximately -40°F dewpoint or 143PPMv of moisture content. Even though this is not a very high moisture level, it is not what would be considered very dry; therefore it is expected that some corrosives will form and thus have an effect on the equipment being used for measuring.

The presence of TEG carryover can cause interference for two major reasons:

  1. slowing down the measurement because of its highly hygroscopic nature

  2. shorting the sensor because of its electrical conductivity (0.002mhos/cm @ 20°C for pure

TEG, and significantly higher in the presence of ionic contaminants). The use of TEG filters actually makes the measurement even slower because the filters buildup much more TEG than the sensing devices would have without any filtration.

Since the gas being measured passes through the filters, it equilibrates with the TEG and slows down the measurement. To understand the importance of the effect of TEG’s electrical conductivity in moisture measuring apparatus, one needs to look at the construction of the moisture sensors.

All of the moisture sensors that use the impedance method of measurement e.g. aluminum oxide, ceramic, polymer etc. have a moisture dependant capacitor.

In all models from all manufacturers except PhyMetrix, both contacts of the capacitor plates are exposed to the gas under measurement. Normally this does not present a problem as the gasses being measured are insulators at the low excitation voltages that are used to measure the capacitance of the sensor.

However when the sensor is coated with a TEG film which is electrically conductive, the currents conducted through the TEG film (from one plate of the sensor capacitor to the other plate) cause the measuring electronics to misread the sensor measurement.

PhyMetrix has designed its sensor construction such that only one of the capacitor plates are exposed to the gas under measurement (the other plate is completely isolated), thus a coating from an electrically conductive film does not effect the measurement. This is a unique feature present only in the PhyMetrix sensor design.

A similar problem is encountered when there is mercury vapor in the gas being measured, where it forms a conductive film and shorts out the sensors of all other manufacturer’s, while the PhyMetrix sensor is not effected by any kind of conductive coating

The presence of particulates may cause abrasive harm to the measuring equipment, thus there should be a particulate filter to remove them.

Of course the desirability of Natural Gas is its energy content, however this property also poses an incendiary hazard; thus the measuring equipment must be certified by appropriate agencies for use in hazardous areas.