Moisture Measurement in Natural Gas

Andrew Benton, Export Sales Manager at Michell Instruments, discusses the issues surrounding natural gas processing, the demands it puts on the humidity measurement industry, and the methods employed to effect a measurement in natural gas.

The presence of water vapour in natural gas, in concentrations above a few tens of parts per million, can be extremely hazardous; corroding pipes and leading to hydrate formation which reduces pipeline flow capacities and may even lead to blockages. As such, it is standard practice for natural gas production facilities, both on- and off-shore, to measure moisture content in natural gas on a continuous, on-line basis, to ensure successful processing and efficient, reliable plant operation.

The successful design, installation and operation of industrial hygrometers for such applications requires special consideration to be given to the particular nature and composition of the gas being measured, as well as the processing techniques being utilised.

Dehydration Processing
The most common processing technique for drying natural gas is that of a simple mechanical separator that divides the gas from the liquids of the two-phase flow coming from the gas field, followed by glycol dehydration.

However, the high velocity of gas through the glycol contactor can lead to glycol mist carrying through. This characteristic must be considered if the hygrometer is to monitor the performance of the dehydration process successfully, as contamination of the moisture sensor, or sample handling system, can result in a serious deadening of response from the sensor.

Working on a bypass flow arrangement and featuring a micro-porous membrane of fluorocarbon material to offer superior protection, membrane type filters provide the best safeguard against sensor contamination in glycol applications.

Conversion from Measured Dew Point to Moisture Content
The pressure of natural gas in processing plant and on-shore transmission is typically 4 to 8MPa, while gas entering off-shore pipelines is compressed to approximately 16MPa or higher. In any dew point analysis, it is essential that the influence of gas pressure is considered.

Absorbing the moisture in equilibrium with the gas sample flow to which it is exposed, the Michell Advanced Ceramic Moisture Sensor exhibits a response to variations in water vapour pressure. Water vapour pressure, directly related to dew point, enables the sensor to be calibrated in the parameter of dew point allowing the relationship between partial pressure of water vapour and dew point to remain consistent, irrespective of total gas pressure and the composition of the dry gas components.

Therefore, sensors calibrated against a traceable calibration reference can be used to accurately determine the dew point of any process gas at any chosen analysis pressure. The parameter of water dew point is the most widely used parameter to stipulate this element of gas quality, in contractual supply specifications between gas producers and pipeline operators, however, some specifications for process plants, such as glycol dehydration contractors, stipulate a maximum permissible moisture content.

For applications requiring units of moisture content to be indicated, the conversion is usually performed in the measurement electronics of the hygrometer, where the measured dew point, at a known pressure, is converted to the measurement unit desired by the natural gas company.

Empirical conversion data, published by recognised gas research institutions, must be used to compensate for the non-ideal behaviour of natural gas. Knowing the pressure of analysis for the primary dew point measurement is critical to the accuracy of the unit conversion. The analyser pressure should therefore either be fixed by a 'peak shaving' pressure regulator, set to the minimum line pressure level, or by on-line measurement of the analysis condition using a pressure transducer.

Sour Natural Gas
Further difficulty may also be experienced in conversion to units of moisture content if the gas composition contains a significant amount of carbon dioxide and/or hydrogen sulphide.

'Sour' natural gas, as it is termed, is found in many gas fields in current production. The maximum concentration of moisture in H²S and C0² is considerably higher than the saturation water vapour pressure of methane or 'sweet' natural gas composition at the same temperature and, as such, the water dew point measured in a sour gas will be significantly lower than for a sweet gas containing the same moisture content.

These difficulties can be overcome by using an enhancement factor to compensate for moisture forming bonds with the sour components of the gas, in addition to free moisture which constitutes the water vapour pressure relating to the dew point actually measured.

The application of a moisture analyser for sour gas measurement requires sensing technology which can perform satisfactorily in these most aggressive of gas media. Michell's Ceramic Moisture Sensor has been successfully used in these applications for many years. The materials selected for the active device of this sensor are exclusively ceramics and base metals, inert by nature and offering good resilience to chemical attack. This, combined with the robust design and careful selection of materials for the construction of the other gas wetted parts of the sensor, affords reliable service in such an application.

Hydrocarbon Dew Point
For natural gas there are two dew-point temperatures of relevance: the water dew point and the hydrocarbon dew point. Hydrocarbon dew point is the temperature at which liquid hydrocarbons condense out of the gas upon cooling. Such liquid hydrocarbons comprise the heavier molecular weight components of the gas composition, typically butane and higher. This parameter, as with water dew point, requires dedicated processing plant, and purpose designed, measurement instrumentation.

A manual visual cooled mirror dew-point hygrometer may suffer because of the difficulty in observing the water dew point separately from that of hydrocarbons, and glycol, which have a high likelihood of condensing on the mirror surface at a higher temperature than the water dew point. The use of a sensor based on a non-condensing measurement principle, such as the Michell Ceramic Moisture Sensor, easily overcomes this problem.

The application of moisture analysers for the measurement of natural gas is not simple and straightforward. There are many aspects to be considered, unique to natural gas, which can greatly effect the reliability of both the instrument in service and the measurement data that it provides. Detailed consultation, between instrument manufacturers and their customers in the natural gas industry, is required in order to devise the best solution for each application.

Back to press releases