Measuring moisture in gas
29 Jan 2007
Measuring the moisture content of natural gas and other hydrocarbon and hydrogen gas mixtures is critical to the efficiency, and arguably the safety, of plant and equipment used throughout their transportation process. Such measurements have to be taken frequently, if not continuously, often in hazardous or otherwise hostile environments.
Two of the most popular technologies are impedance dew point sensors and manual chilled mirror devices.
Impedance dew point sensors are, typically, constructed using thin and thick film techniques, based on the adsorption of water vapour into a porous non-conducting "sandwich" between two conductive layers on a ceramic substrate.
The active sensor layer and the porous top conductor are engineered very thinly so the sensor responds very rapidly to changes in applied moisture, both when being dried (on process start-up) and if there is moisture ingress into a process.
Despite this extreme sensitivity to changes in moisture content, impedance moisture sensors can be incredibly rugged if constructed to a high standard. To protect the sensor further against contaminants and pipe swarf, the sensor should be housed in a protective sintered stainless steel guard.
Ceramic sensors are manufactured from inert materials that resist aggressive chemicals. Such sensors routinely achieve high accuracy and long-term stability. Traditional aluminium oxide sensors can suffer failure on rapid increase or decrease in pressure at start-up or shutdown. This new technology avoids that and can withstand 400barg without any risk of failure due to pressure shock.
Ceramic sensors also have applications in measuring moisture in hydrocarbon liquids such as propane, propylene and hexane.
Some hydrocarbon processes use a manual chilled mirror hygrometer measurement instrument, often known as a dewscope. The mirror is typically cooled using a bottled supply of expandable gas (carbon dioxide) until the operator sees the moisture condensing on the mirror though a viewing window. A direct temperature reading is then taken and this, together with the pressure of the hydrocarbon gas, is used to calculate the moisture content.
Such techniques are, however, difficult to automate in hazardous conditions. Also, 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 tend to condense on the mirror surface at a higher temperature than the water dew point.
A recent development in chilled mirror technology is the Dark Spot optical principle, which is used for hydrocarbon dew-point detection. Using the same principle of condensation on an optical surface, it can detect the formation or drop out of hydrocarbons from complex natural gas mixtures.
Hydrocarbon dew point is again a critical quality parameter in natural gas. The European Association for Streamlining Energy Exchange stipulates a hydrocarbon dew point limit of -2ºC for all cross-border gas transfers.
Sensitivity in the order of 1ppm (molar) of condensate enables the analyser to detect the almost invisible films of condensate that are characteristic of hydrocarbon gases at dew point.
The optical surface is the key element of the sensor cell and comprises an acid-etched, semi-matt surface with a central conical-shaped depression. A well-collimated beam of visible red light is focused onto the central region of the optical surface. In the dry condition most of the incident light beam is reflected from the optical surface to form an annulus ring of light.
The amount of light dispersed is detected. As hydrocarbon condensates form on the optical surface its optical properties are modified - the reflected light intensity of the annulus ring increases and there is a dramatic reduction in the scattered light intensity within the dark spot region. It is this secondary
Two of the most popular technologies are impedance dew point sensors and manual chilled mirror devices.
Impedance dew point sensors are, typically, constructed using thin and thick film techniques, based on the adsorption of water vapour into a porous non-conducting "sandwich" between two conductive layers on a ceramic substrate.
The active sensor layer and the porous top conductor are engineered very thinly so the sensor responds very rapidly to changes in applied moisture, both when being dried (on process start-up) and if there is moisture ingress into a process.
Despite this extreme sensitivity to changes in moisture content, impedance moisture sensors can be incredibly rugged if constructed to a high standard. To protect the sensor further against contaminants and pipe swarf, the sensor should be housed in a protective sintered stainless steel guard.
Ceramic sensors are manufactured from inert materials that resist aggressive chemicals. Such sensors routinely achieve high accuracy and long-term stability. Traditional aluminium oxide sensors can suffer failure on rapid increase or decrease in pressure at start-up or shutdown. This new technology avoids that and can withstand 400barg without any risk of failure due to pressure shock.
Ceramic sensors also have applications in measuring moisture in hydrocarbon liquids such as propane, propylene and hexane.
Some hydrocarbon processes use a manual chilled mirror hygrometer measurement instrument, often known as a dewscope. The mirror is typically cooled using a bottled supply of expandable gas (carbon dioxide) until the operator sees the moisture condensing on the mirror though a viewing window. A direct temperature reading is then taken and this, together with the pressure of the hydrocarbon gas, is used to calculate the moisture content.
Such techniques are, however, difficult to automate in hazardous conditions. Also, 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 tend to condense on the mirror surface at a higher temperature than the water dew point.
A recent development in chilled mirror technology is the Dark Spot optical principle, which is used for hydrocarbon dew-point detection. Using the same principle of condensation on an optical surface, it can detect the formation or drop out of hydrocarbons from complex natural gas mixtures.
Hydrocarbon dew point is again a critical quality parameter in natural gas. The European Association for Streamlining Energy Exchange stipulates a hydrocarbon dew point limit of -2ºC for all cross-border gas transfers.
Sensitivity in the order of 1ppm (molar) of condensate enables the analyser to detect the almost invisible films of condensate that are characteristic of hydrocarbon gases at dew point.
The optical surface is the key element of the sensor cell and comprises an acid-etched, semi-matt surface with a central conical-shaped depression. A well-collimated beam of visible red light is focused onto the central region of the optical surface. In the dry condition most of the incident light beam is reflected from the optical surface to form an annulus ring of light.
The amount of light dispersed is detected. As hydrocarbon condensates form on the optical surface its optical properties are modified - the reflected light intensity of the annulus ring increases and there is a dramatic reduction in the scattered light intensity within the dark spot region. It is this secondary
