This is the final article of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas. Click to read parts one, two, three, four, five, and six.
Sample Conditioning Systems
There are several key requirements to remember when preparing or transporting a sample to water vapor and dew point analyzers. Following is a list of critical components and guides for operating chilled mirror devices in natural gas applications:
- Water is a small polar molecule that will ‘stick’ to most surfaces. The adsorption of water onto surfaces in a sample system can cause a substantial decrease in the response speed of a sample system. The lower the water dew point that is to be measured, the more serious the role of adsorption becomes. Consequently, surfaces are responsible for substantial lag times in measuring both increases and decreases in the moisture concentration of the sample.
- For installation of stationary, automated chilled mirror devices or water vapor analyzers, temperature-controlled sample lines are preferred as the surface coverage of a sample system is a function of temperature. Higher temperatures result in lower surface coverage. Insulating and heating the line to 60°C or higher is recommended. However, most important is temperature stability. Process piping takes up and releases water with ambient temperature changes. A sample line with a varying temperature, will exhibit the same behavior.
- A high-quality stainless-steel sample line should be used. Copper, aluminum, and carbon steel tubes are not recommended because the oxide film on the inside wall surface promotes adsorption. Teflon and plastics are not recommended because these materials will absorb (and later desorb) large quantities of water from the sample gas.
- Filtration to eliminate or reduce the potential for liquids should be considered. It is a common practice to use membrane filters incorporated into the sample system to protect the equipment from liquid slugs and ensure continuous operation of both manually operated, and automated chilled mirror devices.
- Pressure regulation, when required for monitoring of hydrocarbon dew point temperatures at the cricondentherm, should take Joule Thompson cooling into account and be designed to not impact the detection of condensation in the analyzer.
A properly designed sample system reduces or eliminates the presence of contaminants and ensures proper operation of either type of chilled mirror technology.
For more detailed information about this application, refer to our White Paper, “Analytical Devices for the Measurement of Water Vapor and Hydrocarbon Dew-Point in Natural Gas.”