This is part one of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas.
The determination of water vapor (water dew point) is crucial in the processing, custody transfer, and transport of natural gas. High levels of water vapor in a natural gas stream can lead to problems, including the formation of hydrates and the contribution to corrosion of plant and equipment. Furthermore, water vapor present in natural gas streams affects the overall quality of the gas and reduces the energy or heat content (BTU value) of the gas, making the measurement of water vapor in natural gas an important requirement to producers, suppliers and end users in the industry.
Equally important is the need to determine the hydrocarbon dew point temperature in a natural gas sales pipeline network. The formation of hydrocarbon liquids (condensate) due to the presence of heavier hydrocarbons in the natural gas can lead to increased pressure drops in the pipeline system, flooding, and safety hazards associated with liquids such as hot spots on compressor turbine blades.
There are several ways of determining the water vapor concentration in natural gas. In this seven-part series we’ll discuss Water Concentration vs. Water Dew Point Measurement, the different technologies that can be utilized (listed below), and the key requirements for using a Sample Conditioning System.
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Chilled Mirror Hygrometry
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Capacitance Sensors
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Quartz Crystal Microbalance Sensors
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Electrolytic Sensors
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Tunable Diode Laser Absorption Spectroscopy
Water Concentration vs. Water Dew Point Measurement
The dew-point of a gas is a physical property - the temperature at which the sample gas becomes saturated and condensation first begins to appear. At this temperature, the gas exists in equilibrium with a condensed phase. The dew point temperature refers to the equilibrium established over a liquid phase.
There are two classes of measurement devices. Devices capable of determining the physical dew point of either hydrocarbon or water in natural gas, and those devices that can measure the concentration of water in a natural gas sample.
Analyzers capable of measuring the dew point temperature directly are based on chilled mirror technology and are sometimes referred to as ‘first principle’ devices. Expressing water content in terms of dew point originates from this first principle device tracing its history back to the early dates of the natural gas industry when the United States Bureau of Mines developed the manual chilled mirror device.
The other class of measurement devices used for the determination of water vapor in natural gas determine the concentration of water vapor or partial pressure of water in the natural gas sample. Common concentration or content outputs of these devices include mg/Nm3, lbs/mmscf and parts per million either by weight (ppmw) or volume (ppmv). Water vapor measurement technologies most commonly used in the natural gas industry include capacitance-based sensors, quartz crystal microbalance, tunable diode laser absorption spectroscopy, and electrolytic-based sensors.
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.”