ASTM D5454-11 Standard Test Method for Water Vapor Content of Gaseous Fuels Using Electronic Moisture Analyzers

The standard "ASTM D5454-11 Standard Test Method for Water Vapor Content of Gaseous Fuels Using Electronic Moisture Analyzers" developed by the American Society for Testing and Materials (ASTM) describes a test method for determining the water vapor content of gaseous fuels using electronic moisture analyzers. Such analyzers typically use sensing cells based on phosphorus pentoxide, aluminum oxide or silicon sensors, piezoelectric type cells and laser-based technologies.

For the purposes of this standard, the following terms and definitions apply:

• The capacitance type cell uses aluminum coated with aluminum oxide as part of the capacitor. The dielectric aluminum oxide film changes the capacitance of the capacitor according to the water vapor present. Unlike phosphorus pentoxide cells, this type is non-linear in its response. If silicon is used instead of aluminum, the silicon cell gives improved stability and very fast response.
• The electrolytic type cell consists of two noble metal electrode wires coated with phosphorus pentoxide. A bias voltage is applied across the electrodes and the water vapor reacts chemically, producing a current between the electrodes proportional to the water vapor present.
• The piezoelectric cell, the sensor, consists of a pair of electrodes supporting a quartz crystal transducer. When a voltage is applied to the sensor, a very stable oscillation occurs. The surfaces of the sensor are coated with a hygroscopic polymer. As the amount of moisture absorbed onto the polymer changes, a proportional change in the oscillation frequency occurs.
• The laser type cell consists of a sample cell with an optical head attached to one end and a mirror attached to the other. The optical head contains an NIR laser that emits light of a wavelength known to be absorbed by water molecules. A detector sensitive to NIR wavelength light is mounted next to the laser. The light from the laser passes through the far end and returns to the detector in the optical head. A portion of the emitted light is absorbed as the light passes through the sample cell and returns to the detector, in proportion to the water molecules present.
• Water content is usually dew point at atmospheric pressure and is measured in degrees F or C or pound/million standard cubic feet, is expressed in units. The latter term is used in this test method because it is the usual unit of readout for electronic analyzers. One pound per million standard cubic feet is equivalent to 21,1 ppm or 16,1 mgm/m3 of water vapor by volume. Analyzers should cover the range of 0,1 to 50 pounds per million standard cubic feet.
• Water dew point is the temperature at which liquid water will begin to condense from the water vapor present (at a specified pressure). Plots of dew points versus pressure and water content are included in ASTM D1142-95 Standard method of testing water vapor content of gaseous fuels by measuring dew point temperature.

Water content in fuel gas is the most important factor affecting internal corrosion. Hydrates, a semi-solid combination of hydrocarbons and water, form under suitable conditions, causing serious operating problems. Fuel heating value decreases with water concentration. Therefore, water concentration levels are frequently measured in natural gas systems. A common pipeline specification is 4 to 7 pounds per million standard cubic feet. This test method describes the measurement of water vapor content with direct-reading electronic instrumentation.

Moisture analyzer is a device that determines moisture content by the loss in drying method and consists of a weighing and halogen heating unit. It is suitable for quality control and production needs in food, pharmaceutical, chemical and other industries.

In general, contracts governing the pipeline transportation of natural gas include specifications that limit the maximum permissible water vapor concentration. Excessive water vapor can cause corrosive conditions, damaging pipelines and equipment. It can also condense and freeze or form methane hydrates and cause blockages. Water vapor content also affects the heating value of natural gas, thus affecting the quality of the gas. This test method allows the water content of natural gas to be determined.

If an acceptable graph showing the variation of water vapor content with saturation or water dew point temperatures over a suitable pressure range is available for the gas under test, the water vapor content can be calculated directly using the observed water dew point temperature and the pressure at which the determination is made.

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