1、Designation: D7904 15Standard Test Method forDetermination of Water Vapor (Moisture Concentration) inNatural Gas by Tunable Diode Laser Spectroscopy (TDLAS)1This standard is issued under the fixed designation D7904; the number immediately following the designation indicates the year oforiginal adopt
2、ion or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers online determination of vaporphase moisture conce
3、ntration in natural gas using a tunablediode laser absorption spectroscopy (TDLAS) analyzer alsoknown as a “TDL analyzer.” The particular wavelength formoisture measurement varies by manufacturer; typically be-tween 1000 and 10 000 nm with an individual laser having atunable range of less than 10 nm
4、.1.2 Process stream pressures can range from 700-mbar to700-bar gage. TDLAS is performed at pressures near atmo-spheric (700- to 2000-mbar gage); therefore, pressure reduc-tion is typically required. TDLAS can be performed in vacuumconditions with good results; however, the sample conditioningrequir
5、ements are different because of higher complexity and atendency for moisture ingress and are not covered by this testmethod. Generally speaking, the vent line of a TDL analyzer istolerant to small pressure changes on the order of 50 to200 mbar, but it is important to observe the manufacturerspublish
6、ed inlet pressure and vent pressure constraints. Largespikes or steps in backpressure may affect the analyzer read-ings.1.3 The typical sample temperature range is -20 to 65C inthe analyzer cell. While sample system design is not coveredby this standard, it is common practice to heat the sampletrans
7、port line to around 50C to avoid concentration changesassociated with adsorption and desorption of moisture alongthe walls of the sample transport line.1.4 The moisture concentration range is 1 to 10 000 partsper million by volume (ppmv). It is unlikely that one spec-trometer cell will be used to me
8、asure this entire range. Forexample, a TDL spectrometer may have a maximum measure-ment of 1 ppmv, 100 ppmv, 1000 ppmv, or 10 000 ppmv withvarying degrees of accuracy and different lower detectionlimits.1.5 TDL absorption spectroscopy measures molar ratiossuch as ppmv or mole percentage. Volumetric
9、ratios (ppmv and%) are not pressure dependant. Weight-per-volume units suchas milligrams of water per standard cubic metre or pounds ofwater per standard cubic foot can be derived from ppmv at aspecific condition such as standard temperature and pressure(STP). Standard conditions may be defined diff
10、erently fordifferent regions and entities. The moisture dew point can beestimated from ppmv and pressure. Refer to Test MethodD1142 and ISO 18453.1.6 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.7 This standard does not pur
11、port to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Some specifichazards statements are given in
12、 Section 8 on Hazards.2. Referenced Documents2.1 ASTM Standards:2D1142 Test Method for Water Vapor Content of GaseousFuels by Measurement of Dew-Point TemperatureD4150 Terminology Relating to Gaseous FuelsD5503 Practice for Natural Gas Sample-Handling and Con-ditioning Systems for Pipeline Instrumen
13、tationD5454 Test Method for Water Vapor Content of GaseousFuels Using Electronic Moisture Analyzers2.2 ISO Standards:3ISO 10715 Natural Gas Sampling GuidelinesISO 18453 Natural GasCorrelation Between Water Con-tent and Water Dew Point3. Terminology3.1 Definitions: Also refer to Terminology D4150.1Th
14、is test method is under the jurisdiction ofASTM Committee D03 on GaseousFuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-LineAnalysis of Gaseous Fuels.Current edition approved Jan. 1, 2015. Published February 2015. DOI: 10.1520/D7904/D7904152For referenced ASTM standards,
15、visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from International Organization for Standardization (ISO), 1, ch. dela Voie-Creuse
16、, CP 56, CH-1211 Geneva 20, Switzerland, http:/www.iso.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.1 absorption spectroscopy, nrefers to spectroscopictechniques that measure the absorption of electromagneticradiation (such
17、 as light), as a function of frequency orwavelength, because of its interaction with a sample.3.1.2 adsorption, nadhesion of molecules to a solid sur-face forming a molecular or atomic film.3.1.3 chemometrics, nfield of science relating measure-ments made on a chemical system or process to the state
18、 of thesystem via application of mathematical or statistical methods.3.1.4 desorption, nphenomenon whereby a substance isreleased from a surface (the opposite of adsorption).3.1.5 heat trace, nribbon-shaped tape that uses electricalresistance or steam to generate heat.3.1.5.1 DiscussionHeat trace ta
19、pe is attached to sampletubing and other sample conditioning components to avoidcondensation and stabilize the temperature of the wettedcomponents and the gas stream.3.1.6 moisture dew point, nA temperature and pressure atwhich water vapor begins to condense into liquid.3.1.6.1 DiscussionFor a given
20、 concentration of watervapor, the dew point temperature is a function of pressure. Adew point curve is shown in Fig. 1 with the dew pointtemperature on the x-axis and pressure on the y-axis.3.1.7 nanometre, nunit of length;1 nm = 1/1 000 000 000thof a metre.3.1.8 selectivity, nrefers to the extent t
21、o which TDLAScan detect moisture in gas matrices without significant inter-ferences from other components in the mixture.3.1.9 standard condition for temperature and pressure, STP,nstandard set of conditions established to allow the compari-son of different sets of data.3.1.10 tunable diode laser ab
22、sorption spectroscopy, TDLAS,ntechnique for measuring the concentration of a specificcomponent, such as water vapor, in a gaseous sample byabsorption spectrometry using tunable diode lasers.4. Summary of Test Method4.1 A representative sample of the gas is extracted from aprocess pipe or pipeline an
23、d is transferred by a sampletransport line through an appropriately designed samplingsystem to the inlet of a moisture analyzer. The sample must beconditioned with a minimum, preferably negligible, impact onthe moisture concentration. The gas flows continuouslythrough the analyzer and is vented to a
24、tmosphere, or to flare, orback to the process stream depending on application andregulatory requirements.4.2 The gas sample stream flows through the measurementcell. An overall diagram of the system is shown in Fig. 2.Asolid state laser with a narrow wavelength range is used as alight source. Electr
25、onics drive the laser and a thermoelectriccooler, which precisely stabilizes the laser temperature. Thelaser generates a near-infrared beam of light that passesthrough the cell window, is typically reflected using a mirror(or mirrors) within the cell, and then returns back through thewindow and into
26、 a photodiode detector. The photodiode signalis used to determine how much light is absorbed at specificwavelengths.4.3 Fig. 3 is a graph of typical regions in the near-infraredspectrum where water will be absorbed. In the graph, the x-axisindicates the wavelength. The y-axis indicates the “transmis
27、-sion” of light where 1.0 (or 100%) is the maximum. Where thetransmission is less than 1.0, absorbance by water is indicated.The vertical lines within the graph indicate the magnitude ofabsorption at specific wavelengths. Each individual absorptionline can be potentially utilized for TDLAS moisture
28、measure-ment. The actual wavelength used will vary based onmanufacturer, background composition, measurement specifi-cation requirements, and laser availability.FIG. 1 Example of Moisture Dew Point Curve Calculated Using Equations in Test Method D1142D7904 1524.4 The sensitivity of the measurement i
29、s determined by theabsorption as well as the length of the laser beam path (pathlength) within the sample cell. The path length is fixed and canrange from about 30 cm to 30 m depending on the measure-ment range and the wavelength used. By optimizing the pathlength and wavelength, linearity less than
30、 0.1% can be readilyachieved. The TDLAS manufacturer must be consulted foractual linearity specifications.4.5 This test method can be used as a guideline for instal-lation so that good moisture measurement can be achievedusing a TDLAS analyzer. Also, a procedure is outlined forvalidating measurement
31、 integrity.5. Significance and Use5.1 Moisture measurement in natural gas is performed toensure sufficiently low levels for gas purchase contracts and toprevent corrosion. Moisture may also contribute to the forma-tion of hydrates.5.2 The significance of applying TDLAS for the measure-ment of moistu
32、re in natural gas is TDLAS analyzers may havea very high degree of selectivity and minimal interference inmany natural gas streams. Additionally, the sensing compo-nents of the analyzer are not wetted by the natural gas, limitingthe potential damage from corrosives such as hydrogen sulfideFIG. 2 Mai
33、n Components of the TDLAS SystemFIG. 3 Water Transmittance in the Near Infrared (NIR) SpectrumSOURCE: HITRAND7904 153(H2S) and liquid contaminants such as ethylene glycol orcompressor oils. As a result, the TDLAS analyzer is able todetect changes in concentration with relatively rapid response.It sh
34、ould be noted that the mirrors of a TDLAS analyzer maybe fouled if large quantities of condensed liquids enter thesample cell. In most cases the mirror can be cleaned withoutthe need for recalibration or realignment.5.3 Primary applications covered in this method are listed in5.3.1 5.3.3. Each appli
35、cation may have differing require-ments and methods for gas sampling. Additionally, differentnatural gas applications may have unique spectroscopic con-siderations.5.3.1 Raw natural gas is found in production, gatheringsites, and inlets to gas-processing plants characterized bypotentially high level
36、s of water (H2O), carbon dioxide (CO2),hydrogen sulfide (H2S), and heavy hydrocarbons. Gas-conditioning plants and skids are normally used to removeH2O, CO2,H2S, and other contaminants. Typical moistureconcentration after dehydration is roughly 20 to 200 pppmv.Protection from liquid carryover such a
37、s heavy hydrocarbonsand glycols in the sample lines is necessary to prevent liquidpooling in the cell or the sample components.5.3.2 Underground gas storage facilities are high-pressurecaverns used to store large volumes of gas for use during peakdemand. Underground storage caverns can reach pressur
38、es ashigh as 275 bar. Multistage and heated regulator systems areusually required to overcome significant temperature dropsresulting from gas expansion in the sample.5.3.3 High-quality “sales gas” is found in transportationpipelines, natural gas distribution (utilities), and natural gaspower plant i
39、nlets. The gas is characterized by a very highpercentage of methane (90 to 100 %) with small quantities ofother hydrocarbons and trace levels of contaminates.6. Interferences6.1 TDLAS analyzers can be highly selective. They arecapable of measuring the target component with very littleinterference fr
40、om background composition, with some limita-tions. There may be some interference from backgroundcomponents. For example, at some wavelengths, methane mayabsorb at the same wavelength as moisture. If interferencesexist at a particular wavelength, a different wavelength can beemployed and other techn
41、iques such as chemometrics, back-ground compensation, or differential measurements may beutilized. Since hundreds of possible wavelengths are availablein the near-infrared band for measuring moisture, it is notpractical to list the potential interferences.6.2 Background composition changes may also
42、affect themeasurement from TDLAS analyzers because of a phenom-enon called “collisional broadening.” Collisional broadeningchanges the shape of the absorption “peak.” The broadeningeffect may be different at different wavelengths, or at differentpressures and temperatures, or both. TDLAS manufacture
43、rsshould publish the gas concentration ranges of the variouscomponents of the background gas in which the accuracy andrepeatability specifications are valid.7. Apparatus7.1 A TDL analyzer system includes the following subsys-tems: (1) sample extraction, (2) sample transport, (3) sampleconditioning s
44、ystem, (4) TDLAS analyzer, and (5) vent line.7.1.1 Sample extraction is required to obtain a representa-tive sample from the pipeline. To maintain the best speed ofresponse, it is recommended to reduce the pressure at thesample point. To avoid condensation that may occur fromexpanding the gas when i
45、t is depressurized (especially whenthe pipeline pressure is high), it is important to understand thephase diagram of all of the components in the gas (for example,hydrocarbons, alcohols, and water). Use an extraction probeand a regulator as shown in Fig. 4, mounted so that the tip ofthe probe is in
46、the center third of the pipe diameter. If the dewpoint of the gas is lower than the ambient temperature afterconsideration for temperature reduction as a result of gasexpansion through the regulator (approximately 3C per 6 bar),all sampling apparatus such as the probe and regulator mayneed to be hea
47、t traced or enclosed in a heated chamber, or both.According to Practice D5503, “vapor sample must be kept atleast 10C above the hydrocarbon dew point temperature toprevent condensation of the sample.”7.1.2 Sample TransportThe sample transport line carriesthe sample from the sample extraction point t
48、o the analyzer.The length of the sample transport tubing should be as short aspossible. Heat trace is absolutely necessary if the environmen-tal temperature is close to the dew point of the sample gas.Heat trace prevents water condensation and adsorption anddesorption from the walls of the tubing ca
49、used by ambienttemperature changes (Fig. 5). When heat trace is employed, theentire length of tubing must be heated and insulated with nogaps. Electropolished tubing is recommended for use in ana-lytical sample transport to reduce adsorption and desorptionFIG. 4 Heat-Trace Tubing with Self-Regulating Heat Tape Bundled with Insulation and Protective JacketD7904 154effects and to optimize speed of response in the sampletransport. For a moisture concentration below 10 ppmv, thetransport line may be coated with fused silica or an equivalentfor additional resista
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