ASTM_D_7904_-_21.pdf

Designation:D790421Standard 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 adoption 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 concentration 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.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 conditioningrequirements 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 manufacturerspublished 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 65 C inthe analyzer cell.While sample system design is not coveredby this standard,it is common practice to heat the sampletransport line to around 50 C 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 measure 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 ratios(ppmv and%)are not pressure dependent.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 differently fordifferent regions and entities.The dew point can be estimatedfrom ppmv and pressure.Refer to Test Method D1142 and ISO18453.1.6 UnitsThe values stated in SI units are to be regardedas standard.No other units of measurement are included in thisstandard.1.7 This standard does not purport 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,health,and environmental practices and deter-mine the applicability of regulatory limitations prior to use.Some specific hazards statements are given in Section 8 onHazards.1.8 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards,Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade(TBT)Committee.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 Instrumentation(With-drawn 2017)31This 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 Nov.1,2021.Published November 2021.Originallyapproved in 2015.Last previous edition approved in 2015 as D7904 15.DOI:10.1520/D7904-21.2For referenced ASTM standards,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.3The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards,Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade(TBT)Committee.12.2 ISO Standard