ASTM_D_7278_-_11.pdf

Designation:D727811Standard Guide forPrediction of Analyzer Sample System Lag Times1This standard is issued under the fixed designation D7278;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.INTRODUCTIONLag time,as used in this guide,is the time required to transport a representative sample from theprocess tap to the analyzer.Sample system designs have infinite configurations so this guide gives theuser guidance,based on basic design considerations,when calculating the lag time of online sampledelivery systems.Lag time of the analyzer sample system is a required system characteristic whenperforming system validation in Practice D3764 or D6122 and in general the proper operation of anyonline analytical system.The guide lists the components of the system that need to be considered whendetermining lag time plus a means to judge the type of flow and need for multiple flushes beforeanalysis on any sample.1.Scope1.1 This guide covers the application of routine calculationsto estimate sample system lag time,in seconds,for gas,liquid,and mixed phase systems.1.2 This guide considers the sources of lag time from theprocess sample tap,tap conditioning,sample transport,pre-analysis conditioning and analysis.1.3 Lag times are estimated based on a prediction of flowcharacteristics,turbulent,non turbulent,or laminar,and thecorresponding purge requirements.1.4 Mixed phase systems prevent reliable representativesampling so system lag times should not be used to predictsample representation of the stream.1.5 The values stated in inch-pound units are to be regardedas standard.No other units of measurement are included in thisstandard.1.6 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 and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2D3764 Practice for Validation of the Performance of ProcessStream Analyzer SystemsD6122 Practice for Validation of the Performance of Multi-variate Online,At-Line,and Laboratory Infrared Spectro-photometer Based Analyzer Systems3.Terminology3.1 Definitions:3.1.1 continuous analyzer unit cycle timethe time intervalrequired to replace the volume of the analyzer measurementcell.3.1.2 intermittent analyzer unit cycle timethe time intervalbetween successive updates of the analyzer output.3.1.3 purge volumethe combined volume of the full ana-lyzer sampling and conditioning systems.3.1.4 sample system lag timethe time required to transporta representative sample from the process tap to the analyzer.3.1.5 system response timethe sum of the analyzer unitresponse time and the analyzer sample system lag time.3.2 Abbreviations:3.2.1 I.D.Internal Diameter1This guide is under the jurisdiction of ASTM Committee D02 on PetroleumProducts,Liquid Fuels,and Lubricants and is the direct responsibility of Subcom-mittee D02.25 on Performance Assessment and Validation of Process StreamAnalyzer Systems.Current edition approved Oct.15,2011.Published December 2011.Originallyapproved in 2006.Last previous edition approved in 2006 as D727806.DOI:10.1520/D7278-11.2For 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.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 3.2.2 ReReynolds Number4.Summary4.1 The lag time of an analyzer sample system is estimatedby first determining the flow characteristics.The flow isassigned as turbulent or non-turbulent to assign the number ofpurges required to change out the sample.Based on thehardware employed in the sample system an estimation of thelag time can be calculated.5.Significance and Use5.1 The analyzer sample system lag time estimated by thisguide can be used in conjunction with the analyzer output toaid in optimizing control of blender facilities or process units.5.2 The lag time can be used in the tuning of controlprograms to set the proper optimization frequency.5.3 The application of this guide is not for the design of asample system but to help understand the design and toestimate the performance of existing sample systems.Addi-tional detailed information can be found in the referencesprovided in the section entitled Additional Reading Material.6.Basic Design Considerations6.1 Acceptable Lag TimeA one to two minute samplesystem lag time should be maintained to give acceptableperformance.Flow is a key component in the determination ofsample system lag time,and in most systems the desired