1、Designation: D7278 11Standard 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
2、 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 de
3、signs 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
4、 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 cove
5、rs 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
6、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 st
7、ated 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
8、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 Pro-cess Stream Analyzer SystemsD6122 Practice for Validation of the Performance of Mul-tivariate Online,
9、 At-Line, and Laboratory Infrared Spec-trophotometer 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 inter-val between succ
10、essive updates of the analyzer output.3.1.3 purge volumethe combined volume of the fullanalyzer 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 u
11、nitresponse time and the analyzer sample system lag time.3.2 Abbreviations:3.2.1 I.D.Internal Diameter3.2.2 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
12、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.1This guide is under the jurisdiction of ASTM Committee D02 on PetroleumProducts and Lubricants and is the direct responsibility of Subcommittee D0
13、2.25 onPerformance Assessment and Validation of Process Stream Analyzer 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,
14、 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.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5
15、. 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 fre
16、quency.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.
17、 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 desiredsystem lag time is impossible to achieve with maximum
18、allowable sample flow rate to the analyzer. A fast loop orbypass can be ways to improve lag time by increasing samplevelocity. A slipstream is taken from the bypass to feed theanalyzer at its optimum flowrate. Excess sample in the slip-stream is vented to atmosphere, to flare or to the process strea
19、mdependent upon application and regulatory requirements.6.2 Physical State of Sample:6.2.1 Liquid SamplesPressure drop properties often gov-ern the design of a liquid system. This is due for the most parton the close relationship between pressure drop and systemflowrate and the fixed pressure differ
20、ential available from theprocess for sample transport. The sizing of the sample compo-nents is a tradeoff between pressure drop and sample flowrate.High sample flowrates in small sized component systems causehigh-pressure drops and low sample transport times. The sameflowrate in a larger tubing syst
21、em will yield significant im-provements in pressure drop through the system, but will alsosignificantly increase the time for sample transport.6.2.2 Vapor SamplesVapor phase sampling is governedless by pressure drop and more by pressure compressionproperties of gases relative to liquids. In compress
22、ible gasesthe higher the pressure in a given volume, the more sample ispresent in that volume. For this reason, and different fromliquids, the selection and location of pressure regulatingdevices in the vapor sample system has a significant impact onthe overall system design. The optimal location fo
23、r a high-pressure regulator in a vapor sample is immediately down-stream of the sample tap or high-pressure location therebylimiting the volume of the system under high pressure. Sincethe density of a compressible fluid is a function of the pressure,compressible fluid flow rate calculations are some
24、times doneover segmental lengths where average properties adequatelyrepresent the fluid conditions of the line segment.6.2.3 Liquid to Vapor SamplesA change of phase due tosample vaporization can also impact the sample lag time. Thevolume change from the liquid phase to the vapor phase issubstantial
25、. Typical flow rates in gaseous sample lines down-stream of the vaporizer can represent very small liquid feedrates to the vaporizer. Deadheaded sample line lengths up-stream of the vaporizer can, in turn, represent appreciable lagtimes.6.2.4 Phase SeparationThis guide is not intended to dealwith du
26、al phase samples as the volume and flow characteristicsare outside the scope.6.3 Sample TemperatureTemperature also impacts samplesystem lag time but to a lesser degree relative to pressure.Increased temperature of a sample lowers the sample densitythus lowering the amount of sample flow needed to p
27、urge agiven volume. Temperature impact is generally so small that itis ignored in rough estimations of sample system lag time.6.4 Typical Sources of Lag Time to Consider:6.4.1 Process Sample Tap:6.4.1.1 Sample taps can be a significant source of lag time ifa sampling probe is not used, need to know
28、the design insidethe sample stream. See Fig. X1.1.6.4.1.2 Sample taps can present a problem for liquid vapor-izing systems with high volume and low flow on the liquidside. See Fig. X1.2.NOTE 1This refers to the case where the vaporizing regulator islocated at the sample tap and one then has a length
29、 of liquid filled linefrom the probe/process interface to the inlet of the vaporizing regulator.This situation can be mitigated by using a sample probe that takes thepressure drop, and subsequent vaporization, at the probe/process interfaceso that one extracts a gaseous sample only. The sensible hea
30、t of the bulkprocess stream flowing past the tip of the sample probe provides theenergy necessary to vaporize the sample that is extracted.6.4.2 At-Tap Conditioning:6.4.2.1 Filters and Strainers at Sample StreamDependingon design and size these can add large volumes to a nonturbulent sample system.N
31、OTE 2For filters with diameters greater than the sample tubingdiameter calculate the internal volume and use the 3 times the volume ruleto account for the delay attributable to the filter.6.4.2.2 Flow or Pressure RegulatorsInternal volume ofthe regulator(s) are to be included in the system calculati
32、on.6.4.3 Vaporizing RegulatorsInternal volume of the regu-lator are to be included in the system calculation.6.4.3.1 The volume change from a liquid to a gas is on theorder of 300 to 600 volumes of gas per volume of liquid so thelag time of the liquid filled slipstream tubing length from a fastloop
33、to a vaporizing regulator can represent very large lagtimes. See Fig. X1.3.6.4.3.2 A system designed on the basis of a good gasvolumetric flow rate can represent a very small liquid flow rate.6.5 Sample delivery tubing needs to be taken into account inthe system calculation. This can sometimes be a
34、significant runlength depending on the analyzer location to the processstream.6.5.1 Sample Conditioning at Analyzer:6.5.1.1 FilteringDepending on their design and size, fil-ters can add large volumes to a non turbulent sample system.See Note 2.7. Procedure7.1 Determination of Flow Characteristics:D7
35、278 1127.1.1 Calculate the Reynolds number, Re, of each section ofthe sample system using the tubing / pipe internal diameter(I.D.), the flow velocity, density of the sample stream, andviscosity of the sample stream.Re 5I.D.! * Velocity! * Density!# / Viscosity (1)NOTE 3Various forms of this equatio
36、n exist for different units.7.1.2 Use Reynolds number Re to determine whether thesample flow is turbulent or non-turbulent in a particular sectionof the sample system.7.1.2.1 Assume turbulent flow for sections with aRe 4000.7.1.2.2 Assume non-turbulent flow for sections with aRe 2100 to Re 4000. See
37、 Figs. X1.4 andX1.5.7.2.2 Assume three purge volumes are required for adequatesample exchange in systems with non-turbulent flow,Re 93%RID (inches) 2.0000 NA 0.1800 NA 269.12 3 Purge 97%RFlow (SLPM) 5.5000 5.5000 5.5000 0.5000 358.83 4 Purge 98%RVolume (litres) 0.6175 0.0100 0.7502 0.0200Pressure co
38、rrected purge volume 6.1620 0.0998 1.5157 0.0404Single Purge Time (seconds) 67.2223 1.0887 16.5354 4.8490Average Velocity (FPS) 0.0149 NA 9.0714 NAFlow Type (w/o Transition) Non Turbulent Non Turbulent Non Turbulent Non TurbulentNon-Turb Components (Sec) 67.2223 1.1000 16.5354 4.8500 89.71Turbulent
39、Components (Sec) 0.0000 0.0000 0.0000 0.0000 0.00FIG. X1.1 Gas Sample Without Tap ProbeD7278 114Lag Time CalcSS Component Seconds Flow Characteristic Seconds x PurgesLiq Process Tap 21.40 Non-Turbulent 64.20Vaporizing Reg 28.60 Non-Turbulent 85.80Vapor Transport 8.30 Turbulent 8.30Sample Cond 4.80 N
40、on-Turbulent 14.40Total 172.7000Liquid Propane Process Tap Tap Sam Cond Vaporized Propane Sample Transport Analyzer Sam SysApproximate liq density (lbs/cf) 31.6 31.600 Molecular weight of sample gas 44 44Viscosity of liquid (cP) 0.120 0.120 Temperature of sample gas, C 25 25Reynolds Number 822 1 App
41、roximate gas density (lbs/cf) 0.227 0.227L (feet) 3.0 NA Viscosity of sample gas (cP) 0.017 0.017ID (inches) 0.180 NA Reynolds Number 5365 1Flow (LPM) 0.042 0.042 Pressure (PSIG) 15.0 15.0Volume (litres) 0.015 0.020 L (feet) 150.0 NASingle Purge Time (seconds) 21.435 28.571 ID (inches) 0.180 NAAvera
42、ge Velocity (FPS) 0.140 NA Flow (SLPM) 11.00 0.50Flow Type (w/o Transition) Non-Turbulent Non-Turbulent Volume (litres) 0.750 0.020Non-Turb Components (Sec) 21.435 28.571 Pressure corrected purge volume 1.516 0.040Turbulent Components (Sec) 0.000 0.000 Single Purge Time (seconds) 8.268 4.849Average
43、Velocity (FPS) 18.143 NAFlow Type (w/o Transition) Turbulent Non TurbulentNon-Turb Components (Sec) 0.000 4.849Turbulent Components (Sec) 8.268 0.000FIG. X1.2 Vaporizing Regulator Near TapD7278 115Lag Time CalcSS Component Seconds Flow Characteristic Seconds x PurgesLiq Process Tap 0.90 Turbulent 0.
44、90Liq Transport 45.00 Turbulent 45.00Vaporizing Reg 15.00 Non-Turbulent 45.00Sample Cond 0.10 Non-Turbulent 0.30Total 91.2000Liquid Propane Process Tap Sample Transport Analyzer Sam Sys Vaporized Propane Analyzer Sam SysApproximate liq density (lbs/cf) 31.600 31.600 31.600 Molecular weight of sample
45、 gas 44.000Viscosity of liquid (cP) 0.120 0.120 0.120 Temperature of sample gas, C 25.000Reynolds Number 19570.474 19570.474 1.000 Approximate gas density (lbs/cf) 0.227L (feet) 3.000 150.000 NA Viscosity of sample gas (cP) 0.017ID (inches) 0.180 0.180 NA Reynolds Number 1.000Flow (LPM) 1.000 1.000
46、0.040 Pressure (PSIG) 15.000Volume (litres) 0.015 0.750 0.010 L (feet) NASingle Purge Time (seconds) 0.900 45.013 15.000 ID (inches) NAAverage Velocity (FPS) 3.332 3.332 NA Flow (SLPM) 10.500Flow Type (w/o Transition) Turbulent Turbulent Non-Turbulent Volume (litres) 0.010Non-Turb Components (Sec) 0
47、.000 0.000 15.000 Pressure corrected purge volume 0.020Turbulent Components (Sec) 0.900 45.013 0.000 Single Purge Time (seconds) 0.115Average Velocity (FPS) NAFlow Type (w/o Transition) Non TurbulentNon-Turb Components (Sec) 0.115Turbulent Components (Sec) 0.000FIG. X1.3 Vaporizing Regulator Near An
48、alyzerD7278 116Process Tap Tap Sam Cond Sample Transport Analyzer Sam Sys LagtimesMolecular weight of sample gas 30 30 30 30 secondsTemperature of sample gas, C 25 25 25 25Approximate gas density (lbs/cf) 0.7637 0.7637 0.1546 0.1546Viscosity of sample gas (cP) 0.0171 0.0171 0.0171 0.0171Reynolds Num
49、ber 5155 NA 5155 NAPressure (PSIG) 132 132 15 15 11.68 1 Purge 75%RL (feet) 3 NA 150 NA 16.92 2 Purge 93%RID (inches) 0.1800 NA 0.1800 NA 22.16 3 Purge 97%RFlow (SLPM) 15.5000 15.5000 15.5000 0.5000 27.39 4 Purge 98%RVolume (litres) 0.0150 0.0100 0.7502 0.0200Pressure corrected purge volume 0.1497 0.0998 1.5157 0.0404Single Purge Time (seconds) 0.5796 0.3863 5.8674 4.8490Average Velocity (FPS) 5.1757 NA 25.5649 NAFlow Type (w/o Transition) Turbulent Non Turbulent Turbulent Non TurbulentNon-Turb Components (Sec) 0.0000 0.3860 0.0000 4.8500 5.24Turbulen
