1、Designation: D7278 11D7278 16Standard 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 n
2、umber 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 s
3、ystem 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 Practi
4、ce 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. Scope Scope*1.1 T
5、his guide covers the application of routine calculations to estimate sample system lag time, in seconds, for gas, liquid, andmixed phase systems.1.2 This guide considers the sources of lag time from the process sample tap, tap conditioning, sample transport, pre-analysisconditioning and analysis.1.3
6、 Lag times are estimated based on a prediction of flow characteristics, turbulent, non turbulent, or laminar, and thecorresponding purge requirements.1.4 Mixed phase systems prevent reliable representative sampling so system lag times should not be used to predict samplerepresentation of the stream.
7、1.5 The values stated in inch-pound units are to be regarded as standard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to esta
8、blish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D3764 Practice for Validation of the Performance of Process Stream Analyzer SystemsD6122 Practice for Validation of the Performance of Multiv
9、ariate Online, At-Line, and Laboratory Infrared SpectrophotometerBased Analyzer Systems3. Terminology3.1 Definitions:3.1.1 continuous analyzer unit cycle timethe time interval required to replace the volume of the analyzer measurement cell.3.1.2 intermittent analyzer unit cycle timethe time interval
10、 between successive updates of the analyzer output.1 This guide is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of SubcommitteeD02.25 on Performance Assessment and Validation of Process Stream Analyzer Systems.Curre
11、nt edition approved Oct. 15, 2011April 1, 2016. Published December 2011April 2016. Originally approved in 2006. Last previous edition approved in 20062011 asD7278D7278 11.06. DOI: 10.1520/D7278-11.10.1520/D7278-16.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Cu
12、stomer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to
13、the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.*A Summar
14、y of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.3 purge volumethe combined volume of the full analyzer sampling and conditioning systems.3.1.4 sample system lag timethe time
15、required to transport a representative sample from the process tap to the analyzer.3.1.5 system response timethe sum of the analyzer unit response 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 analyze
16、r sample system is estimated by first determining the flow characteristics. The flow is assignedas turbulent or non-turbulent to assign the number of purges required to change out the sample. Based on the hardware employedin the sample system an estimation of the lag time can be calculated.5. Signif
17、icance and Use5.1 The analyzer sample system lag time estimated by this guide can be used in conjunction with the analyzer output to aid inoptimizing control of blender facilities or process units.5.2 The lag time can be used in the tuning of control programs to set the proper optimization frequency
18、.5.3 The application of this guide is not for the design of a sample system but to help understand the design and to estimate theperformance of existing sample systems. Additional detailed information can be found in the references provided in the sectionentitled Additional Reading Material.6. Basic
19、 Design Considerations6.1 Acceptable Lag TimeA one to two minute sample system lag time should be maintained to give acceptable performance.Flow is a key component in the determination of sample system lag time, and in most systems the desired system lag time isimpossible to achieve solely with maxi
20、mum allowable sample flow rate to the analyzer. A fast loop or bypass can be ways toimprove lag time by increasing sample velocity.Aslipstream is taken from the bypass to feed the analyzer at its optimum flowrate.Excess sample in the slipstream is vented to atmosphere, to flare or to the process str
21、eam dependent upon application andregulatory requirements.6.2 Physical State of Sample:6.2.1 Liquid SamplesPressure drop properties often govern the design of a liquid system. This is due for the most part on theclose relationship between pressure drop and system flowrate and the fixed pressure diff
22、erential available from the process forsample transport. The sizing of the sample components is a tradeoff between pressure drop and sample flowrate. High sampleflowrates in small sized component systems cause high-pressure drops and low sample transport times. The same flowrate in alarger tubing sy
23、stem will yield significant improvements in pressure drop through the system, but will also significantly increasethe time for sample transport.6.2.1.1 Users need to perform hydraulic calculations (which are currently outside the scope of this standard) in parallel with thelag time calculcations to
24、ensure that the “design” flow rates from a lag time perspective can actually be achieved with the operatingconditions in the field with some contingency for operational variations.6.2.2 Vapor SamplesVapor phase sampling is governed less by pressure drop and more by pressure compression propertiesof
25、gases relative to liquids. In compressible gases the higher the pressure in a given volume, the more sample is present in thatvolume. For this reason, and different from liquids, the selection and location of pressure regulating devices in the vapor samplesystem has a significant impact on the overa
26、ll system design. The optimal location for a high-pressure regulator in a vapor sampleis immediately downstream of the sample tap or high-pressure location thereby limiting the volume of the system under highpressure. Since the density of a compressible fluid is a function of the pressure, compressi
27、ble fluid flow rate calculations aresometimes done over segmental lengths where average properties adequately represent the fluid conditions of the line segment.6.2.3 Liquid to Vapor SamplesAchange of phase due to sample vaporization can also impact the sample lag time. The volumechange from the liq
28、uid phase to the vapor phase is substantial. Typical flow rates in gaseous sample lines downstream of thevaporizer can represent very small liquid feed rates to the vaporizer. Deadheaded sample line lengths upstream of the vaporizercan, in turn, represent appreciable lag times.6.2.4 Phase Separation
29、This guide is not intended to deal with dual phase samples as the volume and flow characteristics areoutside the scope.6.3 Sample TemperatureTemperature also impacts sample system lag time but to a lesser degree relative to pressure. Increasedtemperature of a sample lowers the sample density thus lo
30、wering the amount of sample flow needed to purge a given volume.Temperature impact is generally so small that it is ignored in rough estimations of sample system lag time.6.4 Typical Sources of Lag Time to Consider:6.4.1 Process Sample Tap:D7278 1626.4.1.1 Sample taps can be a significant source of
31、lag time if a sampling probe is not used, need to know the design inside thesample stream. See Fig. X1.1.6.4.1.2 Sample taps can present a problem for liquid vaporizing systems with high volume and low flow on the liquid side. SeeFig. X1.2.NOTE 1This refers to the case where the vaporizing regulator
32、 is located at the sample tap and one then has a length of liquid filled line from theprobe/process interface to the inlet of the vaporizing regulator. This situation can be mitigated by using a sample probe that takes the pressure drop, andsubsequent vaporization, at the probe/process interface so
33、that one extracts a gaseous sample only. The sensible heat of the bulk process stream flowingpast the tip of the sample probe provides the energy necessary to vaporize the sample that is extracted.6.4.2 At-Tap Conditioning:6.4.2.1 Filters and Strainers at Sample StreamDepending on design and size th
34、ese can add large volumes to a non turbulentsample system.NOTE 2For filters with diameters greater than the sample tubing diameter calculate the internal volume and use the 3 times the volume rule to accountfor the delay attributable to the filter.6.4.2.2 Flow or Pressure RegulatorsInternal volume o
35、f the regulator(s) are to be included in the system calculation.6.4.3 Vaporizing RegulatorsInternal volume of the regulator are to be included in the system calculation.6.4.3.1 The volume change from a liquid to a gas is on the order of 300 to 600 volumes of gas per volume of liquid so the lagtime o
36、f the liquid filled slipstream tubing length from a fast loop to a vaporizing regulator can represent very large lag times. SeeFig. X1.3.6.4.3.2 A system designed on the basis of a good gas volumetric flow rate can represent a very small liquid flow rate.6.5 Sample delivery tubing needs to be taken
37、into account in the system calculation. This can sometimes be a significant runlength depending on the analyzer location to the process stream.6.5.1 Sample Conditioning at Analyzer:6.5.1.1 FilteringDepending on their design and size, filters can add large volumes to a non turbulent sample system. Se
38、e Note2.7. Procedure7.1 Determination of Flow Characteristics:7.1.1 Calculate the Reynolds number, Re, of each section of the sample system using the tubing / pipe internal diameter (I.D.),the flow velocity, density of the sample stream, and viscosity of the sample stream.Re5I.D.!*Velocity!*Density!
39、#/Viscosity (1)NOTE 3Various forms of this equation exist for different units.7.1.2 Use Reynolds number Re to determine whether the sample flow is turbulent or non-turbulent in a particular section of thesample system.7.1.2.1 Assume turbulent flow for sections with a Re 4000.7.1.2.2 Assume non-turbu
40、lent flow for sections with a Re 2100 to Re 4000. See Figs. X1.4 and X1.5.7.2.2 Assume three purge volumes are required for adequate sample 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
41、%RVolume (litres) 0.6175 0.0100 0.7502 0.0200Pressure corrected 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 Compon
42、ents (Sec) 67.2223 1.1000 16.5354 4.8500 89.71Turbulent Components (Sec) 0.0000 0.0000 0.0000 0.0000 0.00FIG. X1.1 Gas Sample Without Tap ProbeD7278 164Lag Time CalcSS Component Seconds Flow Characteristic Seconds x PurgesLiq Process Tap 21.40 Non-Turbulent 64.20Vaporizing Reg 28.60 Non-Turbulent 85
43、.80Vapor Transport 8.30 Turbulent 8.30Sample Cond 4.80 Non-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 Tem
44、perature of sample gas, C 25 25Reynolds Number 822 1 Approximate 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 Purg
45、e Time (seconds) 21.435 28.571 ID (inches) 0.180 NAAverage 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) 0.0
46、00 0.000 Single Purge Time (seconds) 8.268 4.849Average 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 165Lag Time CalcSS Component Seconds Flow Characte
47、ristic Seconds x PurgesLiq Process Tap 0.90 Turbulent 0.90Liq 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
48、(lbs/cf) 31.600 31.600 31.600 Molecular weight of sample 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) 0.1
49、80 0.180 NA Reynolds Number 1.000Flow (LPM) 1.000 1.000 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) 0.000 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 TurbulentN
