ASTM D7278-21

Standard Guide for Prediction of Analyzer Sample System Lag Times

SIGNIFICANCE AND USE
5.1 The analyzer sample system lag time estimated by this guide can be used in conjunction with the analyzer output to aid in optimizing control of blender facilities or process units. A known and constant lag time is key for the use in optimizing control.  
5.2 The lag time can be used in the tuning of control programs to set the proper optimization frequency.  
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 the performance of existing sample systems. Additional detailed information can be found in the references provided in the section entitled Additional Reading Material.
SCOPE
1.1 This guide covers the application of routine calculations to 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 the process sample tap, tap conditioning, sample transport, pre-analysis conditioning and analysis.  
1.3 Lag times are estimated based on a prediction of flow characteristics, turbulent, non turbulent, or laminar, and the corresponding purge requirements.  
1.4 Mixed phase systems prevent reliable representative sampling so system lag times should not be used to predict sample representation of a mixed phase stream.  
1.5 The values stated in inch-pound units are to be regarded as standard. Other units of measurement are included in this standard and Appendix X1 examples where normally seen in industry.  
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Nov-2021
• ICS: 17.220.01 - Electricity. Magnetism. General aspects
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.25 - Performance Assessment and Validation of Process Stream Analyzer Systems

Relations

• Refers: ASTM D6122-23 - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared Spectrophotometer, and Raman Spectrometer Based Analyzer Systems
• Effective Date: 01-Jul-2023
• Refers: ASTM D6122-19b - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems
• Effective Date: 01-Jun-2019
• Refers: ASTM D6122-19a - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems
• Effective Date: 01-May-2019
• Refers: ASTM D6122-19 - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems
• Effective Date: 01-Jan-2019
• Refers: ASTM D6122-18 - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems
• Effective Date: 01-Jul-2018
• Refers: ASTM D3764-15e1 - Standard Practice for Validation of the Performance of Process Stream Analyzer Systems
• Effective Date: 01-Apr-2015
• Refers: ASTM D3764-15 - Standard Practice for Validation of the Performance of Process Stream Analyzer Systems
• Effective Date: 01-Apr-2015
• Refers: ASTM D6122-13 - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems
• Effective Date: 01-May-2013
• Refers: ASTM D6122-10 - Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems
• Effective Date: 01-May-2010
• Refers: ASTM D3764-09 - Standard Practice for Validation of the Performance of Process Stream Analyzer Systems
• Effective Date: 01-Jun-2009
• Refers: ASTM D6122-09 - Standard Practice for Validation of the Performance of Multivariate Process Infrared Spectrophotometers
• Effective Date: 01-Jun-2009
• Refers: ASTM D6122-06 - Standard Practice for Validation of the Performance of Multivariate Process Infrared Spectrophotometers
• Effective Date: 01-Jul-2006
• Refers: ASTM D3764-06e1 - Standard Practice for Validation of the Performance of Process Stream Analyzer Systems
• Effective Date: 01-Jul-2006
• Refers: ASTM D3764-06 - Standard Practice for Validation of the Performance of Process Stream Analyzer Systems
• Effective Date: 01-Jul-2006
• Refers: ASTM D6122-06e1 - Standard Practice for Validation of the Performance of Multivariate Process Infrared Spectrophotometers
• Effective Date: 01-Jul-2006

Overview

ASTM D7278-21 - Standard Guide for Prediction of Analyzer Sample System Lag Times provides a framework for accurately estimating lag time within analyzer sample systems in industrial environments. Developed by ASTM International, this guide is intended to help users calculate the time required to transport a representative gas, liquid, or mixed-phase process sample from the process tap to the analyzer. While not intended for designing new sample systems, this guide supports performance assessments, validation, and optimization of existing sample systems, which are crucial for process control, particularly in facilities such as blenders or process units.

Understanding sample system lag time is essential for tuning process control programs, ensuring reliable analyzer outputs, and facilitating rapid, representative sampling for both batch and continuous process applications. By offering systematic methods for estimating lag time based on flow characteristics and system configuration, ASTM D7278-21 helps operators improve process efficiency and product quality.

Key Topics

• Definition of Lag Time: Lag time is the interval needed for a sample to move from the process tap through transport and conditioning lines to the analyzer. This guide covers lag times for gas, liquid, and mixed-phase systems, but notes that representative sampling in mixed-phase streams is typically unreliable.
• Flow Characterization: The guide emphasizes calculating the Reynolds number for each sample system segment to classify flow as turbulent, transitional, or laminar. Turbulent flow generally allows for a single purge volume, while non-turbulent (laminar or transitional) flow may require multiple (typically three) purge volumes to adequately exchange samples.
• System Components: The lag time calculation requires consideration of multiple sample system components, including sample taps, at-tap conditioning (filters, strainers, regulators), sample transfer lines, vaporizing regulators, and any device contributing to internal sample volume.
• Calculation Procedures: The guide provides a structured method for calculating lag time based on segment volume, number of necessary purge volumes (influenced by flow regime), and segment flow rate. Summing the lag times of each section results in the total system lag time.
• Unit Standards: The standard primarily uses inch-pound units, with alternative metric units also referenced in actual industry applications.
• Safety: The standard notes that it is the user's responsibility to ensure all safety, health, and environmental considerations are addressed in accordance with applicable regulations before applying the guide.

Applications

ASTM D7278-21 finds practical application in industries where online analyzers are used to monitor process streams-especially petroleum, chemical, and gas processing sectors. Key applications include:

• Control Program Optimization: Leveraging accurate lag time data to optimize control strategies and set the appropriate sampling or adjustment frequency for dynamic process management.
• Performance Assessment: Supporting validation of process analyzer systems in accordance with ASTM practices such as D3764, D6122, and D8321, ensuring that analyzer response times reflect true process conditions.
• System Troubleshooting: Diagnosing sources of unexpected delays or variations in analyzer readings by quantifying the contribution of each segment in the sample delivery system.
• System Validation: Essential during commissioning and regular verification of sample systems to confirm that time delays do not impede timely and representative process measurements.

Related Standards

Users of ASTM D7278-21 may also find the following ASTM standards relevant:

• ASTM D3764: Practice for Validation of the Performance of Process Stream Analyzer Systems
• ASTM D6122: Practice for Validation of Multivariate Analyzer Systems
• ASTM D8321: Practice for the Development and Validation of Multivariate Analyses for Predicting Properties of Petroleum Products and Lubricants

Keywords: analyzer sample system lag time, ASTM D7278-21, process analyzer, sample system optimization, process control, lag time calculation, flow characterization, industrial analyzer systems.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7278 − 21
Standard Guide for
Prediction of Analyzer Sample System Lag Times
This standard is issued under the fixed designation D7278; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Lag time, as used in this guide, is the time required to transport a representative sample from the
process tap to the analyzer. Sample system designs have infinite configurations so this guide gives the
user guidance, based on basic design considerations, when calculating the lag time of online sample
delivery systems. Lag time of the analyzer sample system is a required system characteristic when
performingsystemvalidationinPracticeD3764,D6122,orD8321andingeneraltheproperoperation
of any online analytical system. The guide lists the components of the system that need to be
considered when determining lag time plus a means to judge the type of flow and need for multiple
flushes before analysis on any sample.
1. Scope* ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.1 This guide covers the application of routine calculations
mendations issued by the World Trade Organization Technical
to estimate sample system lag time, in seconds, for gas, liquid,
Barriers to Trade (TBT) Committee.
and mixed phase systems.
1.2 This guide considers the sources of lag time from the
2. Referenced Documents
process sample tap, tap conditioning, sample transport, pre-
2.1 ASTM Standards:
analysis conditioning and analysis.
D3764 Practice forValidation of the Performance of Process
1.3 Lag times are estimated based on a prediction of flow
Stream Analyzer Systems
characteristics, turbulent, non turbulent, or laminar, and the
D6122 Practice for Validation of the Performance of Multi-
corresponding purge requirements.
variate Online, At-Line, Field and Laboratory Infrared
Spectrophotometer, and Raman Spectrometer Based Ana-
1.4 Mixed phase systems prevent reliable representative
lyzer Systems
sampling so system lag times should not be used to predict
D8321 Practice for Development and Validation of Multi-
sample representation of a mixed phase stream.
variate Analyses for Use in Predicting Properties of
1.5 The values stated in inch-pound units are to be regarded
Petroleum Products, Liquid Fuels, and Lubricants based
as standard. Other units of measurement are included in this
on Spectroscopic Measurements
standard and Appendix X1 examples where normally seen in
industry.
3. Terminology
1.6 This standard does not purport to address all of the
3.1 Definitions:
safety concerns, if any, associated with its use. It is the
3.1.1 continuous analyzer unit cycle time, n—the time
responsibility of the user of this standard to establish appro-
interval required to replace the volume of the analyzer mea-
priate safety, health, and environmental practices and deter-
surement cell.
mine the applicability of regulatory limitations prior to use.
3.1.2 intermittent analyzer unit cycle time, n—the time
1.7 This international standard was developed in accor-
interval between successive updates of the analyzer output.
dance with internationally recognized principles on standard-
3.1.3 purge volume, n—the combined volume of the full
analyzer sampling and conditioning systems.
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum
Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-
mittee D02.25 on Performance Assessment and Validation of Process Stream
Analyzer Systems. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Dec. 1, 2021. Published December 2021. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2006. Last previous edition approved in 2016 as D7278 – 16. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D7278-21. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7278 − 21
3.1.4 sample system lag time, n—the time required to Highsampleflowratesinsmallsizedcomponentsystemscause
transport a representative sample from the process tap to the high-pressure drops and low sample transport times. The same
analyzer. flowrate in a larger tubing system will yield significant im-
3.1.4.1 Discussion—This includes sample conditioning unit provements in pressure drop through the system, but will also
lagtimeandsamplelooplagtimedescribedinPracticeD3764. significantly increase the time for sample transport.
6.2.1.1 Users need to perform hydraulic calculations (which
3.1.5 total analyzer system response time, n—the sum of the
are currently outside the scope of this standard) in parallel with
analyzerunitresponsetimeandtheanalyzersamplesystemlag
the lag time calculations to ensure that the “design” flow rates
time.
from a lag time perspective can actually be achieved with the
3.2 Abbreviations:
operating conditions in the field with some contingency for
3.2.1 I.D.—Internal Diameter
operational variations.
3.2.2 LPM—liters per minute
6.2.2 Vapor Samples—Vapor phase sampling is governed
less by pressure drop and more by pressure compression
3.2.3 SLPM—standard liters per minute
properties of gases relative to liquids. In compressible gases
3.2.4 Re—Reynolds Number
the higher the pressure in a given volume, the more sample is
present in that volume. For this reason, and different from
4. Summary
liquids, the selection and location of pressure regulating
4.1 The lag time of an analyzer sample system is estimated
devices in the vapor sample system has a significant impact on
by first determining the flow characteristics. The flow is
the overall system design. The optimal location for a high-
assigned as turbulent or non-turbulent to assign the number of
pressure regulator in a vapor sample not in a fast loop system
purges required to change out the sample. Based on the
is immediately downstream of the sample tap or high-pressure
hardware employed in the sample system an estimation of the
location thereby limiting the volume of the system under high
lag time can be calculated.
pressure. Since the density of a compressible fluid is a function
of the pressure, compressible fluid flow rate calculations are
5. Significance and Use
sometimes done over segmental lengths where average prop-
5.1 The analyzer sample system lag time estimated by this
erties adequately represent the fluid conditions of the line
guide can be used in conjunction with the analyzer output to
segment. Dew point of the vapor sample must be taken into
aid in optimizing control of blender facilities or process units.
account to prevent condensation in the sample transport line.
Aknown and constant lag time is key for the use in optimizing
6.2.3 Liquid to Vapor Samples—A change of phase due to
control.
sample vaporization can also impact the sample lag time. The
5.2 The lag time can be used in the tuning of control volume change from the liquid phase to the vapor phase is
programs to set the proper optimization frequency. substantial. Typical flow rates in gaseous sample lines down-
stream of the vaporizer not in a fast loop can represent very
5.3 The application of this guide is not for the design of a
small liquid feed rates to the vaporizer. Deadheaded sample
sample system but to help understand the design and to
line lengths upstream of the vaporizer can, in turn, represent
estimate the performance of existing sample systems. Addi-
appreciable lag times.
tional detailed information can be found in the references
6.2.4 Phase Separation—This guide is not intended to deal
provided in the section entitled Additional Reading Material.
with dual phase samples as the volume and flow characteristics
are outside the scope.
6. Basic Design Considerations
6.3 Sample Temperature—Temperature also impacts sample
6.1 Acceptable Lag Time—In general, a one to two minute
system lag time but to a lesser degree relative to pressure.
sample system lag time should be maintained to give accept-
Increased temperature of a sample lowers the sample density
able performance. Flow is a key component in the determina-
thus lowering the amount of sample flow needed to purge a
tionofsamplesystemlagtime,andinmostsystemsthedesired
given volume. Temperature impact is generally so small that it
system lag time is impossible to achieve solely with maximum
is ignored in rough estimations of sample system lag time.
allowable sample flow rate to the analyzer. A fast loop or
Temperature is paramount to prevent condensation of a vapor
bypass can be ways to improve lag time by increasing sample
sample.
velocity. A slipstream is taken from the bypass to feed the
analyzer at its optimum flowrate. Excess sample in the slip-
6.4 Typical Sources of Lag Time to Consider:
stream is directed to the process stream, to flare, or vented to
6.4.1 Process Sample Tap:
atmosphere, dependent upon application and regulatory re-
6.4.1.1 Sampletapscanbeasignificantsourceoflagtimeif
quirements.
a sampling probe is not used, need to know the design inside
the sample stream. See Fig. X1.1.
6.2 Physical State of Sample:
6.4.1.2 Sample taps can present a problem for liquid vapor-
6.2.1 Liquid Samples—Pressure drop properties often gov-
izing systems with high volume and low flow on the liquid
ern the design of a liquid system. This is due for the most part
side. See Fig. X1.2.
on the close relationship between pressure drop and system
flowrate and the fixed pressure differential available from the
NOTE 1—This refers to the case where the vaporizing regulator is
process for sample transport. The sizing of the sample compo-
located at the sample tap and one then has a length of liquid filled line
nents is a tradeoff between pressure drop and sample flowrate. from the probe/process interface to the inlet of the vaporizing regulator.
D7278 − 21
This situation can be mitigated by using a sample probe that takes the
7.1.2 Use Reynolds number Re to determine whether the
pressure drop, and subsequent vaporization, at the probe/process interface
sampleflowisturbulentornon-turbulentinaparticularsection
so that one extracts a gaseous sample only. The sensible heat of the bulk
of the sample system.
process stream flowing past the tip of the sample probe provides the
7.1.2.1 Assume turbulent flow for sections with a
energy necessary to vaporize the sample that is extracted.
Re > 4000.
6.4.1.3 Sample tap location is key as well.
7.1.2.2 Assume non-turbulent flow for sections with a
(1) Example—Trying to control a distillation tower with a
Re < 4000.
sampletapafterahydrotreatercan,onitsown,causealagtime
7.1.2.3 Traditionally, the break point Re from laminar flow
of possibly hours.
has been 2100. The region of Re > 2100 to Re < 4000 is a
6.4.2 At-Tap Conditioning:
transition region in that in some systems laminar flow could
6.4.2.1 Filters and Strainers at Sample Stream—Depending
exist while in other systems, at the same Re, eddy formation
on design and size these can add large volumes to a non
and turbulent behavior could be observed.
turbulent sample system.
7.1.3 Record the result, turbulent or non-turbulent, for each
NOTE 2—For filters with diameters greater than the sample tubing
section of the sample system.
diametercalculatetheinternalvolumeandusethe3timesthevolumerule
7.2 Number of Purge Volumes Required:
to account for the delay attributable to the filter. This is because the filter
7.2.1 Assume a single purge volume is sufficient for system
needs to be considered as a mixing chamber.
portions with turbulent flow, Re > 4000. See Figs. X1.4 and
6.4.2.2 Flow or Pressure Regulators—Internal volume of
X1.5.
the regulator(s) are to be included in the system calculati
...

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 the previous version. Because
it 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 version
of the standard as published by ASTM is to be considered the official document.
Designation: D7278 − 16 D7278 − 21
Standard Guide for
Prediction of Analyzer Sample System Lag Times
This standard is issued under the fixed designation D7278; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Lag time, as used in this guide, is the time required to transport a representative sample from the
process tap to the analyzer. Sample system designs have infinite configurations so this guide gives the
user guidance, based on basic design considerations, when calculating the lag time of online sample
delivery systems. Lag time of the analyzer sample system is a required system characteristic when
performing system validation in Practice D3764, D6122or, D6122or D8321 and in general the proper
operation of any online analytical system. The guide lists the components of the system that need to
be considered when determining lag time plus a means to judge the type of flow and need for multiple
flushes before analysis on any sample.
1. Scope*
1.1 This guide covers the application of routine calculations to 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 the process sample tap, tap conditioning, sample transport, pre-analysis
conditioning and analysis.
1.3 Lag times are estimated based on a prediction of flow characteristics, turbulent, non turbulent, or laminar, and the
corresponding purge requirements.
1.4 Mixed phase systems prevent reliable representative sampling so system lag times should not be used to predict sample
representation of the a mixed phase stream.
1.5 The values stated in inch-pound units are to be regarded as standard. No other Other units of measurement are included in this
standard.standard and Appendix X1 examples where normally seen in industry.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.25 on Performance Assessment and Validation of Process Stream Analyzer Systems.
Current edition approved April 1, 2016Dec. 1, 2021. Published April 2016December 2021. Originally approved in 2006. Last previous edition approved in 20112016 as
D7278 – 11.D7278 – 16. DOI: 10.1520/D7278-16.10.1520/D7278-21.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7278 − 21
2. Referenced Documents
2.1 ASTM Standards:
D3764 Practice for Validation of the Performance of Process Stream Analyzer Systems
D6122 Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared
Spectrophotometer, and Raman Spectrometer Based Analyzer Systems
D8321 Practice for Development and Validation of Multivariate Analyses for Use in Predicting Properties of Petroleum
Products, Liquid Fuels, and Lubricants based on Spectroscopic Measurements
3. Terminology
3.1 Definitions:
3.1.1 continuous analyzer unit cycle time—time, n—the time interval required to replace the volume of the analyzer measurement
cell.
3.1.2 intermittent analyzer unit cycle time—time, n—the time interval between successive updates of the analyzer output.
3.1.3 purge volume—volume, n—the combined volume of the full analyzer sampling and conditioning systems.
3.1.4 sample system lag time—time, n—the time required to transport a representative sample from the process tap to the analyzer.
3.1.4.1 Discussion—
This includes sample conditioning unit lag time and sample loop lag time described in Practice D3764.
3.1.5 total analyzer system response time—time, n—the sum of the analyzer unit response time and the analyzer sample system
lag time.
3.2 Abbreviations:
3.2.1 I.D.—Internal Diameter
3.2.2 LPM—liters per minute
3.2.3 SLPM—standard liters per minute
3.2.4 Re—Reynolds Number
4. Summary
4.1 The lag time of an analyzer sample system is estimated by first determining the flow characteristics. The flow is assigned as
turbulent or non-turbulent to assign the number of purges required to change out the sample. Based on the hardware employed in
the sample system an estimation of the lag time can be calculated.
5. Significance and Use
5.1 The analyzer sample system lag time estimated by this guide can be used in conjunction with the analyzer output to aid in
optimizing control of blender facilities or process units. A known and constant lag time is key for the use in optimizing control.
5.2 The lag time can be used in the tuning of control programs to set the proper optimization frequency.
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 the
performance of existing sample systems. Additional detailed information can be found in the references provided in the section
entitled Additional Reading Material.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
D7278 − 21
6. Basic Design Considerations
6.1 Acceptable Lag Time—A In general, a 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 is impossible to achieve solely with maximum allowable sample flow rate to the analyzer. A fast loop or bypass can be
ways to improve lag time by increasing sample velocity. A slipstream 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 stream directed to the process stream,
to flare, or vented to atmosphere, dependent upon application and regulatory requirements.
6.2 Physical State of Sample:
6.2.1 Liquid Samples—Pressure drop properties often govern the design of a liquid system. This is due for the most part on the
close relationship between pressure drop and system flowrate and the fixed pressure differential available from the process for
sample transport. The sizing of the sample components is a tradeoff between pressure drop and sample flowrate. High sample
flowrates in small sized component systems cause high-pressure drops and low sample transport times. The same flowrate in a
larger tubing system will yield significant improvements in pressure drop through the system, but will also significantly increase
the 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 the
lag time calculcationscalculations to ensure that the “design” flow rates from a lag time perspective can actually be achieved with
the operating conditions in the field with some contingency for operational variations.
6.2.2 Vapor Samples—Vapor phase sampling is governed less by pressure drop and more by pressure compression properties of
gases relative to liquids. In compressible gases the higher the pressure in a given volume, the more sample is present in that
volume. For this reason, and different from liquids, the selection and location of pressure regulating devices in the vapor sample
system has a significant impact on the overall system design. The optimal location for a high-pressure regulator in a vapor sample
not in a fast loop system is immediately downstream of the sample tap or high-pressure location thereby limiting the volume of
the system under high pressure. Since the density of a compressible fluid is a function of the pressure, compressible fluid flow rate
calculations are sometimes done over segmental lengths where average properties adequately represent the fluid conditions of the
line segment. Dew point of the vapor sample must be taken into account to prevent condensation in the sample transport line.
6.2.3 Liquid to Vapor Samples—A change of phase due to sample vaporization can also impact the sample lag time. The volume
change from the liquid phase to the vapor phase is substantial. Typical flow rates in gaseous sample lines downstream of the
vaporizer not in a fast loop can represent very small liquid feed rates to the vaporizer. Deadheaded sample line lengths upstream
of the vaporizer can, in turn, represent appreciable lag times.
6.2.4 Phase Separation—This guide is not intended to deal with dual phase samples as the volume and flow characteristics are
outside the scope.
6.3 Sample Temperature—Temperature also impacts sample system lag time but to a lesser degree relative to pressure. Increased
temperature of a sample lowers the sample density thus lowering 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. Temperature is
paramount to prevent condensation of a vapor sample.
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 if a sampling probe is not used, need to know the design inside the
sample 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. See
Fig. X1.2.
NOTE 1—This refers to the case where the vaporizing regulator is located at the sample tap and one then has a length of liquid filled line from the
probe/process interface to the inlet of the vaporizing regulator. This situation can be mitigated by using a sample probe that takes the pressure drop, and
subsequent vaporization, at the probe/process interface so that one extracts a gaseous sample only. The sensible heat of the bulk process stream flowing
D7278 − 21
past the tip of the sample probe provides the energy necessary to vaporize the sample that is extracted.
6.4.1.3 Sample tap location is key as well.
(1) Example—Trying to control a distillation tower with a sample tap after a hydrotreater can, on its own, cause a lag time of
possibly hours.
6.4.2 At-Tap Conditioning:
6.4.2.1 Filters and Strainers at Sample Stream—Depending on design and size these can add large volumes to a non turbulent
sample system.
NOTE 2—For filters with diameters greater than the sample tubing diameter calculate the internal volume and use the 3 times the volume rule to account
for the delay attributable to the filter. This is because the filter needs to be considered as a mixing chamber.
6.4.2.2 Flow or Pressure Regulators—Internal volume of the regulator(s) are to be included in the system calculation.
6.4.3 Vaporizing Regulators—Internal 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 lag
time of the liquid filled slipstream tubing length from a fast loop to a vaporizing regulator can represent very large lag times. See
Fig. 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 into account in the system calculation. This can sometimes be a significant run length
depending on the analyzer location to the process stream.
6.5.1 Sample Conditioning at Analyzer:
6.5.1.1 Filtering—Depending on their design and size, filters can add large volumes to a non turbulent sample system. See Note
2.
6.5.1.2 Other Components—Any other components in the conditioning system to the analyzer need to be taken into account.
7. Procedure
7.1 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.
Re 5 @~I.D.!*~Velocity!*~Density!#/Viscosity (1)
NOTE 3—Various forms of this equation exist for different units.
7.1.2 Use Reynolds numb
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