ASTM D7278-06
(Guide)Standard Guide for Prediction of Analyzer Sample System Lag Times
Standard Guide for Prediction of Analyzer Sample System Lag Times
SIGNIFICANCE AND USE
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.
The lag time can be used in the tuning of control programs to set the proper optimization frequency.
The application of this guide is not for the design of a sample system but to estimate the performance of existing sample systems. The principles listed in this guide are there to help understand design concepts and allow the application of this guide for its intended scope. 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, nonturbulent, 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 stream.
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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:D7278–06
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 on-line sample
delivery systems. Lag time of the analyzer sample system is a required system characteristic when
performing system validation in Practice D3764 or D6122 and in general the proper operation of any
on-line 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 D6122 Practice for Validation of the Performance of Mul-
tivariate Process Infrared Spectrophotometer Based Ana-
1.1 This guide covers the application of routine calculations
lyzer Systems
to estimate sample system lag time, in seconds, for gas, liquid,
and mixed phase systems.
3. Terminology
1.2 This guide considers the sources of lag time from the
3.1 Definitions:
process sample tap, tap conditioning, sample transport, pre-
3.1.1 continuous analyzer unit cycle time—the time interval
analysis conditioning and analysis.
required to replace the volume of the analyzer measurement
1.3 Lag times are estimated based on a prediction of flow
cell.
characteristics, turbulent, nonturbulent, or laminar, and the
3.1.2 intermittent analyzer unit cycle time—the time inter-
corresponding purge requirements.
val between successive updates of the analyzer output.
1.4 Mixed phase systems prevent reliable representative
3.1.3 purge volume—the combined volume of the full
sampling so system lag times should not be used to predict
analyzer sampling and conditioning systems.
sample representation of the stream.
3.1.4 sample system lag time—thetimerequiredtotransport
1.5 This standard does not purport to address all of the
a representative sample from the process tap to the analyzer.
safety concerns, if any, associated with its use. It is the
3.1.5 system response time—the sum of the analyzer unit
responsibility of the user of this standard to establish appro-
response time and the analyzer sample system lag time.
priate safety and health practices and determine the applica-
3.2 Abbreviations:
bility of regulatory limitations prior to use.
3.2.1 I.D.—Internal Diameter
2. Referenced Documents 3.2.2 Re—Reynolds Number
2.1 ASTM Standards:
4. Summary
D3764 Practice for Validation of the Performance of Pro-
4.1 The lag time of an analyzer sample system is estimated
cess Stream Analyzer Systems
by first determining the flow characteristics. The flow is
assigned as turbulent or non-turbulent to assign the number of
This guide is under the jurisdiction of ASTM Committee D02 on Petroleum
purges required to change out the sample. Based on the
Products and Lubricants and is the direct responsibility of Subcommittee D02.25 on
hardware employed in the sample system an estimation of the
Performance Assessment and Validation of Process Stream Analyzer Systems for
lag time can be calculated.
Petroleum and Petroleum Products.
Current edition approved July 1, 2006. Published August 2006. DOI: 10.1520/
5. Significance and Use
D7278-06.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.1 The analyzer sample system lag time estimated by this
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
guide can be used in conjunction with the analyzer output to
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. aid in optimizing control of blender facilities or process units.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D7278–06
5.2 The lag time can be used in the tuning of control 6.2.4 Phase Separation—This guide is not intended to deal
programs to set the proper optimization frequency. with duel phase samples as the volume and flow characteristics
5.3 The application of this guide is not for the design of a are outside the scope.
sample system but to estimate the performance of existing 6.3 Sample Temperature—Temperaturealsoimpactssample
sample systems. The principles listed in this guide are there to system lag time but to a lesser degree than pressure. Increased
help understand design concepts and allow the application of temperature of a sample lowers the sample density thus
this guide for its intended scope. Additional detailed informa- lowering the amount of sample flow needed to purge a given
tion can be found in the references provided in the section volume. Temperature impact is generally so minute that it is
entitled Additional Reading Material. ignored in rough estimations of sample system lag time.
6.4 Typical Sources of Lag Time to Consider:
6. Basic Design Considerations
6.4.1 Process Sample Tap:
6.1 Acceptable Lag Time—As a general rule, a one to two
6.4.1.1 Sample taps can be a significant sourceoflagtimeif
minute sample system lag time should be maintained where
a sampling probe is not used, need to know the design inside
possible to give acceptable performance. Flow is a key
the sample stream. See Fig. X1.1.
component in the determination of sample system lag time. In
6.4.1.2 Sample taps can present a problem for liquid vapor-
most systems the desired system lag time is impossible to
izing systems with high volume and low flow on the liquid
achieve with maximum allowable sample flow rate to the
side. See Fig. X1.2.
analyzer.To improve lag time a fast loop or bypass can be used
NOTE 1—This refers to the case where the vaporizing regulator is
to increase sample velocities through the system to a point just
located at the sample tap and one then has a length of liquid filled line
upstream of the analyzer.Aslipstream is taken from the bypass
from the probe/process interface to the inlet of the vaporizing regulator.
to feed the analyzer at its optimum flowrate. Excess sample in
This situation can be mitigated by using a sample probe that takes the
theslipstreamisventedtoatmosphere,toflareortotheprocess pressure drop, and subsequent vaporization, at the probe/process interface
so that one extracts a gaseous sample only. The sensible heat of the bulk
stream dependant upon application and regulatory require-
process stream flowing past the tip of the sample probe provides the
ments.
energy necessary to vaporize the sample that is extracted.
6.2 Physical State of Sample:
6.4.2 At-Tap Conditioning:
6.2.1 Liquid Samples—Pressure drop properties often gov-
ern the design of a liquid system. This is due for the most part 6.4.2.1 Filters and Strainers at Sample Stream—Depending
on the close relationship between pressure drop and system on design and size these can add large volumes to the sample
flowrate and the fixed pressure differential available from the system that may not be turbulent flow.
process for sample transport. The sizing of the sample compo-
NOTE 2—For filters with diameters greater than the sample tubing
nents is a tradeoff between pressure drop and sample flowrate.
diameter calculate the internal volume and use the 3 times the volume rule
Highsampleflowratesinsmallsizedcomponentsystemscause
to account for the delay attributable to the filter.
high-pressure drops and low sample transport times. The same
6.4.2.2 Flow or Pressure Regulators—Internal volume of
flowrate in a larger tubing system will yield significant im-
the regulator(s) needs to part of the system calculation.
provements in pressure drop through the system, but will also
6.4.3 Vaporizing Regulators—Internal volume of the regu-
increase the time for sample transport significantly.
lator needs to part of the system calculation.
6.2.2 Vapor Samples—Vapor phase sampling is governed
6.4.3.1 The volume change from a liquid to a gas is on the
less by pressure drop and more by pressure compression
order of 300 to 600 volumes of gas per volume of liquid so the
properties of gases when compared to liquids. In compressible
lag time of the liquid filled slipstream tubing length from a fast
gases the higher the pressure in a given volume, the more
loop to a vaporizing regulator can represent very large lag
sample present in that volume. For this reason, and different
times. See Fig. X1.3.
from liquids, the selection and location of pressure regulating
6.4.3.2 A system designed on the basis of a good gas
devices in the vapor sample system has a great impact on the
volumetricflowratecanrepresentaverysmallliquidflowrate.
overall system design.The optimal location for a high-pressure
6.5 Sample delivery tubing all needs to be taken into
regulator in a vapor sample is immediately downstream of the
account and this can sometimes be a significant run length
sample tap or high-pressure location thereby limiting the
depending on the analyzer location to the process stream.
volume of the system under high pressure. Since the density of
6.5.1 Sample Conditioning at Analyzer:
a compressible fluid is a function of the pressure, compressible
6.5.1.1 Filtering—Depending on design and size filters can
fluid flow rate calculations are sometimes done over segmental
add large volumes to the sample system that may not be
lengths where average properties adequately represent the fluid
turbulent flow. See Note 2.
conditions of the line segment.
6.2.3 Liquid to Vapor Samples—A change of phase due to
7. Procedure
sample vaporization can impact the sample lag time. The
7.1 Determination of Flow Characteristics:
volume change from the liquid phase to the vapor phase is
7.1.1 Calculate the Reynolds number, Re, of each section of
substantial. Typical flow rates in gaseous sample lines down-
the sample system using the tubing / pipe internal diameter
stream of the vaporizer can represent very small liquid feed
(I.D.), the flow velocity, density of the sample stream, and
rates to the vaporizer. Deadheaded sample line lengths up-
viscosity of the sample stream.
stream of the vaporizer can, in turn, represent appreciable lag
times. Re 5 [~I.D.!3~Velocity!3~Density!# / Viscosity (1)
D7278–06
NOTE 3—Various forms of this equation exist for different units. NOTE 4—Three purge volumes are probably
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