ASTM D6621-21

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of the standard as published by ASTM is to be considered the official document.
Designation: D6621 − 00 (Reapproved 2017) D6621 − 21
Standard Practice for
Performance Testing of Process Analyzers for Aromatic
Hydrocarbon Materials
This standard is issued under the fixed designation D6621; 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.
1. Scope Scope*
1.1 This practice serves as a practical guide for the performance testing of process stream analyzers specifically for measuring
chemical or physical characteristics of liquid aromatic hydrocarbon materials for production or certification of these materials. The
practice may be applicable to other hydrocarbon stream analyzers as well.
1.2 Only external methods (complete substitution of the process stream with a standard) of control sample introduction are
included. Internal methods are beyond the scope of this practice.
1.3 Methods for resetting key operational parameters of analyzers to match predefined limits are provided by vendors and are not
included in this practice.
1.4 Analyzer validation procedures are covered in Practices D3764 and D6122, not in this practice.
1.5 Procedures for statistically interpreting data from automatic sampling process stream analyzers are outlined.
1.6 The implementation of this practice requires that the analyzer be installed according to APIRP-550 (1), and be in agreement
with the analyzer supplier’s recommendations. Also, it assumes that the analyzer is designed to monitor the specific material
parameter of interest, and that at the time of initial or periodic validation, the analyzer was operating at the conditions specified
by the manufacturer and consistently with the primary test method.
1.7 The units of measure used in this practice shall be the same as those applicable to the test primary method used for analyzer
validation.
1.8 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.
1.9 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 practice is under the jurisdiction of ASTM Committee D16 on Aromatic, Industrial, Specialty and Related Chemicals and is the direct responsibility of Subcommittee
D16.09 on On-Line Analysis.
Current edition approved July 1, 2017Nov. 1, 2021. Published July 2017December 2021. Originally approved in 2000. Last previous edition approved in 20122017 as
D6621 – 00 (2012).(2017). DOI: 10.1520/D6621-00R17.10.1520/D6621-21.
The boldface numbers in parentheses refer to the list of references at the end of this practice.
*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
D6621 − 21
2. Referenced Documents
2.1 ASTM Standards:
D3437 Practice for Sampling and Handling Liquid Cyclic Products
D3438 Practice for Sampling and Handling Naphthalene, Maleic Anhydride, and Phthalic Anhydride
D3764 Practice for Validation of the Performance of Process Stream Analyzer Systems
D4177D3852 Practice for Automatic Sampling of Petroleum and Petroleum Productsand Handling Phenol, Cresols, and Cresylic
Acid
D6122 Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared
Spectrophotometer, and Raman Spectrometer Based Analyzer Systems
E456 Terminology Relating to Quality and Statistics
E1655 Practices for Infrared Multivariate Quantitative Analysis
3. Terminology
3.1 Definitions:
3.1.1 accuracy, n—closeness of agreement between a test result and an accepted reference value.
3.1.2 analyzer output, n—signal that is proportional to the quality parameter being measured and suitable for input to readout
instrumentation.
3.1.2.1 Discussion—
It may be pneumatic, electrical, digital, etc., and expressed as psi, mv, sec., etc.
3.1.3 analyzer result, n—numerical estimate of a physical, chemical, or quality parameter produced by applying the calibration
model to the analyzer output signal.
3.1.4 bias, n—the difference between the expectation of the results and an accepted reference value.
3.1.5 control sample, n—material similar to the process stream that is stable over long periods of time so that its parameters may
be measured reproducibly in performance tests to characterize analyzer precision and accuracy.
3.1.5.1 Discussion—
May be a pure compound, standard mixture, or a sample from the process stream. Its parameters are used to plot statistical process
control charts to define analyzer precision in normal operation.
3.1.6 external performance testing, n—procedure involving complete substitution of the process/product stream measured by the
analyzer with a control sample stream to measure the analyzer’s precision and possibly accuracy (if the control sample’s true value
is known).
3.1.7 internal performance testing, n—procedure involving the addition of a known quantity of a standard material homoge-
neously into the process/product stream measured by the analyzer to measure the analyzer’s precision and possibly accuracy (if
the sample material’s true value is known).
3.1.8 linearity, n—parameter ranges where the analyzer’s results do and do not approximate a straight line.
3.1.9 performance testing of an analyzer, n—mechanical and statistical procedure for routinely checking the accuracy and
precision of an analyzer’s results against historical accuracy and precision for a control sample.
3.1.10 precision, n—closeness of agreement of independent test results of the same chemical or physical property of a given
material obtained under stipulated conditions.
3.1.10.1 Discussion—
Expressed in terms of dispersion of test results around the arithmetic mean, usually as variance, standard deviation, repeatability
or reproducibility, or both.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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3.1.11 repeatability of an analyzer, n—difference between two successive analyzer results measured in a short time interval that
would be exceeded in the long run in only 1 case in 20 (5 % of the time) when the analyzer is operated on a flowing sample of
uniform quality.
3.1.12 reproducibility of an analyzer, n—difference between a single result from each of two identical analyzer systems that would
be exceeded in the long run in only 1 case in 20 (5 % of the time) when the two systems are operated at different sites by different
operators, but on identical samples.
3.1.13 rule violation, n—condition when a point value or pattern of points in a statistical process control chart statistically exceeds
the defined probability of its occurrence, as defined by the Western Electric rules (2) being used.
3.1.14 spot sample, n—representative material resembling the stream being monitored, an identical portion of which is analyzed
both in a process analyzer and by a laboratory test on a non-scheduled basis for periodic validation testing.
3.1.14.1 Discussion—
May be the same material as the control sample.
3.1.15 validation of an analyzer, n—process to identify how comparable an analyzer’s results are statistically to results from the
primary method, or to define how the analyzer’s results compare to the primary method’s results in precision and accuracy.
3.1.15.1 Discussion—
Must be done when the analyzer is first configured or reconfigured (initial validation), and then on a periodic basis (periodic
validation), as described in Practice D3764.
3.2 For additional definitions, see Appendix X1.
4. Summary of Practice
4.1 This practice standardizes aromatic hydrocarbon process-analyzer performance testing practices, or processes for maintaining
accurate and precise analyzer measurements. It is used with methods for the measurement and certification of aromatic
hydrocarbon materials applied to continuous on-line analyzers. These methods are generally under the control of Committee D16
on Aromatic, Industrial, Specialty and Related Chemicals. It is meant as a practical guide for persons setting up and maintaining
these analyzers in a process (non-laboratory) environment. They should apply it, with their knowledge of the analyzer’s operation
and of how the process analyzer results are to be used, to maintain and optimize analyzer operation.
5. Significance and Use
5.1 Performance testing of on-line analyzers is critical to their proper performance within predictable levels of precision and
accuracy. This practice can affect production efficiency and certification of aromatic hydrocarbon materials.
6. System Components
6.1 Process analyzers (Fig. 1) for measuring the chemical composition of aromatic hydrocarbons, their purity, or physical
properties often replace existing laboratory test methods, using the same or similar chemical measurement techniques. Fig. 1 shows
several possible analyzer configurations for on-line process testing of aromatic hydrocarbon materials. Aromatic hydrocarbon
stream analyzers are often based on chromatography, but they may also perform physical measurements, wet chemistry, or other
methods described in new or existing Committee D16 methods. This practice is intended to be generally applicable to any of them.
7. Performance Guidelines Before Calibration
7.1 At startup, validate any process analyzer against an existing analytical method, typically in this case, one overseen by
Committee D16.
7.2 The capability measurement (c ) for a given analyzer (3) shall be less than 0.2, as defined in Eq 1:
m
2 2
c 5 σ /σ ,0.2 (1)
m a p
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FIG. 1 Possible Process Analyzer Configurations
where:
σ = standard deviation of the analyzer measurement, and
a
σ = standard deviation of the process.
p
The variance (standard deviation squared) of the analyzer should be less than 20 % of the variance of the process, so that the
analyzer measurement can be useful for detecting changes in the process. The expected capability for a process analyzer
measurement may be available from the vendor for a specific application before installation of the analyzer (advertised analyzer
capability). Actual process stream measurement capability should be measured on the process/product stream, usually after initial
analyzer validation.
7.3 Automated analyzer sampling practices for aromatic hydrocarbon liquid streams shall follow those referenced in Practice-
Practices D4177D3437, D3438, or D3852.
7.4 Determine the linearity of the process analyzer by using at least three calibration standard materials with known
compositions/responses for the components of interest. Each component should be present at a high, low, and medium
concentration/amount level with respect to the concentration/amount range expected for the parameter (analyzer operating range).
A plot of the component concentration/amount versus analyzer response will determine if the analyzer has a linear response over
the concentration range of interest. If analyzer response is nonlinear, additional calibration standards must be analyzed to clearly
determine the nonlinear behavior of each analyzer and component, if the analyzer is to be used in the nonlinear range.
7.5 If a process analyzer is to be used only for trend information, the data generated by it is in a form that does not impart
compositional information, but relative information only, that is, peak area, peak height, counts, millivolts, etc. Initial validation
and frequent performance testing are still required to define precision, as well as to maintain proper analyzer operation.
8. Performance Test Procedure
8.1 Determine analyzer performance using external check samples, which are substituted for the process material stream during
performance test runs.
8.2 Process analyzers are routinely performance tested by using control samples. These may be primary or secondary standard
materials, or actual portions from the process stream. These portions must be representative of normal process conditions, and be
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stored to remain physically and chemically stable over time. The control sample should be repeatedly analyzed by the process
analyzer, and then using statistical process control (SPC), to define the actual analyzer result’s precision.
8.3 Analyzer performance test frequency can be done at a fixed time interval, based on analyzer reliability and operator
experience. Typically, once per shift, day, or week are used, but it may be more or less frequent. Unscheduled control sample
analyses may be performed whenever the unit operator feels that something has changed in the process or process analyzer, or at
a convenient time.
8.4 The control sample material container shall be located at a point in the process to allow for its simple and regular introduction
into the process analyzer’s sample introduction system (if appropriate) by the process operator. A sufficient quantity must be
available for many repetitive analyses.
8.5 Perform an external analyzer performance test by switching the analyzer sample source from the process stream to the control
sample, followed by sampling and analysis of the control sample.
8.6 Monitor the analyzer’s output from the control sample until it stabilizes, that is, the difference between successive readings
is at or below the repeatability of the analyzer,analyzer (which is measured as described in 8.10). If this does not occur, the
repeatabili
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