ISO-6974-2-2001 Natural gas.pdf

Reference number ISO 6974-2:2001(E) ©ISO 2001 INTERNATIONAL STANDARD ISO 6974-2 First edition 2001-02-01 Natural gas Determination of composition with defined uncertainty by gas chromatography Part 2: Measuring-system characteristics and statistics for processing of data Gaz naturel Détermination de la composition avec une incertitude définie par chromatographie en phase gazeuse Partie 2: Caractéristiques du système de mesure et statistiques pour le traitement des données ISO 6974-2:2001(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this area. Adobe is a trademark of Adobe Systems Incorporated. Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below. ©ISO 2001 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's member body in the country of the requester. ISO copyright office Case postale 56 ? CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.ch Web www.iso.ch Printed in Switzerland ii© ISO 2001 All rights reserved -,-,- ISO 6974-2:2001(E) © ISO 2001 All rights reservediii ContentsPage Foreword.iv Introduction.v 1Scope 1 2Normative references1 3Terms and definitions .1 4Symbols and subscripts .2 4.1Symbols2 4.2Subscripts 3 5Procedure.4 5.1Step 1: Determination of the optimal response functions 4 5.2Step 2: Selection of the response functions to be used in practice 8 5.3Step 3: Routine calibration with a working-reference gas mixture10 5.4Step 4: Calculation of non-normalized component mole fractions10 5.5Step 5: Calculation of non-normalized component standard deviations 10 5.6Step 6: Calculation of normalized component mole fractions12 5.7Step 7: Calculation of normalized component standard deviations12 5.8Step 8: Calculation of uncertainty and repeatability of the results13 5.9Relationship between ISO 6974-2 and ISO 6976.13 Annex A (informative) Critical values of t .14 Annex B (informative) Example .15 Annex C (informative) Relative response factors (K) 23 Bibliography24 -,-,- ISO 6974-2:2001(E) iv© ISO 2001 All rights reserved Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this part of ISO 6974 may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. International Standard ISO 6974-2 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis of natural gas. This part as well as the other five parts of ISO 6974 cancel and replace ISO 6974:1984 which specified only one method. ISO 6974 consists of the following parts, under the general title Natural gas Determination of composition with defined uncertainty by gas chromatography: Part 1: Guidelines for tailored analysis Part 2: Measuring-system characteristics and statistics for processing of data Part 3: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C8using two packed columns Part 4: Determination of nitrogen, carbon dioxide and C1to C5and C6+hydrocarbons for a laboratory and on-line measuring system using two columns Part 5: Determination of nitrogen, carbon dioxide and C1to C5and C6+hydrocarbons for a laboratory and on-line process application using three columns Part 6: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and hydrocarbons up to C8using three capillary columns Annexes A, B and C of this part of ISO 6974 are for information only. -,-,- ISO 6974-2:2001(E) © ISO 2001 All rights reservedv Introduction This part of ISO 6974 describes the data processing for the “tailored” analysis of natural gas. This part is to be used in conjunction with part 1 of ISO 6974 which gives the guidelines for “tailored” analysis. Any method of analysis, either one of those in part 3 of ISO 6974 and subsequent parts or another method of choice can only be applied in conjunction with parts 1 and 2 of ISO 6974. The calculation of the composition of the gas using response curves, a working-reference gas mixture and relative response factors are described in part 1 of ISO 6974, while the elaboration of these formulae is described in this part of ISO 6974. The working-reference gas mixture and the gas sample are analysed with the same analytical system under the same set of conditions. Components not measured by this method will influence the accuracy of the method and should therefore be known. If no working-reference gas mixture is used for the regular calibration of the analytical system, a number of equations given in this part of ISO 6974 will change. Such changes are indicated for each of these equations. Once the working ranges of the components have been defined, an evaluation is carried out to determine whether components are to be considered as: ?main components or groups of components to be analysed using direct measurement (directly measured components); ?components or groups of components to be analysed using indirect measurement, as a function of a different, reference component in the calibration gas (indirectly measured components); ?components that are not measured and whose mole fraction can be assumed to be constant (components not measured). The sum of the mole fractions of the main components, the indirect components and the fixed components is equal to 1. INTERNATIONAL STANDARDISO 6974-2:2001(E) © ISO 2001 All rights reserved1 Natural gas Determination of composition with defined uncertainty by gas chromatography Part 2: Measuring-system characteristics and statistics for processing of data 1Scope This part of ISO 6974 describes the data processing for the tailored analysis of natural gas. It includes the determination of the measuring system characteristics and the statistical approach to data handling and error calculation with the aim of defining the uncertainty in the mole fractions of the component measured. This part of ISO 6974 is only applicable in conjunction with part 1 of ISO 6974. 2Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of this part of ISO 6974. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this part of ISO 6974 are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards. ISO 6974-1, Natural gas Determination of composition with defined uncertainty by gas chromatography Part 1: Guidelines for tailored analysis. ISO 6976, Natural gas Calculation of calorific values, density, relative density and Wobbe index from composition. 3Terms and definitions For the purposes of this part of ISO 6974, the following terms and definitions apply. 3.1 response output signal of the measuring system for a component that is measured as peak area or peak height NOTEResponse is expressed in counts. 3.2 uncertainty estimate attached to a measurement result which characterizes the range of values within which the true value is asserted to lie NOTEIn general, the uncertainty of measurements comprises many components. Some of these components may be estimated on the basis of statistical distribution of the results of series of measurements and can be characterized by experimental standard deviation. The estimates of other components can only be based on experience or other information. -,-,- ISO 6974-2:2001(E) 2© ISO 2001 All rights reserved 3.3 certified-reference gas mixture CRM mixture which is used for the determination of the response curves of the measuring system NOTECertified-reference gas mixtures may be prepared gravimetrically in accordance with ISO 61421or ISO 132752or certified and validated by comparison with primary standard gas mixtures of closely related composition in accordance with ISO 61433(see ISO 141114). 3.4 working-reference gas mixture WRM mixture which is used as a working standard for regular calibration of the measuring system NOTEWorking-reference gas mixtures may be prepared by a gravimetric method in accordance with ISO 61421or certified and validated by comparison with CRM of closely related composition in accordance with ISO 61433. 3.5 direct measurement measurement by which individual components and/or groups of components are determined by comparison with identical components in the working-reference gas mixture 3.6 indirect measurement measurement by which individual components and/or groups of components which are themselves not present in the working-reference gas mixture are determined using relative response factors to a reference component in the working-reference gas mixture 4Symbols and subscripts 4.1Symbols ai,bi,ci,dipolynomial coefficients of the component i amc,bmc,etc.polynomial coefficients of the components or groups of components to be analysed using direct measurement as a function of identical components or groups in the calibration gas hsnumber of replicates of the sample hwrmnumber of replicates of the working-reference gas mixture Krelative response factor with respect to the reference component morder of the response function fitted ntotal number of analyses used in the regression qnumber of direct components plus number of indirect components Rinstrument response, expressed as raw data, e.g. area counts or peak height Rfresponse factor (based on a single point calibration) sMSEresidual mean square (MSE) sMSRmean square due to regression (MSR) -,-,- ISO 6974-2:2001(E) © ISO 2001 All rights reserved3 2 SSE sresidual sum of squares (SSE) 2 SSR ssum of squares due to regression (SSR) ( ) s xstandard deviation of the predicted value s(x)standard deviation of a normalized mole fraction s(x?)standard deviation of a non-normalized mole fraction s(xc,WRC)standard deviation of a mole fraction in the WRM (from the certificate of WRM) s(R)standard deviation of the responses at one mole fraction level Tithe difference in slope between the tangent of the optimal response function inRwr,iand calibration line based on the single point method of the WRM only treduced variable of thet-distribution xc,wrmmole fraction of the WRM (given on the certificate) x predicted value of the mole fraction according to a response function x?non-normalized mole fraction xnormalized mole fraction xL,wrlowest mole fraction in the working range xU,wrhighest mole fraction in the working range ?number of degrees of freedom 4.2Subscripts ccertified icomponent i jmole fraction level j hnumber of replicates h Llowest mcmain components or groups of components to be analysed using direct measurement as a function of identical components or groups in the calibration gas MSEresidual mean square MSRmean square due to regression ocother components that are not measured and/or can be regarded as being present at a constant mole fraction ISO 6974-2:2001(E) 4© ISO 2001 All rights reserved rrfcomponents or groups of components to be analysed using indirect measurement as a function of a different reference component in the calibration gas refreference component SSEresidual sum of squares SSRsum of squares due to regression ssample Uhighest (upper limit) wrworking range wrmworking-reference gas mixture 5Procedure 5.1Step 1: Determination of the optimal response functions 5.1.1General remarks Detector response functions are determined using certified reference gas mixtures (CRMs). For each of the direct components, select the most optimal detector response function which gives the best relationship between response signal and component quantity of the CRM. Carry out this step in the following cases: ?initial installation of the system by the supplier; ?return to operation after a maintenance of the system; ?return to operation after replacement of a major part of the system, e.g. injection valve, column, detector; ?return to operation after failure to pass a routine check of the system using test gas(es). NOTEIt may be necessary to determine the best statistical curve fit of the results. Although quadratic and cubic curves may give a better fit to the data, for a series of gases this may not be feasible. It is more important to obtain consistent results for the different gases. In practice, this consistency can often be obtained by simply drawing the relative-response (= absolute response/mole fraction) versus mole-fraction curves. 5.1.2Selection of reference gases To define the response functions, different mole fraction levels of the direct components are analysed. The number of levels needed depends on the order of the response curve that has to be fitted. The minimum number of levels needed is the same as the number of polynomial coefficients (see below) in the