1、BRITISH STANDARD CONFIRMED JUNE1998 BS5857-2.4: 1980 ISO4053-IV: 1978 Methods for measurement of Fluid flow in closed conduits, using tracers Part2: Measurement of gas flow Section2.4 Transit time method using radioactive tracers ISO title: Measurement of gas flow in conduits Tracer methods PartIV:
2、Transit time method using radioactive tracers UDC 532.542:532.574.8:533.6.011:539.16BS5857-2.4:1980 This BritishStandard, having been prepared under the directionof the Industrial-process Measurementand Control Standards Committee, was published under the authority ofthe Executive Board and comesint
3、o effect on 31March1980 BSI12-1999 The following BSI references relate to the work on this standard: Committee referencePCL/2 Draft for comment77/28096DC ISBN 0 580 11250 0 Cooperating organizations The Industrial-process Measurement and Control Standards Committee, under whose direction this Britis
4、hStandard was prepared, consists of representatives from the following Government department and scientific and industrial organizations: British Gas Corporation British Industrial Measuring and Control Apparatus Manufacturers Association British Steel Corporation CBMPE (Council of British Manufactu
5、rers of Petroleum Equipment) Control and Automation Manufacturers Association (BEAMA) Department of Industry (Computers Systems and Electronics) Electrical, Electronic, Telecommunications and Plumbing Union Electricity Supply Industry in England and Wales* Electronic Engineering Association Engineer
6、ing Equipment Users Association* Institute of Measurement and Control Institution of Electrical Engineers Institution of Gas Engineers Oil Companies Materials Association Post Office Engineering Union Scientific Instrument Manufacturers Association Sira Institute The organizations marked with an ast
7、erisk in the above list, together with the following, were directly represented on the committee entrusted with the preparation of this BritishStandard: Department of Energy Gas Standards UnitedKingdom (Atomic Energy Authority) Amendments issued since publication Amd. No. Date of issue CommentsBS585
8、7-2.4:1980 BSI 12-1999 i Contents Page Cooperating organization Inside front cover National foreword ii 0 Introduction 1 1 Scope and field of application 1 2 Reference 1 3 Principle 1 4 Required conditions 1 5 Choice of measuring length 1 6 Procedure 3 7 Selection of tracer 5 8 Estimation of the unc
9、ertainties in flow-rate measurement 5 9 Example of flow-rate calculation 6 Figure 1 Effect of measuring length of straight pipe on ratio of meantransittime to dispersion of tracer 8 Figure 2 Methods for determining characteristic points 9 Table Most commonly used isotopes 6 Publications referred to
10、Inside back coverBS5857-2.4:1980 ii BSI 12-1999 National foreword This BritishStandard has been prepared under the direction of the Industrial-process Measurement and Control Standards Committee and is identical with ISO4053-IV:1978 “Measurement of gas flow in conduits Tracer methods PartIV: Transit
11、 time method using radioactive tracers” published by the International Organization for Standardization (ISO). Terminology and conventions. The text of the International Standard has been approved as suitable for publication, without deviation, as a BritishStandard. Some terminology and certain conv
12、entions are not identical with those used in BritishStandards; attention is especially drawn to the following. The comma has been used throughout as a decimal marker. In BritishStandards it is current practice to use a full point on the baseline as the decimal marker. Wherever the words “Internation
13、al Standard” appear, referring to this standard, they should be read as “BritishStandard”. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not
14、of itself confer immunity from legal obligations. Cross-reference International Standard Corresponding BritishStandard ISO4053-I:1977 BS5857 Methods for measurement of fluid flow in closed conduits, using tracers Part2 Measurement of gas flow Section2.1:1980 General (Identical) Summary of pages This
15、 document comprises a front cover, an inside front cover, pagesi andii, pages1 to10, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS5857-2.4
16、:1980 BSI 12-1999 1 0 Introduction This International Standard is the fourth of a series of standards covering tracer methods of gas flow measurement in conduits. The complete series of standards is as follows: PartI: General; PartII: Constant rate injection method using non-radioactive tracers; Par
17、tIII: Constant rate injection method using radioactive tracers; PartIV: Transit time method using radioactive tracers. 1 Scope and field of application This International Standard specifies the transit time method using radioactive tracers for the measurement of gas flow in conduits. 2 Reference ISO
18、4053-I, Measurement of gas flow in conduits Tracer methods PartI: General. 3 Principle Flow-rate measurement by the transit time method (formerly called the “Allen velocity method”) is based on measuring the transit time of “labelled” particles between two cross-sections of the conduit a known dista
19、nce apart. Labelling of the fluid particles is achieved by injecting a tracer into the flow upstream of the two measurement cross-sections (i.e.detector positions) and the transit time is determined from the difference of the mean arrival times of the tracer at each of the detector positions. Under
20、certain conditions (see clause4), the volume flow-rate q v(see nomenclature in ISO4053-I) in the measurement section is given by: where V is the volume of the conduit between the detector positions; is the transit time of the labelled fluid particles. In general, the theoretical condition for the va
21、lidity of the formula is that the measuring section be “closed to diffusion” i.e.that the ratio of the local velocity to the longitudinal dispersion coefficient be equal at both ends of the measuring section. In practice, this condition is fulfilled when the conduit has a constant cross-section. The
22、 value ofis obtained by measuring the difference in abscissae of characteristic points (in theory, the centre of gravity, but in practice other characteristic points may be found, see6.7) of recorded distributions, corresponding to concentration/time distributions or their integrals, obtained at eac
23、h detector position. The signal from the detectors shall be proportional to the tracer concentration. The exact value of the proportionality constant and hence the concentration need not, however, be known. The mass flow-rate is calculated by determining simultaneously the volume flow-rate and the g
24、as density. 4 Required conditions 4.1 Tracer The tracer shall meet the general requirements defined in clause5 of ISO4053-I. A list of tracers generally used, with their advantages and disadvantages, is also given in the same clause. 4.2 Mixing of tracer The tracer shall be sufficiently mixed with t
25、he flow at the first detector position for the recorded concentration/time distributions at both detectors to be adequately representative of the mean flow (see5.1). The selection of the positions for the injection and the detectors is controlled by the fluid velocity, tracer dispersion, and the con
26、duit layout. The conditions for this selection are dealt with in clause5. At low Reynolds number, Reu5000, the mixing of tracer is not sufficient and no reliable measurement can be made. 4.3 Test procedure The procedure for the preparation and the injection of the tracer gas (which in practice shoul
27、d be injected as rapidly as possible to minimize the longitudinal dispersion of the tracer) is covered in6.3 and6.4. The internal volume of the measuring section shall be determined with sufficient accuracy (see5.7). Other requirements relating to the tests and the calculation of the transit time fr
28、om the data are given in clause6. 5 Choice of measuring length In the transit time method, the measuring length consists of two parts: the length of conduit between the injection point and the position of the first detector; the length of conduit between detectors. q v V t - = t tBS5857-2.4:1980 2 B
29、SI 12-1999 5.1 Length of conduit between injection and first detector In theory, when the tracer concentration, C 2 , in the conduit is measured at only a single point in each measurement cross-section, the length of conduit between the injection and first detector shall be equal to or greater than
30、the mixing distance. The mixing distance is defined as the shortest distance at which the maximum variation ofdt over the cross-section is less than some predetermined value (for example0,5%). (See clause5 of ISO4053-I.) There are, however, insufficient experimental results available to relate varia
31、tions indt at the first detector position, to the overall accuracy of transit time as determined from concentration measurements at single points in the measurement cross-sections. If the measurement of concentration at each detector position represents the mean concentration in the cross-section (f
32、or example by simultaneous measurements at many points or by a detector sensitive to tracer across the cross-section), the degree of mixing required at the first detector position is not as great as that corresponding to the mixing distance. In these circumstances the necessary distance between the
33、injection position and the first detector position can be considerably less than the mixing distance. For example, when using a -emitting tracer centrally injected into a conduit and detecting by three scintillation detectors positioned at each measurement cross-section, and when the distance betwee
34、n injection and the first detector is only12 conduit diameters, no additional error in flow-rate measurement has been observed in practice. The length of conduit between the injection position and the first detector should preferably contain no pipe fittings or sections likely to increase significan
35、tly the longitudinal dispersion of tracer at the detector positions. Examples of such fittings and sections are valves, flow regulators and flow distribution headers. 5.2 Length of conduit between detector positions The length of conduit necessary between the detector positions depends on the axial
36、velocity of the fluid, the spatial dispersion of the tracer at the detector positions and the required accuracy of the measurement of transit time. The length of straight conduit, L, between detector positions, the various ratios, p, of the transit time to the mean duration for the tracer “pulse” to
37、 pass each detector position (i.e.corresponding to the passage of99,7% of the tracer) and the various lengths of conduit, N, between the injection and first detector positions, are related to each other by the formula: where L and N are expressed in conduit diameters. This relationship is shown grap
38、hically in Figure 1. If the concentration/time distributions are recorded on a single-channel recorder, it is necessary for the length of conduit between detectors to be greater than the mean spatial dispersion of the tracer at the detector positions so that the recorded distributions do not overlap
39、. This is achieved when p1. If a multi-channel recorder is used, this distance can be reduced, but it is necessary that for accurate measurement of transit time the length of conduit between detectors is not less than a half of the mean spatial dispersion of the tracer. For guidance, it is recommend
40、ed to use in practice pW 0,5. 5.3 Measuring section For the highest accuracy of flow measurement, the length of conduit between detector positions shall consist of a straight pipe of uniform cross-section and shall contain no pipe fittings or sections where dead zones are likely to affect the concen
41、tration/time distribution measured at the second detector. Examples of such fittings and sections are valves, flow regulators, abrupt changes of cross-sectional area, closed-ended branch pipes or sharp bends. The overall accuracy of the flow measurement depends on the accuracy with which the interna
42、l volume of the measuring section has been determined. 5.4 Losses and additions Additions of fluid upstream of the first detector position, of the same nature as the fluid in the conduit, do not affect the result provided that this fluid is mixed with the main flow at the first detector position. Lo
43、sses of fluid from the conduit before the first detector position do not affect the result but, if the tracer is not completely mixed at the point of loss, the amplitude of the concentration/time distribution at the detector positions may be affected and its value changed by a constant factor. L 4,2
44、5 pp N +() =BS5857-2.4:1980 BSI 12-1999 3 Losses or additions of fluid in the length of conduit between the detector positions would cause serious errors in the measurement of flow-rate. Consequently, it is essential that the conduit between the two detector positions contain no branch connections a
45、nd is free from leaks. 6 Procedure 6.1 Handling of radioisotopes The use of radioisotopes (storing, transportation, handling) shall comply with any existing statutory regulations. 6.2 Location of injection points The number and position of injection points located at the injection cross-section depe
46、nds mainly on the length of conduit between the injection position and the first detector position and the method of measuring the tracer concentration at the detector positions (i.e.“averaging” method or single-point sample). When the available length of conduit between the injection point and the
47、first detector is less than the theoretical mixing distance, it is recommended to proceed as mentioned in clause6 of ISO4053-I. It is advisable to choose procedures which enable the injection of all the tracer to be nearly instantaneous. In particular, a suitable procedure consists in using a single
48、 central injection against the flow or any other device which respects the symmetry of the conduit; injection may also be made upstream of a fan or a turbulence-generating device. If multi-orifice injections are used, the device shall be designed so as to allow a simultaneous injection in every poin
49、t. 6.3 Preparation of the injected gas The concentration of tracer in the injected gas shall be uniform. Homogeneity is generally achieved by molecular diffusion. The required concentration will depend on the volume of gas to be injected for each measurement, the volume flow-rate to be measured, the degree of longitudinal dispersion of the tracer at the detector positions and the sensitivity of the detectors. In the case of a rapid symmetrical injection of tracer, the estimation of its maximum concentration, C m , in curies per cubic metre
