1、BRITISH STANDARD BS5857-1.5: 1980 ISO2975-VII: 1977 Methods for measurement of Fluid flow in closed conduits, using tracers Part1: Measurement of water flow Section1.5 Transit time method using radioactive tracers ISO title: Measurement of water flow in closed conduits Tracer methods PartVII: Transi
2、t time method using radioactive tracers UDC 532.542:532.574.8.082.25:539.16 CONFIRMED JANUARY 2008BS5857-1.5:1980 This BritishStandard, having been prepared under the directionof the Industrial-process Measurementand Control Standards Committee, was published under the authority ofthe Executive Boar
3、d and comesinto effect on 31March1980 BSI12-1999 The following BSI references relate to the work on this standard: Committee reference PCL/2 Draft for comment76/31132 DC ISBN 0 580 11251 9 Cooperating organizations The Industrial-process Measurement and Control Standards Committee, under whose direc
4、tion this BritishStandard 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 B
5、ritish Manufacturers 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 Asso
6、ciation Engineering 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 ma
7、rked with an asterisk 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) United Kingdom Atomic Energy Authority Amendments issued since publication Amd. No. Date of is
8、sue CommentsBS5857-1.5:1980 BSI 12-1999 i Contents Page Cooperating organizations Inside front cover National foreword ii 0 Introduction 1 1 Scope and field of application 1 2 Principle 1 3 Required conditions 1 4 Choice of measuring length 2 5 Procedure 3 6 Selection of tracer 5 7 Estimation of the
9、 uncertainties in flow-rate measurement 6 8 Example of flow rate calculation 7 Figure 1 Effect of measuring length of straight pipe on ratio ofmeantransittime to dispersion of tracer 10 Figure 2 Method of determining characteristic points 11 Publications referred to Inside back coverBS5857-1.5:1980
10、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 ISO2975-VII:1977 “Measurement of water flow in closed conduits Tracer methods PartVII: Transit time method using rad
11、ioactive 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 conventions are not identi
12、cal 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 “International Standard” appear, r
13、eferring to this standard, they should be read as “BritishStandard”. Additional information. In the table in6.2 the SI unit for concentration is the Bq/cm 3where1Bq=3.7 10 10 Ci. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are
14、 responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-reference International Standard Corresponding BritishStandard ISO2975-I:1974 BS5857 Methods for measurement of fluid flow in closed conduits, using tracers
15、 Part1. Measurement of water flow Section1.1:1980 General (Identical) Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to12, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incor
16、porated. This will be indicated in the amendment table on the inside front cover.BS5857-1.5:1980 BSI 12-1999 1 0 Introduction This International Standard is the seventh of a series of standards covering tracer methods of water flow measurement in closed conduits. The complete series of standards is
17、as follows: Part I: General; Part II: Constant rate injection method using non-radioactive tracers; Part III: Constant rate injection method using radioactive tracers; Part IV: Integration (sudden injection) method using non-radioactive tracers; Part V: Integration (sudden injection) method using ra
18、dioactive tracers; Part VI: Transit time method using non-radioactive tracers; Part VII: 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 water flow rate in c
19、losed conduits. 2 Principle Flow-rate measurement by the transit time method (formerly called “Allen velocity method”) is based on measuring the transit time of “labelled” fluid particles between two cross-sections of the conduit a known distance apart. Labelling of the fluid particles is achieved b
20、y 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 certain conditions (see clause3), the flow rate q vis giv
21、en 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 validity of the formula is that the measuring section be “closed to diffusion”: i.e.that the ratio of the local velocity to th
22、e 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 value of is obtained by measuring the difference in abscissae of characteristic points (in theory: centre of gravity, but i
23、n practice other characteristic points may be found, see5.6) of recorded distributions, corresponding to concentration/time distributions or their integrals, obtained at each detection cross-section. The signal from the detectors shall be proportional to the tracer concentration. The value of the pr
24、oportionality coefficient and hence the absolute concentration value need not, however, be known exactly. 3 Required conditions 3.1 Tracer The tracer shall meet the general requirements defined in clause5 of partI, when a list of tracers generally used, with their advantages and disadvantages, is al
25、so given. q v V t - = t tBS5857-1.5:1980 2 BSI 12-1999 3.2 Mixing of tracer The tracer must be sufficiently mixed with the flow at the first detector position for the recorded concentration/time distributions at both detectors tobe adequately representative of the mean flow(see4.1). The selection of
26、 the positions for the injection and the detectors is controlled by the fluid velocity, tracer dispersion and the conduit layout. The conditions for this selection are dealt with in clause4. At low Reynolds number, Reu5000, the mixing of tracer is not sufficient and no measurement can be made. 3.3 T
27、est procedure The procedure for the preparation and injection of the tracer solution, which in practice should be injected as rapidly as possible to minimize the dispersion of the tracer, is covered in5.2 and5.3. The internal volume of the measuring section must be determined with sufficient accurac
28、y (see5.7). Other requirements relating to the tests and the calculation of the transit time from the data are given in clause5. 4 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
29、the first detector; the length of conduit between detectors. 4.1 Length of conduit between injection and first detector When the concentration of tracer C 2at only a single point in each measurement cross-section is measured, the length of conduit between the injection point and first detector shall
30、 be equal to or greater than 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%), C 2being the tracer concentration in the conduit seeclause6 of partI. This dis
31、tance can be calculated theoretically according to 6.2.1 of partI. Figure3 of the latter shows the measured variation of mixing distance, with respect to variations indt across the cross-section, for various injection arrangements. Methods of reducing the mixing distance are described in6.3 of partI
32、. There are, however, insufficient experimental results available to relate variations 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
33、 each detector position represents the mean concentration in the cross-section (for 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 t
34、o the mixing distance. In these circumstances the necessary distance between the injection position and the first detector position may be considerably less than the mixing distance. For example, when using a -emitting tracer centrally injected into a conduit and detected by three scintillation dete
35、ctors positioned at each measurement cross-section, flow rate has been measured accurately with a distance between injection and the first detector equivalent to only twelve conduit diameters. The length of conduit between the injection position and the first detector should preferably contain no pi
36、pe fittings or sections likely to increase significantly the longitudinal dispersion of tracer at the detector positions. Examples of such fittings and sections are valves, flow regulators, and flow distribution headers. 4.2 Length of conduit between detector positions The length of conduit necessar
37、y between the detector positions depends on the linear velocity of the fluid, the spatial dispersion of the tracer at the detector positions and the required accuracy of the measurement of transit time. C 2 0 Z C 2 0 Z C 2 0 Z BS5857-1.5:1980 BSI 12-1999 3 The length of straight conduit (L) between
38、detector positions, the various ratios (p) of the transit time to the mean time for the tracer “pulse” to 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 eac
39、h other by the following formula: where L and N are expressed in numbers of conduit diameters. This relationship is shown graphically 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 grea
40、ter than the mean spatial dispersion of the tracer at the detector positions so that the recorded distributions do not overlap (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 dete
41、ctors is not less than one-half of the mean spatial dispersion of the tracer. For guidance, it is recommended to use in practice p u 0,5. 4.3 Measuring section For the highest accuracy of flow measurement, the length of conduit between detector positions shall consist of a straight pipe of uniform c
42、ross-section and shall contain no pipe fittings or sections where dead water zones are likely to affect the concentration/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
43、 pipes or sharp bends. The overall accuracy of the flow-rate measurement is dependent on the accuracy with which the internal volume of the measuring section is determined. 4.4 Losses and additions Additions of fluid upstream of the first detector position, of the same nature as the fluid in the con
44、duit, do not affect the result provided that this fluid is mixed with the main flow at the first detector position. Losses of fluid from the conduit upstream of 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
45、concentration/time distribution at the detector positions may be affected and its value changed by a constant factor. 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 th
46、e conduit between the two detector positions contain no branch connections and is free from leaks. 5 Procedure 5.1 Handling of radioisotopes The use of radioisotopes (storing, transportation, handling) shall comply with any existing statutory regulations. 5.2 Location of injection points The number
47、and position of injection points located at the injection cross-section depends 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, see4.
48、1,4 thparagraph). When the available length of conduit between the injection point and the first detector is less than the mixing distance, it is recommended to proceed as mentioned in6.3 of partI. In particular, a suitable procedure consists in using a single central injection against the flow or a
49、ny other device which respects the symmetry of the conduit. Injection may be also be made upstream of a pump or a turbulence-generating device. If multi-orifice injections are used, the device shall be so designed as to allow simultaneous injection at all points. 5.3 Preparation of the injected solution The concentration of tracer in the injected solution shall be uniform. Homogeneity can be achieved by means of a mechanical stirrer or closed-circuit pump. L 4,25 p pN + () =BS5857-1.5:1980 4 BSI 12-1999 The required concentration
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