1、BRITISH STANDARD BS5857-1.4: 1980 ISO2975-VI: 1977 Methods for measurement of Fluid flow in closed conduits, using tracers Part1: Measurement of water flow Section1.4 Transit time method using non-radioactive tracers ISO title: Measurement of water flow in closed conduits Tracer methods PartVI: Tran
2、sit time method using non-radioactive tracers UDC 532.542:532.574.8.082.25 CONFIRMED JANUARY 2008BS5857-1.4:1980 This BritishStandard, having been prepared under the direction of the Industrial-process Measurement and Control Standards Committee, was published under the authority ofthe Executive Boa
3、rd and comes into effect on 31March1980 BSI12-1999 The following BSI references relate to the work on this standard: Committee reference PCL/2 Draft for comment77/28092 DC ISBN 0 580 11249 7 Cooperating organizations The Industrial-process Measurement and Control Standards Committee, under whose dir
4、ection 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 Association British Steel Corporation CBMPE (Council of British Manuf
5、acturers 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 Engi
6、neering 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
7、 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 issue Comments
8、BS5857-1.4:1980 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 1 5 Procedure 3 6 Estimation of uncertainties in flow rate measurement
9、5 7 Example of flow rate calculation 6 Annex A Example of application of the transit time method using sodiumchloride for the on-site testing of hydraulic machines 10 Annex B Example of electrodes and circuits which can be used when using sodium chloride 14 Figure 1 Effect of measuring length of str
10、aight pipe on ratio of mean transit time to dispersion of tracer 8 Figure 2 Method of determining characteristic points 9 Figure 3 Example of an arrangement of salt tank and piping for preparation of the tracer solution 12 Figure 4 Cross section of Allen pop-valve 12 Figure 5 Example of injection ar
11、rangement 13 Figure 6 Arrangement of electrodes 14 Figure 7 Grid electrode in a rectangular conduit 15 Figure 8 Bridge circuit 15 Publications referred to Inside back coverBS5857-1.4:1980 ii BSI 12-1999 National foreword This BritishStandard has been prepared under the direction of the Industrial-pr
12、ocess Measurement and Control Standards Committee, and is identical with ISO2975-VI:1977 “Measurement of water flow in closed conduits Tracer methods PartVI: Transit time methods using non-radioactive tracers” published by the International Organization for Standardization (ISO). Terminology and con
13、ventions. 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 identical with those used in BritishStandards; attention is especially drawn to the following. The comma has been
14、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, referring to this standard, they should be read as “BritishStandard”. A British Standard does not purport to
15、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 of itself confer immunity from legal obligations. Cross reference International Standard Corresponding BritishStandard ISO2975-I:1
16、974 BS5857 Methods for measurement of fluid flow in closed conduits, using tracers 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 to16, an inside back cover and a back cover. Thi
17、s 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.BS5857-1.4:1980 BSI 12-1999 1 0 Introduction This International Standard is the sixth of a series of standards covering tracer methods of
18、 water flow measurement in closed conduits. The complete series of standards will be as follows: Part1: General; PartII: Constant rate injection method using non-radioactive tracers; PartIII: Constant rate injection method using radioactive tracers; PartIV: Integration (sudden injection) method usin
19、g non-radioactive tracers; PartV: Integration (sudden injection) method using radioactive tracers; PartVI: Transit time method using non-radioactive tracers; PartVII: Transit time method using radioactive tracers. 1 Scope and field of application This International Standard specifies the transit tim
20、e method using non-radioactive tracers for the measurement of water flow rate in closed 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
21、the conduit aknown distance apart. Labelling of the fluid particles is achieved by injecting a tracer into theflow 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 det
22、ector positions. Under certain conditions (see clause3), the row rate q vis 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 validity of the formula is that the measuring s
23、ection 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 value of is obtained by measuring the differ
24、ence in abscissae of characteristic points (in theory: centre of gravity, but in 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 d
25、etectors shall be proportional to the tracer concentration. The value of the proportionality 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. Sodium
26、 chloride is commonly used as the tracer, mainly because of the linear relationship between the conductivity of its solutions and concentration over a wide range of concentrations, and the ease with which it can be measured with an appropriate electrode system. 3.2 Mixing of tracer The tracer must b
27、e sufficiently mixed with the flow at the first detector position for the recorded concentration/time distributions at both detectors to be adequately representative of the mean flow in the conduit (see4.1). The selection of the positions for the injection and the detectors is controlled by the flui
28、d velocity, tracer dispersion, and the conduit layout. The conditions for this selection are dealt with in clause4. 3.3 Test procedure The procedure for the preparation and injection of the concentrated solution, which in practice should be injected as rapidly as possible to minimize the duration of
29、 the concentration/time function, is covered in5.2 and5.3. The internal volume of the measuring section must be determined with sufficient accuracy (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 len
30、gth 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 tBS5857-1.4:1980 2 BSI 12-1999 4.1 Length of conduit between injection and firs
31、t 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 be equal to or greater than the mixing distance. The mixing distance is defined as the shortest distance at
32、which the maximum variation of C 2dt over the cross-section is less than some predetermined value (for example0,5%) seeclause6 of partI. This distance can be calculated theoretically according to6.2.1 of partI. Figure 3 of the latter shows the measured variation of mixing distance with respect to va
33、riations inC 2dt across the cross-section, for various injection arrangements. Methods of reducing the mixing distance are described in6.3 of partI. There are, however, insufficient experimental results available to relate variations in C 2dt at the first detector position to the overall accuracy of
34、 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 (for example by simultaneous measurements at many points or by a detect
35、or 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 injection position and the first detector position may be considerabl
36、y less than the mixing distance. For example, in the application of the transit time method for the on-site testing of hydraulic machines, the use of a multipoint injection of sodium chloride and a suitable arrangement of electrode detectors can enable flow-rate to be measured accurately with a dist
37、ance between injection and first detector equivalent to only seven conduit diameters (see Annex A). This application of the method has the advantage that only short lengths of conduit are required but has the disadvantage of necessitating the installation of injection points and electrode systems wi
38、thin the conduit. The length of conduit between the injection position and the first detector should preferably contain no pipe fittings or sections likely to increase significantly the longitudinal dispersion of tracer at the detector positions. Examples of such fittings and sections are valves, fl
39、ow regulators and flow distribution headers. 4.2 Length of conduit between detector positions The length of conduit necessary 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 me
40、asurement of transit time. The length of straight conduit (L) between 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)
41、between the injection and first detector positions, are related to each other by the following formula: L=4,25 p (p+) 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-chan
42、nel 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 (p 1). If a multi-channel recorder is used, this distance can be reduced, but it is neces
43、sary 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 recommended to use in practice pW 0,5. Care must be taken that there is no interaction between the detector points in t
44、he conduit during the passage of the tracer. For example, if sodium chloride is used as a tracer and the change in conductivity of the water is the detected parameter, then the “pulse” must not be so long that its presence is sensed simultaneously at both detector electrodes. This is achieved by hav
45、ing p 1 in the case of a common measuring circuit. 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 cross-section and shall contain no pipe fittings or sections where dead water zones are
46、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 pipes or sharp bends. 0 0 0 NBS5857-1.4:1980 BSI 12-1999 3 The overall accuracy of the
47、 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 conduit, do not affect the result provided that this
48、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 concentration/time distribution at the detector po
49、sitions 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 the conduit between the two detector positions contain no branch connections and be free from leaks. 5 Procedure 5.1 Location of injection points The number and position of injection points located at the injection cross-section depend mainly on the length of conduit betwe
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