1、BRITISH STANDARD CONFIRMED JUNE1998 BS5857-2.1: 1980 ISO4053-I: 1977 Methods for measurement of Fluid flow in closed conduits, using tracers Part2: Measurement of gas flow Section2.1 General ISO title: Measurement of gas flow in conduits Tracer methodsPartI: General UDC 532.542:532.574.8:533.6.011BS
2、5857-2.1:1980 This BritishStandard, having been prepared under the directionof the Industrial-process Measurement and Control Standards Committee, was published under the authority ofthe Executive Board and comesinto effect on 31March1980 BSI11-1999 The following BSI references relate to the work on
3、 this standard: Committee reference PCL/2 Draft for comment76/29641DC ISBN 0 580 11246 2 Cooperating organizations The Industrial-process Measurement and Control Standards Committee, under whose direction this BritishStandard was prepared, consists of representatives from the following Government de
4、partment and scientific and industrial organizations: British Gas Corporation British Industrial Measuring and Control Apparatus Manufacturers Association British Steel Corporation CBMPE (Council of British Manufacturers of Petroleum Equipment) Control and Automation Manufacturers Association (BEAMA
5、) Department of Industry (Computers Systems and Electronics Electrical, Electronic, Telecommunications and Plumbing Union Electricity Supply Industry in England and Wales* Electronic Engineering Association Engineering Equipment Users Association* Institute of Measurement and Control Institution of
6、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 asterisk in the above list, together with thefollowing, were directly represented on the
7、 committee untrusted 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 CommentsBS5857-2.1:1980 BSI 11-1999 i Contents Page Cooperating organizations Inside front cover N
8、ational foreword ii 0 Introduction 1 1 Scope and field of application 1 2 Vocabulary and symbols 1 3 Units 1 4 Choice of method 1 5 Choice of tracer 2 6 Choice of measuring length and adequate mixing distance 3 7 Errors 6 Annex Distribution table of # 2(Pearsons law) 9 Figure 1 Theoretical results 1
9、0 Figure 2 Reynolds number effect on mixing distance 11 Figure 3 Experimental results 12 Publications referred to Inside back coverBS5857-2.1:1980 ii BSI 11-1999 National foreword This BritishStandard has been prepared under the direction of the Industrial-process Measurement and Control Standards C
10、ommittee, and is identical with ISO4053-I:1977 “Measurement of gas flow in conduitsTracer methodsPartI: General” published by the International Organization for Standardization(ISO). Terminology and conventions. The text of the International Standard hasbeen approved as suitable for publication, wit
11、hout 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 used throughout as a decimal marker. In BritishStandards it is current practice to use a full point
12、 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 include all the necessary provisions of a contract. Users of British Standards are responsible for
13、their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-reference International Standard Corresponding BritishStandard ISO4006:1977 BS5875:1980 Glossary of terms and symbols for measurement of fluid flow in closed conduits (Ident
14、ical) 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 incorporated. This will be indicated in the amendment table on the in
15、side front cover.BS5857-2.1:1980 BSI 11-1999 1 0 Introduction This International Standard is the first of a series ofstandards covering tracer methods of gas flow measurement in conduits. The complete series of standards will be as follows: PartI: General; PartII: Constant rate injection method usin
16、g non-radioactive tracers; PartIII: Constant rate injection method using radioactive tracers; PartIV: Transit time method using radioactive tracers. 1 Scope and field of application This International Standard deals with the measurement of gas flow in conduits by using tracermethods. In a steady flo
17、w of compressible fluid, the only conservative parameter is the mass rate of flow q m . Therefore the whole of this International Standard will refer to mass rate of flow q m . However, for those fluids for which the composition (and therefore, the mass density) may not be known accurately, the volu
18、me rate of flow q vcould be measured, it being understood that this volume rate of flow is only valid for the conditions of temperature and pressure at which it has been measured. This International Standard applies to flow measurement in conduits into which a tracer can beinjected in such a way tha
19、t effective mixing in single phase with the gas flowing in the pipe can beachieved. The fluid in the conduit can be a mixture of several gases provided the thermodynamic state and conditions of flow of this mixture are well defined. Two fundamental test procedures are used: The first, known as the c
20、onstant rate injection method, is based on the dilution principle: a tracer solution is injected into the conduit and the dilution (ratio) of this tracer in the gas flowing in the conduit is determined, this dilution being proportional to the rate of flow. The second is a method of measurement of th
21、etransit time (formerly called Allen velocity method): the tracer is injected into the conduit and the time taken by the tracer to travel a specified length between two cross-sections in each of which it is detected, is measured. The advantages and disadvantages of these two methods are considered i
22、n clause4. The distance between injection and measuring sections shall be sufficient to achieve mixing of the tracer with the gas flowing in the conduit according to the methods; the adequate mixing distance is considered in clause6. A large number of different tracers may be used, such as radioacti
23、ve or non-radioactive, mineral or organicmaterials. The choice of tracer depends on the circumstances of the measurement (seeclause5). The uncertainty of the measurementsmay be less than1% under good conditions (seeclause7). 2 Vocabulary and symbols The vocabulary and symbols used in this Internatio
24、nal Standard are defined in ISO4006, Measurement of fluid flow-rate in closed conduits Vocabulary and symbols. 3 Units The basic units in this International Standard are SI units. 4 Choice of method 4.1 Comparison between dilution method and method based on transit time measurement 4.1.1 Advantages
25、of the dilution method It is not necessary to know the geometrical characteristics of the conduit. It is not necessary that the conditions of the gas flow rate (p, T) be constant along the measuring length. 4.1.2 Advantages of method based on transit time measurement It is necessary only to determin
26、e the concentration-time distribution at two measuring cross-sections separated by a known volume of pipe. It is not necessary to know volumes, masses or rates of flow of the injected tracer.BS5857-2.1:1980 2 BSI 11-1999 4.1.3 Special recommendation for the method based on transit time measurement F
27、or this method, it is preferable to have a conduit length of constant cross-section between the two measuring points so that the flow parameters be approximately constant in the measuring length. 5 Choice of tracer 5.1 General A large number of different tracers may be used, such as mineral or organ
28、ic, radioactive or non-radioactive, but it is necessary for any tracer to comply with the following requirements: a) it should mix easily with the gas in the conduit; b) it should cause only negligible or known modifications of the rate of flow; c) it should be measurable with sufficient accuracy at
29、 a concentration lower than the highest permissible concentration while taking account of toxicity, corrosion, etc.; d) it should be chemically stable in the conditions of use; e) it should only be present in the gas originally flowing in the conduit at a negligible or constant concentration; f) it
30、should be cheap. In addition, for dilution method, it is important that the tracer: g) should not react with the gas flowing in the conduit or with any other substance with which it may come into contact (in particular, the conduit walls) in such a way as to affect the measurement. Furthermore, for
31、transit time methods, it is recommended that: h) the tracer concentration in the measuring cross-section can be determined, if necessary, at any moment; i) to obtain the greatest precision, the detector signal be proportional to the tracer concentration (ofwhich it is not necessary to know the exact
32、 value) and that its response time be negligible. The following substances are given as examples: 5.1.1 Non-radioactive tracers Helium He Sulphur hexafluoride SF 6 Methane CH 4 Nitrous oxide N 2 O 5.1.2 Radioactive tracers 5.2 Comparison between the different tracers 5.2.1 Advantages of radioactive
33、tracers With tracers emitting* radiation with sufficient energy, it is possible to carry out the measurement by means of probes located outside the conduit. With short half-life tracers, the basic substance of which is chemically inoffensive, any radioactive contamination danger disappears quickly a
34、nd thereis no permanent pollution. 5.2.2 Advantages of non-radioactive tracers It is not necessary for the operators to be specially trained and classified. Tracer Formula (if any) Half-life Argon41 Arsenic76 Bromine82 Krypton85 Sulphur35 Xenon133 76 AsH 3 C 2 H 5 82 Br or CH 3 82 Br 35 SF 6 110min
35、26,5h 36h 10,6years 87days 5,27daysBS5857-2.1:1980 BSI 11-1999 3 Administrative authorizations are not necessary for each measurement and radioprotective means are not required. The substances generally remain stable with time; delays between the supply and the use of the substance do not matter. 6
36、Choice of measuring length and adequate mixing distance 6.1 Introduction When a tracer is used to measure the flow of gas inaconduit, there should be sufficient distance between the region in which the tracer is injected and the region in which concentration or transit time measurements are made. Th
37、e distance which is required in order to allow the tracer to mix with the gas in the conduit is known as the mixing distance. This distance is defined as the shortest distance at which the maximum variation (x), over the cross-section, of for the integration method or the concentration of tracer for
38、 the constant rate injection method is less than some predetermined value (for example,0,5%) whereC 2is the concentration of the tracer in the conduit. Thus, the mixing distance is not a fixed value, but varies according to the allowed concentration variations: the smaller the acceptable variation t
39、he greater the mixing distance. For highest accuracy in flow measurement it is necessary to ensure the smallest possible values of(x) at the measuring cross-section. However, in practice higher values of (x) may have to be tolerated when sufficiently long lengths of conduit are not available. A mult
40、ipoint sampling or detection arrangement should be used where possible, particularly when a systematic variation in concentration or in may exist at the sampling cross-section. Depending on the tracer and the method of detection used, the mixing requirements for the transit time method may not be so
41、 stringent as for dilution methods. Several techniques have been developed to reduce the mixing distance and these should be used whenever it is possible(see6.3). 6.2 Mixing distance 6.2.1 Theoretical derivation of mixing distance Attention is drawn to the fact that the mixing lengthfound in practic
42、e can vary considerably fromthe length predicted theoretically(see6.2.2). Sub-clause6.2.1.1 should not be used as more than a preliminary guide. 6.2.1.1 CENTRAL INJECTION The following equations relating mixing distance (L/D) in terms of the varying concentration of tracer across the conduit, Reynol
43、ds number (Re) and pipe friction have been developed. Equation(1) is derived on the basis of a constant radial diffusion coefficient and uniform flow velocity; equation(2) is derived on the basis of a parabolic distribution of radial diffusion coefficient and uniform flow velocity; equation(3) assum
44、es a parabolic distribution of radial diffusion coefficient and a logarithmic velocity profile. where X is the maximum variation, in%, across the pipe of concentration C 2for the constant rateinjection method, or of for the integration method, at a distance L from the point of injection; D is the di
45、ameter of the conduit; . . . (1) . . . (2) . . . (3)C 2 0 dtC 2 0 dtC 2 0 dtBS5857-2.1:1980 4 BSI 11-1999 2 is the coefficient of resistance of the conduit; 2 0 is the coefficient of resistance of a perfectly smooth conduit. The above equations presented graphically in Figure 1 show the increase in
46、mixing distance with decrease of x for a Reynolds number of Re =10 5and a smooth conduit. The slight dependence of mixing distance on Reynolds number see equation(3) for example is shown in Figure 2. For a change in Re from10 5to10 6 , at x=1% the mixing distance increases by only25%, approximately.
47、 6.2.1.2 RING INJECTION For uniform injection over a ring with a radius of0,63 of the conduit radius, the mixing distances are reduced to about one-third of the values derived for a central injection. 6.2.2 Experimental derivation of mixing distance Values of mixing distance obtained experimentally
48、for a central injection in an unobstructed, straight, circular conduit are about twice the values predicted theoretically. The difference is attributed to several causes but particularly to the difference between the actual flow conditions and those assumed in the theoretical analysis. Care shall th
49、erefore be exercised in the treatment of theoretical results. Details concerning experimental determination of good mixing are given in the parts of the relevant standard. The measured change in mixing distance with (x) fora central injection and for three other methods ofinjection is shown as an example in Figure 3. Itshould be noted that the flow turbulence level influences these results. 6.3 Examples of methods of reducing mixing distance 6.3.1 Multi-orifice injectors When the tracer is injected equally through a number of
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