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1、INTERNATIONAL STANDARD 2975/v INTERNATIONAL ORGANIZATION FOR STANDARDIZATIONMEXAYHAPOAHAX OPI-AHIi3ALlWR I-IO Cl AH ? 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.
2、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 i is obtained by measuring the difference in abscissae of characteris
3、tic points (in theory : centre of gravity, but in practice other characteristic points may be found, see 5.6) of recorded distributions, corresponding to concentration/time distributions or their integrals, obtained at each detection cross-section. The signal from the detec- tors shall be proportion
4、al 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 clause 5 of part I. Sodium chloride is commonly used
5、 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 be sufficiently mixed with
6、the flow at the first detector position for the recorded concentration/time distributions at both detectors to be adequately represen- tative of the mean flow in the conduit (see 4.1). The selection of the positions for the injection and the detectors is controlled by the fluid velocity, tracer disp
7、ersion, and the conduit layout. The conditions for this selection are dealt with in clause 4. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:29:39 MDTNo reproduction or netwo
8、rking permitted without license from IHS -,-,- IS0 2975/W-1977 (E) 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 the concentration/time function, is covered in 5
9、.2 and 5.3. The internal volume of the measuring section must be deter- mined with sufficient accuracy (see 5.7). Other require- ments relating to the tests and the calculation of the transit time from the data are.given in clause 5. 4 CHOICE OF MEASURING LENGTH In the transit time method, the measu
10、ring 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. 4.1 Length of conduit between injection and first detector When the concentration of tracer C, at only a single point in each me
11、asurement 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 which the maximum variation of J,” C, dt over the cross- section is less than so
12、me predetermined value (for example 0,5 %) - see clause 6 of part I. This distance can be calculated theoretically according to 6.2.1 of part I. Figure 3 of the latter shows the measured variation of mixing distance with respect to variations in J,” C, dt across the cross-section, for various inject
13、ion arrangements. Methods of reducing the mixing distance are described in 6.3 of part I. There are, however, insufficient experimental results available to relate variations in Jr C2 dt at the first detector position to the overall accuracy of transit time as deter- mined from concentration measure
14、ments 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 detector sensitive to tracer across the cross-section), the
15、 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 considerably less than the mixing distance. For example, in the
16、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 distance between injection and first detector equivalent
17、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 disadvan- tage of necessitating the installation of injection points and electrode systems within the conduit, The length of conduit between the
18、 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, flow regulators and flow distribution headers. 4.2 Le
19、ngth 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 measurement of transit time. The length of straight c
20、onduit (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 of 99,7 % of the tracer) and the various lengths of conduit (N) between the injection and first detector positio
21、ns, are related to each other by the following formula : L=4,25p(p+j where L and N are expressed in numbers of conduit diam- eters. 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
22、 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 I. If a multi-channel recorder is used, this distance can be reduced, but it is necessary that for accurate measurement of transit
23、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 p 0,5. Care must be taken that there is no interaction between the detector points in the conduit during the passage of the tracer. Fo
24、r 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 elec- trodes. This is achieved by having p 1 in the case of a common measuring cir
25、cuit, 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 likely to affect the concentration/time distr
26、ibution measured at the second detector. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:29:39 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 29
27、75/i/l-1977 (E) 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 rate measurement is dependent on the accuracy with which the internal volume of the measuring sectio
28、n 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 fluid is mixed with the main flow at the first detector position. Losses of fluid from the conduit upstrea
29、m 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 positions may be affected and its value changed by a constant factor. Losses or additions of fluid in the l
30、ength 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 pos- itions contain no branch connections and be free from leaks. 5 PROCEDURE 5.1 Location of injection points The numbe
31、r and position of injection points located at the injection cross-section depend 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 samp
32、le - see 4.1,4th paragraph). 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 in 6.3 of part I. In particular, a suitable procedure is to use a central injection against the flow or any
33、 other symmetrical arrange- ment within the conduit. Alternatively, the injection may be made upstream of a pump or a turbulence-generating device. If several injection points are used, the system shall be so designed as to allow simultaneous injection at all points. 5.2 Preparation of the injection
34、 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. The required concentration will depend on the volume of fluid to be injected for each measurement, the volume flow rate to be measured
35、, the degree of longitudinal dispersion of the tracer at the detector positions and the sensitivity of the detectors. In the case of a symmetrical mode of injec- tion, an estimate of the maximum concentration C, of tracer observed in a straight pipe of diameter D and without obstructions at N condui
36、t diameters downstream of the fast injection point can be obtained from the expression : 3A c, = 403N112 where A is the amount, by mass, of injected tracer. It is of interest to note that the maximum concentration does not depend on the flow rate in the conduit. When a turbulence-generating device i
37、s positioned in the measuring length between the injection position and the first detector, the maximum concentration may be greater than that derived from the above equation. This expression may also be used to estimate the amount of tracer to be injected for each flow measurement from a knowledge
38、of the sensitivity of the measurement detectors. The amount of injected tracer shall be such that the tracer concentration at the detector positions is within the linear range of the detector. 5.3 Injection of concentrated solution In order to minimize dispersion of the measured concen- tration/time
39、 distributions, the tracer shall be injected as rapidly as possible with no “tailing” of the injected solution from the injection tubes within the conduit. This can be achieved by any of the following means : a) by means of injection valves at the extremity of each injection point (for example sprin
40、g-loaded pop- valves); these valves shall open simultaneously, close rapidly and be leak-free; b) by ensuring that the injected solution is flushed into the conduit by a flow of tracer-free water; c) by breaking, with the aid of a suitable device, an ampoule containing tracer introduced into the con
41、duit. The tracer may be injected into the conduit by means of air or liquid pressure according to methods consistent with one of the above requirements. It should be noted that at very high concentrations Of tracer, the density of the injected solution may be significantly different from the density
42、 of fluid in the conduit. This will then affect the symmetry of the injected solution and the distribution of tracer at the detector positions. It has been found that problems of stratification of the injected tracer solution have been avoided by adopting a lower-limit of the mean velocity in the co
43、nduit. The value of this limit for satisfactory measurements to be madecan 3 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/21/2007 10:29:39 MDTNo reproduction or networking permitted w
44、ithout license from IHS -,-,- IS0 2975/W-1977 (E) be estimated from the following expression derived from laboratory tests. where v is the mean velocity, in metres per second, in the conduit; g is the acceleration, in metres per second squared, due to gravity; D is the diameter, in metres, of the co
45、nduit; yr is the density of the injected solution; Yw is the density of the water in the conduit. 5.4 Detection of tracer The tracer concentration may be determined from detec- tors situated within the conduit or from detector flow-cells sampling a flow of fluid from one or more sample points in the
46、 measurement cross-sections. If tracer is detected at only one measuring point in each measurement cross-section, it shall be ensured that results achieved are the same for any pair of selected measuring points. When the measuring length is of uniform section between the two measuring cross-sections
47、, selection of a pair of points on the same radius is liable to yield satisfac- tory results. The difference in the time responses of the detector units, at the two cross-sections, shall be negligible compared to the transit time. Examples of electrode arrangements which may be used within conduits
48、for detecting sodium chloride and of typical electrical circuits are given in annex B. 5.5 Number of injections The number of successive injections required for each measurement of flow rate depends on steadiness of the flow being measured, the random error in determining the transit time and the re
49、quired overall limit of uncertainty on the measurement of flow rate. Because in practice an absolutely constant flow rate is rarely achieved, it is recommended that at least five successive injections of tracer and associated measurements of transit time be made at each flow rate to enable an objective analysis of the random uncertainties of measure- ment to be made (see clause 6). 5.6 Calculation of transit time The transit time of the tracer between detector
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