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1、INTERNATIONAL STANDARD ,kTERNbT,ONAL ORGAN,ZBT,ON FOR STANDARD,ZAT,ON s,Ejh JSH4POJHAJ? OPTAHl13AUMR II0 CTAHAAPT3AUMMORGANISATION INTERNATIONALE DE NORMALISATION Measurement of water flow in closed conduits - Tracer methods - Part II : Constant rate injection method using non-radioactive tracers Me
2、sure de d X is the distance to the injection point; D is the conduit diameter. The fraction of maximum concentration remaining after time t2 as given by this formula is 0,3 %. It is possible, when no preliminary injection is made, to check the appearance of the concentration level, and therefore the
3、 minimum injection time, by taking samples and analysing them during the measurement, with the apparatus used for the measurement. This procedure enables the concentration variation to be determined as a function of time. Timt t Minimum duration of injection t 3 Concentration m Distance to the injec
4、tion cross-section FIGURE -Determination of the duration of injection Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:59:11 MDTNo reproduction or networking permitted without license
5、from IHS -,-,- IS0 2975/11-1975 (El 4 CHOICE OF MEASURING LENGTH 4.1 Mixing distance The mixing distance is defined in 6.1 of part I and may be calculated theoretically according to 6.2.1 of part I. Figure 3 of part I indicates the measured variation of the actual mixing distance according to the re
6、quired accuracy in the. case of a central injection, and in the case of three other inejction methods. Methods for reducing the mixing distance are described in 6.3 of part I. 4.2 Experimental checking It is seldom possible in conduits to check experimentally the homogeneity of the mixture and the d
7、uration of the tracer passage resulting from an instantaneous injection in a large number of cross-sections. In practice, therefore, the experimentation consists of a) determining the minimum period of injection in the measuring length chosen by a preliminary test (see 3.2); b) checking, where possi
8、ble, at the time of the measurement, that the mixture is homogeneous, .by taking samples from at least two points of the measuring cross-section. In the case of high accuracy measurements (for example 0,2 %), it is necessary to check the homogeneity of the mixture, prior to the measurement itself; w
9、hen a doubt exists on the quality of this mixture; c) studying, whenever possible by taking samples from another cross-section further downstream, whether there are any systematic differences between the mean concentrations at the two measuring cross-sections. In particular, this method permits veri
10、fication that the injected tracer is not absorbed in the measuring length either by entrained products in the liquid or by the conduit walls.1 ) 4.3 Losses and additions The losses and additions of the same liquid as that in the measuring length do not affect the results provided certain conditions
11、are met as described in 6.5.1 of part I. 5 PROCEDU.RE 5.1 Preparation of the concentrated solution It is essential for the injected solution to be homogeneous. The homogeneity of the solution can be obtained by vigorous mixing, by means of a mechanical stirrer or a closed circuit pump. It is advisab
12、le to prepare the injection solution in a separate container from the supply container with water filtered using an appropriate procedure. However, if mixing is carried out in the supply container, the latter shall have sufficient capacity so that it is not necessary to add liquid or tracer during t
13、he injection. The solution shall be taken at a certain level above the bottom of the container, and every precaution shall be taken so that undissolved particles of. the tracer are not carried in the solution injected. In the case of an injection of long duration, provision shall be made as required
14、 to avoid a variation of the solution concentration with time (for example by evaporation under the influence of ambient temperature). 5.2 Injection of the concentrated solution The concentrated solution shall be injected into the conduit at a constant rate and for a sufficient duration to ensure a
15、satisfactory period of constant concentration at the measuring cross-section (see 3.2 and 4.2). Several devices may be used for the injection of the concentrated solution. For all devices it must be possible to check the following : - that the injection system is always free from leaks; - that the i
16、njection rate is constant over the whole injection duration. To this end, any entrained impurities which may be contained in the solution shall be eliminated as they could partly or totally block the injection circuits. One of the possible devices is a positive displacement pump driven by a constant
17、 speed motor. 5.3 Measurement of injection flow rate The ,accuracy with which the injection rate can be measured depends on the measuring instruments used. The accuracy of the device shall be taken into account in the estimation of the total error on flow measurement. Various measuring devices can b
18、e used provided they comply with one of the following requirements : a) they satisfy a principle based on the measurement of the basic quantities of mass, length, time, directly involved in the definition of the quantity of flow; b) they are calibrated in the conditions of use by measuring basic qua
19、ntities involved in the flow rate definition (for example calibrated capacity method - weighing method, etc.); c) they are installed and used in conformity with the requirements of a Standard making it possible to calculate the accuracy obtained. In the case of high accuracy measurements (for exampl
20、e 0,2 %I, it is necessary to measure the injection rate in accordance with items a) or b). 1) See 5.1 of part I. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:59:11 MDTNo reproducti
21、on or networking permitted without license from IHS -,-,- IS0 2975/11-1975 (E) The various devices may either give instantaneous values of the injection rate or allow the computation of one or several average values during the injection period. 5.3.1 Amcng the devices satisfying the first requiremen
22、t, positive displacement pumps or calibrated tanks can be mentioned. The former shall preferably be driven by an electric synchronous motor and the constancy of discharge checked by measuring the frequency of the electrical supply. The use of the level drop in a tank as a measurement of the average
23、flow rate requires that the tank be calibrated according to the permissible rules provided for this type of operation. If the dimensions of the tank are measured to estimate the capacity, care shall be taken that no deformation occurs during filling and that horizontal areas are measured over a suff
24、icient number of sections to determine the relationship between height and volume with an error consistent with the required accuracy of flow rate. Another method of calibration consists of determining the relationship between the height and the volume by means of a calibrated container the contents
25、 of which are transferred to the main container, and in measuring the heights of liquid reached each time. An example for the computation of the error is given in 5.5. 5.3.2 Among the devices satisfying the second requirement can be mentioned propeller flowmeters and volumetric meters used under spe
26、cial conditions (for example lack of straight lengths and of straightener for a propeller flowmeter) and therefore needing calibration on site. In this case the mean of two calibrations made before and after testing shall be taken for the value of the injection flow rate, provided the calibrations d
27、o not differ by more than a value consistent with the required accuracy, for example 1 %. 5.3.3 Among the devices satisfying the third requirement, propeller flowmeters and volumetric meters can again be mentioned, when they are installed and used according to the accepted rules and therefore do not
28、 require calibration. 5.4 Samples Samples shall be taken : - from the conduit, to verify that the background concentration of tracer in the flow system is constant; - from the conduit, to determine the tracer concentration in the measuring cross-section, to check that the tracer concentration is hom
29、ogeneous in the sampling cross-section and to check the concentration level (see 4.2); - from the injected solution, to check the homogeneity of the tracer concentration (see 5.1); - from the injected solution, to compare the concentration of tracer in the injected solution with the concentration of
30、 tracer in the samples taken from the conduit. In practice samples shall be taken under the following conditions : - two or three samples of the liquid flowing in the conduit upstream from the injection section during injection or, when variations of the background concentration are suspected, along
31、 the measuring length, in the sampling cross-section before and after the passage of the solution; - three to five samples of the injected solution at the outlet of the injection equipment immediately before and after the injection period; - at least five samples of the diluted solution in the condu
32、it, if possible from at least two points in the measuring cross-section (at least three at the same point), distributed in time during the passage of the solution. 5.5 Example of error computation in injection flow rate In this example, it is assumed that the tracer is injected by means of a positiv
33、e displacement pump driven by a synchronous motor and the frequency of the electrical supply is measured during the injection duration. The injection rate of the pump, 2,09 cma/s at 50 Hz, is determined before and after the flow tests by comparison with a calibrated capacity which determines this fl
34、ow rate to within + 2 cd = 0,005 cma/s (for a 95 % confidence level). During testing, the measured frequency is 49,9 Hz with a tolerance of f 2 CJ = 0,l Hz. The tolerance on flow rate is then : 6 ERRORS IN FLOW RATE MEASUREMENT For the determination of errors, reference shall be made to clause 7 of
35、part I. The application of the specifications described in this International Standard enable an accuracy of flow measurement of about 1 % to be obtained provided the mixture of the tracer in the flow is of equivalent accuracy and the injection rate is measured with a better accuracy. The use of thi
36、s method also enables higher accuracies to be obtained in the best conditions. The estimation of errors as described in clause 7 of part I enables the accuracy of the measurement to be defined for each case. 4 Copyright International Organization for Standardization Provided by IHS under license wit
37、h ISO Licensee=Defense Supply Ctr/9960866100 Not for Resale, 04/24/2007 10:59:11 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 2975/11-1975 (E) 7 ANALYSIS METHODS PRESENTLY USED FOR WATER FLOW RATE MEASUREMENT 7.1 Calorimetric method of analysis 7.1 .l Principle Calor
38、imetric analysis is based on measuring the transmission efficiency of monochromatic light passing through a glass cell containing the sample to be measured and comparing the transmission with that obtained through samples of known dilution ratio (i.e. control solutions). The wavelength of the monoch
39、romatic light normally used corresponds to that for the maximum absorption of light in the particular tracer solution used. The phenomenon is governed by the law of Beer-Lambert : I IO - = 1 O-E C or !I = log- = elc IO I where I is the luminous intensity transmitted after passage through the sample;
40、 IO is the reference luminous intensity (intensity of the incident light); E is the extinction coefficient; this depends on the wavelength of the incident light and on the temperature and nature of the absorbent; c is the concentration of the molecules or ions absorbing the light in the sample; / is
41、 the thickness of the absorbing substance; D is the optical density. 7.1.2 Calibration of the calorimeter Using one of the samples taken at the outlet of the injection apparatus, a set of control solutions (at least four) is made having a known dilution, similar to the dilution obtained in the condu
42、it. For this purpose, a sample of the concentrated injection solution is diluted by a volumetric or gravimetric method with water taken from the conduit upstream of the injection cross-section. These samples are then tested in the calorimeter, the indications of which are plotted on a graph as a fun
43、ction of the dilution ratio. It is advisable to use a calorimeter of which the indication is proportional to the optical density. The homogeneity of the solution injected shall be verified by diluting diffe.rent samples in an identical manner and analysing them in turn. 7.1.3 Use of sodium dichromat
44、e Sodium dichromate (Crz0,Naz.2Hz0) is commonly used for the application of dilution methods. It has a high solubility in water (at least 600 g/l) which is a function of the temperature. It complies with most of the conditions specified in clause 5 of part I. 7.1.3.1 ANALYSIS Calorimetric analysis p
45、ermits the measurement of concentrations of the sodium dichromate which may reach 0,2 mg/l using a reagent of the ion Cr6+ which is diphenylcarbazide. This reagent may be used in the following form : a) diphenylcarbazide (crystalline) (CeHeNH.NH) c is the calorimeter reading. An estimation of the “l
46、inked standard deviation” of the distribution of experimental points about the straight length may be made from the deviations between experimental values of n and calculated values of equation (2) for experimental values of c, i.e. 0,183 X 10e =snC. For a calorimeter reading co = 132,5 (No. 5 sampl
47、e), the 95 % confidence interval of the dilution ratio is therefore : ,- where no=0,144co-11.53 i is the value of the Student variable for (N-2) degrees of freedom; c and oz are respectively the mean and variance of N calorimeter readings used for calibrating the calorimeter, i.e. 7,55 + 0,183 $f a
48、value very near that found above, which justifies the assumptions and constitutes a strong presumption for the good performance of the calorimetric analysis. b) Tolerance on the value of the dilution ratio of a collected sample This tolerance results from the combination of the calorimeter reading r
49、elating to the sample and the error due to the plotting on the experimental curve, i.e. : 2 U“ = 2 40,092 x 1o-e)2 t (0,075 x IO-612 = 0,24 X 1O-6 0,24 that is-= 7,5 3,2 % c) Tolerance on the mean value of the dilution ratio of ten collected samples The tolerance found above is that which corresponds to one single sample. The tolerance on the mean dilution as obtained from several samples will be less. However, it should be noted that variations in the calorimeter reading