1、BRITISH STANDARD Measurement of liquid flow in closed conduits - Method by collection of the liquid in a volumetric tank * m * The European Standard EN IS0 8316: 1995 has the status of a British standard NO COPYING WITHOUT BSI PEBMISSION EXCEPT AS PERMITTED BY COpyE1IGHT LAW BS EN IS0 3316 : 1997 W
2、6199 : Part 2 : 1988 mumbered, ncolpomting Imendment No. 1 BS EN Is0 8316: 1997 Issue 2, January 1997 contents Page National foreword Inside front cover Committees responsible Back cover Mahod 1 Scope and field of application 2 References 3 Symbols and definitions 4 Principie 5 Apparatus 6 Procedure
3、 7 Calculation of flow-rate 8 Calculation of the overall uncertainty in the flow-rate measurement Anmm A Corrections to the measurement of filling time 13 B Density of pure water at standard atmospheric pressure of 101,325 kPa 17 C Example of a volumetric flow-rate installation using the dynamic gau
4、ging method 17 D Example of a volumetric flow-rate installation using the standing start and finish method 18 E Students t-distribution 21 ZA Normative references to international publications with their relevant European publications 22 page Figures 1 Schematic diagram of a volumetric flow-rate 2 S
5、chematic diagram of a volumetric flow-rate installation using the static gauging method installation using the dynamic gauging method 4 3 Examples of diverter design 5 4 Operating law of diverter 6 5 Twin tanks method 8 6 Typical graph used in evaluation of (eR Ip for a diverter system 10 7 Graph wi
6、th plotting of results of diverter time actuator tests, as given in A.12 14 8 Graph of the filling process for the measuring tank 15 9 Diagram of system for measuring the switching time and switching time difference of the diverter 15 10 Graph of volume collected against time at constant flow-rate 1
7、6 11 Schematic diagram of a volumetric installation using the standing start and finish method 18 19 13 Diagram of the flow-rate of liquid entering the tank 20 3 12 Design of volumetric tank Tales 1 Untitled 2 Untitled 3 UntitI b) its viscosity is sufficiently low so as not to alter or delay unduly
8、the measurement of the level in the volumetric tank; ci it is non-toxic and non-corrosive. Theoretically, there is no limit to the application of this method, but, for practical reasons, this method of measurement is nor- mally used for flow-rates less than approximately 1,5 m3/s and is used on the
9、whole in fixed laboratory installations only. However, there is a variation of this method which uses a natural or artificial storage pond as a volumetric tank, but this application is not dealt with in this International Standard. Owing to its high potential accuracy, this method is often used as a
10、 primary method for calibrating other methods or devices for volume flow-rate measurement or for mass flow-rate measurement; for the latter method or device, it is necessary to know the density of the liquid accurately. If the installation for flow-rate measurement by the volumetric method is used f
11、or purposes of legal metrology, it shall be cer- tified and registered by the national metrology service. Such in- stallations are then subject to periodic inspection at stated in- tervals. If a national metrology service does not exist, a certified record of the basic measurement standards (length,
12、 time and temperature), and error analysis in accordance with this International Standard and IS0 5168, shall also constitute certification for legal metrology purposes. Annex A forms an integral part of this International Standard. Annexes 8 to E, however, are given for information only. 2 Referenc
13、es IS0 4006, Measurement of fluid flow in closed conduits - Vocabulary and symbols. IS0 4185, Measurement of liquid flow in closed conduits - Weighing method. IS0 4373, Measurement of liquid flow in open channels - Water level measuring devices. IS0 5168, Measurement of fluid flow - Estimation of un
14、cer- tainty of a flow-rate measurement. 3 Symbols and definitions 1 STD-BSI BS EN IS0 831b-ENGL 1777 = Lb24bbS 0578773 Li52 = BS 6199 : Part 2 : 1988 3.2 Definitions For the purposes of this International Standard, the definitions given in IS0 4006 apply. Only terms which are used with a par- ticula
15、r meaning or the meaning of which might be usefully restated are defined below. The definitions of some of the terms concerned with error analysis are given in IS0 5168. 3.2.1 static gauging: A method by which the net volume of liquid collected is deduced from measurements of liquid levels (.e. gaug
16、ingsl, made respectively before and after the liquid has been diverted for a measured time interval into the gauging tank, to determine the volume contained in the tank. 3.2.2 dynamic gauging: A method by which the net volume of liquid collected is deduced from gaugings made while liquid flow is bei
17、ng delivered into the gauging tank. 1A diverter is not required with this method. 3.2.3 diverter: A device which diverts the flow either to the gauging tank or to its by-pass without changing the flow-rate during the measurement interval. 3.2.4 flow stabilizer: A device inserted into the measuring s
18、ystem, ensuring a stable flow-rate in the conduit being sup- plied with liquid; for example, a constant level head tank, the level of liquid in which is controlled by a weir of adequate length. One variation of this method uses two tanks which are suc- cessively filled see 6.3). A further variation,
19、 given in annex D. uses a valve instead of a diverter mechanism to start and stop the flow into a volumetric tank. Care shall be taken when using a valve instead of a diverter that the flow-rate does not change when the valve is operated. 4.1.2 Dynamic gauging method The principle of the flow-rate m
20、easurement method by volumetric dynamic gauging see figure 2 for a schematic diagram of a typical installation) is - to let liquid collect in the tank to a predetermined initial level (and thus volume), at which time the timer is started; - to stop the timer when a second predetermined final level f
21、and thus volume) is reached and then to drain the liquid collected. The flow-rate is then derived as explained in clause 7. 4.1.3 Comparison of instantaneous and mean flow-rates It should be emphasized that only the mean value of flow-rate for the filling period is given by the volumetric method. In
22、stan- taneous values of flow-rate as obtained on another instrument or meter in the flow circuit may be compared with the mean flow-rate only if the flow is kept stable during the measurement interval, by a flow-stabilizing device, or if the instantaneous values are properly time-averaged during the
23、 whole fillinq period. 4 Principle 4.2 Accuracy of the method 4.1 Statement of the principle 4.1.1 Static gauging method The principle of the flow-rate measurement method by volumetric static gauging see figure 1 for a schematic diagram of a typical installation) is - tank; to determine the initial
24、volume of liquid contained in the - to divert the flow into the volumetric tank, until it is considered to contain a sufficient quantity to attain the desired accuracy, by operation of a diverter which actuates a timer to measure the filling time; - to determine the final volume of liquid contained
25、in the tank. The volume contained at the inhial and at the final times is obtained by reading the liquid levels in the tank and by reference to a preliminary calibration which gives the level-volume relationship. The flow-rate is then derived from the volume of liquid col- lected and the filling tim
26、e as explained in clause 7. 4.2.1 measurement Overall uncertainty in the volumetric The volumetric method gives a measurement of flow-rate which, in principle, requires only level and time measurements. After the weighing method, the static gauging method in a volumetric tank may be considered as on
27、e of the most accurate of all flow-rate measuring methods, particularly if the precau- tions given in 4.2.2 are taken. For this reason, it is often used as a standard or calibration method. When the installation is carefully constructed, maintained and used, an uncertainty of k 0.1 % to f 0,2 % (wit
28、h 95 % confidence limits) may be achieved. 4.2.2 Requirements for accurate measurements The volumetric method gives an accurate measurement of flow-rate provided that ai there is no leak in the flow circuit and there is no unmeasured leakage flow across the diverter; b) the conduit is running full a
29、t the measuring section and there is no vapour or air-lock between the measuring sec- tion and the volumetric tank ; 2 - STD-BSI BS EN IS0 83Lb-ENGL 1777 Lb24bb7 0578779 377 BS 6199 : Part 2 : 1988 c) there is no accumulation (or depletion1 of liquid in a part of the circuit by thermal contraction (
30、or expansion) and there is no accumulation (or depletion) by change in vapour or gas volume contained unknowingly in the flow circuit: d) care has been taken to avoid any leakage from or un- wanted flow into the tank, absorption of liquid by the walls or their coatings, deformation of the walls etc.
31、 ; e) the level-volume relationship in the tank has been established by transferring known volumes, or by calcula- tion from dimensional measurements of the tank, as SDecified in 5.5; Figure 1 fi and stopping the timer achieve the required accuracies; g) the time required by the diverter (for the st
32、atic gauging method) for traversing is short with respect to the filling time, the timer being started and stopped while the diverter is crossing the hydraulic centreline (this position shall be checked and adjusted, if necessary, using the methods described in annex Ai; the level measuring devices
33、and the means for starting h) the temperature of the liquid flowing through the flowmeter under test is either the same as that collected in the volumetric tank or it is corrected accordingly. well Volumetric tank - Schematic diagram of a volumetric fiow.rate installation using the static gauging me
34、thod 3 _ - STD-BSI BS EN IS0 83Lb-ENGL 1997 W ZbZVbb9 0578975 225 BS 6199 : Part 2 : 1988 Levei r Timer sensing Level sensing element Displacement devices Measuring chamber 7 Volumetric tank -b- 7 L Compressed air supply 7 Flow stabilizer Storage tank I 1 - Control valve Flowmeter Figure 2 - Schemat
35、ic diagram of a volumetric flow-rate installation using the dynamic gauging method 4 5 Apparatus 5.1 Diverter The diverter is a moving device used to direct flow alternately along its normal course or towards the volumetric tank. It can be made up of a moving conduit or gutter, or by a baffle plate
36、pivoting around a horizontal or vertical axis (see figure 31. The motion of the diverter shall be sufficiently fast (less than 0.1 s, for example) to reduce the possibility of a significant error occurring in the measurement of the filling time. This is achieved by ensuring, first, that the diverter
37、 travel across the flow is rapid and, second, that the flow is in the form of a thin stream, which is produced by passing it through a nozzle slot. Generally, this liquid stream has a length 15 to 50 times its width in the direction of diverter travel. The pressure drop across the nozzle slot shall
38、not exceed about 20 kPa to avoid splashing, air entrainment) and flow across the diverter and turbulence in the volumetric tank. The movement of the diverter may be generated by an electrical, mechanical or electro-mechanical device, e.9. by a spring or torsion bar, or by Inflow Nozzle Outflow an el
39、ectrical or pneumatic actuator. The diverter shall in no way influence the flow in the circuit during any phase of the measurement procedure. However, for large flow-rates, which could involve excessive mechanical stresses, a diverter with a proportionately longer travel time (1 to 2 s, for example)
40、 may be used provided that the operating law is constant and any variation in flow-rate distribution as a function of diverter stroke is approximately linear and is in any case known and can be verified. Any hysteresis between the two directions of diverter travel shall also be controlled. In the de
41、sign of the mechanical parts of the diverter and its movement device, care shall be taken to ensure that no leakage or splashing of liquid occurs when liquid is either removed from the volumetric tank or allowed to flow from one diverter channel to the other. This condition shall be checked frequent
42、ly during service. Alternatives to a thin flat liquid stream entering the diverter are acceptable provided that corrections to the diversion time, as indicated in annex A, are applied. . Flow to storage tank Figure 3 - Examples of diverter design Flow to volumetric tank 1) circuit. In certain design
43、s of nozzle slot, however, special vents to allow air inyiess to the fluid jet may be necessary to ensure stable flow within the test 5 - STD*BSI BS EN IS0 83Lb-ENGL 1797 1b24bbS 0578977 UT8 BS 6199 : Part 2 : 1988 5.2 Time measuring apparatus The time of discharge into the volumetric tank is normal
44、ly measured by using an accurate electronic timer, e.g. a quartz crystal timer. The diversion period may thus be read to within 0.01 s or better. The error arising from this source may be regarded as negligible provided that the resolution of the timer display is sufficiently high and the equipment
45、is checked periodically against a national time standard, e.g. the frequency signals transmitted by certain radio stations. The timer shall be actuated by the motion of the diverter itself through an optical, magnetic or other suitable switch fitted on the diverter. The time measurement shall be sta
46、rted for stop- ped) at the instant when the hatched areas shown in figure 4, which represent the diverted flow variations with time, are equal. In practice, however, it is generally accepted that this point corresponds to the mid-travel position of the diverter in the fluid stream. The error will ge
47、nerally be negligible provided that the time of passage of the diverter through the stream is very short in comparison with the period of diversion to the tank. If, however, the error in the filling time measurement arising from the operation of the diverter and the starting and stopping of the time
48、r is not negligible, a correction should be made in accordance with the directions given in annex A. 5.3 Volumetric tank The tank into which the liquid flows during each measuring stage is generally but not necessarily cylindrical in form, with the axis vertical, made of steel or reinforced concrete
49、 with a 10O0A 0% leak-proof lining. Attention shall be paid to the construction materials and protective coatings and to the dimensions so that the bottom and walls of the tank are perfectly leak-proof and rigid enough to retain their shape. If the tank is buried in the ground, it is advisable to provide a clear space around the tank so as to avoid any risk of distortion due to the effect of soil pressure and to make any possible leakage obvious. The walls of the tank shall be smooth in order to avoid water retention and to ensure complete drainage
