ISO-8316-1987.pdf

IS0 INTERNATIONAL STANDARD 8316 First edition 1987-10-01 - - INTERNATIONAL ORGANIZATION FOR STANDARDIZATION ORGANISATION INTERNATIONALE DE NORMALISATION MEXJJYHAPOAHAR OPrAHM3AL(MR fl0 CTAHflAPTM3AL(MM Measurement of liquid flow in closed conduits - Method by collection of the liquid in a volumetric tank Mesure de d4bit des liquides dans /es conduites ferm.Ges - M b) its viscosity is sufficiently low so as not to alter or delay unduly the measurement of the level in the volumetric tank; cj 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 I,5 m3/s and is used on the 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 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 for 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, time and temperature), and error analysis in accordance with this International Standard and IS0 5158, shall also constitute certification for legal metrology purposes. Annex A forms an integral part of this International Standard. Annexes B to E, however, are given for information only. 2 References 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 frow in open channels - Water level measuring devices. IS0 5158, Measurement of fluid flow - Estimation of uncer- tainty of a flow-rate measurement. 3 Symbols and definitions 3.1 Symbols (see also IS0 4005) Table 1 Symbol e R 44 % ES qm qv t V Z e Quantity Random uncertainty, in absolute terms Random uncertainty, as a percentage Systematic uncertainty, in absolute terms Systematic uncertainty, as a percentage Mass flow-rate Volume flow-rate Filling time of the tank Discharged or measured volume Liquid level in the tank Density Dimensions * * - MT-1 L3T-1 T L3 L ML-3 SI unit * - * - kg/s ma/s rz3 m kg/ma l The dimensions and units are those of the ouantities in ouestion. Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/18/2007 21:32:47 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6316 : 1667 (E) 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- ticular 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 (i.e. gaugings), 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 being delivered into the gauging tank, (A 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 system, 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. 4 Principle 4,l 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 - to determine the initial volume of liquid contained in the tank; - 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 in the tank, The volume contained at the initial and at the final times is obtained by reading the liquid levels in the tank and by reference to a .preliminaty calibration which gives the level-volume relationship. The flow-rate is then derived from the volume of liquid col- lected and the filling time as explained in clause 7. 2 One variation of this method uses two tanks which are suc- cessively filled (see 6.3). A further variation, 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 measurement 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 (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. Instan- 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 whole filling period. 4.2 Accuracy of the method 4.2.1 Overall uncertainty in the volumetric measurement 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 one 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 carefullv constructed, maintained and used, an uncertainty of f 0,l b) the conduit is running full at the measuring section and there is no vapour or air-lock between the measuring sec- tion and the volumetric tank; Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/18/2007 21:32:47 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO6316:1667(E) c) there is no accumulation (or depletion) of liquid in a part of the circuit by thermal contraction (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.; 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 specified in 5.5; :/ Constant level f) the level measuring devices and the means for starting and stopping the timer achieve the required accuracies; g) the time required by the diverter (for the static 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 A); hl 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. - Pump - _- +-Em _ _ _ _-._- - 4 w.w Volumetric tank Stilling well Figure 1 - Schematic diagram of a volumetric flow-rate installation using the static gauging method Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/18/2007 21:32:47 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6316 : 1667 (El r Timer 1-4 -:_I 1 Q Level sensing element Displacement devices Measuring chamber Volumetric tank- 4 1 I air sUpply Level kensing /elemc mt Flow stabilizer Storage tank . 6 Pump Control valve Flowmeter Figure 2 - Schematic diagram of a volumetric flow-rate installation using the dynamic gauging method 4 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/18/2007 21:32:47 MDTNo reproduction or networking permitted without license from IHS -,-,- ISO6316:1967(E) 5 Apparatus 5.1 Diverter an electrical or pneumatic actuator. The diverter shall in no way influence the flow in the circuit during any phase of the measurement procedure. 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 pivoting around a horizontal or vertical axis (see figure 3). The motion of the diverter shall be sufficiently fast (less than 0,l 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 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 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.g. by a spring or torsion bar, or by 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) 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 design 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 frequently 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. - Nozzle to storage tank Figure 3 - Examples of diverter design Flow to volumetric tank II In certain designs of nozzle slot, however, special vents to allow air ingress to the fluid jet may be necessary to ensure stable flow within the test circuit. 5 Copyright International Organization for Standardization Provided by IHS under license with ISO Licensee=NASA Technical Standards 1/9972545001 Not for Resale, 04/18/2007 21:32:47 MDTNo reproduction or networking permitted without license from IHS -,-,- IS0 6316 : 1967 (EI 5-2 Time measuring apparatus The time of discharge into the volumetric tank is normally measured by using an accurate electronic timer, e.g. a quartz crystal timer. The diversion period may thus be read to within 0,Ol 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 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 started (or stop- pedl 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 generally 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 timer 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 with a 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 of the tank. The tank shall be large enough to ensure that any errors in tim- ing and in level measurements are negligible; moreover, it is necessary for the ratio of