# ISO 9300-2005-08

Reference numberISO 9300:2005(E)©ISO 2005INTERNATIONAL STANDARD ISO9300Second edition2005-08-15Measurement of gas flow by means of critical flow Venturi nozzles Mesure de débit de gaz au moyen de Venturi-tuyères en régime critique Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) © ISO 2005 – All rights reserved iiiContents Page Foreword iv 1 Scope . 1 2 Terms and definitions. 1 2.1 Pressure measurement 1 2.2 Temperature measurement 2 2.3 Venturi nozzles 2 2.4 Flow 2 3 Symbols . 5 4 Basic equations 6 4.1 State equation . 6 4.2 Flow-rate under ideal conditions 6 4.3 Flow-rate under real conditions 6 4.4 Critical mass flux 7 5 Applications for which the method is suitable 7 6 Standard critical flow Venturi nozzles (CFVN).7 6.1 General requirements. 7 6.2 Design 8 7 Installation requirements . 11 7.1 General. 11 7.2 Upstream pipeline. 11 7.3 Large upstream space 12 7.4 Downstream requirements 12 7.5 Pressure measurement 12 7.6 Drain holes 13 7.7 Temperature measurement 13 7.8 Density measurement. 13 7.9 Calculated density 14 8 Calculation methods. 14 8.1 Mass flow-rate. 14 8.2 Discharge coefficient, Cdc14 8.3 Critical flow function, C, and real gas critical flow coefficient, CR15 8.4 Conversion of measured pressure and temperature to stagnation conditions. 15 8.5 Maximum permissible downstream pressure 16 9 Uncertainties in the measurement of flow-rate . 17 9.1 General. 17 9.2 Practical computation of uncertainty . 18 Annex A (normative) Venturi nozzle discharge coefficients . 19 Annex B (normative) Tables of values for critical flow function C— Various gases 21 Annex C (normative) Computation of critical mass flux for natural gas mixtures 28 Annex D (normative) Mass flow correction factor for atmospheric air 32 Annex E (normative) Computation of critical mass flux for critical flow nozzles with high nozzle throat to upstream pipe diameter ratio, E ! 0,25 33 Bibliography . 36 Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) iv © ISO 2005 – All rights reservedForeword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 9300 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits, Subcommittee SC 2, Pressure differential devices. This second edition cancels and replaces the first edition (ISO 9300:1990), which has been technically revised. Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.INTERNATIONAL STANDARD ISO 9300:2005(E)© ISO 2005 – All rights reserved 1Measurement of gas flow by means of critical flow Venturi nozzles 1 Scope This International Standard specifies the geometry and method of use (installation in a system and operating conditions) of critical flow Venturi nozzles (CFVN) used to determine the mass flow-rate of a gas flowing through a system. It also gives the information necessary for calculating the flow-rate and its associated uncertainty. It is applicable to Venturi nozzles in which the gas flow accelerates to the critical velocity at the throat (this being equal to the local sonic velocity), and only where there is steady flow of single-phase gases. At the critical velocity, the mass flow-rate of the gas flowing through the Venturi nozzle is the maximum possible for the existing upstream conditions while CFVN can only be used within specified limits, e.g. Iimits for the nozzle throat to inlet diameter ratio and throat Reynolds number. This International Standard deals with CFVN for which direct calibration experiments have been made in sufficient number to enable the resulting coefficients to be used with certain predictable limits of uncertainty. Information is given for cases where the pipeline upstream of the CFVN is of circular cross-section, or it can be assumed that there is a large space upstream of the CFVN or upstream of a set of CFVN mounted in a cluster. The cluster configuration offers the possibility of installing CFVN in parallel, thereby achieving high flow-rates. For high-accuracy measurement, accurately machined Venturi nozzles are described for low Reynolds number applications. 2 Terms and definitions For the purposes of this document, the following terms and definitions apply. 2.1 Pressure measurement 2.1.1 wall pressure tapping hole drilled in the wall of a conduit in such a way that the edge of the hole is flush with the internal surface of the conduit NOTE The tapping is achieved such that the pressure within the hole is the static pressure at that point in the conduit. 2.1.2 static pressure of a gas actual pressure of the flowing gas which can be measured by connecting a pressure gauge to a wall pressure tapping NOTE Only the value of the absolute static pressure is used in this International Standard. 2.1.3 stagnation pressure pressure which would exist in a gas in a flowing gas stream if the stream were brought to rest by an isentropic process NOTE Only the value of the absolute stagnation pressure is used in this International Standard. Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) 2 © ISO 2005 – All rights reserved2.2 Temperature measurement 2.2.1 static temperature actual temperature of a flowing gas NOTE Only the value of the absolute static temperature is used in this International Standard. 2.2.2 stagnation temperature temperature which would exist in a gas in a flowing gas stream if the stream were brought to rest by an isentropic process NOTE Only the value of the absolute stagnation temperature is used in this International Standard. 2.3 Venturi nozzles 2.3.1 Venturi nozzle convergent/divergent restriction inserted in a system intended for the measurement of flow-rate 2.3.2 normally machined Venturi nozzle Venturi nozzle machined by a lathe and surface polished to achieve the desired smoothness 2.3.3 accurately machined Venturi nozzle Venturi nozzle machined by a super-accurate lathe to achieve a mirror finish without polishing 2.3.4 throat section of minimum diameter of a Venturi nozzle 2.3.5 critical flow Venturi nozzle CFVN Venturi nozzle for which the nozzle geometrical configuration and conditions of use are such that the flow-rate is critical at the nozzle throat 2.4 Flow 2.4.1 mass flow-rate qmmass of gas per unit time passing through the CFVN NOTE In this International Standard, the term flow-rate always refers to mass flow-rate. 2.4.2 throat Reynolds number Rentdimensionless parameter calculated from the gas flow-rate and the gas dynamic viscosity at nozzle inlet stagnation conditions NOTE The characteristic dimension is taken as the throat diameter at stagnation conditions. The throat Reynolds number is given by the formula: nt04mqRedP SDocumento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) © ISO 2005 – All rights reserved 32.4.3 isentropic exponent N ratio of the relative variation in pressure to the corresponding relative variation in density under elementary reversible adiabatic (isentropic) transformation conditions NOTE 1 The isentropic exponent is given by the formula: 2sddpcppUUNU §·¨¸©¹where p is the absolute static pressure of the gas; U is the density of the gas; c is the local speed of sound; s signifies “at constant entropy”. NOTE 2 For an ideal gas, N is equal to the ratio of specific heat capacities J and is equal to 5/3 for monatomic gases, 7/5 for diatomic gases, 9/7 for triatomic gases, etc. NOTE 3 In real gases, the forces exerted between molecules as well as the volume occupied by the molecules have a significant effect on the gas behaviour. In an ideal gas, intermolecular forces and the volume occupied by the molecules can be neglected. 2.4.4 discharge coefficient Cdcdimensionless ratio of the actual flow-rate to the ideal flow-rate of non-viscous gas that would be obtained with one-dimensional isentropic flow for the same upstream stagnation conditions NOTE This coefficient corrects for viscous and flow field curvature effects. For each type of nozzle design and installation conditions specified in this International Standard, it is only a function of the throat Reynolds number. 2.4.5 critical flow maximum flow-rate for a particular Venturi nozzle, which can exist for the given upstream conditions NOTE When critical flow exists, the throat velocity is equal to the local value of the speed of sound (acoustic velocity), the velocity at which small pressure disturbances propagate. 2.4.6 critical flow function Cdimensionless function which characterises the thermodynamic flow properties of an isentropic and one-dimensional flow between the inlet and the throat of a Venturi nozzle NOTE It is a function of the nature of the gas and of stagnation conditions (see 4.2). 2.4.7 real gas critical flow coefficient CRalternative form of the critical flow function, more convenient for gas mixtures NOTE It is related to the critical flow function as follows: RCCZ Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) 4 © ISO 2005 – All rights reserved2.4.8 critical pressure ratio rratio of the static pressure at the nozzle throat to the stagnation pressure for which the gas mass flow-rate through the nozzle is a maximum NOTE This ratio is calculated in accordance with the equation given in 8.5. 2.4.9 back-pressure ratio ratio of the nozzle exit static pressure to the nozzle upstream stagnation pressure 2.4.10 Mach number Ma ¢at nozzle upstream static conditions² ratio of the mean axial fluid velocity to the velocity of sound at the location of the upstream pressure tapping 2.4.11 compressibility factor Z correction factor expressing numerically the deviation from the ideal gas law of the behaviour of a real gas at given pressure and temperature conditions NOTE It is defined by the formula: pMZRTU where R, the universal gas constant, equals 8,314 51 J/(mol·K). 2.5 uncertainty parameter, associated with the results of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) © ISO 2005 – All rights reserved 53 Symbols Symbol Description Dimension SI unit A2Cross-sectional area of Venturi nozzle exit L2m2AntCross-sectional area of Venturi nozzle throat L2 2CdcCoefficient of discharge Dimensionless CRCritical flow coefficient for one-dimensional flow of a real gas Dimensionless CCritical flow function for one-dimensional flow of a real gas Dimensionless C iCritical flow function for one-dimensional isentropic flow of a perfect gas Dimensionless D Diameter of the upstream conduit L m d Diameter of Venturi nozzle throat L m M Molar mass M kg mol1Ma1Mach number at the location of the upstream pressure tapping Dimensionless p1Absolute static pressure of the gas at nozzle inlet ML1T2Pa p2Absolute static pressure of the gas at nozzle exit ML1T2Pa p0Absolute stagnation pressure of the gas at nozzle inlet ML1T2Pa pntAbsolute static pressure of the gas at nozzle throat ML1T2Pa piAbsolute static pressure of the gas at nozzle throat for one-dimensional isentropic flow of a perfect gas ML1T2Pa (p2/p0)iRatio of nozzle exit static pressure to inlet stagnation pressure for one-dimensional isentropic flow of a perfect gas Dimensionless qmMass flow-rate MT1kg·s1qmiMass flow-rate for one-dimensional isentropic flow of an inviscid gas MT1kg·s1R Universal gas constant M L2T241J·mol1K1RentNozzle throat Reynolds number Dimensionless rcRadius of curvature of nozzle inlet L m rCritical pressure ratio pnt/p0DimensionlessUc Relative uncertainty Dimensionless T1Absolute temperature of the gas at nozzle inlet 4 K T0Absolute stagnation temperature of the gas at nozzle inlet 4 K TntAbsolute static temperature at nozzle throat 4 K vntThroat sonic flow velocity; critical flow velocity at nozzle throat LT1m·s1Z Compressibility factor Dimensionless E Diameter ratio d/D DimensionlessJ Ratio of specific heat capacities Dimensionless G Absolute uncertainty a aN Isentropic exponent Dimensionless P0Dynamic viscosity of the gas at stagnation conditions ML1T1Pa·s PntDynamic viscosity of the gas at nozzle throat ML1T1Pa·s U0Gas density at stagnation conditions at nozzle inlet ML3kg·m3UntGas density at nozzle throat ML3kg·m3M mass L length T time 4 temperature a Same as the corresponding quantity. Documento contenuto nel prodotto UNIEDIL IMPIANTI edizione 2006.2E vietato l uso in rete del singolo documento e la sua riproduzione. E autorizzata la stampa per uso interno.ISO 9300:2005(E) 6 © ISO 2005 – All rights reserved4 Basic equations 4.1 State equation The behaviour of a real gas can be described by the formula: pRTZMU§· ¨¸©¹(1) 4.2 Flow-rate under ideal conditions For ideal critical flow to exist, three main conditions