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1、 Reference number ISO 3966:2008(E) ISO 2008 INTERNATIONAL STANDARD ISO 3966 Second edition 2008-07-15 Measurement of fluid flow in closed conduits Velocity area method using Pitot static tubes Mesurage du dbit des fluides dans les conduites fermes Mthode dexploration du champ des vitesses au moyen d
2、e tubes de Pitot doubles ISO 3966:2008(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performi
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5、etariat at the address given below. COPYRIGHT PROTECTED DOCUMENT ISO 2008 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writ
6、ing from either ISO at the address below or ISOs member body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2008 All rights reserved ISO 3966:
7、2008(E) ISO 2008 All rights reserved iii Contents Page Foreword iv 1 Scope . 1 2 Normative references. 1 3 Symbols and definitions 2 4 Principle. 4 5 Design of Pitot tubes 7 6 Requirements for use of Pitot tubes. 8 7 Positioning of Pitot tube 11 8 Velocity computation 11 9 Determination of the disch
8、arge velocity by graphical integration of the velocity area 14 10 Determination of the discharge velocity by numerical integration of the velocity area . 17 11 Determination of the discharge velocity by arithmetical methods 19 12 Corrections of local velocity measurements. 23 13 Errors . 28 Annex A
9、(normative) Pitot tubes 34 Annex B (normative) Correction to the measuring position of Pitot tubes used in a transverse velocity gradient . 39 Annex C (normative) Study concerning turbulence correction 41 Annex D (normative) Damping of pressure gauges . 44 Annex E (normative) Measurements with a Pit
10、ot tube in a compressible fluid. 46 Annex F (normative) Determination of coefficient m for extrapolation near the wall . 50 Annex G (normative) Example of calculation of the uncertainty on the flow-rate measurement by means of Pitot tubes. 51 Bibliography. 54 -,-,- ISO 3966:2008(E) iv ISO 2008 All r
11、ights reserved Foreword 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 subjec
12、t 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)
13、 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 committee
14、s 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 he
15、ld responsible for identifying any or all such patent rights. ISO 3966 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits, Subcommittee SC 5, Velocity and mass methods. This second edition results from the reinstatement of ISO 3966:1977 which was withdrawn in
16、 2003 and with which it is technically identical. INTERNATIONAL STANDARD ISO 3966:2008(E) ISO 2008 All rights reserved 1 Measurement of fluid flow in closed conduits Velocity area method using Pitot static tubes 1 Scope This International Standard specifies a method for the determination in a closed
17、 conduit of the volume rate of flow of a regular flow: a) of a fluid of substantially constant density or corresponding to a Mach number not exceeding 0,25; b) with substantially uniform stagnation temperature across the measuring cross-section; c) running full in the conduit; d) under steady flow c
18、onditions. In particular, it deals with the technology and maintenance of Pitot static tubes, with the calculation of local velocities from measured differential pressures and with the computation of the flow rate by velocity integration. 2 Normative references The following referenced documents are
19、 indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 2186, Fluid flow in closed conduits Connections for pressure signal transmissions b
20、etween primary and secondary elements ISO 7194, Measurement of fluid flow in closed conduits Velocity-area methods of flow measurement in swirling or asymmetric flow conditions in circular ducts by means of current-meters or Pitot static tubes -,-,- ISO 3966:2008(E) 2 ISO 2008 All rights reserved 3
21、Symbols and definitions 3.1 Symbols Symbol Quantity Dimensions SI unit A cross-sectional area of the conduit L2 m2 a, a distance of the extreme measuring point to the nearest wall L m D pipe diameter L m d head diameter L m d steam diameter L m di total pressure tapping hole diameter L m H rectangul
22、ar conduit height L m h height of a particular point above the bottom L m kb blockage coefficient of a cylindrical stem kg coefficient depending on the nose shape kt coefficient of turbulence correction L rectangular conduit width L m l distance from a particular point to the side-wall L m M molar m
23、ass of fluid M kg/mol m roughness coefficient Ma Mach number p absolute static pressure of the fluid ML1T2 Pa qV volume flow rate L3T1 m3/s Rg molar constant of gas ML2T11 J/molK R pipe radius L m r measuring circle radius L m Re Reynolds number S frontal projected area of the stem inside the condui
24、t L2 m2 T absolute temperature K U discharge velocity LT1 m/s u mean velocity along a circumference or a measurement line LT1 m/s v local velocity of the fluid LT1 m/s X pipe dimension L m y distance of a measuring point to the wall L m Z gas law deviation factor calibration factor of the Pitot tube
25、 ratio of the specific heat capacities p differential pressure measured by the Pitot tube ML1T2 Pa expansibility factor (1 ) compressibility correction factor universal coefficient for head loss dynamic viscosity of the fluid ML1T1 Pas kv kinematic viscosity of the fluid L2T1 m2/s head loss ML1T2 Pa
26、 density of the fluid ML3 kg/m3 Pitot tube inclination -,-,- ISO 3966:2008(E) ISO 2008 All rights reserved 3 3.2 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.2.1 Pitot static tube “Pitot tube” a tubular device consisting of a cylindrical head
27、attached perpendicularly to a stem allowing measurement of a differential pressure from which the flow rate of the fluid in which it is inserted can be determined, and which is provided with static pressure tapping holes (drilled all around the circumference of the head at one or more cross-sections
28、) and with a total pressure hole (facing the flow direction at the tip of the axially symmetrical nose of the head) 3.2.2 static pressure tapping a group of holes for the measurement of fluid static pressure 3.2.3 total pressure tapping a hole for the measurement of fluid stagnation pressure (the pr
29、essure produced by bringing the fluid to rest without change in entropy) 3.2.4 differential pressure the difference between the pressures at the total and static pressure taps 3.2.5 stationary rake a set of Pitot tubes, mounted on one or several fixed supports, which explore the whole diameter or me
30、asuring section simultaneously 3.2.6 peripheral flow rate the volume flow rate in the area located between the pipe wall and the contour defined by the velocity measuring points which are the closest to the wall 3.2.7 discharge velocity the ratio of the volume rate of flow (integral of the axial com
31、ponent of local velocities with respect to the cross- sectional area) to the area of the measuring cross-section 3.2.8 relative velocity the ratio of the flow velocity at the considered point to a reference velocity measured at the same time and being either the velocity at a particular point (e.g.
32、the centre of a circular conduit) or the discharge velocity in the measuring section 3.2.9 straight length a conduit section, the axis of which is rectilinear and the surface and cross-section of which are constant NOTE The shape of this section is usually circular, but it may be rectangular or annu
33、lar. 3.2.10 irregularity any element or configuration of a conduit which makes it different from a straight length NOTE For the purpose of this International Standard, those irregularities which create the most significant disturbances are bends, valves, gates and sudden widening of the section. ISO
34、 3966:2008(E) 4 ISO 2008 All rights reserved 4 Principle 4.1 General principle The principle of the method consists of: a) measuring the dimensions of the measuring section, which must be normal to the conduit axis this measurement is necessary for defining the area of the cross-section (see 4.2); b
35、) defining the position of the measuring points in the cross-section, the number of measuring points having to be sufficient to permit adequate determination of the velocity profile; c) measuring the differential pressure existing between the total and static pressures of the Pitot tube placed at th
36、ese measuring points (see 4.3) and determining the density of the fluid in the test conditions; d) determining the local velocity of the flow, from given formulae, on the basis of previous measurements (see Clause 8); e) determining the discharge velocity from these values; f) calculating the volume
37、 rate of flow equal to the product of the cross-sectional area and the discharge velocity. Errors in the techniques described in a) to f) contribute to the error in the flow-rate measurement; other sources of error (such as the shape of the velocity distribution and the number of measuring points) a
38、re discussed in Clause 13. The method of measurement and the requirements defined in this International Standard aim at reaching, at the 95 % confidence level, an uncertainty in flow rate not greater than 2 %. To attain this result, it may be necessary, according to measurement conditions, to take i
39、nto account the corrections given in Clause 12. If any of the requirements of this International Standard are not fulfilled, this method may still be applied in special cases but the uncertainty on flow rate will be larger. This International Standard presents three types of methods for determining
40、the discharge velocity. 4.1.1 Graphical integration of the velocity area (see Clause 9) This method consists in plotting the velocity profile on a graph and evaluating the area under the curve which is bounded by the measuring points closest to the wall. To the value thus obtained is added a calcula
41、ted term which allows for the flow in the peripheral zone (the area between the wall and the curve through the measuring positions closest to the wall) on the assumption that the velocity profile in this zone satisfies a power law. For this method, the measuring points may be located at whichever po
42、sitions are required in order to obtain a satisfactory knowledge of the velocity profile. 4.1.2 Numerical integration of the velocity area (see Clause 10) The difference between this method and 4.1.1 lies in the fact that the graphical velocity profile is replaced by an algebraic curve and the integ
43、ration is carried out analytically. 4.1.3 Arithmetical methods (see Clause 11) The arithmetical methods assume that the velocity distribution follows a particular law and the mean velocity in the conduit is then given by a linear combination of the individual velocities measured at the locations spe
44、cified by the method. For the arithmetical methods described in Clause 11, the assumption is made that in the peripheral zone the velocity distribution follows a logarithmic law as a function of the distance from the wall. -,-,- ISO 3966:2008(E) ISO 2008 All rights reserved 5 4.2 Measurement of the
45、measuring cross-section 4.2.1 Circular cross-sections The mean diameter of the conduit is taken as equal to the arithmetic mean of measurements carried out on at least four diameters (including the traverse diameters) at approximately equal angles to each other in the measuring section. Should the d
46、ifference between the lengths of two consecutive diameters be greater than 0,5 %, the number of measured diameters shall be doubled. 4.2.2 Rectangular cross-sections The conduit width and height shall both be measured at least on each straight line (at least four) passing through the measuring point
47、s. Should the difference between the widths (or heights) corresponding to two successive measuring lines be greater than 1 %, the number of measured widths (or heights) shall be doubled. 4.3 Measurement of local velocities 4.3.1 Method of exploring traverse section It is sometimes proposed that seve
48、ral Pitot tubes be mounted on a stationary rake in order to explore simultaneously the whole measuring cross-section. However, the experimental data available at the time of publication are insufficient to allow the design of certain details (such as shape of head and of stem) which would ensure that measurements by a rake would achieve the accuracy required by this International Standard. Therefore, this International Standard deals only with velocity area methods using a single Pitot tube placed successively at each measuring point. 4.3.2 Reference measur