BS ISO 7194-2008 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 cu.pdf

1、BS ISO 7194:2008ICS 17.120.10NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBRITISH STANDARDMeasurement offluid flow in closedconduits Velocity-area methods of flowmeasurement inswirling or asymmetricflow conditions incircular ducts bymeans of current-meters or Pitot staticstu

2、besThis British Standardwas published under theauthority of the StandardsPolicy and StrategyCommittee on 31 August2008 BSI 2008ISBN 978 0 580 62862 7Amendments/corrigenda issued since publicationDate CommentsBS ISO 7194:2008National forewordThis British Standard is the UK implementation of ISO 7194:

3、2008. Itsupersedes BS 1042-2.3:1984 which is withdrawn.The UK participation in its preparation was entrusted to TechnicalCommittee CPI/30/5, Velocity based methods.A list of organizations represented on this committee can be obtained onrequest to its secretary.This publication does not purport to in

4、clude all the necessary provisionsof a contract. Users are responsible for its correct application.Compliance with a British Standard cannot confer immunityfrom legal obligations.BS ISO 7194:2008Reference numberISO 7194:2008(E)ISO 2008INTERNATIONAL STANDARD ISO7194Second edition2008-07-15Measurement

5、 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 Mesurage de dbit des fluides dans les conduites fermes Mesurage de dbit dans les conduites circulaires dans le cas

6、dun coulement giratoire ou dissymtrique par exploration du champ des vitesses au moyen de moulinets ou de tubes de Pitot doubles BS ISO 7194:2008ISO 7194:2008(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewe

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9、r use by ISO member bodies. In the unlikely event that a problem relating to it is found, please inform the Central Secretariat 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

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11、 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2008 All rights reservedBS ISO 7194:2008ISO 7194:2008(E) ISO 2008 All rights reserved iiiContents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 1 3 Symbols . 2 4 Principle. 2 5 Choice of meas

12、uring plane 3 6 Devices for improving flow conditions. 3 7 Measurement of local velocities 7 8 Determination of mean flow velocity 14 9 Accuracy of flow-rate estimation 14 Annex A (normative) Detection and removal of regular pressure fluctuations. 17 Annex B (normative) Damping of manometers. 18 Ann

13、ex C (normative) Calculation of Pitot static tube locations for method B. 20 Annex D (normative) Corrections to be applied when a Pitot static tube is used. 22 Annex E (normative) Corrections to be applied when a current-meter is used 23 Annex F (normative) Errors due to non-axisymmetrical velocity

14、distribution 26 Bibliography . 27 BS ISO 7194:2008ISO 7194:2008(E) iv ISO 2008 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 ca

15、rried 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

16、. 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 Intern

17、ational 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

18、 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 7194 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits, Subcommittee SC 5, Velocity and mass methods. This se

19、cond edition results from the reinstatement of ISO 7194:1983 which was withdrawn in 2003 and with which it is technically identical. BS ISO 7194:2008ISO 7194:2008(E) ISO 2008 All rights reserved vIntroduction In order to carry out measurements of the flow-rate of single phase fluids in closed pipes

20、by velocity-area methods, using either current-meters or Pitot static tubes, with satisfactory accuracy (e.g. of the order of 2 %), it is usually necessary to choose a measuring plane where the velocity distribution approaches that of fully developed flow (see ISO 3354 and ISO 3966). There are, howe

21、ver, some cases where it is practically impossible to obtain such a flow distribution, but where as good as possible a measurement of the flow-rate is desirable. BS ISO 7194:2008BS ISO 7194:2008INTERNATIONAL STANDARD ISO 7194:2008(E) ISO 2008 All rights reserved 1Measurement of fluid flow in closed

22、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 1 Scope This International Standard specifies velocity-area methods for measuring flow in swirling or asymmetric flow conditions in circular

23、 ducts by means of current-meters or Pitot static tubes. It specifies the measurements required, the precautions to be taken, the corrections to apply, and describes the additional uncertainties which are introduced when a measurement in asymmetric or swirling flow has to be made. Only flows with a

24、negligible radial component are considered, however. Furthermore, it is not possible to make a measurement in accordance with this International Standard if, at any point in the measuring cross-section, the local velocity makes an angle of greater than 40 with the axis of the duct, or where the inde

25、x of asymmetry Y (defined in Annex F) is greater than 0,15. This International Standard deals only with instruments for measuring local velocity as defined in ISO 3354 and ISO 3966. If Pitot static tubes are used, this International Standard applies only to flows where the Mach number corresponding

26、to local velocities does not exceed 0,25. 2 Normative references The following referenced documents are 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 amend

27、ments) applies. ISO/TR 3313, Measurement of fluid flow in closed conduits Guidelines on the effects of flow pulsations on flow-measurement instruments ISO 3354:2008, Measurement of clean water flow in closed conduits Velocity-area method using current-meters in full conduits and under regular flow c

28、onditions ISO 3455:2007, Hydrometry Calibration of current-meters in straight open tanks ISO 3966:2008, Measurement of fluid flow in closed conduits Velocity area method using Pitot static tubes ISO 4006, Measurement of fluid flow in closed conduits Vocabulary and symbols ISO 5168, Measurement of fl

29、uid flow Procedures for the evaluation of uncertainties BS ISO 7194:2008ISO 7194:2008(E) 2 ISO 2008 All rights reserved3 Symbols For the purposes of this document, the symbols given in ISO 4006, and the following, apply. Symbol Description Dimension SI unit D Pipe diameter L m d Diameter of the head

30、 of a Pitot static tube Diameter of holes or tubes of a straightener L m E Uncertainty, as a relative value e Uncertainty, as an absolute value aakDirectional calibration coefficient l Length of the head of a Pitot static tube L m R Pipe radius L m r Measuring circle radius L m U Mean axial fluid ve

31、locity LT1m/s UiMean velocity along the ith radius LT1m/s v Local velocity of the fluid LT1m/s vxComponent of the local velocity parallel to the pipe axis LT1m/s Y Index of asymmetry of the flow y Distance between the heel of a Pitot static tube and the wall L m y1Distance between the nose of a Pito

32、t static tube and the wall L m Calibration factor of a Pitot static tube p Differential pressure registered by a Pitot static tube ML1T2Pa Expansibility factor Angle of the local velocity with the pipe axis rad b Mass density of the fluid ML3kg/m3 Angle of the local velocity with the metering device

33、 axis rad baThe dimensions and units are those of the quantity to which the symbol refers. bAlthough the radian is the SI unit, for the purposes of this International Standard, angles are expressed in degrees. 4 Principle This International Standard describes methods which minimize the errors in car

34、rying out a traverse in swirling or asymmetric flow; corrections which should be applied for certain sources of error; methods of determining the increase in uncertainty in the flow-rate measurement when it is not possible to compensate for a particular source of error. BS ISO 7194:2008ISO 7194:2008

35、(E) ISO 2008 All rights reserved 3The origins of the errors giving rise to the uncertainties considered in this International Standard are a) errors in the determination of local velocities, due to the behaviour of the instruments in a disturbed flow; b) errors in the calculated mean pipe velocity,

36、due to the number and position of the measuring points and the methods of integration used. Corrections are possible for some of these errors, but, in general, the limiting uncertainty in the flow-rate measurement has to be increased according to the characteristics of the flow. Although velocity-ar

37、ea integration techniques to measure flow-rate under conditions where there is swirl and/or asymmetry in the flow are described, a measuring section in the pipe where the swirl or asymmetry is as small as possible is preferred. 5 Choice of measuring plane When the configuration of the pipe and any f

38、ittings installed in it is such that any changes of directions of the flow are all in the same plane (e.g. a single bend, a single valve, or two bends in an S-shape), no significant bulk swirl is introduced and the disturbance to the flow results in an essentially asymmetric velocity distribution. I

39、f, however, the pipe configuration is such that the flow changes direction in two or more different planes in rapid succession (e.g. two bends at 90 to each other), a bulk swirl is introduced in addition to the asymmetry which the individual fittings introduce. Unlike asymmetry, swirl has a big effe

40、ct on the response of Pitot static tubes and current-meters, and also persists for very much longer distances; whenever possible, therefore, the traverse plane should not be downstream of a swirl-inducing configuration. Care should also be taken to avoid locating the traverse plane downstream of any

41、 adjustable fitting for which the geometry may change (e.g. a flow control valve), especially if several different flow-rates have to be measured. 6 Devices for improving flow conditions 6.1 Where asymmetric or swirling flow is to be measured, a device (straightener) for improving flow conditions sh

42、ould be used, if possible. It should be installed as shown in Figure 1. The lengths L1, L2, L3shall fulfil the conditions: L1W 3D; L2W 5D; L3W 2D. These distances should be increased whenever possible, and, where a total straight length of more than 10D exists upstream of the traverse plane, it is b

43、etter to increase the distance between the pipe fitting and the straightener than to increase the distance between the straightener and the traverse plane. 6.2 The choice of straightener is dependent on the nature of the velocity distribution which has to be corrected and on the head loss which can

44、be tolerated. Five types of straightener are described below. 6.2.1 Type A Zanker straightener (see Figure 2) The purpose of this device is to eliminate both swirl and asymmetry. It has a head loss of approximately five velocity heads. The various plates should be chosen to provide adequate strength

45、, but should not be unnecessarily thick. BS ISO 7194:2008ISO 7194:2008(E) 4 ISO 2008 All rights reserved6.2.2 Type B Sprenkle straightener (see Figure 3) The Sprenkle straightener consists of three perforated plates in series, and is particularity effective in eliminating asymmetry. It does, however

46、, have a high head loss (about 15 velocity heads) but two plates or even one plate (with head losses of about 10 and five velocity heads, respectively) can be used if such a high head loss is not acceptable. Although they cannot completely eliminate such severe asymmetry as can the three plates, the

47、y are often sufficient for disturbances such as a single bend. Perforated plate straighteners have some effect in reducing swirl, but are not designed for this; if, therefore, swirl is the dominant type of irregularity in the velocity distribution, one of the other straighteners should be used. 6.2.

48、3 Type C Tube bundle straightener (see Figure 4) The basic purpose of the tube bundle straightener is to eliminate swirl, but it also has some effect in reducing asymmetry. There shall be a minimum of 19 tubes, with a length of at least 20 times the diameter of the tubes, and each tube shall have a

49、maximum diameter of D/5. The head loss of this straightener depends on the size and length of the individual tubes, but is typically about five velocity heads. 6.2.4 Type D AMCA straightener (see Figure 5) The AMCA straightener is useful only in eliminating swirl; it does not improve asymmetric velocity distributions. Its dimensions are given in Figure 5, and it has a very low head loss, normally about 0,25 times the velocity head. 6.2.5 Type E toile straighte