BS ISO 7194-2008 en_5756 Measurement of fluid flow in closed conduits - Velocity-area methods of flow measurement in swirling or asymmetric flow conditions in circular ducts by means.pdf

1、BS ISO 7194:2008 ICS 17.120.10 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW BRITISH STANDARD 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 Pi

2、tot statics tubesThis British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 August 2008 BSI 2008 ISBN 978 0 580 62862 7 Amendments/corrigenda issued since publication Date Comments BS ISO 7194:2008 National foreword This British Standard is the UK im

3、plementation of ISO 7194:2008. It supersedes BS 1042-2.3:1984 which is withdrawn. The UK participation in its preparation was entrusted to Technical Committee CPI/30/5, Velocity based methods. A list of organizations represented on this committee can be obtained on request to its secretary. This pub

4、lication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard cannot confer immunity from legal obligations.BS ISO 7194:2008Reference number ISO 7194:2008(E) ISO 2008INTERNATIONAL STANDARD ISO 71

5、94 Second edition 2008-07-15 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 Mesurage de dbit des fluides dans les conduites fermes Mesurage de dbit dan

6、s les conduites circulaires dans le cas dun coulement giratoire ou dissymtrique par exploration du champ des vitesses au moyen de moulinets ou de tubes de Pitot doubles BS ISO 7194:2008 ISO 7194:2008(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing

7、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 performing the editing. In downloading this file, parties accept therein the responsibility of not infringing Adobes licensing policy. The ISO Cent

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9、en to ensure that the file is suitable for 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

10、publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing 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

11、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 reservedBS ISO 7194:2008 ISO 7194:2008(E) ISO 2008 All rights reserved iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 1 3

12、Symbols . 2 4 Principle. 2 5 Choice of measuring 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

13、B (normative) Damping of manometers. 18 Annex 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)

14、 Errors due to non-axisymmetrical velocity distribution 26 Bibliography . 27 BS ISO 7194:2008 ISO 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 prep

15、aring 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

16、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

17、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

18、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 7194 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits, Subcommit

19、tee SC 5, Velocity and mass methods. This second edition results from the reinstatement of ISO 7194:1983 which was withdrawn in 2003 and with which it is technically identical. BS ISO 7194:2008 ISO 7194:2008(E) ISO 2008 All rights reserved v Introduction In order to carry out measurements of the flo

20、w-rate of single phase fluids in closed pipes 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 flo

21、w (see ISO 3354 and ISO 3966). There are, however, 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 7194:2008 INTERNATIONAL STANDARD ISO 7194:2008(E) ISO 2008 All rights

22、 reserved 1 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 1 Scope This International Standard specifies velocity-area methods for measuring flow in sw

23、irling or asymmetric flow conditions in circular 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

24、swirling flow has to be made. Only flows with a 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

25、 40 with the axis of the duct, or where the index 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 on

26、ly to flows where the Mach number corresponding 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

27、 of the referenced document (including any amendments) 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-

28、 meters in full conduits and under regular flow conditions 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 V

29、ocabulary and symbols ISO 5168, Measurement of fluid flow Procedures for the evaluation of uncertainties BS ISO 7194:2008 ISO 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

30、SI unit D Pipe diameter L m d Diameter of the head 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 a ak Directional calibration coefficient l Length of the head of a Pitot static tube L m R Pipe radius L m

31、 r Measuring circle radius L m U Mean axial fluid velocity LT 1m/s U iMean velocity along the ith radius LT 1m/s v Local velocity of the fluid LT 1m/s v xComponent of the local velocity parallel to the pipe axis LT 1m/s Y Index of asymmetry of the flow y Distance between the heel of a Pitot static t

32、ube and the wall L m y 1Distance between the nose of a Pitot static tube and the wall L m Calibration factor of a Pitot static tube p Differential pressure registered by a Pitot static tube ML 1T 2Pa Expansibility factor Angle of the local velocity with the pipe axis rad b Mass density of the fluid

33、ML 3kg/m 3 Angle of the local velocity with the metering device 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 Internatio

34、nal Standard describes methods which minimize the errors in carrying 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

35、for a particular source of error. BS ISO 7194:2008 ISO 7194:2008(E) ISO 2008 All rights reserved 3 The 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

36、 disturbed flow; b) errors in the calculated mean pipe velocity, 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 a

37、ccording to the characteristics of the flow. Although velocity-area 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 Choic

38、e of measuring plane When the configuration of the pipe and any fittings 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 f

39、low results in an essentially asymmetric velocity distribution. If, 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 indi

40、vidual fittings introduce. Unlike asymmetry, swirl has a big effect 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 als

41、o be taken to avoid locating the traverse plane downstream of any 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 m

42、easured, a device (straightener) for improving flow conditions should be used, if possible. It should be installed as shown in Figure 1. The lengths L 1 , L 2 , L 3shall fulfil the conditions: L 1W 3D; L 2W 5D; L 3W 2D. These distances should be increased whenever possible, and, where a total straig

43、ht length of more than 10D exists upstream of the traverse plane, it is better 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 veloci

44、ty distribution which has to be corrected and on the head loss which can 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 velocit

45、y heads. The various plates should be chosen to provide adequate strength, but should not be unnecessarily thick. BS ISO 7194:2008 ISO 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,

46、 and is particularity effective in eliminating asymmetry. It does, however, 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 cann

47、ot completely eliminate such severe asymmetry as can the three plates, they 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

48、velocity distribution, one of the other straighteners should be used. 6.2.3 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 o

49、f at least 20 times the diameter of the tubes, and each tube shall have a 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,