ISO TR 12767-2007 Measurement of fluid flow by means of pressure differential devices - Guidelines on the effect of departure from the specifications and operat.pdf

1、 Reference number ISO/TR 12767:2007(E) ISO 2007TECHNICAL REPORT ISO/TR 12767 Second edition 2007-09-01 Measurement of fluid flow by means of pressure differential devices Guidelines on the effect of departure from the specifications and operating conditions given in ISO 5167 Mesurage du dbit des flu

2、ides au moyen dappareils dprimognes Lignes directrices relatives aux effets des divergences par rapport aux spcifications et aux conditions de fonctionnement donnes dans lISO 5167 ISO/TR 12767:2007(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing po

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6、blication 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 Ge

7、neva 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 2007 All rights reservedISO/TR 12767:2007(E) ISO 2007 All rights reserved iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 1 3 Terms and defin

8、itions. 2 4 Symbols and abbreviated terms . 2 5 Effect of errors on flowrate calculations 3 5.1 General. 3 5.2 Quantifiable effects. 3 6 Effects of deviations in construction 4 6.1 Orifice-plate edge sharpness 4 6.2 Thickness of orifice edge. 5 6.3 Condition of upstream and downstream faces of orifi

9、ce plate 6 6.4 Position of pressure tappings for an orifice 6 6.5 Condition of pressure tappings 7 7 Effects of pipeline near the meter . 7 7.1 Pipe diameter 7 7.2 Steps and taper sections . 8 7.3 Diameter of carrier ring 8 7.4 Undersize joint rings 11 7.5 Protruding welds. 11 7.6 Eccentricity 11 8

10、Effects of pipe layout 14 8.1 General. 14 8.2 Discharge coefficient compensation 14 8.3 Pressure tappings. 16 8.4 Devices for improving flow conditions. 17 9 Operational deviations . 17 9.1 General. 17 9.2 Deformation of an orifice plate 17 9.3 Deposition on the upstream face of an orifice plate. 19

11、 9.4 Deposition in the meter tube . 23 9.5 Orifice-plate edge sharpness 23 9.6 Deposition and increase of surface roughness in Venturi tubes 24 10 Pipe roughness . 26 10.1 General. 26 10.2 Upstream pipe . 27 10.3 Downstream pipe 30 10.4 Reduction of roughness effects 30 10.5 Maintenance 30 Bibliogra

12、phy . 32 ISO/TR 12767:2007(E) iv ISO 2007 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 commi

13、ttees. 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 I

14、nternational 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 Internationa

15、l 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. In exceptional circumstances, when a technical committee has collected data of a different

16、kind from that which is normally published as an International Standard (“state of the art”, for example), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature and does not have to be reviewed until t

17、he data it provides are considered to be no longer valid or useful. 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/TR 12767 was prepared by Tech

18、nical 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/TR 12767:1998), which has been technically revised. ISO/TR 12767:2007(E) ISO 2007 All rights reserved v Introduct

19、ion ISO 5167 (all parts) specifies methods for flowrate measurement using pressure differential devices. Adherence to ISO 5167 (all parts) results in flowrate measurements whose uncertainty lies within specified limits. If, however, a flow-metering installation departs, for whatever reason, from the

20、 conditions specified in ISO 5167 (all parts), the specified limits of uncertainty may not be achieved. Many metering installations exist where these conditions either have not been or cannot be met. In these circumstances, it is usually not possible to evaluate the precise effect of any such deviat

21、ions. However, a considerable amount of data exists which can be used to give a general indication of the effect of non-conformity to ISO 5167 (all parts), and it is presented in this Technical Report as a guideline to users of flow-metering equipment. TECHNICAL REPORT ISO/TR 12767:2007(E) ISO 2007

22、All rights reserved 1 Measurement of fluid flow by means of pressure differential devices Guidelines on the effect of departure from the specifications and operating conditions given in ISO 5167 1 Scope This Technical Report provides guidance on estimating the flowrate when using pressure differenti

23、al devices constructed or operated outside the scope of ISO 5167. Additional tolerances or corrections cannot necessarily compensate for the effects of deviating from ISO 5167 (all parts). The information is given, in the first place, to indicate the degree of care necessary in the manufacture, inst

24、allation and maintenance of pressure differential devices by describing some of the effects of non-conformity to the requirements; and in the second place, to permit those users who cannot comply fully with the requirements to assess, however roughly, the magnitude and direction of the resulting err

25、or in flowrate. Each variation dealt with is treated as though it were the only one present. Where more than one is known to exist, there may be unpredictable interactions and care has to be taken when combining the assessment of these errors. If there is a significant number of errors, means of eli

26、minating some of them have to be considered. The variations included in this Technical Report are by no means complete and relate largely to examples with orifice plates. An example with Venturi tubes has been placed at the end of its section. There are, no doubt, many similar examples of installati

27、ons not conforming to ISO 5167 (all parts) for which no comparable data have been published. Such additional information from users, manufacturers and any others may be taken into account in future revisions of this Technical Report. 2 Normative references The following referenced documents are indi

28、spensable 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 5167-1:2003, Measurement of fluid flow by means of pressure differential devices inserte

29、d in circular cross-section conduits running full Part 1: General principles and requirements ISO 5167-2:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full Part 2: Orifice plates ISO 5167-3:2003, Measurement of fluid flo

30、w by means of pressure differential devices inserted in circular cross-section conduits running full Part 3: Nozzles and Venturi nozzles ISO 5167-4:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full Part 4: Venturi tubes

31、 ISO/TR 12767:2007(E) 2 ISO 2007 All rights reserved3 Terms and definitions For the purposes of this document, the terms and definitions given in ISO 5167-1 and the following apply. 3.1 square edge angular relationship between the orifice bore of the flow-measurement device and the upstream face, wh

32、en the angle between them is 90 0,3 3.2 sharpness radius of the edge between the orifice bore of the flow-measurement device and the upstream face NOTE The upstream edge of the orifice bore is considered to be sharp when its radius is not greater than 0,000 4d, where d is the diameter of the orifice

33、 bore. 4 Symbols and abbreviated terms For the purposes of this Technical Report, the symbols given in Table 1 apply. Table 1 Symbols and units Symbol Quantity represented Dimensions M: mass L: length T: time SI units c Percentage change in discharge coefficient 100( / ) CC dimensionless C Discharge

34、 coefficient dimensionless C cContraction coefficient dimensionless d Diameter of orifice or throat of primary device at operating conditions L m D Upstream internal pipe diameter at operating conditions L m D 1Carrier ring diameter L m D 2Orifice-plate support diameter L m e Relative uncertainty di

35、mensionless E Orifice-plate thickness L m E eOrifice thickness L m k Uniform equivalent roughness L m L 1Distance of upstream pressure tapping from upstream face of plate divided by pipe bore, D dimensionless 2 L Distance of downstream pressure tapping from downstream face of plate divided by pipe b

36、ore, D dimensionless q mMass rate of flow MT 1kg/s r Orifice-plate edge radius L m Re dReynolds number based on throat bore of device dimensionless Re DReynolds number based on upstream pipe diameter dimensionless u Local axial velocity LT 1m/s u CLCentreline axial velocity LT 1m/s U Mean axial velo

37、city LT 1m/s Y Modulus of elasticity of orifice-plate material ML 1 T 2Pa Diameter ratio, (= d/D) dimensionless p Differential pressure ML 1 T 2Pa p yDifferential pressure required to reach orifice-plate yield stress ML 1 T 2Pa ISO/TR 12767:2007(E) ISO 2007 All rights reserved 3 Table 1 (continued)

38、Symbol Quantity represented Dimensions M: mass L: length T: time SI units Expansibility (expansion) factor dimensionless Friction factor dimensionless Fluid density ML 3kg/m 3 1Fluid density at the upstream pressure tapping ML 3kg/m 3 yYield stress of orifice-plate material ML 1 T 2Pa 5 Effect of er

39、rors on flowrate calculations 5.1 General In this Technical Report, the effects of deviations from the conditions specified in ISO 5167 (all parts) are described in terms of changes in the discharge coefficient, C, of the meter. The discharge coefficient, C, of a pressure differential device is give

40、n by Equation (1): 4 2 1 4( 1) (2 ) mq C = d p (1) The sharp edge of an orifice plate ensures separation of the flow and consequently contraction of the fluid stream to the vena contracta. Defining the contraction coefficient, C c , as the ratio of the flow area to the geometric area the orifice pro

41、duces C c 0,6, which mainly accounts for the discharge coefficient, C 0,6. The effect of change in the discharge coefficient is illustrated by the following example. Consider an orifice plate with an unduly rounded edge. The result of this will be to reduce the separation and increase C c , leading

42、in turn to reduced velocities at the vena contracta. The observed differential pressure will therefore decrease. From Equation (1), it can be seen that the discharge coefficient would therefore increase. Alternatively, as C cincreases, so does C. If no correction is made for this change in C, the me

43、ter reading will be less than the actual value. It can therefore be concluded that: a) an effect which causes an increase in discharge coefficient will result in a flowrate reading lower than the actual value if the coefficient is not corrected; and conversely, b) an effect which causes a decrease i

44、n discharge coefficient will result in a flowrate reading higher than the actual value if the coefficient is not corrected. 5.2 Quantifiable effects When the user is aware of such effects and they can be quantified, the appropriate discharge coefficient can be used and the correct flowrate calculate

45、d. However, the precise quantification of these effects is difficult and so any flowrate calculated in such a manner should be considered to have an increased uncertainty. Except where otherwise stated, an additional uncertainty factor, equivalent to 100 % of the discharge coefficient correction, sh

46、ould be added arithmetically to that of the discharge coefficient when estimating the overall uncertainty in the flowrate measurement. ISO/TR 12767:2007(E) 4 ISO 2007 All rights reserved6 Effects of deviations in construction 6.1 Orifice-plate edge sharpness Orifice plates that do not have the speci

47、fied sharpness of the inlet edge (edge radius r u 0,000 4d in accordance with 5.1.7.2 of ISO 5167-2:2003), will have progressively increasing discharge coefficients as the edge radius increases. Tests have shown that the effect on the discharge coefficient, C, is to increase it by 0,5 % for r/d of 0

48、,001, and by about 5 % for r/d of 0,01. This is an approximately linear relationship (see Figure 1 and Reference 1). These values apply particularly to Re dvalues above 300 000 and for values below 0,7, but they can be used as a general guide for other values. Measurement techniques for edge radius

49、are available, but in general it is better to improve the edge sharpness to the required value rather than to attempt to measure it and make appropriate corrections. The effect of nicks in orifice plates has also been measured in Reference 1. Key 1 National Engineering Laboratory (NEL, UK) tests D = 300 mm 2 ISO limit r = 0,000 4d 3 others 4 NEL 5 D = 50 mm (Reference 56) 6 D = 100 mm (Reference 56) 7 D = 150 mm (Reference 34) 8 D = 75 mm (Reference 57) 9 D = 1