# PD ISO TR 12767-2007

PUBLISHED DOCUMENT PD ISO/TR 12767:2007 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 ICS 17.120.10 PD ISO/TR 12767:2007 This Published Document was published under the authority of the Standards Policy and Strategy Committee on 28 September 2007 © BSI 2007 ISBN 978 0 580 55215 1 National foreword This Published Document is the UK implementation of ISO/TR 12767:2007. It supersedes BS ISO/TR 12767:1998 which is withdrawn. The UK participation in its preparation was entrusted by Technical Committee CPI/30, Measurement of fluid flow in closed conduits, to Subcommittee CPI/30/2, Differential pressure methods. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Amendments issued since publication Amd. No. Date Comments Reference number ISO/TR 12767:2007(E)TECHNICAL 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 débit des fluides au moyen d appareils déprimogènes — Lignes directrices relatives aux effets des divergences par rapport aux spécifications et aux conditions de fonctionnement données dans l ISO 5167 PD ISO/TR 12767:2007ii iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references . 1 3 Terms and definitions. 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 orifice 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 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 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 Bibliography . 32 PD ISO/TR 12767:2007iv 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 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. In exceptional circumstances, when a technical committee has collected data of a different 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 the 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 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/TR 12767:1998), which has been technically revised. PD ISO/TR 12767:2007v Introduction 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 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 deviations. 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. PD ISO/TR 12767:2007 blank 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 differential 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, installation 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 error 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 eliminating 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 installations 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 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 5167-1:2003, Measurement of fluid flow by means of pressure differential devices inserted 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 flow 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 PD ISO/TR 12767:20072 3 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, when 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 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 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 dimensionless 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 bore, 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 velocity LT −1m/s Y Modulus of elasticity of orifice-plate material ML −1 T −2Pa β 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 PD ISO/TR 12767:2007 3 Table 1 (continued) Symbol Quantity represented Dimensions M: mass L: length T: time SI units ε Expansibility (expansion) factor dimensionless λ Friction factor dimensionless ρ Fluid density ML −3kg/m 3ρ 1Fluid density at the upstream pressure tapping ML −3kg/m 3σ yYield stress of orifice-plate material ML −1 T −2Pa 5 Effect of errors 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 given by Equation (1): 4 2 1 4( 1) (2 ) mq C = d p β ε ρ − π∆(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 produces 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 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 meter 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 in 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 calculated. 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, should be added arithmetically to that of the discharge coefficient when estimating the overall uncertainty in the flowrate measurement. PD ISO/TR 12767:20074 6 Effects of deviations in construction 6.1 Orifice-plate edge sharpness Orifice plates that do not have the specified 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 0,001, 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 β values below 0,7, but they can be used as a general guide for other values. Measurement techniques for edge radius 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 = 100 mm (Reference [58]) c change in discharge coefficient r/d radius ratio Figure 1 — Effect of edge radius on discharge coefficient PD ISO/TR 12767:2007 5 6.2 Thickness of orifice edge For orifice plates, the increase in discharge coefficient due to excessive thickness of the orifice edge (see 5.1.5 of ISO 5167-2:2003) can be appreciable. With a straight-bore orifice plate in a 150 mm pipe, the changes in discharge coefficient shown in Figure 2 were obtained (see Reference [2]). Key 1 section of an orifice plate 2 symbol 3 limit of standard c change in discharge coefficient E e /D orifice thickness to upstream in