1、 ISO 2017 Measurement of fluid flow in closed conduits Guidance for the use of electromagnetic flowmeters for conductive liquids Mesurage du dbit des fluides dans les conduites fermes Lignes directrices pour lutilisation des dbitmtres lectromagntiques dans les liquides conducteurs INTERNATIONAL STAN
2、DARD ISO 20456 First edition 2017-09 Reference number ISO 20456:2017(E) ISO 20456:2017(E)ii ISO 2017 All rights reserved COPYRIGHT PROTECTED DOCUMENT ISO 2017, Published in Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwis
3、e in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below or ISOs member body in the country of the requester. ISO copyright office Ch.
4、 de Blandonnet 8 CP 401 CH-1214 Vernier, Geneva, Switzerland Tel. +41 22 749 01 11 Fax +41 22 749 09 47 copyrightiso.org www.iso.org ISO 20456:2017(E)Foreword iv Introduction v 1 Scope . 1 2 Normative references 1 3 Terms and definitions . 1 4 Symbols 3 5 Theory and basic formulae 4 6 Construction a
5、nd principle of operation . 4 6.1 General . 4 6.2 Sensor . 5 6.3 Transmitter 7 6.3.1 General 7 6.3.2 Alternating magnetic field in the measuring system 7 6.3.3 Measuring system with applied pulsed DC excitation (simplified model) 7 6.3.4 Measuring system with applied AC excitation (simplified model)
6、 . 8 6.4 Flowmeter/Transmitter output . 9 7 Equipment marking 9 7.1 Recommended data 9 7.1.1 Sensor 9 7.1.2 Transmitter 10 8 Installation design and practice 10 8.1 Sensor 10 8.1.1 Sizing .10 8.1.2 Mounting conditions .11 8.1.3 Potential equalization General requirements.12 8.1.4 Electrical connecti
7、ons 13 8.1.5 Sensor mounting .13 8.1.6 Installation dimensions for flanged connections 14 8.2 Transmitter location .15 8.3 Operational considerations .16 8.3.1 General.16 8.3.2 Effect of the liquid conductivity .16 8.3.3 Reynolds number effect 16 8.3.4 Velocity profile effect.16 9 Flowmeter calibrat
8、ion, validation, and verification .16 9.1 Flowmeter calibration .16 9.2 Flowmeter verification ( in-situ electronic verification) 16 10 Evaluation of flowmeter performance 17 10.1 General 17 10.2 Applications within the scope of other standards.17 11 Uncertainty analysis 17 Annex A (informative) Mat
9、erials for construction of sensors .19 Annex B (informative) Practical considerations for measuring system with AC and DC excitation 22 Annex C (informative) Cathodic protection 23 Annex D (informative) Conversion of nominal diameters from metric to US units 24 Annex E (informative) Manufacturers ac
10、curacy specifications25 Bibliography .29 ISO 2017 All rights reserved iii Contents Page ISO 20456:2017(E) 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 nor
11、mally 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
12、the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular t
13、he different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives). Attention is drawn to the possibility that some of the elements of this doc
14、ument may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.o
15、rg/patents). Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement. For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as informat
16、ion about ISOs adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL: www.iso.org/iso/foreword.html. This document was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits, Subcommittee SC 5, Veloc
17、ity and mass methods. This first edition of ISO 20456 cancels and replaces ISO 6817:1992, ISO 9104:1991 and ISO 13359:1998, which has been technically revised.iv ISO 2017 All rights reserved ISO 20456:2017(E) Introduction Clauses 3 to 7 cover the definitions, symbols and basic theory of electromagne
18、tic flowmeters. This document does not cover insertion type meters, partially filled meters or meters for non-conductive and highly conductive fluids. Clause 8 covers installation types and practice, the different types of meter construction, transmitters, lay lengths and sizing, in order to achieve
19、 the best performance of the electromagnetic flowmeter in the field. Clauses 9 to 11 cover some methods of calibration, verification, evaluation, and uncertainty analysis, which can be useful for users or independent testing establishments to verify manufacturers relative performance and to demonstr
20、ate suitability of application The tests specified in this document are not necessarily sufficient for instruments specifically designed for unusually difficult duties. Conversely, a restricted series of tests may be suitable for instruments designed to perform within a limited range of conditions.
21、This document is for users and manufacturers. ISO 2017 All rights reserved v Measurement of fluid flow in closed conduits Guidance for the use of electromagnetic flowmeters for conductive liquids 1 Scope This document applies to industrial electromagnetic flowmeters used for the measurement of flowr
22、ate of a conductive liquid in a closed conduit running full. It covers flowmeter types utilizing both alternating current (AC) and pulsed direct current (DC) circuits to drive the field coils and meters running from a mains power supply and those operating from batteries or other sources of power. T
23、his document is not applicable to insertion-type flowmeters or electromagnetic flowmeters designed to work in open channels or pipes running partially full, nor does it apply to the measurement of magnetically permeable slurries or liquid metal applications. This document does not specify safety req
24、uirements in relation to hazardous environmental usage of the flowmeter. 2 Normative references There are no normative references in this document. 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. ISO and IEC maintain terminological databases for
25、use in standardization at the following addresses: IEC Electropedia: available at http:/ /www.electropedia.org/ ISO Online browsing platform: available at http:/ /www.iso.org/obp 3.1 electromagnetic flowmeter flowmeter which creates a magnetic field perpendicular to the direction of flow, so enablin
26、g the flowrate to be deduced from the induced voltage, U v , produced by the motion of a conducting fluid through the magnetic field Note 1 to entry: The electromagnetic flowmeter consists of a sensor (3.2) and a transmitter (3.3). 3.2 sensor device containing at least the following elements: an ele
27、ctrically insulating meter tube through which the conductive fluid to be measured flows; one pair of electrodes across which the signal generated in the fluid is measured; an electromagnet for producing a magnetic field in the meter tube (3.4) Note 1 to entry: The sensor produces a signal proportion
28、al to the flowrate and, in some cases, a reference signal (3.9). See 6.2. Note 2 to entry: For a sensor, the wording primary device or flowtube has previously been used. INTERNATIONAL ST ANDARD ISO 20456:2017(E) ISO 2017 All rights reserved 1 ISO 20456:2017(E) Note 3 to entry: In some cases, further
29、 electrodes are used such as grounding electrodes, full pipe detection electrodes (empty pipe detection) (see 3.5). 3.3 transmitter equipment which contains the circuitry which drives the field coils and extracts the flow signal Note 1 to entry: This equipment may be mounted directly onto the sensor
30、 (3.2) or remotely, connected to the sensor by a cable. Note 2 to entry: For a transmitter, the wording secondary device, converter or electronic unit has previously been used. 3.4 meter tube pipe section of the sensor (3.2) through which the liquid flows, at least part of whose inner surface is ele
31、ctrically insulating 3.5 measuring electrodes one or more pairs of electrical contacts or capacitor plates by means of which the induced voltage is detected 3.6 lower range value lowest value of the measured variable that a device is set to measure 3.7 upper range value highest value of the measured
32、 variable that a device is set to measure 3.8 span difference between the upper and lower range values (3.6) 3.9 reference signal signal which is proportional to the magnetic flux created in the sensor (3.2) and which is compared in the transmitter (3.3) with the flow signal 3.10 output signal signa
33、l from the transmitter (3.3) which is a function of the flowrate 3.11 Reynolds number dimensionless parameter expressing the ratio between the inertial and the viscous forces Note 1 to entry: For closed pipe flow through an electromagnetic flowmeter (3.1), Reynolds number should be based on the nomi
34、nal diameter of the meter and corresponding mean velocity through a section of that size. 3.12 accuracy closeness of the agreement between the result of a measurement and the (conventional) true value of the measurement Note 1 to entry: The quantitative expression of accuracy should be in terms of u
35、ncertainty (see Annex E). Note 2 to entry: The use of the term precision for accuracy should be avoided.2 ISO 2017 All rights reserved ISO 20456:2017(E) 3.13 uncertaintyrange within which the true value of the measured quantity can be expected to lie with a specified value and confidence level Note
36、1 to entry: See Clause 11. 3.14 calibration factor number, determined by liquid calibration, that enables the output signal (3.10) to be related to the volumetric flowrate 3.15 calibration operation that, under specified conditions, in a first step, establishes a relation between the quantity values
37、 with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication 3.16 verificationmeans of verifying tha
38、t an electromagnetic flowmeter (3.1) is operating correctly, normally with a poorer uncertainty than under controlled laboratory conditions 3.17 calibration validation number of runs (one or more) at flowrates between zero and the upper range value (3.7) in order to verify that the flowmeter does pe
39、rform in the expected way and within the manufacturers specification 3.18 measuring window period of time during which the voltage representing the flow velocity is measured 3.19 ideal flow conditions conditions that exist when a pipe is infinitely long and straight with no internal disturbances Not
40、e 1 to entry: For electromagnetic flowmeters (3.1), it may, in addition, also be assumed that the metering liquid has a viscosity and density similar to water. Under these conditions, the flow is axisymmetric and will be fully developed and turbulent at flowrates and pipe sizes most often found in i
41、ndustry. 4 Symbols Symbol Quantity Units (SI) B magnetic field strength Tesla ( T ) B mean magnetic field strength Tesla ( T ) d inside diameter of meter tube metres (m) a E electric field strength volt per metre (V/m) U c electrochemical voltage volt (V) U t transformer voltage volt (V) U v velocit
42、y related voltage volt (V) F Lorentz Lorentz force newton (N) k 1 constant dimensionless () k 2 constant dimensionless () ISO 2017 All rights reserved 3 ISO 20456:2017(E) Symbol Quantity Units (SI) L e distance between measuring electrodes metres (m) q V volumetric flowrate of the liquid cubic meter
43、s per second (m 3 /s) v mean axial liquid velocity metres per second (m/s) Nabla or Del operator dimensionless () aSee Annex D for a conversion table of nominal diameters from metric to US units. 5 Theory and basic formulae When a conductive liquid moves through a magnetic field, voltage(s), U v , a
44、re generated in accordance with Faradays law (see Formula 2). The strength of the induced voltages is given by the simplified expression shown in Formula (1): (1) ; Spatial integration of Formula (1) results in Formula (2): (2) The volume flowrate in the case of a circular pipe is given in Formula (
45、3): (3) Which, combined with Formula (2), gives Formula (4): (4) Or Formula (5): 2(5) Formula (5) may be interpreted in various ways to produce a calibration factor which in practice is usually determined by wet calibration, as described in 9.1. 6 Construction and principle of operation 6.1 General
46、As indicated schematically in Figure 1, the magnetic field is so placed with respect to a lined meter tube that the path of the conductive liquid, flowing in the meter tube, is normal to the magnetic field. In accordance with Faradays law, motion of the liquid through the magnetic field induces a vo
47、ltage, U v , in the liquid in a path mutually normal both to the field and the direction of liquid motion. By placing electrodes which contact the liquid in insulated mountings or by using insulated electrodes with capacitance-type coupling in the meter tube in a diametrical plane normal to the magn
48、etic field, a 4 ISO 2017 All rights reserved ISO 20456:2017(E) voltage proportional to the flow velocity is produced which can be processed by a transmitter. Meters based on this principle are capable of measuring flow in either direction through the meter tube. Key 1 coil system 2 lined meter tube
49、3 measuring electrodes B magnetic flux density L e distance between measuring electrodes U v flow signal (velocity related voltage) v mean axial liquid velocity Figure 1 Principle of Faradays law The electromagnetic flowmeter consists of a sensor through which the process liquid flows and a transmitter which converts the flow signal generated by the sensor into a stand
