1、Designation: E2337/E2337M 10Standard Guide forMutual Inductance Bridge Applications for Wall ThicknessDeterminations in Boiler Tubing1This standard is issued under the fixed designation E2337/E2337M; the number immediately following the designation indicates the yearof original adoption or, in the c
2、ase of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This guide describes a procedure for obtaining relativewall thickness indications in fer
3、romagnetic and non-ferromagnetic steels using the mutual inductance bridgemethod. The procedure is intended for use with instrumentscapable of inducing two substantially identical magnetic fieldsand noting the change in inductance resulting from differingamounts of steel. It is used to distinguish a
4、cceptable wallthickness conditions from those which could place tubularvessels or piping at risk of bursting under high temperature andpressure conditions.1.2 This guide is intended to satisfy two general needs forusers of industrial Mutual Inductance Bridge (MIB) equip-ment: (1) the need for a tuto
5、rial guide addressing the generalprinciples of Mutual Inductance Bridges as they apply toindustrial piping; and (2) the need for a consistent set of MIBperformance parameter definitions, including how these per-formance parameters relate to MIB system specifications.Potential users and buyers, as we
6、ll as experienced MIBexaminers, will find this guide a useful source of informationfor determining the suitability of MIB for particular examina-tion problems, for predicting MIB system performance in newsituations, and for developing and prescribing new scan pro-cedures.1.3 This guide does not spec
7、ify test objects and test proce-dures for comparing the relative performance of different MIBsystems; nor does it treat electromagnetic examination tech-niques, such as the best selection of scan parameters, thepreferred implementation of scan procedures, the analysis ofimage data to extract wall th
8、ickness information, or theestablishment of accept/reject criteria for a new object.1.4 Standard practices and methods are not within thepurview of this guide. The reader is advised, however, thatexamination practices are generally part and application spe-cific, and industrial MIB usage is new enou
9、gh that in manyinstances a consensus has not yet emerged. The situation iscomplicated further by the fact that MIB system hardware andperformance capabilities are still undergoing significant evo-lution and improvement. Consequently, an attempt to addressgeneric examination procedures is eschewed in
10、 favor ofproviding a thorough treatment of the principles by whichexamination methods can be developed or existing onesrevised.1.5 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefor
11、e, eachsystem shall be used independently of the other. Combiningvalues from the two systems may result in non-conformancewith the standard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard t
12、o establish appro-priate safety and health practices and determine the applica-bility of regulatory requirements prior to use.2. Referenced Documents2.1 ASTM Standards:2E1316 Terminology for Nondestructive Examinations3. Terminology3.1 DefinitionsThe definitions of terms relating to con-ventional ma
13、gnetic examination methods can be found inTerminology E1316.3.2 Definitions of Terms Specific to This Standard:3.2.1 inductancethe property of an electric circuit ordevice whereby an electromotive force is created by a changeof current in it or in a circuit near it.3.2.2 mutual inductancethe electri
14、cal property of circuitsthat enables a current flowing in one conductor (or coil) toinduce a current in a nearby conductor (or coil).3.2.3 mutual inductance bridge (MIB)a nondestructiveexamination method, which employs a magnetic inductionmethod for the detection and assessment of variations of wall
15、thickness in tubular vessels. In this procedure, an appropriate1This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.07 on Electro-magnetic Method.Current edition approved June 1, 2010. Published July 2010. Origina
16、lly approvedin 2004. Last previous edition approved in 2004 as E2337 - 04. DOI:10.1520/E2337_E2337M-10.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards
17、 Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.magnetic field is first induced into two identical sections offerromagnetic o
18、r non-ferromagnetic tubing through two iden-tical coils, and then a bridge circuit between the two coils isconstructed and balanced from a voltage measurement. Al-though, the two coils are identical, one is designated as thereference coil and is left in place with the other (probe) coilbeing moved t
19、o a section of pipe with unknown thickness. Theelectrical effect of the tubing is to modify the inductance of thecoil used to generate the field, and the resulting voltage readingbecomes proportionate to a change in mass of steel in the field.Based on this comparison the section of tubing is judged
20、to beeither acceptable or unacceptable.4. Summary of the Technology4.1 IntroductionA method was needed to rapidly makeadequate relative wall thickness measurements for a widevariety of steel piping without any removal of surface contami-nants. The Mutual Inductance Bridge (MIB) described heremeets t
21、hese requirements.4.1.1 The MIB has been used successfully as an appliednondestructive testing tool. This non-destructive examinationtechnique is based on a generic electrical circuit. The MIB iscapable of detecting many larger flaws in large, metallicsystems with repeating elements with somewhat le
22、ss than100 % reliability. However, it uses the system under examina-tion to provide in-situ standardization, eliminating a commonproblem. It is very robust, portable and safe, making roughhandling by unskilled operators acceptable, and it is fast toapply compared to competing techniques. It can, the
23、refore, beuseful in detecting non-life-threatening flaws in systems wherea substantial but incomplete reduction in failures is beneficialand 100 % accuracy is not required. There are systems in usein industry today where the consequences of in-use failures ofloss of life or personal injury. The syst
24、ems often occur in verylarge industrial installations where inexpensive components arestrictly limited to costly down time. Such failures rarely resultin nondestructive examination techniques (eddy-current, dye-penetrant, ultrasound, X-ray and more) that might easily detectnearly 100 % of incipient
25、problems in time to prevent systemfailures are usually not cost-effective because they are orders ofmagnitude too slow, use delicate instruments unlikely tosurvive in many industrial environments, or require veryexpensive equipment and highly skilled operators. The overallsituation can be summarized
26、 as follows: It is not cost-effectiveto perform near-100 %-effective tests for some flaws in somelarge industrial systems using existing technology, while at thesame time, such flaws are nearly 100 % certain to induce verycostly failures. The purpose of the MIB is to access the middleground. The MIB
27、 system is substantially, but not 100 %effective in locating relatively large flaws in industrial systemsthat exhibit spatially repetitive or translationally invariantstructures, the simplest example of which is an array of tubesthat might be used in a heat exchanger, and for which theexample we dis
28、cuss here is optimized. Note that although thefollowing description uses heat exchangers as a specificexample, the MIB is by no means limited to either ferromag-netic steel tubing or heat exchangers, but may be applied tomany systems. The measurements are average values takenover the volume of the g
29、enerated magnetic fields, and shouldnot be considered as point values. The system described herewas created to measure the mass variance of identical materialsin two identical magnetic fields.4.1.2 This guide is intended to provide a practical introduc-tion to MIB-based nondestructive examination, h
30、ighlightingsuccessful applications and outlining failures, limitations, andpotential weaknesses. MIB voltage signals are considered fromthe perspective of flaw detection in 4.2.In4.2.2, reviews ofsome of the types of MIB measurements are presented.4.2 Operating PrinciplesFor a satisfactory understan
31、dingof the relevant physics behind the MIB, consideration must begiven to inductance. Faradays Law for a coil tells us that thevoltage induced in a conductor is given by:Vinduced5 NdF/dtwhere:V = the amount of induced voltage in volts,N = the number of turns of wire, anddF/dt = the rate of change of
32、 flux cutting the conductor orcoil in webers/second.In addition, self inductance, usually referred to as simplyinductance L, is the property of a circuit whereby a change ina current causes a change in voltage. This is given by:VL5 di/dtwhere:VL= the induced voltage in volts,L = the value of self in
33、ductance in henries, H, anddi/dt = the rate of change in current in amperes per second.We also need to consider mutual inductance as the electricalproperty of a circuit enabling a current flowing in oneconductor (or coil) to induce a current in a nearby conductor(or coil). This is given by:M 5 k=L1L
34、2where:M = the mutual self inductance in henries, H,k = the coefficient of coupling between the twoconductors, andL1and L2= represent the values of the two inductances.Two conductors are said to be coupled when they arearranged so that a changing magnetic field created by one ofthe coils can induce
35、a current in the other coil or conductor.Finally, a significant physical element underpinning the MIB isthe “skin depth” of the current in effective electrically conduct-ing component. The skin depth reflects the exponential decayof magnetic field intensity into the conducting component andis define
36、d by:d5=2/vswhere: = the magnetic permeability,v = the angular frequency, ands = the electrical conductivity.This shows that the penetration of the magnetic field into theconducting material is reduced when the frequency, permeabil-ity, or conductivity is increased. Since the complex geometryof the
37、materials under examination, such as the webbing onE2337/E2337M 102tubes, and the nonlinear dependence of the magnetic perme-ability on magnetic field intensity also affects the field distri-bution in the material, the effective skin depth is best foundempirically and the skin depth relation is most
38、 useful for notingthe dependence on the various physical parameters. By usingan appropriate frequency, the ac magnetic field can approxi-mately penetrate the wall thickness and the electrical effect ofthe wall material is to modify the self inductance of the coilthat is used to generate the magnetic
39、 field. For example, at 60Hz, the field will fully penetrate a low carbon steel wall ofapproximately 2.5 mm 0.100 in. The change in self induc-tance L is a complex variable that can be expressed in real andimaginary parts (mathematical notation) and which depends onthe total volume of metal in the e
40、ffective region of the coil,including its geometry. For our purposes, it is the sensitivity ofL to the total volume and geometry of metal in the region ofsensitivity of the coil that will enable detection of wall erosionor major voids. Unfortunately, substantial (that is, large enoughto indicate rep
41、lacement at a scheduled maintenance) changes inwall thickness arising from flame erosion or significant internalcorrosion might change L by only a few percent. To reliablydetect a 1 % change in L is simple in a laboratory, butimpossible in a coal fired power generation boiler. A problemexists from a
42、n environment where accurate instruments aresubject to rough handling and temperature changes. Anotherimportant factor is that the tubes often come from manydifferent lots, including different manufacturers, and so “know-ing” a good value of L would turn into a bookkeepingnightmare, as the value of
43、L for every lot of tubing, and itslocation in the plant, would need to be tracked. It might,therefore, seem simple to measure a known “good” section oftube at each location, something that can be found reliably, andcompare readings of suspect sections. This process still re-quires very accurate read
44、ings, something that must be avoidedif the defects a plant operator needs to find are to be detected,and a probable reason that such techniques have not been inuse.4.2.1 The Bridge Circuit:4.2.1.1 There is, however, another approach that eliminatesthe need for an accurate measurement system. This ap
45、proach isa “bridge” circuit, where variation in inductance of each of twoidentical coils reflects differences in the tubing inside the coils,illustrated in Fig. 1. Because the circuit is sensitive only todifferences, various external perturbations, disastrous for thedirect precision measurement of L
46、, affect examination andreference tubes equally, so that the bridge measurement be-comes insensitive to these problems. The circuit elementsperform the following functions:(1) Two resistors (R1) and a potentiometer (P1) provide“real” or dissipative balance adjustment so that a very smallresidual sig
47、nal is observed when both the reference and probecoils are placed on good tubing.(2) An additional two resistors (R2) and another potenti-ometer (P2) provide “reactive” or inductive balance adjustmentso that a very small residual signal is observed when both thereference and probe coils are placed o
48、n good tubing.(3) Each coil should be fabricated using several turns ofcopper wire meeting the specifications of the instrumentmanufacturer.(4) Two high-current, low-frequency power transformersare employed. These enable the very low impedance of thecoils to be increased 100 fold, thereby greatly re
49、ducing thesensitivity of the system to stray magnetic fields and electricalnoise. The transformers provide several amperes of ac to coilsto ensure adequate excitation of ferromagnetic steel. Forstainless steel and other non-ferromagnetic metals, lowerexcitation may be used, but there is no real advantage to this,since signal/noise ratio could be degraded at low drive levels.(5) A means of generating an alternating current signal, forexample a 120V to 60V 60400 Hz power transformer isemployed.(6) Remember that the application is focused on powerplant boilers, where electri
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