1、Designation: E 2337 04Standard Guide forMutual Inductance Bridge Applications for Wall ThicknessDeterminations in Boiler Tubing1This standard is issued under the fixed designation E 2337; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revis
2、ion, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide describes a procedure for obtaining relativewall thickness indications in ferromagnetic a
3、nd 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 acceptable wa
4、llthickness 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 tutorial guide a
5、ddressing 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 well as experi
6、enced 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 specify test obj
7、ects 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 thickness info
8、rmation, 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 enough that in m
9、anyinstances 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 favor ofpro
10、viding a thorough treatment of the principles by whichexamination methods can be developed or existing onesrevised.1.5 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 to establish appro-priate
11、safety and health practices and determine the applica-bility of regulatory requirements prior to use.2. Referenced Documents2.1 ASTM Standards:2E 1316 Terminology for Nondestructive Examinations3. Terminology3.1 DefinitionsThe definitions of terms relating to con-ventional magnetic examination metho
12、ds can be found inTerminology E 1316.3.2 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 electrical property of circuit
13、sthat 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 wallthickness in tubular ve
14、ssels. In this procedure, an appropriatemagnetic field is first induced into two identical sections offerromagnetic or 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 coil
15、s are identical, one is designated as thereference coil and is left in place with the other (probe) coilbeing moved to a section of pipe with unknown thickness. Theelectrical effect of the tubing is to modify the inductance of the1This guide is under the jurisdiction of ASTM Committee E07 on Nondest
16、ruc-tive Testing and is the direct responsibility of Subcommittee E07.07 on Electro-magnetic Methods.Current edition approved January 1, 2004. Published February 2004.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual
17、Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.coil used to generate the field, and the resulting voltage readingbecomes propo
18、rtionate to a change in mass of steel in the field.Based on this comparison the section of tubing is judged 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 p
19、iping without any removal of surface contami-nants. The Mutual Inductance Bridge (MIB) described heremeets these 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
20、 iscapable of detecting many larger flaws in large, metallicsystems with repeating elements with somewhat less 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 roughhand
21、ling by unskilled operators acceptable, and it is fast toapply compared to competing techniques. It can, therefore, 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 i
22、n usein industry today where the consequences of in-use failures ofloss of life or personal injury. The systems often occur in verylarge industrial installations where inexpensive components arestrictly limited to costly down time. Such failures rarely resultin nondestructive examination techniques
23、(eddy-current, dye-penetrant, ultrasound, X-ray and more) that might easily detectnearly 100 % of incipient 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 environme
24、nts, or require veryexpensive equipment and highly skilled operators. The overallsituation can be summarized 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 nearl
25、y 100 % certain to induce verycostly failures. The purpose of the MIB is to access the middleground. The MIB system is substantially, but not 100 %effective in locating relatively large flaws in industrial systemsthat exhibit spatially repetitive or translationally invariantstructures, the simplest
26、example of which is an array of tubesthat might be used in a heat exchanger, and for which theexample we discuss 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 exch
27、angers, but may be applied tomany systems. The measurements are average values takenover the volume of the generated 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.
28、4.1.2 This guide is intended to provide a practical introduc-tion to MIB-based nondestructive examination, highlightingsuccessful applications and outlining failures, limitations, andpotential weaknesses. MIB voltage signals are considered fromthe perspective of flaw detection in 4.2. In 4.2.2, revi
29、ews ofsome of the types of MIB measurements are presented.4.2 Operating PrinciplesFor a satisfactory understandingof 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/dtwher
30、e:V = the amount of induced voltage in volts,N = the number of turns of wire, anddF/dt = the rate of change of 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
31、a change in voltage. This is given by:VL5 di/dtwhere:VL= the induced voltage in volts,L = the value of self inductance 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 fl
32、owing in oneconductor (or coil) to induce a current in a nearby conductor(or coil). This is given by:M 5 k=L1L2where: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 sai
33、d to be coupled when they arearranged so that a changing magnetic field created by one ofthe coils can induce 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 s
34、kin depth reflects the exponential decayof magnetic field intensity into the conducting component andis defined 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
35、 is reduced when the frequency, permeabil-ity, or conductivity is increased. Since the complex geometryof the materials under examination, such as the webbing ontubes, and the nonlinear dependence of the magnetic perme-ability on magnetic field intensity also affects the field distri-bution in the m
36、aterial, the effective skin depth is best foundempirically and the skin depth relation is most 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 w
37、all material is to modify the self inductance of the coilE2337042that is used to generate the magnetic field. For example, at 60Hz, the field will fully penetrate a low carbon steel wall ofapproximately 0.100 inches. The change in self inductance L isa complex variable that can be expressed in real
38、and imaginaryparts (mathematical notation) and which depends on the totalvolume of metal in the effective region of the coil, including itsgeometry. For our purposes, it is the sensitivity of L to the totalvolume and geometry of metal in the region of sensitivity ofthe coil that will enable detectio
39、n of wall erosion or majorvoids. Unfortunately, substantial (that is, large enough toindicate replacement 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 simp
40、le in a laboratory, butimpossible in a coal fired power generation boiler. A problemexists from an environment where accurate instruments aresubject to rough handling and temperature changes. Anotherimportant factor is that the tubes often come from manydifferent lots, including different manufactur
41、ers, and so “know-ing” a good value of L would turn into a bookkeepingnightmare, as the value of 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 r
42、eliably, andcompare readings of suspect sections. This process still re-quires very accurate readings, 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
43、is, however, another approach that eliminatesthe need for an accurate measurement system. This approach 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 t
44、odifferences, various external perturbations, disastrous for thedirect precision measurement of L, 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 po
45、tentiometer (P1) provide“real” or dissipative balance adjustment so that a very smallresidual signal 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
46、that a very small residual signal is observed when both thereference and probe coils are placed on 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 emplo
47、yed. These enable the very low impedance of thecoils to be increased 100 fold, thereby greatly reducing 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
48、 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) Re
49、member that the application is focused on powerplant boilers, where electricity contains many harmonics, andthe use of a sine-wave inverter and storage battery could renderthe system portable and insensitive to the harmonic content.4.2.2 Application Example:4.2.2.1 To perform an examination, a reference coil isplaced around a tube, or aligned flush with the surface andcentered over the joint between two of the water wall tubes.The second or probe coil is placed around or on a similarsection of pipe and the bridge circuit is balanced. The referencecoil is kept in place while th
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