1、Designation: E2337/E2337M 10 (Reapproved 2015)Standard 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 ado
2、ption or, in the case 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. Scope1.1 This guide describes a procedure for obtaining relativewall thickness i
3、ndications in ferromagnetic 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
4、 to distinguish acceptable 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) th
5、e need for a tutorial 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
6、and buyers, as well 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 gu
7、ide does not specify test objects and test proce-dures for comparing the relative performance of different MIBsystems; nor does it treat electromagnetic examinationtechniques, such as the best selection of scan parameters, thepreferred implementation of scan procedures, the analysis ofimage data to
8、extract wall thickness 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 applicationspecific, and industrial MIB usag
9、e is new enough 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 i
10、s eschewed in 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 equivale
11、nts; therefore, 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 th
12、is standard to 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
13、ven-tional magnetic examination methods can be found in Termi-nology 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 induct
14、ancethe electrical 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 induction1This guide is under the jurisdiction of ASTM
15、Committee E07 on Nondestruc-tive Testing and is the direct responsibility of Subcommittee E07.07 on Electro-magnetic Method.Current edition approved June 1, 2015. Published June 2015. Originallyapproved in 2004. Last previous edition approved in 2010 as E2337 - 10.DOI:10.1520/E2337_E2337M-10R15.2For
16、 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 Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
17、 West Conshohocken, PA 19428-2959. United States1method for the detection and assessment of variations of wallthickness in tubular vessels. In this procedure, an appropriatemagnetic field is first induced into two identical sections offerromagnetic or non-ferromagnetic tubing through two iden-tical
18、coils, and then a bridge circuit between the two coils isconstructed and balanced from a voltage measurement.Although, the two coils 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. Theelect
19、rical 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 to beeither acceptable or unacceptable.4. Summary of
20、 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 these requirements.4.1.1 The MIB has been used succes
21、sfully 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 less than100 % reliability. However, it uses the syste
22、m 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, therefore, beuseful in detecting non-life-threatening f
23、laws 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 systems often occur in verylarge industrial installation
24、s 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 problems in time to prevent systemfailures are usual
25、ly 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 as follows: It is not cost-effectiveto perform near
26、-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 system is substantially, but not 100 %effective in
27、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 discuss here is optimized. Note that although thefollow
28、ing 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 generated magnetic fields, and shouldnot be considere
29、d 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, highlightingsuccessful applications and outlining fai
30、lures, 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 understandingof the relevant physics behind the MIB, consider
31、ation must begiven to inductance. Faradays Law for a coil tells us that thevoltage induced in a conductor is given by:Vinduced5 Nd/dtwhere:V = the amount of induced voltage in volts,N = the number of turns of wire, andd/dt = the rate of change of flux cutting the conductor orcoil in webers/second.In
32、 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 inductance in henries, H, anddi/dt = the rate of change
33、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=L1L2where:M = the mutual self inductance in henries, H,k
34、= 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 a current in the other coil or conductor.Finally, a si
35、gnificant 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 defined by: 5 =2/where: = the magnetic permeability, = the a
36、ngular frequency, and = the electrical conductivity.E2337/E2337M 10 (2015)2This shows that the penetration of the magnetic field into theconducting material is reduced when the frequency,permeability, or conductivity is increased. Since the complexgeometry of the materials under examination, such as
37、 thewebbing on tubes, and the nonlinear dependence of themagnetic permeability on magnetic field intensity also affectsthe field distribution in the material, the effective skin depth isbest found empirically and the skin depth relation is mostuseful for noting the dependence on the various physical
38、parameters. By using an appropriate frequency, the ac magneticfield can approximately penetrate the wall thickness and theelectrical effect of the wall material is to modify the selfinductance of the coil that is used to generate the magneticfield. For example, at 60 Hz, the field will fully penetra
39、te a lowcarbon steel wall of approximately 2.5 mm 0.100 in. Thechange in self inductance L is a complex variable that can beexpressed in real and imaginary parts (mathematical notation)and which depends on the total volume of metal in the effectiveregion of the coil, including its geometry. For our
40、purposes, itis the sensitivity of L to the total volume and geometry of metalin the region of sensitivity of the coil that will enable detectionof wall erosion or major voids. Unfortunately, substantial (thatis, large enough to indicate replacement at a scheduled main-tenance) changes in wall thickn
41、ess arising from flame erosionor significant internal corrosion might change L by only a fewpercent. To reliably detect a 1 % change in L is simple in alaboratory, but impossible in a coal fired power generationboiler. A problem exists from an environment where accurateinstruments are subject to rou
42、gh handling and temperaturechanges. Another important factor is that the tubes often comefrom many different lots, including different manufacturers,and so “knowing” a good value of L would turn into abookkeeping nightmare, as the value of L for every lot oftubing, and its location in the plant, wou
43、ld need to be tracked.It might, therefore, seem simple to measure a known “good”section of tube at each location, something that can be foundreliably, and compare readings of suspect sections. This pro-cess still requires very accurate readings, something that mustbe avoided if the defects a plant o
44、perator needs to find are to bedetected, and a probable reason that such techniques have notbeen in use.4.2.1 The Bridge Circuit:4.2.1.1 There is, however, another approach that eliminatesthe need for an accurate measurement system. This approach isa “bridge” circuit, where variation in inductance o
45、f 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, affect examination andreference tubes equally, so that the
46、 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 signal is observed when both the reference and probecoils are p
47、laced 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 on good tubing.(3) Each coil should be fabricated using sever
48、al 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 reducing thesensitivity of the system to stray magnetic fields
49、 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 electricity contains many harmonics, andthe use of a sine-wa
