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IS 15539 (2004): Recommended practice for magnetic particleexamination of steam turbine rotor blades [MTD 21:Non-Destructive Testing]

-

IS 15539:2004

Indian Standard

RECOMMENDED PRACTICE FORMAGNETIC PARTICLE EXAMINATION OF

STEAM TURBINE ROTOR BLADES

ICS 27.040; 77.040.20

@ BIS 2004

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

December 2004 Price Group 3

Non-destructive Testing Sectional Committee, MTD21

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the Non-destructive Testing Sectional Committee had been approved by the Metallurgical Engineering Division Council.

Magnetic particle flaw detection is used to locate cracks, discontinuities at orjust below the surface of ferromagneticturbine rotor blades.

This standard does not prescribe any acceptance criterion for turbine rotor blades, however, that after indicationshave been produced, they must be interpreted or classified and then evaluated according to the specific requirement.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value,observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance with1S2: 1960 ‘Rules for rounding off numerical value (revised)’. The number of significant places retained in therounded off value should be the same as that of the specified value in this standard.

IS 15539:2004

Indian Standard

RECOMMENDED PRACTICE FORMAGNETIC PARTICLE EXAMINATION OF

STEAM TURBINE ROTOR BLADES

1 SCOPE

This standard describes the general procedures for wetfluorescent magnetic particle examination, a non-destructive test method for detecting surface andnear surface cracks and other discontinuities inferromagnetic turbine rotor blades. This recommendedpractice would produce repeatable results and alsoprecise enough to use as a basis for the preparation ofspecific procedure to suit the individual type of turbineblades.

This standard does not indicate, suggest, or specifyacceptance standards for turbine rotor blades inspectedit shouIdbe pointed out, however, that after indicationshave been produced, they must be interpreted orclassified and then evaluated according to the specificrequirement. For this purpose, there should be aseparate standard to define the type, size, location, areaof concentration and orientation of indications that areunacceptable in each type of turbine blade.

2 SUMMARY OF METHOD

The magnetic particle test method is based on theprinciple that magnetic lines of force when present ina ferromagnetic material will be distorted by a changein material continuity, such as sharp dimensionalchange or a discontinuity. If the discontinuity is openor near to the surface of magnetized material, flux lineswill be distorted at the surface, a condition termed fluxleakage. When fine magnetic particles are distributedover the area of the discontinuity while the flux leakageexists, they will be held in place and the accumulationof particles will be visible under proper lightingconditions.

3 EQUIPMENT

3.1 Types

There are various types of magnetic particle equipmentavailable such as yokes, portable units, mobile units,stationary units, etc. Selection of a specific type ofequipment should depend on the intended application,the type and magnitude of the magnetizing currentrequired and the desired speed of inspection.

Various control test given in the Annex A are

recommended for ensuring the reliable magneticparticle testing.

4 MAGNETIC PARTICLE MATERIALS

4.0 For the inspection of turbine rotor blades, sincevery fine service induced discontinuities such as tightfatigue cracks open to surface are expected, wetmagnetic particle examination shall be employed. Wetmagnetic particle examination technique uses fineferromagnetic particles which have been treated toimpart colour (fluorescent and non-fluorescent) inorder to make them highly visible against thebackground of the surfaces being examined. Themagnetic particles generally have high permeabilityyto allow ease of magnetization and low retentivity sothat they will not be attracted to each other. Wetmagnetic particles are designed to be suspended in avehicle such as water or oil at a given concentrationfor application to the test surface by flowing, sprayingor pouring. The wet particles are available in bothfluorescent and nonfluorescent concentrates. The liquidvehicles should not be used under the extremeenvironmental conditions such as ternperature above135 ‘C. Some sort of mixing equipment is usuallyrequired to keep wet particles in suspension (desirableproperties of a suspension vehicle are given in AnnexA). The wet fluorescent method usually is performedindoors or in areas where shelter and proper applicat ionequipment are available. Fluorescent wet particles glowwith a bright yellow-green colour when viewed underblack light. Non-fluorescent particles are usually b!ackor reddish brown, although particles with other coloursare available. The colour often chosen for any givenexamination should be one that contrasts maximumwith the blade surface.

4.1 Particle Concentration

For fluorescent particles, the recommended settlingvolume is from 0.1 to 0.5 ml in a 100 ml bath sampleand from 1.2 to 2.4 ml per 100 ml of vehicle for non-fluorescent particles unless otherwise specified by theparticle manufacturer.

5 PART PREPARATION

The surface of the rotor blades to be examined should

IS 15539:2004

be essentially clean, dry and free of contaminates suchas oil, grease, loose rust, loose scale, etc. Cleaning ofthe test surface may be accomplished by detergents,organic solvents or mechanical means.

6 SEQUENCE OF OPERATION

The sequence of operation in magnetic particleexamination is related to the application of particlesand magnetizing current. In practice, it involves bathingthe blade with the inspection medium to provide anabundant source of suspended particles on the surfaceof the test surface and terminating the bath applicationsimultaneously with the initiation of the magnetizingcurrent flowing. The duration of the magnetizingcurrent is typically of the order of 0.5s.

The three basic types of current used in magneticparticle examination to establish magnetization arealternating current a.c., single phase half waverectified alternating current (HW) and three phase fullwave rectified alternating current (FWRC). Theinductance associated with a.c. results in a skin effectthat confines the magnetic field to the surface of apart Hence a.c. can be advantageously used for thedetection of surface breaking defects such as fatiguecracks. In contrast, both HW and FWDC producea magnetic field having maximum penetratingcapabilities and are used when near surfacediscontinuities are also of concern.

7 MAGNETIZATION TECHNIQUES

A set of turbine blades or a single blade can bemagnetized either directly or indirectly. For directmagnetization, the magnetizing current is passeddirectly into the part creating a circular magnetic fieldin the part. With indirect magnetization techniques, amagnetic field is induced in the blade which can createa circular, longitudinal or multi-directional magneticfield in the blade.

For the inspection of turbine blades, indirect methodof magnetization using coils and cables should be usedto avoid any possible arcing that can happen as in thedirect method of magnetization. In the coil or cablewrap technique, the magnetic field strength isproportional to the current and inversely proportionalto the thickness of the blade section being inspected.

Alternating current electromagnetic yokes provideeffective means of magnetization of a confined regionof interest that is the root region of the blade, for thedetection of surface discontinuities. This method isvery sensitive for the detection of service inducedfatigue type discontinuities. Half wave rectified directcurrent electromagnetic yokes, however, provide aneffective means for near surface discontinuities aswell.

8 DIRECTION OF MAGNETIC FIELDS

Since indications are not normally obtained when,discontinuities are parallel to the magnetic field, andsince discontinuities may occur in various or unknowndirections in a blade each blade must be magnetized atleast in two directions approximately at right angles toeach other.

9 MAGNETIC FIELD STRENGTH

9.1 To produce satisfactory indications, the magneticfield in the turbine rotor blade must have sufficientstrength. For the indications to be consistent, the fieldstrength must be controlled within reasonable limits,usually 25 percent. Factors that affect the strength ofthe field are size, shape and material of the blade andthe technique of the magnetization. Since these factorsvary widely, it is diftlcult to establish rigid rules forfield strength for different geometries. For newgeometries it is best to experiment with a blade havingknown discontinuities and by determining the actualrequirements. In general, the field indicator can beeffectively used for finding the direction and adequacyof magnetic field for detection of discontinuities of allpossible orientations.

9.2 Overall Magnetization

Overall magnetization in the longitudinal directionis produced by passing a current through a multi turncoil encircling (wraping) the rotor shaft. Thisproduces a magnetic field on all blades of the rotorstage(s) covered and also the surfaces of rotor androtor to blade root region. The direction of themagnetic field produced is parallel to the axis of thecoil. The unit of measurement is ampere turns (N I).The effective field extends on either side of the coilto a distance approximately equal to the radius of thecoil being employed. Turbine rotors with many stagesshould be examined, examination shall be carried outin a shot of a few stages at a time. The basic formulaemployed for determining the ampere turns whencable wrap technique is used to produce overallmagnetization is:

NI =K

(L ID)+2

where

I=~=

D=

L=

K=

NI =

coil current, ampere;

number of turns in the coil or cable wrap;

part diameter, mm;

part length, mm;

35000 ampere turns; and

ampere turns.

For magnetizing the blades in the circumferential

2

direction, cable loop in the form of a garland shall beused. Yoke may also be used for establishing field atany required direction by properly positioning theyoke. The field strength of yokes can be empiricallydetermined by measuring its litling power.

10 APPLICATION OF WET MAGNETICPARTICLES

Wet fluorescent magnetic particles suspended in avehicle at a recommended concentration may beapplied either by spraying or flowing over the areas tobe inspected during the application of the magnetizingfield current (continuous technique) or after turningoff the current (residual technique). Since fine orweakly held indications on highly finished or polishedsurfaces may be washed away, care must be taken toprevent high-velocity flow over critical surfaces andto cut-off the bath application before removing themagnetic field. Since a residual field has a lowermagnetic flux intensity, for sensitive defect detection,continuous method shall be used rather then residualmethod of magnetization.

11 1NTERPRETATION OF INDICATIONS

Indications formed as a result of magnetic flux leakagefields may be relevant or non-relevant indicationscaused by particles being held by reasons other thanleakage fields are false.

Relevant indications are produced by flux leakage fieldswhich are the result of discontinuities that may or maynot be acceptable. Relevant indications should beevaluated with regard to the acceptable limits asapplicable.

Non-relevant indications can occur singly or in patternsas a result of flux leakage fields created by conditionssuch as changes in section, inherent material properties,etc.

12 RECORDING OF INDICATIONS

When required, permanent record of the indicationsmay be made by one or more of the following means.

12.1 Sketching the Indication(s)

indications are recorded by hand.

12.2 Covering the Indication(s)

a) With transport adhesive-backed tape,removing the tape with the magnetic particleindication(s) adhering to it, and placing it onpaper or other appropriate backgroundmaterial, and

b) With a spray-on strippable film that fixes theindication(s) in place. When the film isstripped from the part, the magnetic particleindication(s) will adhere to it.

IS 15539:2004

12.3 Photographing

The indications themselves, the tape, or the strippablefilm reproductions of the indications is photographed.Discontinuities along with relative locations on thecomponent are indicated in the photograph.

12.4 Recording the Location, Length and Numberof Indications

A record of the parameters listed below and asapplicable should accompany the inspection resu Its:

a) Magnetic particle technique,

b) Magnetizing process,

c) Magnetizing current,

d) Magnetic current strength, and

e) Direction of magnetic field.

13 DEMAGNETIZATION

13.1 All ferromagnetic materials will retain someresidual magnetism, the strength of which is dependenton the retentivity of the turbine blade material. Aresidual field may permit chips to adhere to thesurface affecting subsequent clean ing process. Whenrequired, an acceptable level of residualmagnetization and its measurement method shall alsobe specified. In general demagnetization isaccompanied by subjecting the turbine blade to a fieldequal to or greater than that used to magnetize, thencontinuously reversing the field direction whilegradually decreasing it to zero.

13.2 Decreasing Alternating Current

In this technique, the turbine blade is subjected to thefield and gradually reducing its strength to a desiredlevel.

13.3 Reversing Direct Current

In this technique, the blade is subjected toconsecutive steps of reversed and reduced directcurrent magnetization to a desired level. Thistechnique requires special equipment for reversingthe current while simultaneously reducing it in smallincrements.

Effectiveness of the demagnetizing operation can beindicated by the use of appropriate magnetic tieldstrength indicators. For complete demagnetization, itis advisable to carry out circular magnetization beforelongitudinal magnetization.

14 POST-INSPECTION CLEANING

Post-inspection cleaning is necessary where magneticparticle materials could interfere with subsequentprocessing or with service requirements. Typical post-cleaning techniques employed are:

3

IS 15539:2004

a) Drying of wet particlesremoval by brushing orand

b) Removal of wet particlessolvent.

and subsequent 15 ACCEPTANCE STANDARDScompressed air,

The acceptability of the turbine rotor blades examined

by flushingby this method is not specified herein. Acceptance

with standard shall be detem”ined taking in to account thedesign requirements.

ANNEX A

(Clause 3.1)

VARIOUS CONTROL TEST FOR RELIABLE MAGNETIC PARTICLE TESTING

A-1 PERFORMANCE TESTING OF MAGNETICPARTICLE TESTING EQUIPMENT

The following tests are recommended for ensuringjhereliable performance of the magnetizing equipment andaccessories.

A-1. 1 Ammeter Accuracy Tests

The equipment meter readings should be verified withthose of a control test meter incorporating a shunt orcurrent transformer connected to monitor the outputcurrent. The equipment reading shall not deviate bymore than ~ 10 percent of the current values as shownby the test meter.

A-1.2 Equipment Current Output Check

To assure the continued accuracy of the equipment,ammeter readings at each transformer tap should be madewith a calibrated ammeter shunt combination. Variationsexceeding ~ 10 percent from the equipment ammeterreadings indicate the necessity for its service or repair.

A-1.3 Internal Short Circuiting Check

Magnetic practice equipment should be checkedperiodically for internal short-circuiting. With theequipment set for maximum amperage output, anydeflection of the ammeter when the current is activatedwith no conductor between the contacts is an indicationof an internal short circuit.

A-1.4 Electromagnetic Yoke Lifting Force

The magnetizing force of a yoke can be tested bydetermining its lifting power on a steel plate. The liftingforce relates to the electromagnetic strength of theyoke. Alternating current electromagnetic yokes shouldhave a lifting force of at least 4.5 kg and for dc yokesit shall be 18 kg at the maximum pole spacing wherethey will be used.

A-1.5 Black Light Intensity Control Test

The black light intensity at the examination surface(380 mm from the face of the light lens filter) shouldnot be less than 800 p W/sq cm when measured with asuitable backlight meter.

A-1.6 Maintenance and Calibration of Equipment

The magnetic particle equipment employed should bemaintained in proper working order at all times. Thefrequency of calibration, usually once a year orwhenever a malfunction is suspected should be specifiedin the general procedures of the testing facility.Calibration tests should be conducted in accordance withany specification that may be applicable.

A-2 DESIRABLE PROPERTIES OF SUSPENSIONVEHICLEGenerally the particles are suspended in a low viscosityoil or conditioned water. Low viscosity, kerosinepetroleum hydrocarbon vehicles are ideal forsuspending both fluorescent and non-fluorescentmagnetic particles and are commonly employed.

Two significant advantages for the use of oil vehiclesare:

a)

b)

magnetic particles are suspended anddispersed in oil vehicles without the use ofconditioning agent, and

oil vehicles provide a measure of corrosionprotection to turbine blades.

Oil vehicles to be used in wet magnetic particleexamination should possess the following:

a) Low viscosity in order not to impede particlemobility;

b) Minimum flash point of 1350C in order tominimize fire hazards;

c) Odourless, not objectionable to user

d) Low inherent fluorescence, if used withfluorescent particles; that is, it should notinterfere significantly with the fluorescentparticle indications; and

e) Non-reactive, that is it should not degradesuspended particles.

Water may be used as a vehicle for wet magneticparticles provided suitable conditioning agents areadded which provide proper particle dispersion, inaddition to corrosion protection for the turbine bladebeing tested and the equipment in use.

4

1S 15539:2004

A-3 LOW VISCOSITY A-5 NON-REACTIVE

The conditioned water should not exceed a maximum The conditioned water should not cause deteriorationviscosity of45 cSt at 90”C. of the suspended magnetic particles.

A-4 NON-FLUORESCENT A-6 ALKALINITY

The conditioned water should not be fluorescent if The alkalinity of the conditioned water should notintended for use with fluorescent particles. exceed a pH of 10.5.

T --

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This Indian Standard has been developed from Doc : No. MTD21 (4303).

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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