NON DESTRUCTIVE EVALUATION ON TURBINE BLADES OF POWER PLANT

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 1, Issue 2007, Jan Dec 2007, pp Available online at Journal Impact Factor (2007): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication NON DESTRUCTIVE EVALUATION ON TURBINE BLADES OF POWER PLANT Dr. S. Ravichandran CEO & Chief Scientist, Trimentus Technologies T. Nagar, Chennai, India 1. INTRODUCTION The turbine is a vital component in a power station. Hence for the continuous and uninterrupted power generation the turbine inspection, particularly the blade inspection becomes very important in a power plant. This paper narrates the applications of Non Destructive Testing methods in evaluation of steam turbines. 2. STEAM TURBINE DETAILS a. Pressure kg/cm 2 b. Temperature C c. Moisture content 0.26% d. Type Two turbine coupled H.P cylinder and double flow L.P. cylinder e. H.P turbine Total 5 nos of Impulse Reaction Design f. L.P turbine 2x5 Stages of impulse Reaction design g. Blade materials H.P Moving S.S %Cr H.P Fixed L.P Moving Low carbon S.S S.S %Cr L.P Fixed Low Carbon S. S. 3. NON DESTRUCTIVE TESTING Non destructive evaluation is a method of engineering analysis in which the detection of material flaws and defect is combined with a prediction of the remaining life of a component with due consideration of the flaws. The detection of flaws by using a variety of physical probing techniques is known as NON-DESTRUCTIVE TESTING (NDT). The role of NDT is to guarantee with a level of confidence that cracks corresponding to a critical size for a fracture at the design load are absent form a component when the component is used in service. One obvious and clear benefit which can be derived from the judicious use of NDT is the identification of defects which, if they remained undetected, could result in a catastrophic failure which would be very costly in money and possibly in lives. NDT can detect, locate, characterize and size the defects without impairing the part under test. 8 editor@iaeme.com

2 Non Destructive Evaluation on Turbine Blades of Power Plant Figure 1 stages in penetrant testing: (a) material surface clean and grease free; (b) penetrant absorbed into defect; (c) excess penetrant removed, but liquid remains in defect; (d) developer applied to surface; (e) penetrant absorbed into developer indication of defect 3.1 TYPES OF NDT Visual Examination (VE) Liquid penetrant testing (LPT) Magnetic particle testing (MPT) Radiography testing (RT) Ultrasonic testing (UT) Eddy current testing (ET) From above mentioned NDT techniques, the tests which are widely used for turbine blade inspection are highlighted below LIQUID PENETRANT TESTING (LPT) LPT is a technique which can be used to detect the defects in wide range of components (whether large or small, of simple or complex configuration, ferrous or non-ferrous metals or alloys), provided that the defect breaks the capillary action in to the defect and after subsequent development, any surface breaking defects, may be rendered visible to human eye. In order to achieve good defect visibility, the penetrating liquid will either be coloured with a bright and persistent dye or else contain a fluorescent compound. In former type the dye is generally red and developed surface is viewed in natural or artificial light, but in later case the component must be viewed under Ultra Violet light if identification of defects are to be seen. The LPT involves five steps: 1. Surface preparation - The component must be cleaned dried before subjected to inspection. 2. Application of penetrant - dwell time 10 minutes. 3. Removal of excess penetrant 4. Application of developer (not applicable for fluorescent penetrant) 9 editor@iaeme.com

3 Dr.S. Ravichandran 5. Observation and inspection MAGNETIC PARTICLE TESTING (MPT) MPT is a sensitive method of locating surface and sub surface defects in ferro-magnetic components only. When a ferro magnetic component is magnetized, magnetic discontinuities that lie in a direction approximately perpendicular to the field. This leakage field is present at and above the surface of the magnetized component and its presence can be visibly detected by the utilization of finely divided magnetic particles. The application of dry particles or wet particles in liquid carrier over the surface of the component results in a collection of magnetic particles at a discontinuity. The "Magnetic bridge" so formed indicates the location, size and shape of the discontinuity. Magnetization may be induced by passing high current through or around the component. The later technique is widely used as high intensity magnetic f ie ld s c an b e ge ner ated wit h in components. Hence good sensitivity in flaw detection is attained. FIGURE 2.1 (a) Current passed through workpiece, including circular magnetisation. (b) Longitudinal magnetisation induced by placing workpiece within a coil FIGURE 2.2 Bar Containing several surface defects editor@iaeme.com

4 Non Destructive Evaluation on Turbine Blades of Power Plant FIGURE 2.3 Magnetic flaw detection. Detectable surface leakage fields produced by defects A and B. Defect C likely to remain undetected ULTRASONIC TESTING (UT) Ultrasonic inspection beams of high frequency (1-25 MHz) are used to detect surface and sub-surface flaws. Flaws are detected by monitoring one or more of the following signals: 1. The reflection of energy from the surfaces of internal discontinuities, metal-gas interfaces or metal-liquid interfaces. 2. The time of transmit of sound wave through a part. 3. Attenuation of a beam of sound waves by absorption and scattering within the metal. 3.2 JUSTIFICA TION OF MPT FOR THE TURBINE BLADE INSPECTION The advantages of MPT over other methods of turbine blades; MPT is very simple as compared to UT. It works with cracks plugged with debris etc. It is a simple procedure to adopt, no sophisticated instruments are required for this. It can show the surface cracks of blades upto a depth of 5mm which is sufficient for turbine blade inspection. And on the contrary LPT can bring information about only surface flaws. UT cannot be used effectively in turbine blade inspection since the blade is of complex profile. This makes the identification difficultto know whether a signal is coming from its geometry or from any flaws. e. Radiography test, cannot be used because of high cost, exposure of personnel in radiation and processing time for film. 4. NDT PROCEDURES FOR TURBINE BLADES 4.1. VISUAL EXAMINATION The visual examination is conducted to determine condition of the part, surface condition such as cracks, wear, corrosion, erosion, pit marks, and physical damage on the surface of the test object. The components subjected to visual examination of turbine components viz: blade root, blade cavity, erosion shield, disc, shaft and Blades. Equipment required are Hand Lamp, Magnifying lens l0x and mirror STEPS 1. Surface preparation Prior to examination, the surface to be examined and adjacent areas within atleast 25mm shall be dry and free of all dirt, grease, lint, scale etc. Cleaning may be accomplished by using a solvent (acetone). Oxide film present may be removed by using 600 emery sheet. However 11 editor@iaeme.com

5 Dr.S. Ravichandran no grinding or filing is permitted to remove hard scale. Sunless otherwise approved specifically. 2. Inspection Direct visual inspection may be conducted when the access is sufficient to place the eye with in 24" of the surface to be examined andat an angle not less than 30 Deg, to the surface. Mirrors may be used to improve the angle of vision and aids such as a magnifying lens with 10% magnification may be used to assist examinations. The inspection areas may be illuminated if necessary with an auxiliary lighting to attain a minimum of 50 ft candle (500 Lux) for general examination. All relevant indications may be evaluated for sizing using in-situ metallography, crack depth meter and/or UT. Recording is done by hand sketch, or Photograph using high contrast technique. 3. Acceptance criterion Any cracks and relevant linear indications are acceptable. Rounded indications are to be reviewed for acceptance or otherwise. 4.2 LIQUID PENETRANT TESTING LPT on turbine shaft, disc, blades and associated components at all accessible locations with the blades in assembled state on disc or disassembled state if opportunity or need arises is done. LPT is intended to serve as a complimentary to magnetic particle testing which precedes LPT. Hence LPT is required to be performed only in such areas where MPT could not be performed due to geometric limitations or inability to magnetize in the required orientation. It is done in areas like, lacing rod, erosion shield, and blades METHOD OF EXAMINATION By using solvent removable fluorescent dye penetrant EQUIPMENT Specification Dye : Magnaflux / Flaw check - ZL 27A Developer : Magnaflux/Flaw check - SKD S2 Cleaner : Flaw check SKC S (NOTE : No cleaning agent for penetrant or developer shall be used which contain halide substances more than 25 ppm level and sulphur content 1% by weight.) PROCEDURE 1. Surface preparation:- Same as described in visual inspection. 2. Temperature: - The temperature of penetrant and part to be inspected shall be maintained between 16 c, and 52 C. If the temperature of the test surface is more than 52 C, dwell time is to be decreased or if it is less than 16 C it shall be increased. 3. Application of penetrant :- The surface to be tested should be thoroughly coated with appropriate penetrant by spraying or brushing. The surface shall be kept wetted for a minimum time of 15 minutes. Whenever the dye becomes dry, reapply. 4. Removal of excess penetrant:- The excess penetrant should be removed with clean, lint free cloth wiping with SKC-S until all the penetrant has been removed. Excessive 12 editor@iaeme.com

6 Non Destructive Evaluation on Turbine Blades of Power Plant application of the cleaner shall be avoided to prevent the possibility of reducing the sensitivity of the test by removing penetrant in discontinuities. 5. Application of developer:- Wet developer shell be uniformly applied to the surface by spraying. Examination and evaluation of indications shall not commence until atleast 30 minutes have elapsed after application of developer. 6. Observing formation of relevant indications under black light:- Use high intensity black light. Appropriate clean filters which will pass near ultraviolet (3650A range) but which filters out short wave length burning rays (which are harmful to the eyes) and most of the visible light, shall be used. To provide adequate contrast, a partially shaded or darkened area shall be used. The background white light should not exceed 20 Lux. The black light intensity should be minimum 1000uW/cm 2 Before using the light allow atleast 5 minutes warm time for the lamp to attain rated intensity LIMITATIONS The LPT is intended to bring out only surface discontinuities and is not meant to reveal any sub-surface defects. To locate such defects complementary examination using MT must be performed. Even with very careful application of dye, it is possible that the interface between disc and blade and the root cavity may act as receptors of dye which will lead out during developing process masking out relevant indications in critical locations like root interface where crack indication is highly probable dueto stress concentration. Hence LPT for root area is not done FINAL CLEANING When the inspection is over the penetrant and developing material shall be removed as soon as possible by means of approved solvents. Having used fluorescent dye, to confirm complete removal of dye, examination with black light is essential PRECAUTIONS While carrying out LPT, the examination is to be done in the good ventilated area. It should be done in the area where no inflammable substances kept. Only from approved supplier the consumable for testing like, penetrant, cleaning agent and developer so that good quality of testing consumable can be used for testing. Normal room temperature recommended for carrying out this testing is 50 deg.c. The test results can be photographed for ensuring permanent record for future reference and evaluation as when required. 4.3 MAGNETIC PARTICLE TESTING This procedure described the technique for wet Fluorescent Magnetic Particle Testing of Turbine blades and associated components. Wet Magnetic Particle is chosen because of its higher sensitivity as compared to Dry Powder Technique editor@iaeme.com

7 Dr.S. Ravichandran APPLICABILITY This procedure applies to MPT of the following components with the blades in assembled state on the disc or disassembled state if opportunity or need arises; FIGURE 3 Cracks in base of blades of different shapes a Possibly detectable while assembled b. in firtree root, detection very difficult. FIGURE 4 Probe for detecting fatigue cracks in turbine discs 1. Turbine Blades, root, lacing rod and shroud. 2. Dise-including at the blade root location editor@iaeme.com

8 Non Destructive Evaluation on Turbine Blades of Power Plant METHOD OF EXAMINA TION Examination shall be done by the continuous method, that is, the magnetising current remains on while the examination medium is being applied MAGNETIZING TECHNIQUES 1. Longitudinal Magnetization Encircling coil method. 2. Multi-directional Magnetization by multi turn coil. 3. Longitudinal Magnetization by Yoke. Magnetization current : 3200 Ampere turns. (800 A X 4 turns) EQUIPMENTS 1. Mobile MPT equipment 2. YOKE (Longitudinal Magnetization) Input: 230 V 50 Hz one phase Input current: 2 Amps Magnetic field: AC or HWDC-pulse infinitely variable. Sub-surface crack detection detection capability AC : - upto 2mm (depth) DC:- upto 4mm 3. Wet Flourescent Magnetic Particle : Product No. i. WT211 ii. 14A Medium : Kerosine with low viscosity of 5 est Particle size : 6 microns 4. Black Light 5. KETO'sTest ring This is a tool used in evaluating and comparing overall performance and sensitivity of both Dry and Wet flourescent technique using central conductor magnetization technique. 6. Field indicator and Magnetic Flux Meter 7. Lux Meter and Residual Field Strength Meter PROCEDURE 1. Surface preparation - Same as that of visual examination. 2. Prerequisites : editor@iaeme.com

9 Dr.S. Ravichandran a. Preparation of wet magnetic powder : The wet magnetic powder WT-211 is mixed in kerosine to obtain a concentration of 1.25 g/lit. b. System performance check Check the overall performance and sensitivity of the system using the test ring specimen. c. Pre-examination demagnetization Prior to inspection residual magnetization check shall be carried out using residual field strength meter which otherwise will reduce the reliability of the test. 3. Longitudinal magnetization encircling coil method: In this 'method the magnetic flux lines would be running radially, and only circumferentially oriented flaws can be detected. a. Set Up : 1. Wind two turns of magnetizing coil on either side of the stage to be tested. 2. Calibrate the magnetic flux density meter as per standard procedure. 3. Switch on black light and allow it to warm up. 4. Switch off the lights in the area so that the background light does not exceed 10 lux. 5. Keep the field indicator ready. 6. Switch on the mobile MT equipment and allow it to warm up for atleast 10 minutes. b. Magnetization Current 1. Check that the MT equipment is switched on. 2. Confirm that the cables are connected for HWDC operation. 3. Increase the current to 8000 A 4. Record magnetic flux distribution in the test area at the beginning of the work and whenever the setup is disturbed. c. Application of wet magnetic flourescent powder: 1. Place the magnetic particle field indicator on the surface to be examined. 2. Spray the powder over the filed indicator uniformly. 3. Confirm that indications appear in the desired orientation. 4 Spray the wet powder carefully and slowly to avoid washing away of fine weakly held indications editor@iaeme.com

10 Non Destructive Evaluation on Turbine Blades of Power Plant d. Observing the formation of indications : 1. Use high intensity black light, (min 1000 uw/cm 2) 2. Appropriate clean filter should be sued. 3. Background white light should not be more than 10 lux. 4. Look carefully all over the test area. 4. Multi directional magnetization by multi turn coil: The magnetic lines of force in this case run in multi direction covering flaws in any orientation. This is achieved by placing the coil over a segment and bending it to form an "U" covering either sides of the stage. The procedure is same as explained above. 5. YOKE method: a. Inspection of blades in assembled state : An AC electromagnetic yoke shall be used for the detection of defect lying parallel to the blade root serrations and also those same plane in the end faces. 1. Turn the supply On 2. Spray the wet magnetic powder uniformly over the entire test area. 3. Place the yoke from one end of the test area and switch On. 4. Place the field indicator on the test area,spray wet magnetic powder and confirm formation of indications on all orientations. 5.Record magnetic flux distribution in the test area at the beginning of the work and whenever setup is disturbed 6.Carefully look of formation of indications. 7.Change the orientation of the Yoke by Again look for formation of indications. 9. Shift the Yoke by another pitch, giving adequate overlap and check for indications. 10. Repeat step 2 to Repeat the process to cover the test area. b. Inspection of blades in dis-assembled state: This is done by placing the Yoke on the surface so that the pole pieces straddle the area to be examined (i.e.,) one pole is placed on the flat bottom of the root and the other on the edge 17 editor@iaeme.com

11 Dr.S. Ravichandran of the blade root packer just above the root serrations. On the convex side of the root this latter location is a wide face giving a good seating for the pole piece, but on the concave side is somewhat narrower. Maintain good pole contact LIMITATIONS This procedure with the use of multi techniques for magnetizing is expected to cover flaws in alt orientations. For counter checking LPT may be performed FINAL CLEANING AND DEMAGNETIZATION Thoroughly clean the test object free of magnetic particle powder etc. by flushing with solvent (Acetone) Confirm it by black light. Demagnetization can be done by passing the part through a high intensity alternating current coil equal to or greater than the Ampere turns applied for the test and then slowly withdrawing the part from the field of the coil. AC Yokes may be used for local demagnetization by placing the poles on the surface, moving them around the area and slowly withdrawing the Ac Yoke while it is still energized. 5.0 NDT EXECUTION 5.1 LP TURBINE INSPECTION TESTS DONE ON BLADE Following examinations and testing were done as per the standard procedures. 1. Visual examination 2. Dye penetrant examination 3. Magnetic particle examination 4. Ultra Sonic examination for crack depth measurement. 5. Crack depth measurement using DC differential voltage VISUAL EXAMINATION Visual examination both Pre-cleaning and Post cleaning were done on LP turbine assembly and no relevant defects were visually seen except foreign particles hit marks 0.5 to 1.0 mm on 5th stage LP blades editor@iaeme.com

Turbine Blade FIGURE 6 Photograph of Magnetic Particle testing

12 Non Destructive Evaluation on Turbine Blades of Power Plant FIGURE 5 Photograph view of liquid Penetrant Testing of Turbine Blade FIGURE 6 Photograph of Magnetic Particle testing of Turbine Blade 19 editor@iaeme.com

13 Dr.S. Ravichandran LIQUID PENETRANT TESTING LPT was performed covering about 50% of the area Identified. It was observed that the fluorescent dye penetrant was entering inside blade root-disc interface and the root cavity. This was causing excessive bleeding out of the dye from these interfaces and forming a thick red smear, masking possible relevant indications at the fir tree edges of the root WET FLUORESCENT MAGNETIC PARTICLE TESTING (WPMT) a. Magnetization technique 1. Longitudinal magnetization by encircling coil 2. Multi - directional magnetisation by multi - turn coil b. Interpretations and evaluation Details of relevant indications observed during WFMT Sl. Stage Front/Rear and Outlet Position Approx. Size inmm Depth length 1. 5 F (0) > F (0) > F (0) F (0) (UT) Depth CDM CRA CK DEPTH MEASUREMENT: (CDM) Crack depth as given in the above table was measured using ultrasonic method and crack depth meter. The readings obtained by these two methods were found to vary considerably, perhaps due to the unfavourable crackorientation and complex geometry of the test object. Hence these readings are to be taken as indicative only CONCLUSION From the relevant indications it was concluded that the cracks noticed in the 4 blades (3 in the front and 1 in the rear) are not accepable and they were replaced. 5.2 INSPECTION OF HP TURBINE The above mentioned tests were carried out on this turbine. No relevant indications were noticed and as such the equipment is considered fit for normal service. 6. DISCUSSION ON TURBINE INSPECTION The Turbine design & technology is ever growing field and right from the initial stage many modifications have been carried out. Keeping in phase with the technology development and from the knowledge gained by the past experience. The operating experience and the problems related to turbine suggested many modifications in existing technology in order to avoid such problem thereafter. Though the computer applications such as simulation techniques, Finite Element Analysis gives a great deal of accuracy in design and stress calculations, also the modern developments in manufacturing methods can be applied for its trouble free operation.here are the few suggestions to reduce the down time of the unit which can be considered in the future both during inspection editor@iaeme.com

14 Non Destructive Evaluation on Turbine Blades of Power Plant Instead of the kerosine based "Magnetic inks" water based inks can be used in WFMT. The UT for blades can be standardized by getting the response signal from the known crack positions and form the healthy blade. This can be used to evaluate the cracks in blades quickly. In modern design of turbines the casing itself is having some provision to inspect the LP last stage blades and roots with the help of some special tools, without removing the casing. This reduces the inspection outages drastically. Such arrangements may be possible in our existing turbine sets also. The signals from the good blades free from defects and the blades with the artificial defects of the required sizes can be recorded through signal analysers and evaluation of blades can be done through the signal processing and analysing. Later the software can be developed for detection of flaws. Periodic measurement of moisture at turbine on steam to ensure moisture separation in steam. REFERENCES [1] Barry Hull, Venron John, Non-Destructive Testing ELBS edition. [2] Modem Power Station Practice. Volume 'E' Chemistry and Metallurgy. British Electricity International, London 3rd edition [3] Non Destructive Inspection and Quality Control, American Society for Metals, Vol [4] Krautkrammer, Testing of Materials by Ultrasonic Testing, Fourth ed., Narosa Publishers, New Delhi. AUTHOR PROFILE Dr S. Ravichandran is a Software Quality Professional. He is also a Six Sigma Master Black Belt with academic qualifications ME,MBA,Ph.D.He has carried out software quality appraisals, consultancy and training services in India and over 20 countries such as China, Brazil, Singapore, Malaysia, US, Europe, Middle East etc He is Presently the Chairman, CEO and Chief Scientist of Trimentus Technologies, Chennai & US. He also well known expert in engineering quality including NDT techniques editor@iaeme.com

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