ROLE OF NON-DESTRUCTIVE EXAMINATION (NDE) FOR THE WELDING QUALITY ASSURANCE (QA) IN THE ENGINEERING INDUSTRIES

INTRODUCTION:
NDE - Non-destructive examination or Non-destructive evaluation plays a very significant role in almost all the Engineering industries (employing weld fabrication as a major manufacturing process) as a Quality Assurance - QA measure to ultimately cut down the Quality Cost and thus to enhance the profitability. Right from the raw material selection, all through the entire manufacturing cycle (including all the bought outs procurement) - components to assembly - and up to the final despatch stage of the engineering product, the role played by NDE is highly important. Infact, the professional design engineer uses the confidence provided based on NDE in order to reduce ‘factors of safety’ even though the added advantage thus ensured comes with additional cost element (by multiple NDE methods deployment for welding / manufacture). All the advancements happening in typical weld fabrication industries, viz, in steam boiler manufacturing industry which has gone from 30MW sub-critical boilers (40 years back) to the present day 800MW supercritical boilers and in the immediate future to advanced ultra supercritical boilers of 1000MW, in Nuclear Industry wherein 220MW Steam Generators are no longer constructed due to the advancements in 4 decades and the present standard Indian version of Pressurized Heavy Water Reactor type steam generator is of 700MW, etc. and all these developments have been made feasible mainly because of the QA programme which endorsed additional safety of operation, environmental protection and enhanced operating parameters (like increased thermal efficiency or reduction of auxiliary power consumption) through the adoption of appropriate NDE methods in the respective manufacturing industries and also based on the use of advanced and newer welding compatible raw materials certified using multiple and the state of art NDE methods for the equipment’s construction by weld fabrication. An overview of different NDE methods in the Welding Industries’ QA programme is discussed in this article.

WHAT IS NDE :
Non-destructive examination is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability or further use (after NDE) of the part or system. Discontinuities and differences in material characteristics are more effectively found by NDE. This will ensure product integrity, its performance and reliability by serving as a control element for manufacturing processes. As internal and external failure costs are significantly brought down based on NDE, ultimately lower production costs are achieved and savings by way of reduced after sales services cost also becomes a reality. Besides, uniform Quality level of the product is guaranteed to enhance the customer satisfaction. Basically, some fundamental principles of physics and their consequent effects on materials is used along with the association of some detection methodology (using the principles of physics) are followed in NDE in order to segregate ‘good’ and ‘bad’.

WHY NDE IS CONDUCTED :
Fundamentally, there are 3 main reasons for conducting NDE in order to gain confidence on materials, manufacture and assembly (without affecting or causing deterioration to the part) in an Industry. First reason is to detect the defects or discontinuities beyond the acceptance levels or leakages of operational fluids exceeding the safe limits - on materials, part processed items or systems respectively and in case, if this detection is not taking place, it will lead to impairment of the use or operation or performance of the concerned item (undergoing the test) in its operation. Off course, the next step after detection is repair or rectification or replacement so that ultimate technical requirements would be fulfilled. The second reason for conducting NDE is to ensure the reliability of operation / performance (over longer periods of time) of the material or part or the assembly or the system without the appearance or occurrence of any incipient problems or defects or discontinuities or leakages growing to a stage of non acceptance level (due to continued operation / performance). The third reason is to ensure environmental protection. By conducting NDE of appropriate methods, potential failures or accidents at later date are avoided. Release of poisonous gases, radio active substances, emission levels exceeding the Governmental norms, contamination of water due to industrial releases, pressure vessel failures / explosions,etc. and many more would be under control and thus protecting environment, plants, animals, public and surroundings (includes property also) based on the adequate confidence offered by NDE. The accidents in operational plants or critical equipment are highly catastrophic like Bhopal Gas Tragedy (which resulted in 20000 deaths approximately), Challenger Space Shuttle failure leading to an estimated loss of 3 billion US dollars worth of equipment and 7 lives, Titanic Ship Breakage which killed around 1500 people, Chernobyl Radio Active Accident - affecting 5 lakh people and this is considered as the worst nuclear disaster, etc. and hence in the present era of International Codes Reviews and Up-dates continuously happening, number of additional mandatory Code requirements are being demanded for materials and equipment manufacture which involves extensive use of different welding processes for heavy fabrication and the same are satisfactorily fulfilled using specific NDE methods. Besides, for operating plants / Industries, NDE methods are appropriately adopted for ensuring the operational stability of plant and machinery after the maintenance and precautionary shutdown cum up-keeping activities. The objective of this activity is to determine the residual life of the existing plant along with installation of some replacements or carrying out systematic weld repairs. This Residual Life Assessment (RLA) uses NDE methods for its significant part and professional engineers gain more confidence for the future life of the plant based on these NDE data straight away (if they are maintained equivalent to original data) or based on replacing certain parts of the plant (out of necessity) while comparing the In-Service NDE data (NDE data obtained from critical welds of an already running plant) with the Base Line NDE data (NDE data of the corresponding critical welds obtained at the end of manufacture of the equipment and all associated performance and functional tests, but before taking up equipment operation or its dynamic loading by fluid flows under pressures).

DIFFERENT NDE METHODS :
As per ASNT (American Society for Non-destructive Testing) which is the most significant authority in the field of NDE - based on consolidation of NDE knowledge, administering rules for NDE personnel and methods Qualification and Certification, maintaining the compendium on various methods of NDE in a comprehensive and structured manner, Internationally well known society in this field and constantly maintaining all sorts of updates on NDE theory and practices for the use of Industry and Research, etc. classifies NDE in to 16 different methods. A brief summary of these 16 methods is given in the following TABLE 1.

5.0. VISUAL TESTING (VT) : Visual examination basically uses unaided or aided eyes for the first stage examination every where. Typical visual inspection aids include : Lenses or magnifying glasses, mirrors (dentist mirrors), binoculars, telescopes, optical microscopes, electron microscopes, boroscopes (straight and flexible and are called as fiberoscopes), use of digital cameras and multiple viewing by personnel of different geographical locations, use of optical theodolites and software using the principle of triangulation for accurate determination of dimensions including geometric features on huge jobs (in their manufacture and site assembly), etc. In most of the aids (including unaided eye) good quality light is used for examination and all the principles of light like diffraction, refraction and reflection are extensively used in VT. Normally, this examination is qualitative but nowadays, quantitative examination is conducted accurately on a plan (facing the objective lens in a microscope) but there are advanced equipment used for depth direction measurement also using focussing principle. Few measuring gadgets like tape, vernier, profile gauges, radii gauges and including specific type of high low gauges (as per the guidelines of AWS for the measurement of welds mismatch on the root side) are conveniently used along with Visual Examination of the welds in order to determine dimensional aspects pertaining to the welds and welded assemblies which include welding distortion and welding shrinkage also. As per ASNT, steps involved in the VT process are : surface preparation, inspection, evaluation and reporting. The report making covers simple accepted / not accepted type of Qualitative statements to descriptive Quantitative statements giving the size and orientation of defects and upto the stage of the most advanced forms of visual and dimensional data recording by means of photography and videography and on line recording by using computers with a provision for multiple persons viewing (from geographically different places) and making interpretations simultaneously.

The primary objectives of VT are : detecting surface defects, dimensional verification, welding, forging and casting examinations quickly for gross defects, providing complimentary information supporting other NDE methods, etc. Advantages of VT are : simplicity and accessibility mostly with minimal support of equipment, cost-effectiveness, immediate results, non-invasive as it does not change the material and highly versatile finding its application from simple areas of common life to advanced areas like aerospace applications wherein most advanced remote inspections are followed. Some of the limitations of VT are : subsurface flaws are normally not detectable, effective VT requires direct line of sight, proper lighting is crucial in VT, the examination is subjective which demands experienced and skilled operators, extreme environmental conditions like under water, radiation or high temperature will affect VT results, prior to VT surface preparation and cleaning are required for obtaining reliable results, dependence on visual acuity and other senses of operator, etc.

NDT professionals use VT in the oil and gas industry to inspect pipelines, pressure vessels, heat exchangers, storage tanks, oil rigs and off shore structures for corrosion, cracks, visible leaks, damages to seals, etc. and this covers welds, materials, assemblies and openings. VT is used in power generation industry (where weld fabrication is the most significant production activity) to inspect boilers, turbines, heat exchangers and many other critical equipment for cracks, wear, corrosion, damage etc. Similarly, in the Nuclear industry, VT is used for the observation of cracks, corrosion, erosion, etc on reactor components, steam generators, pressure vessels, pipelines and turbine. VT is used in aerospace industry for the inspection of air craft structure, engines and many other critical items for cracks, corrosion, alignment issues and other surface defects. In the automotive industry, VT is used for inspecting welds, castings and machined parts for defects like cracks, blow holes, porosity, surface irregularities, etc. In marine industry, ship hulls, under water structures, offshore platforms, etc. are examined for all sorts of visual defects affecting safety and longevity of marine items. In railways, VT is used for examining rails, axles, wheels and many other components in a systematic manner to detect all sorts of surface defects to ensure safe riding of trains and public comfort in travel. In manufacturing industries, as an in-process Quality control check, VT is used on welds, forgings, castings, tubes, plates, pipes, raw materials, bought-out items, semi-finished and finished items to ensure compliance to specifications in order to eliminate defective parts going to customers. VT is used to ensure integrity and safety of buildings,bridges, dams, roads and other structures incorporating welds, concrete surfaces, wood works etc. for cracks, misalignments and other surface defects.

Some of the typical examples (but not exhaustive) where VT has played its most significant role in product certification in a welding industries are highlighted here. The observation of machining chip colour and its type (continuous long vs discrete short) of an item (made of alloy steel) is quite likely to throw light about the strength and ductility of the welded part getting machined and instances are common in industries to identify material mix-up by adopting this approach. In the nuclear steam generators or process plant heat ex-changers, tube to tube sheet welding is very critical and the quality level of these welds decide the functional performance of the equipment. Normally, these welds are single pass type circular (all round) ones and manual or semi automatic or automatic welding is done between tiny tubes of few millimeters diameter and a thick tube sheet. In these weds, heat input control during welding is very important especially at weld terminations in order to avoid ‘weld craters’. Professional NDE person resorts to higher magnification VT (of the order of 30X to 50X) using hand held eye pieces observing at craters and presence of star like cracks would indicate higher heat input and the hence the formation of crater cracks. Isolated crater cracks will get repaired and systematic crater cracks would demand revisit of welding parameters to minimize heat input (and hence crater cracks). Application of contrast coatings on critical pressure boundary welds of vessels undergoing proof test is common visual aid for quick and accurate observation of water seepage during pressurization. Cleanliness Certification on ferritic surfaces (material plus welds) and on specially made tube to tube sheet welds (which are of stainless steel or monel or inconel or titanium in Nuclear equipment fabrication industries) is practically done by adopting a simple visual test called as white cloth wipe test (on the completely rust and debris / dirt / dust cleaned surface, gentle rubbing of a lint free white cloth is not supposed to pick up any soil or colouration), Similarly, painting is partly certified using VT by observing initial surface prior to painting visually and peel off free condition after painting. In very critical Nuclear products undergoing hydro static tests at higher pressures, wherein there are no feasibilities for the observation of water leakage due to equipment design / configuration, the code approved engineering practice adopts VT in a convenient manner for the reliable certification of satisfactory hydro test and it is based on continuing the hydro test (equipment under pressure) for 24+ hours and during this period, pressure and temperature measurements (at multiple points) along with certain critical dimensions measurement are carried out.

LIQUID PENETRANT TESTING (PT) :

The basic principle of liquid penetrant testing is that when a very low viscosity (highly fluid) liquid (the penetrant) is applied to the surface of a part, it will penetrate into fissures and voids open to the surface due to capillary action. Once the excess penetrant is removed, the penetrant trapped in those voids will flow back out due to reverse capillary action, creating an indication on the developer applied (which aids reverse capillary action and also serves for the contrast to the penetrant coming out). As per ASNT, the basic steps involved in PT process are surface cleaning cum preparation, penetrant application, excess penetrant removal, developer application, inspection and post cleaning. Here also, similar to VT, all the methods mentioned in VT are followed for reporting of results from the visual observation of PT indications.

Penetrant testing can be performed on magnetic and non-magnetic materials, but does not work well on porous materials. Penetrants may be "visible", meaning they can be seen in ambient light, or fluorescent, requiring the use of a "black" light. Penetrants may be water washable, post emulsifiable or solvent removable considering the method of removal of excess penetrant. Versatility, sensitivity to finer defects detection (with respect to naked eye), cost effective, visual clarity of indications and portability are some of the advantages of PT. Limitations of PT are : detectability for only open to surface defects, non-suitability on porous materials, need to address concerns on health and safety due to chemicals, specific efforts needed for surface preparation prior to PT and post PT cleaning of developer, etc. Salient applications of PT include : in the oil and gas industry - to inspect pipe lines, storage tanks, etc., in power plants - for the inspection of turbines, boilers, etc., in aerospace industry - for the inspection of landing gear, turbine blades, stress corrosion cracking on aluminium skins of air craft, etc., in transportation industry - to inspect engine components, cast aluminium engines, etc., in manufacturing industries - for the inspection of metallic parts, plastic parts, especially stainless steel and other non magnetic materials, etc., in the infrastructure industry - to examine bridges, dams, parts of buildings, etc. Thus, PT is used in a versatile manner in almost all the welding industries for ensuring the structural integrity of components, welded parts (multiple times - at pre-weld stage for inspecting the weld edges, root welding stage and at welding completed stage) and assemblies, as part of in-process QC check and also for detecting fatigue and service induced cracks on critical welds.

Some of the typical examples (but not exhaustive) where PT has played its most significant role in product’s reliability certification are highlighted here. In Nuclear Steam Generators and heat exchangers and very critical process plant equipment, the tube to tube sheet welds are made up of superior materials like stainless steel, monel, inconel or titanium. For the Quality assessment of these welds (which are considered as the most critical items separating the primary and secondary sides in the heat exchange process and absolute leak tightness is demanded for these welds to avoid media mix-up). 100% surface PT is followed for these welds as part of QA programme. To enhance the test sensitivity in PT as the penetrants’ blots grow in size over time and also to detect very fine delayed fissures, after the application of developer, an evaluation time of 4 hours is followed for the observation of penetrant bleed out, if any. Systemmatically, for every 30 minutes, evaluation will be done using suitable illuminators and magnifiers as visual aids and will go on up to 4 hours and this higher dwell time cum evaluation (unlike 30 minutes in normal cases) is one of the specific technical requirements of critical items (like Nuclear) manufacture. To enable, this longer duration evaluation, penetrants and developers are suitably established in a PT procedure qualification which will demonstrate the fluidity of penetrant without air drying and the spatial stability of developers without disturbance during the longer evaluation times. Besides, the test chemicals, viz. cleaner, penetrant and developer are ensured for the control of Sulphur and Halogens. Similar to tube to tube sheet welds, the tube sheet cladding of critical heat exchangers (materials like stainless steel, monel or inconel are weld deposited - called as cladding - on ferritic tube sheets in order to ensure material compatibility between tubes (of respective materials) and tube sheet in the process of tube to tube sheet welding) is also subjected to 100% PT with the evaluation time of 4 hours ensuring higher sensitivity in the test. Another method of enhancing the test sensitivity is to go for fluorescent PT wherein examination is done using ultra violet or black light instead of white light. This sort of longer duration penetrant test is also followed in dissimilar welding between very closely varying chemical composition materials and with the adoption of multiple types of welding fillers (of closely varying chemistry amongst the fillers), for the detection of material mix-ups and for finalization of successful welding parameters. Also, PT is used as a very convenient and mandatory test for the detection of pressure induced defects by carrying out PT on critical pressure boundary welds after the hydro static proof tests. In residual life assessment programme of plant and machinery also, PT plays a very significant role by bringing out service induced surface defects on welds and materials for taking up applicable repair plans.

MAGNETIC PARTICLE TESTING (MT) :
MT involves magnetizing a ferromagnetic material and then applying fine ferromagnetic particles to the surface. Discontinuities in the material such as cracks or voids disrupt the magnetic field creating a leakage field which attract the sprinkled ferromagnetic particles forming a visible indications of the discontinuities. The primary objectives of MT are : to detect and measure the surface and near surface defects (within a depth of 4 to 5mm from surface) and to ensure the integrity and safe operation of critical components and welds in different industries in a versatile manner - like aerospace, automobile and manufacturing and this makes MT very invaluable NDE method. Advantages of MT are : MT is highly effective for surface and subsurface inspection, quick and simple, cost effective compared to many NDE methods, versatile by being capable of offering its usage on ferromagnetic materials including iron, nickel, cobalt, etc and their alloys, various shaped parts and various sized parts, capable of giving results / judgement on item under inspection quickly from the visual scene of MT indications and MT equipment and process offer portability as an important benefit. Limitations of MT are : it is restricted to only ferromagnetic materials, it requires surface preparation, MT’s limiting depth of examination is restricted to few millimeters, post MT de-magnetization of part is required, environmental conditions like higher temperature or humidity are likely to affect results in MT, MT particles pose health and safety concerns as they affect operating personnel demanding proper handling and disposal procedures, etc.

In the MT, magnetic field is introduced into the part being tested using equipment like yokes, coils or prods and the magnetic field is applied by using alternating current or direct current or half wave rectified alternating current based on inspection needs. When magnetism is in place, the sprinkled ferromagnetic particles in solid dry or wet suspension would form a visible coloured line (visibility will be enhanced by contrast coating on parts some times) of ferromagnetic particles clinging at the locations of leakage fields (sites of discontinuities) and then the skilled technician will evaluate the indications and interpret the results. Normally, discontinuities oriented perpendicular to the magnetic field are easily detected in MT. Accordingly, circular magnetization generated around the part by passing current through the part (or a central conductor) is suitable for the detection of longitudinal defects. Longitudinal magnetization produced by placing the part in a coil or using yoke will create magnetic field running along the length of the part and this is suitable for the detection of transverse defects. Normally alternating current is suitable for surface defects detection and direct current is

suitable for sub-surface defects detection. Similar to PT, in MT also ferromagnetic particles are coloured ones used in direct visual examination in normal light or fluorescent particles used with black light offering higher sensitivity. As per ASNT, the basic steps involved in MT process are surface cleaning, Magnetization, ferromagnetic particles application, inspection, demagnetization and post cleaning. Here also, similar to VT, all the methods mentioned in VT are followed for reporting of results from the visual observation of MT indications. As part of huge items MT, when testing a item between the heads in the “head shot method” of MT, the item is placed between the heads, the moveable head is moved up so that the item being tested is held tightly between the heads, the item is wetted down with the bath solution containing the ferromagnetic particles and the current is applied while the wet stream of particles is flowing over the item. Since the current flow is from head to head and the magnetic field is oriented 90 degrees to the current, indications oriented parallel to a line between the heads will be visible. Similarly, when testing hollow items such as pipes, tubes and fittings, a conductive circular bar can be placed between the heads with the item suspended on the bar (the "central conductor"). The item is then wetted down with the bath solution of ferromagnetic particles and the current is applied, travelling through the central conductor rather than through the item. The Inner Diameter and Outer Diameter sides of the item can then be inspected. As with a head shot, the magnetic field is perpendicular to the current flow, wrapping around the test piece, so indications running axially down the length of the item can be found using this technique.

Some of the typical examples (but not exhaustive) where MT has played its most significant role in product’s reliability certification are highlighted here. In oil field equipment manufacture as per API codes, depending upon the progressively increasing criticality and operational requirements of the product, as part of QA programme, for raw materials and for critical in process stages during weld fabrication, dry powder MT, visible wet suspension MT and fluorescent suspension MT are respectively carried out and records of such inspections are preserved for 10 years. In Nuclear Steam Generators, after the completion of all manufacturing activities, final functional tests and proof tests, all the pressure boundary welds are examined using MT and no indications of unacceptable discontinuities are expected. Even, if there are any acceptable indications due to geometric features or any other assignable reasons like surface issues or permeability differences, such details (on indications) are recorded and preserved as part of PSI - Pre-Service Inspection and till the life of the steam generators (in domestic versions, it is 40 years presently), in all the ISI - In-Service Inspection programmes conducted in a planned and systemmatic manner on such pressure boundary welds, MT results comparison between corresponding ISI stages and the initial PSI stage will be made for the maintenance of Quality level of the pressure weld joints in operation - as part of QA programme. For multi layer higher thickness weld joints made at operating sites of power generating plants on high strength low alloy steel materials, during the erection and commissioning activity, volumetric NDE by RT (Radiographic Testing) is not feasible due to radiation safety considerations in open site fields. Full volumetric coverage by UT (Ultrasonic Testing) to detect all the oriented flaws may not be ensured due to non access of these joints from inner diameter side and non availability of longer straight portions of the material on both sides (on outer diameter side) of the weld joint - due to design constraint. Under this circumstances, weld layer wise MT is followed (as an acceptable QA measure) during the welding of the joint. Satisfactory MT in all welding layers is equivalent to satisfactory volumetric NDE by RT or UT. To facilitate this layer wise MT, nowadays, higher temperature MT powders and provisions for magnetization at higher temperature are available and practically MT is conducted at temperatures of the order of 200 to 250 degree C based on a successful and satisfactory procedure Qualification. Lot of creep resistant high strength steels are now being used like T91, T92, T23, etc. for critical pressure parts like super heaters and re-heaters, when thermal power generating plants are going for super critical versions. To establish and certify these materials’ and welds’ Quality level and satisfactory NDE performance, such items (both material and weld) are subjected to wet MT and fluorescent MT as part of QA programme and also as per the governing code guidelines.

RADIOGRAPHIC TESTING (RT) :
Radiographic Testing or Industrial Radiography is a method of viewing the internal conditions and structure of a component using ionizing electro magnetic radiation of X rays or gamma rays to produce visible images on a detection media which is a film by and large. Like doctors seeing damages inside the organs using X rays, NDT inspectors observe volumetric defects like slag in welds, blow holes, shrinkage and cavities in castings and other flawed internal arrangements of assemblies using RT. Being able to detect voluminar and sub-surface defects also, RT is capable of informing about the overall internal soundness of the components even though RT comes with radiation safety related risks demanding detailed protocols and mandatory adherence for operator, environment and public safety. The main objectives of RT are : locating internal defects using visual comparison with known geometric features of the object having internal fabrication errors - void in a casting or slag in a weld, etc. universally accepted volumetric NDE for the certification of internal integrity and the basic RT process itself is capable of creating permanent record of internal condition on radiographic films or digital displays along with sensitivity of inspection. Some of the other distinguishing advantages of RT are : RT is versatile to adopt on most types of materials and welds and offering very fine sensitivities to detect even 1% thickness and density variations along the path of X ray beam. Limitations of RT are : expensive, complex geometries may not get radiographed fully, very small isolated defects of less than 2% of thickness

are usually not detected by RT, requirement of access on both sides of part for RT-one side for radiation exposure and other side for film placement, defects of thin layers present perpendicular to radiation path are not detectable and special requirements on safety, operator training and experience.

The basic steps involved in RT process are : Selection of X ray or gamma ray (using either Iridium 192 or Cobalt 60 radio-active isotopes) radiation at the test part, placement of film or digital sensor behind the part, allowing radiation to pass through the part to the sensor for a set time (decided based on the strength of radiation - energy level and thickness / material type of the part), processing of film / sensor and examination of the image. X ray production requires electricity where as gamma radiation is emitted by small pellet of radio active isotope. Based on detection medium, there are 4 types in RT. They are : film radiography which comes with Image Quality Indicators (IQI) for the determination of radio graph quality level which is used to estimate the sensitivity of RT process, Computed Radiography (CR) which uses reusable phosphor plate or imaging plates which contain photostimulable phosphors that store radiation as latent images and these images are scanned by laser for production of electrical signals and then the electrical signals are converted to a digital image for viewing through high resolution monitors. Digital Radiography (DR) which uses Flat Panel Detectors (FPDs) to capture images directly by converting radiation into electric charges and the electric charges are sent to a processor for assembling the images as digital file and DR is also called as Real Time Radiography (RTR) as radiation falling and display take place almost simultaneously. Computed Tomography (CT) which uses detectors similar to DR but multiple radiographic exposures are taken at different angles to create cross-sectional images of the part and based on this multiple imaging, 3D map of the conditions of the part under NDE is obtained. RT techniques are : single wall single image, double wall single image, double wall double image and panoramic exposure based on the radiographic shot and choice of technique is decided based on cost economics and application.

RT finds its versatile use on various materials, viz. metals, composites and concrete. RT is used in oil and gas industry for inspecting pipe lines, offshore structures and storage tanks for the assessment of corrosion damage and to ensure compliance to safety and environment protection standards. RT is used for the in-process Quality, integrity and reliability certification of Nuclear equipment (during manufacture) and Nuclear plants (during operation). In the Aviation industry, RT is used to ensure the structural integrity of aircraft components, engines and air frames - mainly using CT to certify

compliance to stringent safety standards. In transportation industry, RT is vital for the inspection of ships, submarines, rail engines, etc. for the certification of critical castings (made of copper nickel) for

internal soundness. In manufacturing industries of different categories, like pressure vessels, automobiles, boilers, process equipment, etc., as part of vital certification of internal soundness for materials and inprocess items (like welds and assemblies), RT is used. In infrastructure and construction, RT is used to inspect structural steel fabrication, load bearing welds and many critical items during their construction and maintenance in order to detect cracks, voids and inclusions. Primarily, RT is capable of detecting cluster porosity, blow holes, slag inclusions, shrinkage, lack of penetration, lack of fusion, suck back, internal undercut, external undercut, offset or mismatch, cold or hot cracks, inadequate weld reinforcement, excess weld reinforcement, tungsten inclusions, burn through, etc.on weldments. Blow holes, shrinkage cavities, cold shuts, cold shots, misruns, slags, voids, hot tears, segregation, inclusions, porosity, etc. are the casting defects detectable by RT.

Some of the typical and special examples (but not exhaustive) where RT has played its most significant role in product’s reliability certification are highlighted here. Micro focal radiography is a recent development for critical tube to tube sheet welds of Nuclear heat exchangers and Steam generators. By establishing X ray focal spots of the order of 10 microns using special types of rod anode used in micro focal radiography and with the capability to achieve upto 5X magnification on the detection media, RT sensitivity of the order of 20 microns are achieved. Critical low alloy steel and titanium welds are made between tubes (of thickness of the order 1.5 to 2.3mm) and tube sheets - either in the form of butt weld (where in at weld locations tube sheet is in the form of specially machined spigot with matching thickness as that of tube) or in the form of an end fillet weld (all around the tube fusing the full thickness of tube and tube sheet part surrounding the tube). These tiny welds are radiographed using micro focal method in order to meet the stringent Quality requirements. Similarly, in thermal power plant equipment manufacture, boiler portion consists of many kilometers of tubes of different materials, viz. carbon steel, low alloy steels of different types, stainless steel and tubes of super alloys with thickness ranging from 4 mm to 10 mm and the choice of tube material and thickness are related to temperature and pressure conditions to be faced during the operation of the boiler at the corresponding locations. Tube to tube joining is primarily done by straight tube butt welding in an automatic machine. As part of the total process in the production line, immediately after welding, Real Time Radiography (RTR) is carried out and defective welds are segregated. The sensitivity as low as 0.20 mm are achieved in this RTR process. Also, there is an auto feed back system for effecting changes in welding parameters as and when there are rejections in RTR. RT offers an important advantage of preserving the data - as found - on weld discontinuities for years together (upto 40 plus years in the case of Nuclear products) either on films or on digital mode and are retrievable. Review of RT data is convenient at any time for the clients as well as for manufacturers in case if there are requirements for analyzing the root causes for operational failures. Even digitization of RT films has come in a big way for compact storage of film RT contents with very high resolutions. RT is the very convenient volumetric NDE approved by international codes for adoption in fields and sites during the erection stages for the examination of pressure welds but
off-course there is a caution to adhere on radiation safety for the public and environment. Accordingly, specific and site RT gadgets with radiation protection (which gets subjected to satisfactory Qualification for use) and norms for qualification and certification of personnel doing field radiography are well established.

welds as an in-process NDE check in order to positively ensure the removal of defects from flawed portions wherein during the RT defect location repair (usually done by gouging or grinding), at many stages, RT is taken in order to confirm the positive removal of defect and this is one of the QA measure to ensure sound metal deposition after repair completion. Similarly, on high thickness critical weld joints, the practice of Inter-stage RT is followed by taking multiple RT shots (during the progression of welding) for every 15 to 20 mm of weld deposition. This practice is a cautionary one to avoid weld repairs at deeper locations (if only final stage RT is followed) by progressively ensuring the Quality weld deposition and is considered as good engineering practice for preserving the mechanical properties of material besides cost and cycle time cutting aspects related to fabrication. Also, if there are any material mix-ups in weldments due to change of welding consumables in the midst of welding for few beads / layers, RT is a capable technique to bring out such Quality issues based on density variations on films. Welding distortions especially in critical tube to tube welds leading to flow area changes or offset levels exceeding the limits are detectable in RT and such deviations are perused by design engineers for taking critical decisions on otherwise defect free joints. In Nuclear Steam Generators, an important QA requirement mandatorily practiced is FME - Foreign Materials Exclusion - and this is mainly meant for ensuring that all the flow passages, nozzles, dams, holes, annular gaps, etc. are free in order not to have any foreign objects like welding electrode bits or other unwanted materials (which may lead to performance issues on the steam generator). In the FME

programme, RT plays a crucial role by exposing the adjacent portions of the weld joints undergoing RT. The RT shots are devised in such a manner to obtain a vital information towards FME also besides certifying the joint for NDE quality. There are occasions wherein, such planned RT and corresponding results have added greater value to the FME programme. Successful RT is specified as the Mandatory Qualifying Criteria for the Welders and Welding Operators for their personnel Qualification as per ASME codes and IBR regulations and welders demonstrate their skill in the particular welding position and welding process by making RT Quality welds in pipe / tube and plate.

ULTRASONIC TESTING (UT) :
In UT, high frequency sound waves are sent into a material and the received sound waves or echoes that pass the material or that comes back from the material are used to analyse the structural condition of the material and serious defects like cracks get detected accurately in this method. Skilled and experienced UT technician, by adopting various techniques carries out UT to fulfill the objectives, viz. By comparing front surface echoes with the back surface echoes, UT will measure the part thickness, by detecting the returned signal or reflected signal, internal flaws are detected based on the interaction of discontinuity to the sound beam and UT finds its unique applications in residual stress or bolting stress measurements on engineering materials. Based on the volumetric inspection capability, UT is very much useful in critical welding industries like aerospace, Nuclear and Oil & Gas based on bringing out surface and subsurface defects in order to guarantee integrity and safety of components, welds and structure. In the forging, plates and tubes & pipes (raw materials) manufacturing industries, on line automatic UT or manual UT is performed for 100% volume of materials to detect all sorts of manufacturing defects of raw materials so that the possibilities of these raw material issues becoming triggers for fabrication defects (at later date during weld fabrication as a part of manufacture of a pressure part) is completely avoided. Salient advantages of UT are : ability to detect internal defects unlike VT, PT and MT, UT is harmless to operating personnel and environment unlike RT, UT is highly sensitive with its detectability for critical planar defects lying parallel to surface or perpendicular to sound beam, versatility, portability and speed of inspection are other distinguishing benefits of UT, now with advancements in this field, UT technicians are highly supported for informed decision making about UT indications for the defect sizing and defect’s severity assessment, etc. Some of the limitations of UT are : it is a challenge to inspect complex shapes and geometries by UT, UT generally requires good surface contact between transducer (emitter and receiver of sound waves) and test surface and hence warrants good surface preparation, a gel or liquid medium in-between transducer and surface is needed for effective passage of sound waves into material and this becomes a challenge on logistics some times, on materials which scatter sound waves like concrete, stainless steel, soft plastics, rubber, etc. UT is less effective, normally defects in particular orientation are detected easily and this is the limitation demanding other NDE methods also for detecting defects present in non-preferred orientations, UT requires a high level of expertise and technicians need to be familiar with the subject and possessing special skills, Linear defects oriented parallel to sound beam (along the direction of sound wave) may go undetected, Reference standards are required for both equipment calibration and characterization of flaws, etc.

Ultra high frequency sound waves are generated in the transducer (devices made of piezoelectric materials) by conversion of electrical impulses to acoustic energy and they pass through the material. When the sound waves encounter a discontinuity in their path they take a diversion and these returned sound waves are caught by the same or different transducer and get converted into electrical impulses which are displayed on a screen indicating the discontinuity. When sound waves encounter a reflector - a material with different density and acoustic velocity - (reflector is other name given to discontinuity in UT) - they bounce back or change direction to reach the transducer. By analyzing the time and amplitude along with its pattern of these bounced echoes, skilled operators can determine the distance to the reflector (location of defect) and identify the type of reflector / discontinuity such as slag, porosity, crack, inclusion, etc. There are different sound wave types, transducer types and

display options to enable UT operators for the comprehensive understanding and analysis of the materials undergoing UT for flaws detection. UT operates on the principles of sound reflection (similar to reflection of light by a mirror when discontinuities are perpendicular to sound waves), refraction (sound waves change direction when flaws are not perpendicular to sound wave) and penetration (similar to light passing through the glass) which all happen at the media interface and these principles help to accurately locate and identify the discontinuities. There are different transducers to create different modes of sound waves for propagation into materials for various applications. Longitudinal waves where particle motion is same as direction of wave propagation and they are having the highest velocity and longest wave length and are capable of travelling through solids, liquids and gases. In shear waves, particle motion is perpendicular to wave propagation and can pass through only solids and they are the most suitable candidates for weld inspection because of their superior sensitivity. Surface waves or Rayleigh waves have particle motion in an elliptical manner and are confined to surfaces and hence effective for inspecting the surface discontinuities. Plate waves are similar to surface waves but used for thin materials and bonded composites for defects detection over long distances using pitch catch method (which has two transducers as set with a fixed distance between them - one as transmitter and other one as receiver).

There are different UT testing modes. In Pulse - Echo method, single transducer sends and receives sound waves. Discontinuities are identified from the amplitude and location of the echo received back or reflected from the material. In Through - Transmission type, there are two transducer one for sending and other one for receiving sound energy. Discontinuities are detected partial or total loss of the received signal. There are three UT techniques, viz. Contact UT is of 2 types - straight beam testing (in which transducer sends sound waves perpendicular to surface) and angle beam testing (in which sound waves enter material at an angle to surface using a wedge and the method is effective for catching flaws at different orientations on welds), Immersion UT which is conducted in water using water proof transducers and finds applications for raw materials testing in automatic and non contact manner, Air coupled transducers UT wherein sound waves are transmitted through air as medium and is effective for composites testing (where contact method and immersion method are not feasible), etc.

Some of the typical and special examples (but not exhaustive) where UT has played its most significant role in the certification of welded items are highlighted here. For the critical pressure boundary welds of Nuclear Steam Generators, Nuclear Reactor Components and Aerospace products, at the time of pre-despatch stage (after the completion of all manufacturing and functional testing), UT is carried out using multiple angle probes and normal probes for the detection of transverse and longitudinal defects (in all the possible orientations) and also the location wise thickness scanning is also carried out. Weld thicknesses of the order of 15mm to 120mm and a weld circumferences of the order 4000mm to 10000mm are covered in this exercise. Weld joint wise, a very detailed report is generated as a
pre-service or pre-operational data for the critical equipment and this data serve as the initial signature before dynamic loading or fluids admission into the equipment. During periodic intervals, say once in 2 to 5 years (the applicable QA programme for the operation of the concerned equipment dictates this periodicity) of operation, these critical welds will be again subjected to UT adopting the initial methods and techniques follwed as part of In-Service Inspection (ISI) and the ISI data thus obtained will be compared with the corresponding initial Pre-Service Inspection (PSI) data. Based on the comparison of these two, useful information on defect enlargement, corrosion and erosion pattern and generation of new but acceptable defects (possibly due to dynamic loading) are obtained and this data serve as a feedback to design engineer for making changes to take care of the ultimate life of the equipment and also to QA engineer for taking up site weld repairs, if needed. Even though, the situations demanding design and QA actions are very very rare, as a part of robust QA measure, periodic inspection as part of ISI (to compare with PSI data) is carried out by UT. The plotting of initial PSI data and the periodic ISI data in the form of a trend chart is of greater importance for the monitoring of the continuing successful performance of the specialized equipment.

For the enhancement of sensitivity, defects detection in all the orientations and for increased resolution in flaws detection, there are number of advanced UT techniques which are followed for critical pressure parts welds in Nuclear and Boiler industries. Phased Array Ultrasonic Testing (PAUT) employs multiple elements in a transducer to form and focus the beam of an ultrasonic wave. It provides the ability to record data and display a discontinuity image in three dimensions increasing the reliability of inspections. In one of the high thickness weld joint made up of high strength low alloy steel on a Nuclear Vessel (of 4 m circular diameter and 90 mm weld thickness), initial regular UT (due to the specific nature of material and its processing in welding, the UT is normally done on this material only after completion of post weld heat treatment and UT in the as welded condition is ruled out on this material as the material / weld joint goes to furnace post weld heat treatment directly with weld heat and maintenance of pre-heating and hence cooling down to room temperature for conducting as welded condition UT is not possible) has revealed weld defects of unacceptable length with respect to customer requirement in-spite of the best efforts taken to complete the weld without any defect. Repair of the weld to remove the defect was not acceptable to customer as it would demand one more post weld heat treatment after repair welding and the material’s mechanical properties did not have a provision for one more post weld heat treatment (after repair). The only option was to reject the entire assembly (with very high value addition both in terms of cost - from raw material stage as forging to the stage of weld completion and cycle time, etc) and sourcing the newer imported raw material forging is also a long time consuming issue besides incurring huge cost. Plus lot of further processing cycle time to reach the present stage of this critical product was giving night mare to the industry and the important and prestigious customer’s confidence on the industry was getting shattered. At this juncture, using PAUT, the single normal UT defect was resolved into two different defects (in different planes) with each of the individual lengths acceptable as per the customer requirements. Thus, by avoiding repair, a costly welded forging and hence the critical equipment were accepted by customer and finally by adopting PAUT for the finer resolution of defects, it became a win-win situation to industry by maintaining the customer’s satisfaction and to customer to meet the planned project schedules committed to Government of India. Further, PAUT also finds wide applications (by using Cobra Scanners) in erection sites of boilers for the Quality assessment of different diameter and different thickness tube to tube weld joints for volumetric NDE instead of RT and this provision helps for faster and safer NDE (as RT is radiation hazardous NDE).

Time Of Flight Diffraction (TOFD) method of UT uses two transducers - one to transmit and one to receive. Measures tip diffracted signals providing high sensitivity and accuracy for discontinuity sizing. Produces a two dimensional image and hence this method requires verification by another technique for the determination of discontinuity location. TOFD is used along with PAUT for welds examination in a comprehensive manner. Full Matrix Capture (FMC) + Total Focusing Method (TFM). Here, data are captured from multiple elements and synthetic focusing of data is performed in post-processing. Allows flexibility in the selection of a focal point providing a highly representative image of discontinuity size and location. Electro Magnetic Acoustic Transducer (EMAT). This is a non-contact and couplant free technique using interacting magnetic fields to induce ultrasonic waves into the test part. Suitable for high temperature and high speed inspections with the involvement of special instrumentation and a larger probe compared to usual probe. Guided wave UT. Here, Ultrasonic waves can detect discontinuities at the material surface, away from the location of the transducer. Similar to fiber-optic guided wave inspection but provides the advantage of enabling inspection in remote and non-accessible areas such as buried pipelines. But complexity in signal patterns requires specialized data analysis training requirements. Due to the presence of number of variables in this method, this method requires additional techniques for accurate defect sizing. In petrochemical plants, NDT practitioners use daily to measure the thickness of the pressure vessel walls including weld locations and piping to look for the signs of corrosion and wear. The operation of such plants tend to produce cracks on material and welds and UT is capable of such cracks detection. ASME Section VIII (requirements for pressure vessels), API 510 (for pressure vessels inspection) and API 570 (for piping inspection) are the different codes demanding UT for the operational damage assessment and integrity testing of petrochemical plants, vessels and piping.

ELECTRO MAGNETIC TESTING (ET) :
Electromagnetic testing is an NDT method that uses electromagnetic fields to detect and measure discontinuities in industrial components. ET operates on the principle of inducing electric currents or magnetic fields in a material and analyzing the resulting electromagnetic response to gather information about the internal structure of the test part. Versatility of this method allows it to be adopted in many industries and environments using various techniques. Operator skill, knowledge, training and experience are needed for the selection of appropriate technique and for reporting of the results in proper formats for the interpretation. The primary objectives of ET in engineering industries include : detection of surface and subsurface discontinuities on materials, welds and components by analyzing the changes in the electromagnetic field. Material properties such as conductivity and permeability on critical components are assessed using ET as part of inprocess Quality control. ET can measure the thickness of non conductive coatings on the conductive substrates. ET’s capability to flaw detection without direct contact makes the method superior for ensuring the integrity and safety of critical products like Nuclear and process industries’ tubing. Other distinguishing advantages of ET are : high speed testing as measurements are made in fraction of a second and thus advantageous in high speed production lines, suitability of ET for versatile applications like determination of material composition and evaluation of material’s heat treatment conditions, modern ET methods are pretty low cost ones and this aspect compared to the high speed and large volume testing quickly on production lines of metallic materials is economically beneficial to industries, ET is amenable for automation and hence a feature on mass testing of similar parts at high rates enhances testing efficiency at reduced labour costs for testing, quick testing tool for maintenance machinery and condition monitoring to reveal surface and subsurface flaws due to operation / service and ET also offers an advantage of its ability for testing with the help of small portable equipment for in-situ or local applications in order to track deterioration of material in service. However, there are some limitations of ET. They are : non conductive materials like plastics, ceramics or composites can not be examined by ET as this method is suitable only for conductive materials like metals, test surface condition especially roughness, surface coatings and surface contamination can interfere with the accuracy of the examination, ET is not effective for the detection of certain defects like small cracks or defects in complex geometries and for which other NDE methods are needed, also, ET is not effective for defects at deeper

subsurface defects, normally ET signals are complex and skilled and experienced operator is needed for the interpretation of signals into discontinuities / flaws as variations in material properties, geometry and surface conditions may pose difficulty for correct interpretation, ET is sensitive to electromagnetic noise and interference from environment and this demands proper shielding and filtering of these effects in order to assure reliability in testing and the final limitation is due to the need for expensive investments for advanced ET equipment with its complexities for operation and maintenance besides the challenges on specialized training needs for sophisticated equipment’s usage.

In the ET method, alternating current is introduced into the coil creating a varying electromagnetic field. When an eddy current probe (simulating coil) is placed on the material under test, this electromagnetic field of varying nature can penetrate the material or test part directly. Due to this eddy currents are generated in the part under inspection (being a conductive material). These eddy currents create their own secondary magnetic field opposing the primary field created by the probe. When eddy currents encounter a discontinuity (material with different electrical conductivity or magnetic permeability), they are disrupted causing changes in the secondary magnetic field. By analyzing these changes, presence of discontinuities like cracks, inclusions and corrosion are determined by Qualified technicians. By having different probe types, frequencies and display options, ET is enabled as a proven NDE method for various applications and environments. Eddy currents are named for their circular and swirling motion similar to water currents in a stream. Eddy currents are stronger on the material surface and become weaker with respect to depth. Therefore, defect detection (due to disruption of eddy currents) is better on surface and progressively, this capability diminishes with depth. Eddy currents are influenced by magnetic permeability of the material also. Changes in the material variables like thickness or conductivity affect the interaction of magnetic fields and the same is detected. Normally, defect display pattern is on impedance plane. The depth at which eddy currents penetrate a material depends on factors like frequency, conductivity and permeability. Higher frequencies reduce the penetration depth of eddy currents (test part’s effective depth for examination using ET) but increase sensitivity. Frequency is adjusted to get the desired balance between penetration and sensitivity. Materials with higher electrical conductivity create stronger eddy currents but with less penetration. Lower conductivity results in weaker currents but deeper penetration. Sensitivity of detection is proportional to eddy current. Materials with higher magnetic permeability generate strong magnetic fields that keep eddy currents near the surface which limits inspection depth.

Basically, there are five methods of ET. Alternating Current Field Measurement (ACFM) : uses an alternating current magnetic field conditioned to be homogeneous in undamaged areas, detects discontinuities by measuring the disturbance in the magnetic field and this method is ideal for sizing cracks in offshore structures and other applications where surface breaking defects need to be accurately determined. Electro Magnetic Acoustic Transducer (EMAT) : It is a non contact couplant free technique using interacting magnetic fields to induce ultrasonic waves into the test object, suitable for high temperature and high speed inspections and this method demands special instrumentation and larger probe for inspection. Eddy Current Testing (ECT) : The most traditional technique, uses a single coil probe to generate and detect eddy currents, ideal for detecting surface and near surface examinations on conducting materials normally of non ferromagnetic (bobbin method for inner diameter and close to inner diameter examination and encircling coil method for outer diameter and close to outer diameter examination on tubes) and the method provides immediate feed back on the presence of defects. Remote Field Testing (RFT) : uses a probe placed inside a tube to generate a magnetic field (by using low frequency alternating current) that travels through the material, ideal for ferromagnetic\ tubes and pipes like carbon steels and this detects both internal and external defects (for the full thickness) by analyzing the magnetic field that returns to the probe.
Pulsed Eddy Current Testing (PECT) : uses a pulsed DC magnetic field to generate eddy currents, measures the decay of the eddy currents over time to detect corrosion and other discontinuities, effective for inspecting thick materials and detecting corrosion under insulation and the method is ideal for testing through intermediate objects like insulation or concrete. Heat exchangers are crucial in power plants and chemical processing facilities transferring heat between fluids through multiple tubes (either in the form of U tubes bundle or long tubes panels or coils). Over time, these tubes can degrade due to corrosion, erosion and cracking and ECT is commonly used to inspect the condition of heat exchangers’ tubes. During the inspection, the heat exchangers are taken offline and the tubes are cleaned to remove any deposits or fouling. An ECT probe is inserted into each tube in order to detect changes in the tube wall’s thickness or the presence of defects. The recorded signals are then analyzed to identify and characterize any discontinuities, allowing the maintenance team to decide whether to replace the affected tubes, preventing potential leaks or media mix-ups and thus ensuring the safe and efficient operation of the heat exchangers, In the case of Nuclear power plants also, there are many heat exchangers including the vital equipment of the plant, viz. Steam Generators. Here also, ECT is used for tubes examination during shut downs in an in-situ manner (without physically offline shifting of the disconnected equipment to a test one). But, here there will be sophisticated equipment participating in these shutdown inspections from remote areas along with robots (due to the presence of radio active environment) and in these plants, tubes revealing unacceptable flaws will be plugged instead of replacement (as replacement of tubes in Nuclear plant) is almost an impossible proposition.

Some of the typical and special examples (but not exhaustive) where ET has played its most significant role in the certification of critical items are highlighted here. The tubes of Nuclear Steam Generators of Pressurized Heavy Water type are of Incoloy 800 and are U tubes. They are thin tubes of 1mm or 1.1mm with diameter 16mm or 19mm respectively depending upon the power rating of the steam generators (viz. 235MW or 500MW/700MW respectively). In the tubes manufacturing mill, just before U bending in the final tube sized condition, 100% ECT is carried out using an encircling coil method by passing the tube through the coil from one end to other end for its full length (which is of the range 24m to 26m). This ECT is performed as a part of production on line activity. ECT ensures (based on parameters selection for ECT) freedom from defects on both outer surface and inner surface of the tube. Also, the full thickness is ensured for sound condition of tube material and the tube thickness is measured for meeting the requirements. ECT is complimented with UT also in the tube mill. Before final sizing, atleast 3 to 4 times at various stages of tubes sizing, ECT is carried out to control process induced defects. In the steam generator manufacturing plant, after all manufacturing and testing activities, 100% ECT is carried out using bobbin method wherein a probe is inserted through the tubes and ECT is carried out with inner diameter of tube has closer proximity to probe where as in the ECT carried out inn tubes mill, outer diameter of tube was having closer proximity to the probe (coil). But in both the stage tests, the test variables are chosen in such a manner that the sensitivities are equivalent in both the stages and results of both the stages are replicating to each other (after analysis due to the presence of ECT signatures at the final stage due to tube supports at many locations and tube insertion induced acceptable scratch marks). The base line ECT signatures obtained at the final stage of the Nuclear Steam Generator at manufacturer works is used as a reference for tracking the defects growth or tube thickness reductions observed during shut down stages’ ECT of the operating Nuclear Steam Generator (normally this ECT is performed once in two years) in order to decide about tubes plugging. The advanced ECT methods like RFT are practised in thermal power plants on tubes of different zones like economiser, water wall, super heater and re heater in order to monitor corrosion damages and to track biologically induced corrosion.

LEAK TESTING (LT) :

Leak Testing is a form of NDE for detection and location of leaks and measurement of leakage in different pressurized or vacuum systems. Leak is a physical hole or crevice whereas leakage is flow rate of fluid through the leak. LT is primarily conducted for the three reasons, viz. 1. For the prevention of material (operating fluid) leakage - EX: In space crafts, operating fluid (vacuum in space craft) leakages are prevented, 2. For the prevention of environmental

contamination - EX: In Nuclear equipment, by avoiding hazardous nuclear or radio active leakages, the environment is protected and 3. For the detection of unreliable components - EX: Many critical industries like TV picture tube manufacturers, space components manufacturers, critical heat exchangers manufacturers, etc. Adopt leak testing for the reliability certification of their products by conducting LT as in-process or final functional test in the manufacturing cycle. Essential requirements for conducting leak test are : Creation of pressure gradient across the leak and Availability of Leak measuring system on the low pressure side based on leaking medium’s property. During the leak test, leak in the metal wall is measured by measuring the rate of flow of fluid (leakage) through the leak and this is achieved by measuring actual flow rate or by measuring drop in pressure / vacuum level in constant volume systems or by measuring change in volume to maintain the pressure constant. Measuring unit for leakage is Std.CC/sec of air ( Leakage of 1 Std.CC/sec is equal to leakage of one cubic centimeter volume of air in one second and the pressure rating of this leaking air is one standard atmosphere). Basically, there are 4 methods of LT. 1. Pressure change method where in leak rate or leakage is calculated as dp.v/t or p.dv/t, v - volume, p - pressure, dv - change in volume, dp - change in pressure of the system (pressurized or evacuated) undergoing leak test and t - time for which these measurements are made. For this method, Sensitivity = 1 x 10-1StdCC/sec of air. 2. Bubble testing method where in visible observation of bubbles due to leaking gas (normally air) from high pressure side to low pressure side is used for leaks detection. For this method, Sensitivity = 1 x 10-3 StdCC/sec of air. 3. Halogen diode method in which for leak rate measurement, use of halogenated gas (as tracer gas on high pressure side) and specific equipment called Halogen Diode for its measurement (on low pressure side) are practiced. For this method, Sensitivity = 1 x 10-9StdCC/sec of air. 4. Helium leak testing method wherein use of Helium gas (as tracer gas on high pressure side) and specific equipment called Mass Spectrometer Helium Leak Detector for its measurement (on low pressure side) are used. For this method, Sensitivity = 1 x 10-11StdCC/sec of air (the best of all the methods). The distinguishing reasons for selecting Helium in the most sensitive method are : Helium is the lightest gas next to Hydrogen and hence it can pass through crevices (very fine leaks) easily, it is an Inert gas and chemically non reactive, it is Non toxic and does not cause harm to people or environment, it is Non flammable and above all its presence in atmosphere is 4 ppm only which makes it the most ideal one (with the least background noise) for tracking very fine leakages of Helium gas from the system.

In bubble testing (also called as air leak testing), the parameters of testing normally followed are :

Pressure gradient - 1kg/sqcm to Maximum 1.25 times design pressure of the equipment, Probing medium - gas - nitrogen / air and Detection liquid - soap solution. Three types of bubble testing are practiced. 1. Liquid immersion technique - pressurized test object is fully immersed in detection liquid and is practiced for smaller items in a practical manner offering the highest sensitivity. 2. Liquid film application - here, on the pressurized test part, at test areas, soap solution will be sparyed or applied industrial products of huge sizes are tested for leak tightness using this method. 3. Foam application - it is a very gross test to detect bigger holes (blow holes) which will be visible as black holes in the foam background. In bubble testing, illuminators and magnifiers are used for enhancing visibility and sensitivity. Other ways of improving bubble testing sensitivity are : allowing more time for bubbles observation, pressure gradient increase, reduction of pressure on the soap solution side, decrease of surface tension of detection liquid (that is why instead of water, soap solution is used for detection liquid) and heating of the job. Typical detection liquids are water (with soap added), silicone oil and mineral oil. Typical composition of soap solution is 1 part of liquid soap, 1 part of glycerine and 4.5 parts of water. Here, glycerine is added for increasing the wettability of bubbles and hence stability (sustenance) of bubbles is increased by reducing air drying tendencies of bubbles. The most advanced method of bubble testing in industries for testing the running meter lengths of butt welds is called vacuum box method. In this method, on one side of the weld, a vacuum box is constructed using quick vacuum clamps and ‘O’ rings and the box will be held. Before placement of vacuum box, soap solution will be applied on the test portion (weld). Evacuation of the contents of vacuum box (one side volume above the weld within the vacuum box) will result in bubbling if there are leaks in the weld - due to the atmospheric air from the other side of the weld leaking to the vacuum side (due to the presence of vacuum box and its evacuation). Desirable features of vacuum box are : provision of a viewing window of glass for visual examination, ability to admit light, ability to create vacuum (to a level of 20 mm of mercury - absolute pressure) using vacuum pumps, portability and keeping vacuum tightness while isolating the pump and also with a provision for vacuum gauge installation.

In the case of Helium leak testing, there are 3 different techniques. 1. Pressure Technique or Sniffer Probe method or Detector Probe method in which product undergoing Helium leak test is pressurized with Helium gas or a mixture of Helium and air (depending upon the desired test sensitivity) and the typical pressures range from 0.25 bar (g) to as high as 20 bar (g). Through the leaks, if any, in the pressure boundary, Helium leak will take place from the inner volume of the product (typically a vessel) to the outer side environment (atmospheric pressure side) due to pressure gradient. A sniffer probe connected to the working Helium leak detector (kept out side the part undergoing test) will be moved over the product (on the atmospheric pressure side) at a particular scanning speed and maintaining a particular range of proximity (between probe tip and the product’s surface). Both the scanning speed of the probe and the proximity level (distance) are calibrated using standard leaks releasing Helium from higher pressure to atmospheric pressure prior to scanning on the product and the calibrated scanning speed and proximity are used for the actual testing. If there are Helium leaks, the location of leaks and the leakage rate at the leak location are determined in the scanning process by moving the sniffer probe all around the product giving the complete coverage of the part. There are three sub-types of pressure method. 1. Normal sniffing method (where a bare sniffer probe without any attachment is used for scanning). 2. Encapsulator method - especially for tube to tube sheet welds (in which probe tip is made in the form of an in-situ cup to locally place the probe for a known time (which is calibrated prior to testing using known leaks) covering a single tube to tube sheet weld and this makes the provision for totally collecting all the leaking Helium, if any, from multiple points of the circular weld and weld wise leakage rates are thus quantified). 3. Sniffing after accumulation method (in which, longer collection times (which is a pre-calibrated one using known leaks prior to

testing) are followed for leaking Helium to get accumulated in polythene pouches made on atmospheric pressure side and these typical pouch volumes are reasonably standardized and hence pre-calibrated using known leaks prior to testing). After the collection time, the contents of the pouches are pierced using the sniffer probes connected to the leak detector for the identification of pouches wherein leakage rate exceeds the acceptance criteria. Then, the pouches identified as unacceptable are subjected to normal sniffing to identify the leak locations exactly. By making multiple pouches and allowing Helium collection into pouches for overnight, total test is performed with higher sensitivity and at a faster pace compared to normal sniffing (especially on large scanning area products under test). Test variables affecting the sensitivity in pressure method are : pressure gradient between vessel side and atmospheric pressure side, Helium concentration in the test gas, scanning speed of the sniffer probe, proximity of probe to test surface and volume of representative pouch or Encapsulator and collection time (in accumulation and Encapsulator methods).

Second technique is Vacuum technique or Dynamic Testing or Tracer Probe method. In pressure technique, Helium flow takes place normally from inside the vessel volume (pressurized) to outside (atmospheric pressure) whereas in Vacuum method, during the test, if there are any leaks in the pressure boundary, then flow of Helium will take place from outside (atmospheric pressure) to vessel volume inside and therefore to create a pressure gradient, the inner volume of the vessel is evacuated to a vacuum pressure of 10-4 milli bar or better using a combination of roto dynamic and diffusion / turbo-molecular high vacuum pumps. Flow takes place across a pressure gradient of 1 bar (maximum) always in vacuum method (atmospheric pressure to vacuum). While conducting the vacuum type test, after the evacuation of inner volume, Helium will be sprayed on the test portions at atmospheric pressure side (on the outer portion of the vessel). Through the leaks, if any, Helium flows to vacuum side where it is dynamically measured and the total leak rate is quantified by the leak detector (which is connected parallel to the vacuum pumping system). Prior to Helium spray, system sensitivity check is carried out by sending known leakage rate signals from a standard leak calibrator into the vacuum side and proportion of signal realization is measured using the leak detector (already in parallel connection to vacuum pumping) and this proportion of signal is used for the calculation of system sensitivity. During the actual testing (while Helium is sprayed) also, the same proportion signal realization is expected and hence, from the signal realized on the leak detector during Helium spray (proportion of the actual), the actual global leak rate is calculated. In vacuum tests, bare spraying is normally followed which will ensure 10% Helium concentration normally. To enhance the Helium concentration and thereby to achieve higher test sensitivities, hoods of Helium are constructed on the test part on its outer side by making closely fitting polythene envelopes. Helium is sprayed into these envelopes and enveloping along with periodic spray of Helium into the envelopes - for every 10 minutes - will retain the Helium for longer duration at higher concentrations (to the level of 60-80%)

and thus this Hood method offers enhanced test sensitivities. Test variables affecting the sensitivity in vacuum method are : vacuum level inside the vessel, Helium concentration in the test gas (either in bare spraying or spraying into the Helium hood on the atmospheric pressure side) and the product’s cleanliness especially on the high vacuum side - as cleaner vacuum side helps for comparatively higher and faster signal pick up (by the leak detector connected to the high vacuum side).

The third method is Pressure Vacuum Method which is the combination of pressure and vacuum methods. In this method, Helium gas pressurized test part is placed in a vacuum chamber and then the chamber is evacuated to create high vacuum. Leak detector is connected parallel to high vacuum pumping of the vacuum chamber. Global leakage of Helium from higher pressure side (inner volume of the test part) to its outer side (into the vacuum chamber) is measured by the leak detector. Leak location cannot be found out in the pressure vacuum method as the total part (both of its sides) are not visible to the test personnel. In case of leakage situation in pressure vacuum tests based on leakage rate reported in the test is exceeding the acceptance criteria, one of the other two methods, viz. Pressure method or Vacuum method will be attempted for leak location identification. Typical test sensitivities achieved by pressure method, vacuum method and pressure vacuum methods are 1x10-5StdCC/sec of air, 1x10-8StdCC/sec of air and 1x10-10StdCC/sec of air respectively.

The equipment used for Helium leak detection is called as MSLD - Mass Spectrometer Leak Detector. Basically, it is similar to Cathode Ray Tube. It has an emitter to release electrons by thermionic emission from a tungsten filament heated in the high vacuum chamber, ionization chamber where ions of different elements of the leaking gas (coming from the product leakage and consequently entering into the leak detector) are formed based on interaction of gas molecules with the electrons, ions travel and separation system (using magnetization) based on charge to weight ratio of ions and the final signal generation part in which the lightest ions separated (Helium) are made to hit on a collector plate to generate a current and this is amplified and readout on the display (analogue or digital) and this is proportional to Helium leak signal. Based on calibrated display, leakage rate is thus measured and quantified. Sensitivity of MSLD is of the order of 2x10-12StdCC/sec of air to 6x10-11StdCC/sec of air.

There are few limitations for LT. This method is capable of detecting only through and through defects like the ones present for the total thickness of the pressure boundary from one side to the other and it can be a straight or zigzag passage of continuous nature (with uniform or varying cross sectional area of passage) allowing fluid flow from one side to the other side during the test and during the operation of the equipment. The defects present for a fraction of thickness only (either surface or subsurface defects restricting their presence to part thickness with defect free remaining thickness or inner defects in mid thickness only leaving defect free zones on both the sides) cannot be defected. Even if a wafer like defect free part thickness of the order of fraction of a millimeter is present with the balance portion of thickness as completely defective, this method is incapable of detection of such gross defects. All LT methods generally demand higher order cleaning requirements on both the high pressure and low pressure sides as contaminants like traces of water due to prior operations or testing (hydro test), rust, dirt and dust have the tendencies to clog or mask the finer leaks. Some NDE methods like RT and UT will equally certify for the product’s structural integrity also based on successful NDE by ensuring totally defect free pressure boundary for full thickness where as

this is not feasible with LT. LT is advantageous only for the reliability certification of the product for its functional / operational leak free performance and product’s load with standing or pressure holding capability cannot be certified by LT. LT demands the requirement of skilled and experienced operators (especially for finer methods like Helium leak testing) for the accurate location of closely spaced finer leaks and the Quantification of leakage rates is also sometimes becoming a specialized job. While dealing with high vacuum systems, practical issues like Helium capture within the vacuum system and out gassing of captured Helium after some time are common and fluctuations in leak meter readings are to be properly interpreted by the specialist while quantifying the test results. Normally, in finer LT methods like Helium leak testing, leak location identification is a tedious process (especially in vacuum method) where as in pressure method, leak location identification is relatively easier but the quantification of the total leakage rate is an experience / skill demanding task.

Some of the typical and special examples (but not exhaustive) where bubble testing and Helium leak testing have played their most significant role in the certification of critical items are highlighted here. All the process plant heat exchangers are subjected to bubble testing for the leak tightness of tube to tube sheet weld joints. In this, after the completion of tubes insertion into tube sheet and tube supports, shell course welding and tubes to tube sheet welding, nitrogen is pressurized (at 1 bar (g) inside the shell course) so that the tubes are having nitrogen pressure on the shell side at the back of their tubes to tube sheet welds. If there are any leaks in the tube to tube sheet welds, release of nitrogen will happen through such locations and the same will be observed as bubbles when the tube sides (all around portions of tube to tube sheet welds) are sprayed / brushed with soap solution. These weld joints are qualitatively examined and accepted in the bubble testing, if there are no bubbles (leaks) on the welds during soap solution spray. Similarly, all the reinforcement pads of nozzles are examined by bubble testing in a qualitative manner for the pads welding (both on shell portion and nozzle portion). Here, for the bubble testing, nitrogen is pressurized in the gap (metal to metal space) existing between inner part of pad (concave portion) and the matching outer part (convex portion) of respective shell.

Tube to tube sheet welds of Nuclear steam generators are Helium leak tested by vacuum method. Shell side volume of the order of 40 to 50 cubic meter is evacuated and at a vacuum level of 10-4 millibar or better, vacuum pumps are isolated. Helium is sprayed into a polythene hood covering all the tube to tube sheet welds on their tube side. In one shot, globally thousands of tube to tube sheet welds are Helium leak tested against the acceptance criteria of 1x10-6StdCC/sec of Helium (for all the welds together) and 1x10-7StdCC/sec of Helium (for a single weld). The leak detector connected to the high vacuum side (in pumping isolated condition) indicates the leakage rate. All the nozzles’

openings are Helium leak tested for their effective closure using gaskets in a Nuclear steam generator by vacuum method. The acceptance criteria is 1x10-4StdCC/sec of Helium per gasket joint. In a large stainless steel pressure vessel (for fast breeder Nuclear project) constructed using petals, the running meter lengths of petal to petal joints were Helium leak tested by vacuum method using Vacuum Jackets. On one side of the weld, the vacuum jacket gets placed which has a provision for doing system sensitivity check using known leaks. Once evacuation is done within the jacket volume, Helium will be sprayed into a polythene envelope on other side of weld for leak detection. In multiple

placements of vacuum jackets (multiple variants fabricated to suit to the profile conditions of the vessel constructed by welding of petals), the total length of the butt welds gets successfully Helium leak tested against the acceptance criteria of 1x10-7StdCC/sec of Helium. For a critical pressure vessel used in Defence Application, pressure vacuum Helium leak test was successfully established for the complete set of weld joints of vessel. A stainless steel vacuum chamber was exclusively made for this test. To enable this specialized testing, all the openings of vessel were closed using double gaskets (in order to ensure Helium leak tightness from higher pressures to high vacuum) with a provision to monitor inter gasket leakages, if any, during pressurization. Also, a separate pump out was provided for these inter gasket collections. The product vessel was pressurized with Helium at 10 bars (g) after placing the vessel (product) inside the vacuum chamber. Then, the vacuum chamber was evacuated and after reach of 10-4 milli bar or better vacuum in this chamber, evacuation gets isolated. Leak detector connected parallel to high vacuum line of chamber indicates global leakage rate of Helium coming out of all the weld joints and the material surfaces of vessel (from 10 bars (g) of inner volume of vessel to high vacuum of chamber). The acceptance criteria is
1x10-7StdCC/sec of Helium.

CONCLUSIONS :

Out of the sixteen different methods of NDE as per ASNT, the seven important ones widely deployed in the heavy engineering, power plant and process equipment manufacturing, Aerospace, Defence and Nuclear equipment manufacturing and infrastructure industries have been discussed in a detailed manner along with salient novel applications pertaining to these methods. Actually, in industries especially for the welding, as a part of Quality assurance measure, more than one NDE method is prescribed in Quality Assurance Plans. This is because of the distinguishing method specific advantages and method specific limitations. Normally, the different methods chosen for a particular stage / application are complementary to each other. There are basically number of pre-requisites as part of QA measure while choosing these methods in a complementary manner. Structural integrity (static and dynamic loading and creep resistance), functional performance (in terms of criticality of operation of the product, leak tightness requirements, pressure and temperature holding in operation,etc.) and environmental protection by avoiding release of hazardous substances are the striking demands for an Industrial product to be fulfilled and to meet these aspects, appropriate methods are selected. To understand the concept of NDE methods selection versus the product’s manufacturing and operational requirements, few examples are cited. For lower thickness materials and welds of less critical nature, VT along with surface / subsurface NDE like PT and MT are followed. For pressure parts’ tube to tube welds of power plant equipment - for tube panels and coils, VT, MT/PT and RT/UT are followed. Normal thumb rule here is to have one surface NDE method and one volumetric NDE method. In higher thickness multi layer weld joints of pressure parts of power plant equipment like pipe welds, header welds, welds connecting fittings, welds on structural items, etc. along with surface NDE and volumetric NDE (similar to tube to tube joints), inter stage RT or UT using multiple angle beam probes or welding layer wise MT are followed. In the case of Nuclear application equipment manufacture, multiple surface NDE (PT and MT) and multiple volumetric NDE (RT and UT) are practiced for the welds with much more stringent processing aspects for conducting NDE and the acceptance criteria. In UT, the practice is very elaborate using 18 directions scanning (multiple angle probes and normal probes are used) to detect

all the defects of different types (transverse and longitudinal) and different orientations. In addition, all the pressure boundary welds and tube to tube sheet welds are subjected to Helium leak test. Thus, the prudent and complete Quality Assurance Approach for welding appropriately selects the required NDE methods in line with customer requirements and as per mandatory codes for equipment manufacture and equipment operation. A detailed Quality Assurance Plan (QAP) is drawn out detailing various stages of Quality checks during product manufacture and during equipment operation. This covers appropriate NDE methods required for the welds also. Typically, for all welds, NDE is performed at many sub stages of welding, like, prior to welding for the weld edges (to ensure weld edges are free from defects and PT is normally done), after root welding (normally VT and surface NDE are done) and after back gouging followed by back grinding (in case of ferritic welds) or after back grinding (in case of austenitic or non ferrous welds), surface NDE is performed. In addition, inter stage welding NDE (especially in high thickness welds of multi layers) and final welding completed stage NDE (which are combinations of surface NDE and volumetric NDE) are performed.

Based on adoption of appropriate NDE methods at multiple stages (as per the robust QAP) of welding, the welds also tend to attain the strength equivalent to parent material ultimately. The higher thickness butt welds attain the joint efficiency of one. The NDE methods are suitably deployed during shut downs and maintenance of plants for the Quality Health Tracking in operation of the item and this data gives an alarm for the needed repairs or replacements during shut downs. The confidence given by NDE methods in this type of Quality Health Tracking episodes has resulted in enhancing the predicted operating lives of critical process plants and Nuclear equipment. Welding and NDE are two co-existing technical fields and both of which demand strict requirements for procedure and personnel qualification as part of QA measure. In any manufacturing Industry, the most significant portion of welding history is only based on NDE reports (demanded by customers and codes) other than recorded welding parameters preserved, if any. Corrosion mapping and thickness survey on welds during equipment operation and post proof test NDE results of critical welds (at manufacturer’s works) serve as vital triggers for newer material developments to meet the demands of Industry and Customer.