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TIP 0402-18 ISSUED – 1993 REVISED – 2001 WITHDRAWN – 2008 REVISED AND REINSTATED – 2010 REVISED – 2015 2015 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published. TIP Category: Automatically Periodically Reviewed (Ten years) TAPPI Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones 1. Scope This TIP is a review of general considerations related to UT thickness testing of black liquor recovery boiler tubes: Part I of this TIP provides guidelines and procedures for acquiring accurate ultrasonic thickness readings on boiler tubes, where the tube material and condition are conducive to such non-destructive thickness testing. Part II describes how tube thickness surveys are commonly laid out for different zones of a typical recovery boiler. It also stresses that a customized survey is essential for each boiler. 2. Limits of responsibility The material presented in this document is intended to be for information only. It is subject to change as more information is developed or becomes available. The authors and those individuals or companies that have provided assistance in the preparation of these guidelines assume no liability for the accuracy or completeness of the material presented. These contributing parties shall not be held liable for any direct or consequential loss or damage of any nature whatsoever arising from or in connection with the use of these guidelines or the information contained therein. The content of this document makes no deliberate attempt to reflect or represent the requirements of any relevant federal, state, or local laws, codes, and regulations. Awareness of and compliance with the requirements of any such laws, codes, and regulations are the responsibility of anyone inspecting recovery boilers. These guidelines do not set, and should not be construed as setting, standards for acceptable practices, policies, procedures, limits or goals. Conversely, not following the procedures contained herein shall not necessarily constitute improper or negligent practice. 3. Safety precautions All parties involved in thickness testing work are responsible for ensuring that a safe work environment is always maintained. Keys to safe boiler inspection include assuring that: Applicable safe entry procedures are followed. Boiler lockout/tag-out procedures are diligently followed. Protection from falling material is assured.

TIP 0402-18 Ultrasonic testing (UT) for tube thickness in ...Accurate monitoring of boiler tube corrosion or erosion is an important element of a program to assure safe and reliable

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  • TIP 0402-18 ISSUED – 1993

    REVISED – 2001 WITHDRAWN – 2008

    REVISED AND REINSTATED – 2010 REVISED – 2015

    2015 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

    TIP Category: Automatically Periodically Reviewed (Ten years)

    TAPPI

    Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones 1. Scope This TIP is a review of general considerations related to UT thickness testing of black liquor recovery boiler tubes: Part I of this TIP provides guidelines and procedures for acquiring accurate ultrasonic thickness readings on boiler tubes, where the tube material and condition are conducive to such non-destructive thickness testing. Part II describes how tube thickness surveys are commonly laid out for different zones of a typical recovery boiler. It also stresses that a customized survey is essential for each boiler. 2. Limits of responsibility The material presented in this document is intended to be for information only. It is subject to change as more information is developed or becomes available. The authors and those individuals or companies that have provided assistance in the preparation of these guidelines assume no liability for the accuracy or completeness of the material presented. These contributing parties shall not be held liable for any direct or consequential loss or damage of any nature whatsoever arising from or in connection with the use of these guidelines or the information contained therein. The content of this document makes no deliberate attempt to reflect or represent the requirements of any relevant federal, state, or local laws, codes, and regulations. Awareness of and compliance with the requirements of any such laws, codes, and regulations are the responsibility of anyone inspecting recovery boilers. These guidelines do not set, and should not be construed as setting, standards for acceptable practices, policies, procedures, limits or goals. Conversely, not following the procedures contained herein shall not necessarily constitute improper or negligent practice. 3. Safety precautions All parties involved in thickness testing work are responsible for ensuring that a safe work environment is always maintained. Keys to safe boiler inspection include assuring that:

    Applicable safe entry procedures are followed. Boiler lockout/tag-out procedures are diligently followed. Protection from falling material is assured.

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 2 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones

    The air in the boiler furnace and drums meets the mill’s requirements for safe entry. All access platforms and scaffolding conform to regulatory and industry standards. Devices are in place to ensure electrical safety and function correctly. Personnel wear the prescribed personal protective equipment.

    4. General considerations 4.1 Objectives of tube thickness testing

    Accurate monitoring of boiler tube corrosion or erosion is an important element of a program to assure safe and reliable operation of a black liquor recovery boiler (BLRB). Tube failures due to corrosion or wall thinning have caused explosions in BLRB's. Such explosions have caused costly repairs, economic losses, and in some cases, injuries and fatalities.

    Surveying the thickness of boiler tubes, primarily using nondestructive ultrasonic testing (UT) methods, is an essential part of any program for quantitatively monitoring and trending tube corrosion. Trends derived from thickness surveys can be used to optimize intervals between inspections. They also can be used to plan tube replacement or corrosion protection measures during regularly scheduled outages. Bauer and Sharp’s papers provide useful discussions about trending tube thickness data (1, 2). Surveys of boiler tube thickness on their own do not constitute adequate inspection of a boiler. Surveys should always be complemented by a thorough visual inspection to locate and evaluate corrosion and other potential tube problems. This TIP aims to inform recovery boiler operators how to obtain valid UT thickness readings at each measurement location for the various kinds of boiler tubes most commonly used in BLRB’s. It also offers advice on how to lay out a general thickness survey in a recovery boiler based on the Working Group members’ collective experience with tube corrosion phenomena in the various parts of such boilers. Thorough visual inspection always should precede and accompany any UT thickness survey. The visual findings may influence the layout of the thickness survey, and vice versa. A practical, step-by-step approach to visually inspecting a recovery boiler and conditions to look for are described in Vol. 1 of the Recovery Boiler Reference Manual (3) and in a NACE report (4). Localized corrosion and distress of boiler tubes may be manifested as cracks, bulges, dents, flat spots, discoloration, etc. of the tubes. When such conditions are observed, the thickness of the affected tubes may need to be accurately determined to assess their suitability for continued service. (In some cases, one or more of the affected tubes may need to be removed for metallurgical analysis. Removed tubes also can serve as standards for UT technician qualification and instrument calibration for thickness testing.) 4.2. Instruments and techniques 4.2.1 Instruments for manual and semi-automated testing UT for thickness measurements involves sending and receiving low energy, ultrasonic frequency, sound pulses into the component whose thickness must be measured. This technique is called “straight-beam” UT because the pulses are sent perpendicular to the accessible surface. The thickness is characterized by the time it takes the pulse(s) to travel in the material from the probe and be reflected back to the probe from the opposite (hidden) surface of the component, along with the velocity of sound through the material. A calibration standard should be used to ensure the accuracy of the measurements being taken in the boiler. Many types and brands of devices are available for UT thickness measurements. Manual instruments can be as basic as small, hand-size units that display a simple digital reading of the thickness at a point. There also are more sophisticated instruments which display both the time signature of the ultrasonic reflection (called the A-scan) and the digital thickness reading for the test location. Instruments with both digital thickness readout and the A-scan presentation of the UT signal provide the most information in measuring the thickness of recovery boiler tubes, provided the testing is done by a qualified technician who can interpret A-scan information. Instruments with a digital thickness readout and A-scan

  • 3 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones presentation should be used to limit the acquisition of erroneous data that can be obtained using digital only UT instruments. Instruments with A-scan capability also are essential for thickness scanning over an area of tube. Semi-automated UT equipment has been developed for special purposes. Examples include rotary UT testing for generating bank tube corrosion or erosion, as well as semi-automated equipment to measure corrosion especially adjacent to the mud or steam drums (See Section 7.2.8.2). There are also rapid-scanning, electro-magnetic acoustic testing (EMAT) systems designed to screen tubes for metal wastage. These techniques may or may not involve direct coupling of the sensor to the surface. They typically involve programmed (encoded) scanning of a surface and rapid, computerized accumulation of the thickness data. Specially trained technicians must operate the equipment. As discussed in Section 6.4, all technicians must be trained, certified and qualified on the same type of instrument they will use for the UT work. 4.2.2 Probe (transducer) selection Probably the most common probe for manual UT thickness testing on boiler tubes and headers is a 6.4 mm (0.25 in.) diameter dual element transducer with a frequency of 2.5-5 MHz. Smaller diameter (“pencil”), or specially shaped, probes are available to access the tube surface between studs, in wall openings or when the tubes are in closely spaced platens or rows. A small diameter transducer can increase the amount of detail obtained when an area of tube is scanned. The designs and sizes of probes for semi-automated UT systems are purpose-driven. 4.2.3 Instrument calibration All ultrasonic thickness instruments should be calibrated according to ASTM E-797 (5) requirements. Calibration standards should be similar in diameter and material to the tubes to be tested. There should be discrete calibration thickness steps with the value legibly stamped on each step that covers the range of thickness expected. Calibration with dual element transducers should “bracket” the thickness range to be examined to ensure accuracy in the range to be tested. Calibration of manual UT instruments should be checked regularly (typically every 30 minutes) during the data gathering. This should also be done at the beginning and end of each technician’s shift and whenever the technician or any of the equipment is changed. An instrument is considered “in calibration” if the calibration check reading is within +/- 0.1 mm (+/- 0.004 in) of the selected calibration standard thickness. If an instrument is not in calibration, all readings taken since the last time it definitely was in calibration should be repeated. The purpose behind frequent, ritualistic calibration is to avoid accumulating a large population of “off-calibration” readings. A calibration check is also recommended when any "below flag or below minimum" thickness is encountered, so as to increase the confidence in the accuracy of these specific readings. Boiler tubes have a “minimum” wall thickness requirement as per the original code of construction. In many cases, companies use a “flag” value, which is a percentage above the “minimum” requirement to help track, trend and develop repair options for the boiler before the tube reaches the “minimum” thickness value. Calibration techniques and frequency for automated/specialized UT thickness instruments should be established ahead of the job with the contractor providing the service. Whenever practical, a tube “standard” containing similar features to those being tested should be used to refine and calibrate the testing procedure. This type of standard is also recommended for on-site calibrations and technician qualification. (See Section 6.4 for more on technician qualification.) 4.2.4 Spot readings and area scanning Most thickness data consist of spot readings taken at positions on the tube predetermined in the thickness survey plan. For wall and floor tubes, the thickness is most often measured in three spots at each test location, such as left, right, and center (Fig. 1). The left and right readings should be taken as close as possible to the membrane weld or adjacent tube. These readings should be made within 15 mm (0.6 in) of the membrane weld to be valid “crotch” readings. The area scanning technique is useful for finding where a tube is thinnest in the scanned area. Area scanning can also help determine the extent of any thinning that is detected either by visual inspection by spot reading or grid surveys. Area scanning is useful to detect thinning patterns in the curved tubes that form airport openings. It can also detect waterside thinning or pitting, such as in super-heater tube loops. Area scanning can be done manually or with

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 4 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones semi-automated procedures that scan tube thicknesses with UT probes that rotate inside the tubes. Generally the semi-automated procedures capture and present UT data better than manual procedures.

    Fig. 1. Crown and crotch locations for boiler tube thickness testing. 5. Data management and review Boiler tube thickness surveys generate large amounts of data. To get the most value from the data, they should be recorded, analyzed, and reported in an effective manner. Data recording can be either manual or electronic, as discussed below. The original, nominal wall thickness should be documented for the boiler tubes. Tube thickness survey reports in the form of many pages filled with thousands of measurements make effective analysis of the data impractical and time consuming. Computer-based data analysis and reporting methods that utilize color-coded tables or colored graphic data presentations are readily available and are preferable. These “tools” are useful for rapidly identifying both tube wastage areas and systematic thickness measurement or calibration errors. Systems that provide color-coded, condensed graphical presentations of the thickness data, preferably presented as they were taken in the boiler, allow rapid evaluation of boiler tube thicknesses and may be used to estimate wastage rates. Three or four sets of readings obtained over a period of several years may be analyzed statistically to determine wastage rates in different regions in the boiler. When the wastage rates are known, the projected remaining life of a given area or section of the boiler can be calculated. This value should be the primary basis for the number of survey points and the frequency at which the tubes in that area of the boiler are tested for thickness. The data from the inspections can be trended and used for condition-based inspections for the boiler. A visual inspection test (VT) is needed even if the number of measurements is reduced to look for evidence of wastage. The operational characteristics of the boiler should be considered when determining wastage rates. Changes in operation of the boiler, such as adding an over-fired air system, changes in the liquor solids for burning, burning non-condensable gases, changes in liquor chemistry, etc., can cause changes in historical wastage rates in the boiler. The value of tube thickness data depends entirely on the diligence with which the data is reviewed and acted upon by mill management. 6. PART I - Guidelines for accurate tube thickness testing The key elements in obtaining accurate thickness readings on boiler tubes are:

  • 5 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones

    Know what type of tube is involved Ensure access is adequate Ensure adequate surface preparation Qualify the test method (standards, scan vs. spot, etc.) and technicians Take the reading at the correct place and record the data error-free Verify the accuracy of some of the readings before the survey is concluded.

    6.1 What type of tube is involved? Many recovery boilers use mostly plain carbon steel tubing. Densely studded carbon steel tubes may be also used in the lower parts of the boiler (such as floor and walls). Some generating tubes and many economizer tubes are finned carbon steel tubes. Other materials with superior high temperature properties or corrosion resistance have been widely used where tube temperatures or heat flux are highest. These include tubes of low-alloy carbon steels, nickel alloys or stainless steels, composite tubes with stainless steel or nickel alloy outer layers, pack-diffusion coated (e.g. chromized) tubes or weld overlaid tubing. Thermal spray coatings have also been used to protect carbon steel tube against corrosion and erosion. Figure 2 shows schematically the different types of tube configurations used in recovery boilers. The UT testing company must know in advance of doing a thickness survey what type and sizes of tubing they will be asked to test for thickness in each part of the boiler. This knowledge helps them prepare adequately for the job, especially with regard to having appropriate qualification and calibration standards. UT thickness testing of plain tubing is straightforward. This covers all carbon steel tubes, corrosion-resistant alloy tubes or (carbon steel) tubes with a diffusion layer. Pin studs on tubes present challenges in getting adequate access with the UT probe to the tube surface between the studs, especially on the sides of the tubes. Getting the tube surface clean enough for UT testing is important for accurate measurements. Special, smaller diameter, “pencil” probes are often used to access the tube surface between the studs, but these probes require more highly skilled technicians. (Section 6.3 contains more information about cleaning pin-studded tubes.)

    Fig. 2. Different types of recovery boiler tubes

    The total thickness of composite tubes and weld-overlaid tubes can be tested as if they are typical tubes, because their external, corrosion-resistant layer is supposed to be fully metallurgically bonded to the carbon steel core everywhere and the velocity of sound in the corrosion-resistant layer is similar to that in the carbon steel substrate. UT thickness testing is also used to check the continuity and integrity of the metallurgical bond at the interface between the composite layer and the substrate. If the outer layer of a composite or a thermal sprayed coating is not ferro-magnetic (e.g. it is stainless steel), the separate thickness of the layer can be measured with a magnetic lift-off (MLO) gauge or eddy current coating

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 6 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones thickness (ECT) gauge (MLO may not be accurate on weld overlaid tubes). The separate thickness of a ferro-magnetic or partially magnetic external layer, including the diffusion layer on a chromized tube, cannot be measured reliably using non-destructive test methods. In addition to UT testing, chromized tubes should be inspected visually for rust bleed-through, which indicates the chrome layer has been compromised. Weld overlaid tubes can be inspected with UT. Calibration procedures for these types of tubes and measurements must be reliable and easily transferable to in-boiler testing. 6.2 What access is available? The UT probe must be properly positioned on the tube to acquire a valid UT reading. The technician should not be straining or stretching uncomfortably to place the probe in the assigned location. Proper use of maintenance beams, pole scaffolding, sky-climbers, pick boards, etc., are critical to good safety and satisfactory job progress. Careful installation and removal of scaffolding is necessary to avoid damaging the tubes. Platforms must provide convenient and safe access to the test lines and locations. Adequate lighting also is essential. Other work, usually including welding and grinding, often is also going on in the boiler during an outage. Visual inspection work and UT thickness testing must be dovetailed and coordinated with other work in the various parts of the boiler. 6.3 What surface preparation is necessary? Salt cake deposits and buildup must be washed away both for safety and to expose the boiler tubes for inspection. This typically involves burning out the smelt bed as the boiler is taken out of service and water washing the boiler from the top down before it is entered. Special attention should be paid during washing to eliminate large salt cake deposits, which may dislodge from the upper areas of the boiler and pose a safety hazard to anyone below. If the boiler is force-dried, firing with natural gas usually dries the boiler tubes more rapidly than drying with hot air. Firing with oil-burners can cover the tubes with a layer of soot that may impede inspection. This is not a good practice and is not allowed at many mills. All inspection locations must be clean for accurate UT thickness testing. Very thin, tightly adhering scales can be left in place. Cleaning can be done with abrasive blasting, with power tools (e.g., wire-brushes, sanders or grinders), or with hand tools. Water (Hydro-) blasting is often used to clean recovery boilers. Blast cleaning of the tubes can be especially disruptive of other work and must be scheduled at the most opportune time. Blast cleaning is essentially the only way to remove residues from the tube surfaces between the studs on studded tubes, especially if the studs are longer than 12 mm (0.5 in.). The appearance of rust bloom after surface cleaning is not a cause for concern. The tube surface must be smooth enough to properly seat the manual probe on the surface without excessive rocking. Corroded surfaces should be ground carefully to the bottom of the corrosion roughness, pits or fissures in an area big enough to make a repeatable thickness reading. High viscosity couplant can be used to obtain more accurate UT readings of rough surfaces. Another option is the use of small diameter (“pencil”), or specially shaped, probes to measure the remaining thickness in the corroded area. Thermal spray coated tubes are conventionally cleaned by controlled “brush blasting” that does not thin the coating significantly, or by alternative abrasive blast cleaning procedures recommended by the thermal spray coating applicator. If aggressive mechanical cleaning methods (e.g., jackhammers) are used to remove frozen smelt or refractory from floor tubes, careful visual inspection of the tubes after cleaning is essential so that any significant damage produced by the cleaning can be assessed for repair. Use of jackhammers must be done without damaging the tubes. Less aggressive cleaning methods such as abrasive or hydro-blasting are preferred whenever possible. 6.4 Are the technicians and the test procedure qualified? An NDT technician obtains certification by meeting the education, training, examination and practical work experience requirements of the certifying organization. Only certified NDT technicians should perform recovery boiler tube thickness surveys.

  • 7 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones NDT technicians should be tested, qualified and certified in accordance with a procedure meeting the recommended guidelines of the American Society for Nondestructive Testing (ASNT) Recommended Practice, SNT-TC-1A (6) or ASNT CP-189, “ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel” (7). The Canadian General Standards Board (CGSB) Standard 48.9712, “Certification of Nondestructive Testing Personnel” fills the same role in Canada (8) These documents outline the requirements with respect to general education, relevant classroom training and practical experience a technician must meet to obtain certification at different levels. SNT-TC-1A is administered by each NDT company for its own employees. NDT companies are responsible for establishing certification requirements for their own technicians. (Certification of NDT technicians through the CGSB is more uniformly administered.) The NDT contractor’s certification program should be reviewed with this in mind. In addition to certification, a technician’s hours of experience doing UT thickness testing on boiler tubes should be taken into consideration. Technicians who do UT thickness testing of recovery boiler tubes should be certified to at least SNT-TC-1A, Level II in thickness testing, or the CGSB equivalent (6-8). Due to the various training, experience and qualification programs among NDT vendors, UT thickness should be qualified at the job site by the boiler owner to evaluate their competence. TAPPI TIP 0402-21 describes a method the boiler owner can use to evaluate a technician’s skill at properly calibrating an UT instrument and measuring the thickness of boiler tubes (9) Whether a manual or semi-automated thickness testing procedure or technique (i.e. spot or scanning) is used, the procedure should be available in written form and should be verified effective, using appropriate tube standards, as discussed in Section 4.2.3. 6.5 Are the readings taken exactly at the assigned locations? The NDT contractor delivers many numbers – the tube thickness readings – to the boiler “owner.” These readings must be accurately recorded with respect to the precise test location and the test reading obtained there. Two primary issues are important – the precise location where the thickness is determined and error-free handling of the large amounts of thickness data. The first issue associated with location of thickness readings is precisely where on the tube the “crown” and “crotch” readings are taken. As stated in Section 4.2.4, crown readings should be taken within the centermost 15 - 20 mm (0.6 - 0.8 in.) of the membrane or tangent tubes. Left and right “crotch” readings should be taken within 15mm (0.6 in.) of the membrane weld, or as far away from the crown as possible on a tangent tube. Crotch readings must be taken in these specific zones of the tube to qualify as “crotch” readings. Another location and data related issue is ensuring the map position of every test location is correct. Manual methods of recording UT readings with a two-man crew, in which a technician and the scribe call locations and thickness readings back and forth so the scribe can record the data, can be inefficient and produce errors. Modern UT thickness instruments can both integrate the test location information into the data-handling system and collect electronic data instantly and directly when the (lone) technician pushes a button. Use of these modern instruments, with their ability to download their collected data directly to a computer, can help to ensure both that the reading is taken at the correct location in the boiler and that all the data is transferred without error. Trending accuracy of wastage is enhanced if a method is used to return to the same thickness testing location for subsequent surveys. Common methods to do this involve keying on the wall openings and using permanent boiler height markers (studs, washers, pins, etc.) that may be welded to the membrane between the tubes (usually in the corners of the boiler) and snapping chalk strings between these height markers. 6.6 How can the data be verified? To ensure the quality of the data, it is recommended that a procedure be established prior to the job to verify the reproducibility of the vendor's data. This procedure typically would aim to have an NDT supervisor or a technician qualified to ASNT Level II or Level III demonstrate the reproducibility of a small percentage of the total data set. Doing verification testing with a different instrument and transducer from that used to collect the initial data helps eliminate systematic errors that could arise if only one set of equipment is used and has been consistently but erroneously calibrated. Discrepancies must be thoroughly resolved to maximize confidence in the accuracy of the final reported readings.

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 8 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones Data can be verified “blindly” or “specifically”. Blind verification involves obtaining a second set of readings and statistically comparing them with the first set. Specific verification involves verifying specific readings, with the boiler owner representative physically seeing if the initial and verifying readings match up within the required tolerances. Data should be verified for tubes in different parts of the boiler, e.g., walls, floor, screens, superheater, etc., and not focus entirely on the lower furnace zone. It is difficult to obtain precise UT measurements on rough tube surfaces, so the inclusion of these locations into any verification exercise should be defined in the verification process. Verification performance is directly linked to the confidence in the validity of the data. Some paper companies have even used this as the basis for “pay-for performance” contracts with the NDT contractor. Where verified thickness data indicate thinning below the “minimum” or “flag” limits established ahead of time for the various tubes in the boiler, or where readings fall below the expected values predicted by corrosion rate projections, additional readings should be taken to define the size of the thinned area. These extra measurements may be made by spot readings around the low readings or by area scanning, as described in Section 4.2.4. 7. PART II - Default layouts for tube thickness surveys in various boiler zones 7.1 Getting started The first step in the process for planning a tube thickness survey should be to assemble and carefully review:

    Pertinent previous tube thickness data; The recent operating history of the boiler; Other inspection-related information, such as bulletins from BLRBAC, TAPPI, AF&PA or the boiler OEM.

    The next step should be to interact directly with whoever will be involved in preparing for and subsequently doing the tube thickness survey and inspection to clearly define the following:

    Safety requirements and procedures in effect. (See Section 3) Details of how the technicians will be provided safe and convenient access to the inspection locations. The actual hours available for the inspection. Types of equipment and instrumentation to be used. Level of training and certification required for NDT technicians. (See Section 6.4) Scope and administration of on-site UT technician performance demonstrations. Type and method for cleaning the tubes (by area, if needed). Assignment of responsibilities for items such as lighting, scaffolding, cleaning of tubes, safe access, and

    safety personnel. An area-by-area description of the number of readings to be taken, or scope of work, i.e., elevations or

    positions of test lines on walls; number of tubes to be tested per line; number of readings per location, etc. (See Section 7.2)

    The minimum or “flag” thickness values for each tube included in the survey. (These are defined by the boiler owner and may not be Code-defined values.)

    Data should be tested for repeatability (See Section 6.6) Specific reporting procedures, verification procedures and/or actions to be followed when readings are found

    below “minimum” or “flag” values (see Section 6.6). Nature of reports and report formats to be delivered, number of copies to be provided, and to whom and by

    what date they are to be delivered. (See Section 5) How to deal with such issues as additional work stimulated by visual and UT thickness test findings, stand-by

    time and other changes to the scope of work and schedule. 7.2 Default survey patterns To be most effective, inspection scopes and tube thickness survey patterns should be customized for each boiler.

  • 9 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones Default layouts are described below for tube thickness surveys in the various zones or parts of a recovery boiler. These are examples of survey layouts used for periodic outage inspections when the survey layout has not yet been customized, either because little is known about or because insufficient consideration has been taken of the specific condition and corrosion experience in those zones of a particular boiler. Boilers subjected to changes in operating conditions or boilers with zones or components that have known wastage problems or tube integrity concerns may require more extensive inspection than the examples described below. Conversely, a boiler that has a well-established inspection history based on high quality thickness data could have both its inspection scope and frequency reduced, especially in parts of the boiler where little or no wastage has been experienced over time. Most tube thickness surveys are done in a grid pattern of some type. The grid pattern selected should be designed to locate general areas of tube thinning. The chances of finding the thinnest tube location with a conventional grid pattern are low, because thinned areas are typically much smaller than grid spacings. It would be coincidence for a survey spot to coincide with the thinnest tube location. Additional measurements are needed to confirm the location of the lowest thinned tubes is found using a grid pattern test method. Experience and/or the condition of the boiler dictates that the grid spacing should be small enough to find any general area of thinning that may reasonably be expected to occur, based on previous inspection history. For example, if general thinning in a lower water wall typically occurs over a 3 m by 3 m (10 ft by 10 ft) area, testing every fifth tube every 1.7 m (5 ft) should produce at least 10 “low” readings to indicate this condition. Areas including thinned tubes should be tested using a tighter grid spacing, to help determine the extent and severity of the condition. Conversely, areas with tubes that thin at a consistently low rate may be tested with larger grid spacing. 7.2.1 Orientation of the boiler The furnace wall opposite the flue gas outlet is conventionally designated as the front wall. The other walls are designated typically by looking at the gas outlet side from the front wall. The sidewalls are typically termed the left and right walls respectively, and the gas outlet wall is the rear wall. The smelt spouts are not always in the rear wall but the nose arch always protrudes from the upper rear wall. In dual-drum boilers, such as shown in Fig. 3, the tubes extending upwards from the nose arch may form a generating bank screen and may form a portion of the roof. Alternatively the tubes may lead directly into the steam drum. In single-drum boilers, the tubes extending up from the nose arch form the generating bank screen. The superheater tubes are suspended in platens above the nose-arch. There may be water bearing screen tubes in front of the superheater section that are in the furnace. The next tubes “downstream” in the gas flow are the generating bank tubes. A boiler may have either a horizontal or vertical (long flow) economizer section and may have a direct contact evaporator or a cyclone system to reclaim the final flue gas heat.

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 10 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones

    Stack

    Precipitator

    Economizer Section

    GeneratingSection

    SuperheaterSection

    NoseArch

    Tertiary Airports

    Liquor Guns

    Secondary Airports

    Primary Airports

    Floor

    SmeltSpouts

    RearWall

    FrontWall

    ScreenTubes

    Fig. 3. General sloped bottom boiler for dual-drum unit with economizer.

    7.2.2 Floor

    Furnace floors may be sloped or decanting (flat). Most floors are of membrane construction. These tubes may be composite, plain carbon steel, pin-studded or weld overlaid (See Section 6.1). Floor tube thickness surveys are typically done along lines parallel to the front wall, as shown in Fig. 4.

    For single-sloped floor boilers, the first (reference) survey line is usually approx. 0.3 m (1 ft) from the rear wall, and the other lines are spaced approx. 1.2 m -1.5 m (4-5 ft) apart across the floor from there. It is common to test the thickness up to every fifth tube along each line. More testing can be done as needed based on test results during the inspection. Inspecting alternating tubes at subsequent shutdowns can be done, but trending of data must be done on only those tubes tested.

    Many sloped floor boilers have experienced more rapid wastage on tubes within 1.5 m (5 ft) of the sidewalls or the non-spout (front or rear) wall, or directly in front of the spouts. This area can be tested on every tube at 0.3-0.6 m (1-2 ft) intervals to ensure there is no excessive wastage.

    Rear Wall Tubes

    Side Wall Tubes

    Floor Tubes Inspection Lines

    Fig. 4. Survey locations on floor tubes.

  • 11 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones

    Experience has shown that floor tubes in decanting units are less prone to tube wastage than sloped bottom units. This is attributed to the buildup of a thick layer of frozen smelt and lime or refractory over the floor tubes in this type of floor. Therefore, the frequency of inspection in this type of floor can be lower than for sloping floors. A similar inspection grid should be used on decanting floors after the frozen smelt bed has been removed for inspections. 7.2.3 Waterwalls

    The lower waterwalls and floor form a box around the liquor combustion zone and the smelt bed. These walls have many openings: primary, secondary, and tertiary air ports, manways, camera ports, observation doors and burner openings. The tubes here may be composite, pin-studded or plain carbon steel, diffusion-coated or weld-overlaid. Carbon steel tubes may be thermal spray coated. Most walls are of membrane construction, but some boilers have tangent tubes (with or without seal welds) or flat studs covered with refractory.

    Wall tubes are typically numbered from left to right when facing the wall from inside the furnace. Tubes on each wall are numbered separately, starting with the left-most tube. The numbering system must be consistent and clearly documented. Tubes should be visibly numbered at several elevations to avoid confusion about any tube’s number. It is good practice also to clearly label the openings inside and outside the boiler and in a consistent way from outage to outage to facilitate all types of communication about tube location and identification for inspection and repair.

    UT thickness testing of carbon steel waterwall tubes (with or without studs) is most commonly done in a grid pattern. The line elevations are usually 0.6–1.5 m (2–5 ft) apart below the secondary air ports; 1.5–3.0 m (5–10 ft) apart between secondary and tertiary air ports and 3–5 m (10–16 ft) apart above the tertiary air ports.

    It is common to test up to every fifth tube along each grid line, similar to floor tubes, alternating the tubes at the next shutdowns. It is conventional to test only every fourth or fifth tube on the lines above the tertiary air ports. Thickness surveys at elevations above the tertiary air ports are also typically conducted less often than on the tubes below this level.

    Wall tubes that have the liquor sprayed onto them (as with perimeter or wall-dry firing) should be tested more frequently than tubes in units where the liquor is sprayed in the center of the boiler floor. Sections of tubes that are bent to form, particularly primary airports, may be tested more frequently in order to detect thinning of the tubes. Composite tubes can suffer wastage exposing carbon steel areas that need to be monitored.

    Carbon steel tubes above the composite (or otherwise protected) tubes should be tested regularly in the first (lowest) 25 mm (1 in.) above the transition or “cut” line. These measurements can determine whether the composite layer or other protective barrier terminates too low down on the wall. If thinning is detected, the grid lines above this location should be adjusted to determine the extent of wastage.

    The thickness of composite tubes and tubes with other protective barriers is usually tested less frequently and over wider spacing than plain or studded carbon steel tubes. Corrosion of the composite or protective barrier on tubes can occur rapidly. As long as the composite layer or protective barrier shows negligible corrosion from a rigorous visual inspection, extensive UT surveys are not warranted.

    UT thickness testing measurements can be conducted on tubes protected with a composite (co-extruded) or thermal sprayed coating, as long as the coating is adhered to the base metal. MLO measurements can be taken on thermal sprayed and composite tubes to monitor the corrosion of the protective barrier over time. UT measurements on weld overlaid tubes are needed because MLO measurements are inaccurate for this type of tube material. MLO measurements also do not work well on chromized tubes.

    As discussed in PART I, waterwall thickness survey grids should be customized for each boiler and permanent survey line markers (e.g., studs or nuts welded on corner membranes) can maximize the repeatability of survey line placement at each elevation.

    Special attention should be paid to the visual inspection for localized corrosion on tubes forming the openings in the lower furnace since this corrosion is seldom detected with a standard grid pattern survey. Corrosion can occur toward the cold side of the tubes. Both visual examination and UT testing of smelt spout openings and lower airports should be done, in addition to the general survey grid, to ensure localized corrosion here is not missed, such as balding in airports where the carbon steel substrate is exposed on composite tubes.

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 12 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones 7.2.4 Roof tubes

    Roof tubes are typically plain carbon steel tubes. A combination of membrane and flat studded construction is commonly used to provide a gas tight seal at the openings for the superheater tubes that pass through the roof. Roof tubes are inspected less frequently than waterwall tubes.

    In some boilers, roof tubes have been thinned by rubbing against superheater pendant or screen tubes that penetrate through the roof. This type of thinning is examined using oblique illumination in these areas. Obtaining UT measurements can be difficult, or impossible, due to lack of access.

    For tube thickness surveys, the zero reference line should be the top of the bend at the top of the front wall. Tube numbering is a continuation of the front wall. As shown in Fig. 5, four or five survey lines are typically spaced evenly across the roof, from the zero reference line up to the first screen or superheater tubes. Thickness testing is usually done on every fourth or fifth tube.

    Fig. 5. Survey line layout on roof tubes

    7.2.5 Nose arch

    Nose arch tubes are typically plain carbon steel with lateral membranes or flat studs. On the fireside, nose arch tubes may also be subjected to gas impingement, fly ash abrasion, and potential corrosion from dripping of molten deposits from superheater loops. Within the arch cavity, nose arch and furnace sidewall tubes can experience aqueous corrosion from water washing residues if wash water enters the cavity. This is particularly likely if the arch tube panels are not of welded membrane construction.

    Nose arch tubes are numbered as a continuation of the rear wall. Typical survey lines and locations are shown in Fig. 6. One survey line should be at the center of the convex bends in the nose. The other lines are usually spaced at about 0.3 m (1 ft) intervals above and below this base line. Thickness tests may be made on every third to fifth tube.

    If visual inspection of the tubes on the top slope of the nose arch or any other clues or inspection information such as thinning of the superheater loops indicates that molten carryover deposits have dripped on the nose arch tubes, thickness testing should be done at the superheater drip lines. (Nose arch tubes dented by falling deposits should be carefully evaluated for fitness for continued service.)

    678910

    Tube Numbers

    InspectionLines

    Corner Tube

    Roof Tubes

    Front Wall

    Nose Arch

    Rear Wall

  • 13 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones 7.2.6 Screen tubes

    Screen tubes are typically plain carbon steel. The screen tubes normally are in uniformly spaced platens of tubes that penetrate the rear wall or nose arch and extend to the roof. Some screens may penetrate the front wall. A line across the main bend in the outer-most screen tubes should be designated as the zero reference point.

    Example test locations are shown in Fig. 7. The outermost portion of each screen tube bend may also be scanned to determine its minimum thickness.

    R e a r W a ll

    1 2 3 4T u b e n u m b e r s

    N o s eA r c h

    I n s p e c t io nL in e s

    C o r n e rT u b e

    Fig. 6. Survey locations on nose arch tubes.

    1 2T u b e o r P la t e n n u m b e r s

    N o s eA r c h

    R e a r W a l l

    I n s p e c t io nL in e s

    Fig. 7. Survey locations on screen tubes.

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 14 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones 7.2.7 Superheat section Area scanning of the outermost bend of each loop in the superheater platens can reveal whether there is internal pitting and thinning due to the bending operation during original fabrication of the tubes. Internal pitting can be detected with ultrasonic scans, but quantifying the extent of the pitting can be very difficult. The use of profile radiography is a useful tool in determining the pitting and for detecting preferential corrosion at welds. This testing is usually done on the outermost carbon or low-alloy steel tube in the platens. If the outermost tube is not carbon steel or low alloy steel, the outermost loop of these materials should be tested. All sootblower lanes should be inspected and tested as described in Section 7.2.10.

    A

    B

    C D E F

    G AB

    C D EF

    N o s eA r c h

    Fig. 8. Survey locations on superheater loops.

    7.2.8 Generating section 7.2.8.1 General considerations

    Generating section tubes are typically plain carbon steel. In dual-drum boilers, they are swaged to a reduced diameter for insertion into the drums. In single-drum boilers, they may or may not be swaged for insertion into the platen headers and usually have longitudinal fins welded in the fore and aft positions. Generating section tubes in sootblower lanes are often shielded; most commonly with sheet metal covers but also sometimes with thermal spray coatings.

    Sootblower lane tubes in the generating section should be regularly tested for erosion-assisted wastage from the sootblowing operation. Testing is typically done first where visual inspection reveals any evidence of wastage on the impinged surfaces of the generating tubes. In such cases, a default survey tests the thickness around the half of the tube that faces the sootblower cavity, at the elevation of the sootblower and 0.3 m (1 ft.) above and below that level (Section 7.2.10). Shielded tubes are typically only visually inspected to ensure the shielding is intact.

    Contact points with vibration ties or baffles may experience wear and should be inspected. The sidewall tubes at the generating bank in dual-drum boilers are particularly prone to corrosion on the cold side of the tubes because these tube panels often are not gas-tight. A typical survey layout for the generating bank tubes is shown in Fig. 9.

    Generating bank tube thickness testing can also be done from within the tubes, using semi-automated immersion UT or electromagnetic testing (ET) methods. These specialized methods were mentioned in Section 4.2.1, but detailed explanation of them is beyond the scope of these guidelines. A significant advantage of these

  • 15 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones methods is that relatively rapid thickness testing along the entire tube length is possible, with the exception of tight bends in the tubes. Section 6.4 emphasizes the importance of properly qualifying these semi-automated methods. 7.2.8.2 Near-drum corrosion. (NDC)

    Specialized UT equipment, transducers (probes), and procedures are required to locate a wastage problem that can occur on generating bank tubes adjacent to the outside surface of the mud drum. Near-drum corrosion (NDC) is localized and is usually most severe in the line of sight from the sootblower nozzle. NDC has been found in tubes on the hot (furnace) side of the boiler, as well as the cold side (exit side) of the generating bank tubes. Figure 10 shows a tube thinned by NDC. Simple, manual, contact UT testing does not provide adequate characterization of NDC and has often failed to detect it.

    M u d D r u m

    In sp e c tio nL in e s

    E co n o m iz e r S id e F u rn a c e S id e

    G a s F lo w

    Fig. 9. Thickness testing locations on generating bank tubes.

    Fig. 10. Tube thinned by near-drum corrosion at tip of arrow

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 16 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones

    To obtain a statistically significant sample of the severity of NDC, at least 40% of the first two rows of generating tubes astride each sootblower lane should be scanned at the mud drum. Inspections for NDC are also recommended on the tube in the cold side of the generating bank. The scan inside the tube usually extends from approximately 12 mm (0.5 in.) before to 50 mm (2 in.) beyond the outer surface of the drum wall, as shown in Fig. 11. It is difficult and usually unnecessary to acquire data where the diameter of the swaged tube starts to increase.

    The lowest thickness of each tube and its clock position should be recorded. The standard tube for calibration should have the same radius as the tubes being inspected.

    The specialized, semi-automated techniques for accurately defining NDC require specially trained technicians. Section 6.4 discusses the importance of properly qualifying these semi-automated methods. Some paper companies have used a mud-drum mock-up with rolled-in tubes with known defects to qualify the various commercial systems for NDC testing.

    Near-drum thinning has occasionally been detected adjacent to the steam drum. This wastage is not associated or oriented with soot blowing. It is most likely to be found where the steam drum has been externally counter-bored. UT similar to that for NDC can be used to test for this condition.

    7.2.9 Economizer section

    Economizer section tubes are typically plain carbon steel. The tubes often have heat transfer fins longitudinally welded in the fore and aft positions with respect to the gas flow.

    The tubes in economizer modules usually are visually inspected and thickness tested at accessible bends, in each sootblower lane and at contact points with anti-vibration or baffle hardware. The coldest tubes in the economizer may experience “dew-point corrosion,” i.e., attack by acidic condensates from the flue gas. Areas that commonly accumulate salt-cake, such as the access cavities and bends in bent tube economizers, can experience unusual corrosion, especially if water washing does not completely remove the deposits in these areas.

    Fig. 11. Scanned area inside (swaged) tubes for NDC: usually starts 12 mm (0.5 in) before and ends about 50 mm (2 in) beyond the outer surface of the drum.

    It is common practice to test every fourth or fifth tube in the center of all accessible bends and 0.3 m (1 ft) above and below these points. Owing to the wide variety of economizer module designs, a reference drawing showing the test point locations should be included in the test report. An example test layout is shown in Fig. 12.

    Economizer tubes may also thin internally due to flow-accelerated corrosion (FAC) above the inlet header. The thickness of the tubes can be checked by UT at 25 mm (1 in.) above the internal headers. FAC can also affect feed-water supply piping and the inlet header (10). 7.2.10 Sootblower lanes

    A sootblower is a rotating, steam-ejecting lance which traverses the boiler to remove “soot” (deposits) from the tubes. The steam jets can cause accelerated wastage where the steam impinges on the tubes, especially where the

  • 17 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones steam is wettest (i.e., when the sootblower starts its run). Sootblowers are located in the superheater, generating and economizer sections.

    Visual inspection testing (VT) using oblique illumination can find erosion patterns or the presence of erosion or corrosion. Thinned zones obviously should be tested. If thinning is found, it typically decreases away from the center of the lane. If specific corrosion zones are present, thickness testing is typically done facing the sootblower, at the elevation the sootblower operates, and 0.3 m (1 ft.) above and below the sootblower elevation. An example survey layout for a typical sootblower lane is shown in Fig. 13. Tubes behind those in the rows framing the sootblower lane are not usually tested.

    Tubes in sootblower lanes are typically numbered according to the sootblower number and the section of the boiler, plus their location across the boiler. A drawing showing where each sootblower is located should be included in the test report.

    The first five tubes away from the wall at each end of each sootblower lane should all be tested. Only every fourth or fifth tube is commonly tested beyond that, unless an erosion pattern is present. If the sootblowers overlap in the center of the unit, tubes exposed to both sootblowers should be regularly tested. The tubes forming the openings for sootblowers should be tested.

    G a s F l o w

    G a s B a f f l e s

    S o o tB l o w e r

    S o o tB l o w e r

    S o o tB l o w e r G a s F l o w

    G a s F l o w

    I n s p e c t i o nL i n e s

    G a s F l o w

    G a s B a f f l e s

    S o o tB l o w e r

    S o o tB l o w e r

    S o o tB l o w e r G a s F l o w

    G a s F l o w

    I n s p e c t i o nL i n e s

    Fig. 12. Survey locations in the economizer section.

  • TIP 0402-18 Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers:/ 18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones

    Fig. 13. Default test locations for a sootblower lane with no erosion-corrosion pattern.

    Fig. 14. Openings in recovery boilers and typical test locations.

  • 19 / Ultrasonic testing (UT) for tube thickness in black liquor recovery boilers: TIP 0402-18 Part I - Guidelines for accurate tube thickness testing Part II - Default layouts for tube thickness surveys in various boiler zones 7.2.11 Boiler openings

    There are several openings in a recovery boiler, such as air openings, smelt spout openings, soot blower openings and observation openings. Accelerated corrosion has been found in openings, especially primary airports, smelt spouts and sootblower openings. UT and MLO measurements should be made to monitor wastage. Figure 14 shows openings and typical inspection grids used during inspections. A typical inspection grid is center-line of the opening and ± 0.3 m (1 ft).

    Keywords Recovery boilers, Inspection, Nondestructive tests, Black liquor, Tube, Thickness, Ultrasonic tests, Corrosion Additional information Effective date of issue: April 10, 2015 Working Group:

    Michael L. Lykins, Chair, Packaging Corporation of America Dennis Beggs, Rock-Tenn Mark LeBel, Andritz Margaret Gorog, Weyerhaeuser Corporation Mark Gilkey, Factory Mutual Global Justin Rauch, Acuren Larry Richardson, Hi-Tech Testing

    Literature cited 1. Bauer, D. G. and Sharp, W. B. A., “The inspection of recovery boilers to detect factors that cause critical leaks,”

    Tappi J. 74, (9), 92 ff. (1991). 2. Bauer, D. G. and Sharp, W. B. A., “Interpreting recovery boiler thickness data,” TAPPI J, 79 (11), 161-168.

    (1996). 3. The Recovery Boiler Reference Manual for Owners and Operators of Kraft Recovery Boilers, 2nd Ed., American

    Paper Institute, Recovery Boiler Committee - Operations and Maintenance Subcommittee, 1992. 4. Inspection of Composite Tube Kraft Recovery Boilers for Corrosion, NACE Technical Committee Report, 1992. 5. ASTM E-797-01 “Standard practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact

    Method”, published by American Society for Testing and Materials, West Conshohocken, Pennsylvania. 6. ASNT Standard CP-189, “Standard for Qualification and Certification of Nondestructive Testing Personnel”,

    published by the American Society for Nondestructive Testing, Columbus, Ohio. 7. Recommended Practice No. SNT-TC-1A (2006), published by American Society for Nondestructive Testing,

    Columbus, Ohio. 8. Canadian National Standard CAN/CGSB-48.9712-2000 “Non-destructive Testing – Qualification and

    certification of personnel (ISO 9712:1999, MOD), published by Canadian General Standards Board, Ottawa, Canada.

    9. TAPPI TIP 0402-21, “Ultrasonic technician performance test for boiler tube inspection,” 2008 10. Petric, G.W. and Ksiazek, P.E., “Flow-Accelerated Corrosion in Industrial Steam and Power Plants,” TAPPI

    Engineering and Papermakers Conference, 1997.