AUTOMATED UT CORROSION MAPPING
Automated Ultrasonic Testing (AUT) covers a range of ultrasonic inspection techniques using powered, mechanical scanners to locate inherent defects
within a given material. AUT is the term used to describe corrosion mapping inspections, pulse-echo weld inspection, Phased Array and Time of Flight Diffraction.
NDT APPLIES AUT
- Measure flaws in the base material and show their position, extent and depth
- Detect pitting, general corrosion and laminar defects including inclusions, plate laminations and blistering
- Obtain routine and repeatable corrosion maps from room temperature to 350 ⁰C (662 ⁰F) and higher
- To assess pressure vessels and piping for blistering, Hydrogen Induced Cracking (HIC also referred to as stepwise cracking) and Stress Oriented Hydrogen Induced Cracking (SOHIC) damage; for these inspections we can use ‘Tri-element’ probes
- To differentiate between laminations, blistering and stepwise cracking
ADVANTAGES
- High degree of repeatability
- Position and size data for every flaw can be compared for repeat scans of the same area to track flaw growth or corrosion rates both generally and for individual pits
LIMITATIONS
In-service inspections, specially high temperature examinations, require extensive preparation and cooperation between pressure containing operating personnel and AUT personnel.
PHASED ARRAY PA
NDT APPLIES PA TO INSPECT
- Welds and weld overlays, piping, pressure vessels, clad material, storage tanks and structural steel
- Raised face flanges for damage on the ID bore and sealing face without having to disassemble them. This technique can be used during shutdowns, or on operating flanges
- Complex geometries with a single probe
ADVANTAGES
- High degree of repeatability. Position and size data for every flaw can be compared for repeat scans of the same area to track flaw growth or corrosion rates both generally and for individual pits
- Sound beams of many angles can be generated sequentially, inspecting a large portion of the component’s cross-section. Electronic adjustment of beam focus and angle, and sweeping of UT beam through the entire weld volume
- The ultrasonic beam may be both steered (change of angle), focused and scanned without moving the probe.
- Portable, battery operated testing device
- Inspection can be performed on in-service equipment
- Can be combined with TOFD scan
LIMITATIONS
- Probes are often larger than those used for conventional UT
- Higher temperature probes are in development
- Combines a phased array probe with water from the probe to the vessel surface to allow scanning of surfaces unsuitable for contact probes
- Allows the probe to scan rough and uneven surfaces while maintaining consistent coupling of the ultrasonic beam through the water column
- Works with semi-automated and full mechanized scanners or it can be used manually to produce encoded line scans
TIME OF FLIGHT DIFFRACTION (TOFD)
This ultrasonic inspection is commonly performed on welds and weld overlays, piping, pressure vessels, clad material, storage tanks and structural steel.
As fabricated heavy wall vessels and piping can be thoroughly assessed for fabrication flaws with TOFD.
NDT APPLIES TOFD
- To verify compliance with fabrication requirements for piping and vessels in accordance to standards such as ASME Section I, Code Case 189,or ASME VIII Division 1 and 2
- For in-service defect monitoring
- For detecting defects during the manufacturing process
ADVANTAGES
- High degree of repeatability
- Position and size data for every flaw can be compared for repeat scans of the same area to track flaw growth or corrosion rates both generally and for individual pits
- High penetrating power for detection of defects deep in the material
- Portable, battery operated testing device
- On-line, volume inspection
- Set-up independent of weld configuration
- Amplitude insensitive, acoustical coupling less critical
LIMITATIONS
- Weld must be accessible from both sides
- There is a dead zone for detection near the surface of the material
- Gathers large volumes of information which takes experienced technicians to interpret
TOFD FUNDAMENTALS
Time of flight diffraction (TOFD) inspection is an ultrasonic technique which allows for the location and sizing of defects. Two probes are used in a transmitter-receiver arrangement,
with the transmitter introducing ultrasonic sound waves at an angle into the material, and the receiver picking up diffracted signals from any defects present. The amplitude of the signal
is not displayed, but the position of the signals on the time scale is shown. This enables the technician to determine the defect location, length, and defect height.
The scanners and UT systems can perform both conventional pulse-echo (angle beam) and Time of Flight Diffraction (TOFD) scans simultaneously if required. Data is recorded in blocks (scans)
anywhere from 305 mm (12 inch) to 5.3 m (210 inch) in length. Successive scans began at the endpoint of the previous scan. To ensure complete coverage of the examination volume, a data point and
depth reading can be taken every 1 mm (0.04 inch) in the axial (Y) scan direction before indexing by 5 mm (0.200 inch) in the circumferential or longitudinal (X) direction to begin the next scan stroke.
This overlap of the search unit while indexing ensures that small flaws can be detected reliably.
ADVANCED ULTRASONIC BACK SCATTER AND SPECTRUM ANALYSIS (AUBT/ABSA)
NDT is a licensee of the technology and procedures developed by Shell for High Temperature Hydrogen Attack HTHA detection.
The Advanced Ultrasonic Backscatter Technique (AUBT) and the associated Angle Beam Spectral Analysis (ABSA) technique for welds were developed for detecting and quantifying damage from high temperature hydrogen attack.
NDT APPLIES AUBT TO INSPECT pressure vessels and piping for High Temperature Hydrogen Attack (HTHA). This damage can develop in steels exposed to high pressure hydrogen at elevated temperatures. Examinations incorporate:
- The backscatter technique used primarily to determine damage progression through the wall. It works with materials of any geometry with or without cladding
- Spectrum analysis which helps to determine the degree of HTHA; it is sensitive to fissures and is independent of the measurement system.
- The velocity ratio measurement which differentiates between fissures and other internal defects. It is not affected by material geometry, back wall surface condition, or the measuring system.
AUBT FUNDAMENTALS
The technique uses high frequency, broadband UT probes and a digital oscilloscope to provide both an A-Scan display and frequency analysis. Variables such as the ID surface condition, geometry and other internal defects
have little effect on backscatter measurements. The method is consistent and reliable so monitoring of hydrogen attack and its progression over time provides meaningful information. Nevertheless, the surfaces to be inspected
must have a buffed finish at each measurement location, typically 60 – 120 grit.
The backscatter techniques were developed because conventional UT techniques do not detect micro-fissuring and older methods such as attenuation measurement proved unreliable in the field. The ABSA procedure for weld HAZ
inspection utilises the Spectrum Analysis technique
In high pressure hydrogen at elevated temperatures hydrogen atoms can diffuse into steels and react with carbides. The reaction leads to formation of methane and, subsequently, intergranular fissuring and losses of material strength and toughness.
FLANGE INSPECTION
IRISNDT inspects in-service raised face flanges with phased array ultrasonic testing (PAUT); we detect bore wall loss and flange face damage without having to disassemble flanges.
The technique detects flange ID bore and raised face wall losses.
Typical applications include:
- Raised face flange corrosion losses inspections.
- Most raised face flanges from Class 150 to Class 2500 can be inspected.
- Flange pre-shutdown inspections to identify maintenance requirements.
- High-wear service monitoring of flanges to determine remaining lifetime.
ADVANTAGES
- Flanges can be inspected without needing to disassemble, which is a significant cost savings for large diameter and/or high class flanges.
- The data is permanently recorded to allow audit or future damage monitoring.
- Both sides of the flange are typically inspected at the same time.
LIMITATIONS
- The skin temperature of the OD surface of the flange must be below 100 °C to perform PAUT consistently.
- Ultrasonic testing requires access to the OD of the flange and an ultrasonic surface quality
- This technique does not identify flange face isolated worm-type corrosion reliably.
GUIDED WAVE TESTING GUL
Guided wave ultrasonic testing detects corrosion damage and other defects over long 10 m – 50 m (33 ft – 165 ft) distances in piping. A special tool (transducer ring) is clamped around the pipe and transmits
guided waves in both directions along the pipe. Reflected signals from defects and pipe features such as welds are received by the transducer ring and sent to the main unit.
NDT APPLIES GWT
- To inspect sleeved road crossings
- To detect riser soil to air interface losses
- To detect corrosion under insulation
- To detect pipe rack support losses in-situ
- To inspect buried or insulated lines
- Difficult to access locations
ADVANTAGES
- Inspects difficult to access components without extensive excavation or insulation removal
- Portable, battery powered equipment
- Sophisticated software routines help identify and classify pipe signals
- Rigorous operator training and certification with individual electronic keys which activate the system and track its use by each operator
- Embedded reporting software allows the operator to analyze the results and produce a report on the spot
LIMITATIONS
- Coatings: bitumen wrap and similar heavy coatings cause high attenuation
- Pipe condition: corroded line scatters UT signals and reduces range
- Features such as welds: each weld typically reflects 20% of the signal. Without other limiting factors, six welds set the maximum span
- Bends and ‘T’ junctions: these features distort the signal, one generally cannot test beyond a bend
- Pipe contents: high viscosity liquids or waxy deposits attenuate the signal
- Special soil conditions can cause additional attenuation and are usually associated with wrapped pipe
- Complementary inspection methods are needed to map out and size flaws found with GWT; typically UT and RT are used
GWT FUNDAMENTALS
Ultrasonic signals are swept over a full frequency range, in both directions from the GWT tool, in a single shot. In addition to faster, more efficient data acquisition, having all the data in a single file makes analysis faster and more certain. Sophisticated processing and analysis software allows trained operators to interpret these signals and report their findings. Special, wide frequency probes are available for cases such as buried pipe where attenuation is too high over the standard frequency range.
Enhanced Focusing Capability (EFC) rings (pipes 100 mm (4 inch) and above) improve defect characterization and provides color coded C-scan type maps of the pipe. In effect, the EFC processing focuses on all reflectors over the entire range of the shot. This improves sensitivity as well as making the guided wave data more understandable to the end user.
EMAT – BULK WAVE
NDT uses EMAT bulk waves to track piping metal losses while the part is in service at high temperature (up to 1200°F). This non-contact method does not need a couplant. It can inspect through coatings and needs minimal (if any) surface preparation. EMAT can generate longitudinal or shear waves to assess thickness. IRISNDT operators integrate different sets of magnets and radio frequency (RF) coils to obtain thickness measurements at various ranges.
APPLICATIONS
- Ideal for dry (no couplant) thickness measurement applications
- Ideal for internal thinning wall measurements i.e. sulfidation corrosion
FEATURES
- Measures the thickness of parts up to 5 in.
- Does not need wedges and couplant making the inspection easy and fast
- Is less sensitive to surface conditions such as dirt or coating than traditional ultrasound tests
- Can display A, B and C scans
- Implements changes in sound velocity to correct for temperature changes
LIMITATIONS
- Can only be used for thickness spot checks
- Surface preparation might be needed if external corrosion/pits are present. Rough surfaces can impede good sound transmission by attenuating the signal
EMAT – GUIDED WAVE
NDT uses EMAT guided waves to inspect piping for metal loss. More specifically, we use the technique to inspect inaccessible areas such as that under pipe supports.
The guided waves can travel in the circumferential or axial direction and are sensitive to thickness changes (corrosion losses) on entire volumes of pipe (internally and externally).
To assess the location and extent of corrosion damage, NDT operators use EMAT guided wave ultrasonic wave arrival time (velocity change) and amplitude information.
APPLICATIONS
- Detects touch/ contact point corrosion. The volumetric wall loss is estimated with a ±15% error margin when using combinations of circumferential and axial scanning.
- Detects corrosion under supports.
- Is used to identify soil to air interface corrosion.
- Detects corrosion, erosion and cracking in ground piping.
- Detects corrosion, erosion and cracking in furnace tubes.
FEATURES
- This dry and non-contact method does not require an acoustic couplant. It allows up to 0.1 inch (2.5 mm) lift-off.
- Allows coverage of up to 3 feet (1 meter) with short range guided wave.
- Allows for the inspection of physically inaccessible areas.
- Allows real time analysis and storage of inspection data. The data is represented in A, B and C scans.
- The high frequency guided wave results in better accuracy and resolution than low frequency guided wave application (long range guided waves).
- Allows us to inspect pipe of diameters greater than 2 inch using axial guided waves and pipe of diameters ranging from 6 inch to 24 inch using circumferential guided waves.
- Can be used at up to 200 °C (392 °F).
ALTERNATING CURRENT FIELD MEASUREMENT ACFM
ACFM is used for in-service inspection as an alternative to MT / PT. ACFM is ideal for detecting cracks in welds, through coatings and with less extensive cleaning. In addition, it can be used to estimate crack depths.
NDT APPLIES ACFM TO INSPECT
- Through coatings including: paint, epoxy coatings, oxide layers, fire protection layers and marine growth
- Piping, Pressure vessels, pipelines, and drillpipe, structural supports in petrochemical facilities
- Pipe for ovality deviations
- And size growing fatigue cracks in –in service pressure vessels.
- Equipment operating at temperatures as high as 500 ⁰C
- During welding without having to allow each bead to cool down and without subsequent reheating
- Wind turbines
ADVANTAGES
- As a non-contact inspection method, ACFM can be used to inspect through coatings of various thickness (including dirt), saving on coating removal and reapplication costs
- Equipment is highly portable, easy to deploy through rope access
- High temperature applications allow for in-service inspections, avoiding the cost of equipment shutdown
- Highly reproducible electronic data for reference/audits and integration into plant software
- In-depth defect morphology allows for prioritization of resources
- ACFM sizing models currently only sizes accurately crack depths in steel. The length assessment is not greatly affected in any material. When sizing cracks in other materials, a calibration chart is needed based on defects of known depth. The chart can then be used to convert the ACFM indicated depths into true depths
- ACFM sizing models be readily applied to cracks with a simple morphology (fatigue); they are less suitable for networks of branched cracks (stress corrosion cracking)
- Identify CUI on piping (≥ 2” diameter), vessel and tanks
- Inspect sphere legs above concrete fireproofing
- Inspect vessel skirts above concrete fireproofing
- Inspect I-beams, piping above concrete fireproofing
- Inspect underneath corrosion scale to determine remaining thickness
- Serves to test carbon steel of thickness values up to 1.5”
- Serves to test at temperatures from -238°F to 932°F
- Serves to test through up to 6” thick insulation (liftoff)
- Serves to inspect piping of diameter greater than 2”
- Acquires data at up to 15 points/second
- Performs dynamic scans at up to 3”/second
- Is a volumetric technique (covers ID and OD)
- Has a sizing accuracy of ±10% when defects are larger than the probe footprint (check limitation section)
- Can be used to examine the following weather jackets: stainless steel up to 0.06”; aluminum up to 0.04”; galvanized steel up to 0.02”
- Can only be used to inspect carbon steel material
- The detection sensitivity is dependent on the footprint of the probe (liftoff / insulation thickness) and the thickness of the material (isolated pitting can be difficult to detect)
- Cannot differentiate between Internal or external defects
- Variation in measurements when close to edges (i.e. nozzles, flanges or the end of structure)
- Scanning speed depends upon the thickness of the material
LIMITATIONS
PULSED EDDY CURRENT (PEC)
NDT uses state-of-the-art high-performance Pulsed Eddy Current (PEC) systems to inspect for corrosion under insulation (CUI). We use this fast and cost effective electromagnetic inspection technique
(along with guided wave testing, real time radiography and digital radiography) for honing in on CUI suspect areas without stripping insulation. Our portable high-performance fast data acquisition PEC system has
dynamic scanning modes and real-time C-scan imaging. The grid maps obtained facilitate data presentation and long cables give us flexibility during our set-up.
NDT APPLIES PEC TO
FEATURES
LIMITATIONS