Advanced NDT Applications
EDDY CURRENT TUBE INSPECTION
Eddy Current Inspection is well-established and widely used in Petrochemical Refinery, Chemical Processing and Power Generation facilities, in the examination of Tubing in System Heat Exchangers, Air Coolers, Power Plant Steam Turbine Generator Condensers and Feedwater Heaters.
Systems are reliant on these critical components and hence avoidance of failure is of high importance. Heat exchangers are typically made-up of ‘bundles’ of tubes, small in diameter and thin walled, which can extend to 1000’s in number.
The Eddy Current Tube Test Method has numerous application variables, and method selection is dependent upon tube material, tube dimension and equipment/tubing type. The inspection is used to detect typical tubing associated defects such as; pitting, corrosion, longitudinal and circumferential cracking, fretting, wall thinning and erosion at tube-support plates. Early detection of these defects can prevent failure and the determination of potential failure mechanisms, providing plant engineers not only with a tool to assess condition from a digitally recorded data set, but also facilitating the estimation of remaining life. The assessment results provide valuable data, from which to plan strategic plant operational and maintenance programs and ongoing unit monitoring.
At Integrity NDT we have experienced personnel, using the latest technology for Eddy Current Tube inspection, with the following method/material type capability:
ECT – Conventional Eddy Current Inspection (Nonferrous metals)
PSEC – Partial Saturation Eddy Current Inspection (Ferrous Metals)
RFT – Remote Field Testing (Ferrous Metals and Heavy Wall Thickness)
NFT – Near Field Testing (Finned Tubes)
IRIS – Internal Rotary Ultrasonic Inspection*
*IRIS is an ultrasonic application; however, as it is used in tube inspection, it is shared here under Eddy Current section
Having secured long-term contracts to support continual refinery shutdown programs, our equipment and probe portfolio is wide ranging.
Integrity NDT also manufacture Eddy Current Tube Inspection Probes ‘in-house’, widening the inventory to suit each project workscope.
POWER PLANT CONDENSER TUBE INSPECTION
Integrity NDT provide Eddy Current Condenser Tube inspection and comprehensive reporting, and alongside the operational assessment of Power Plant Steam Turbine Generator Condensers, we work with our PP clients to utilise this data in establishing a programmed approach to Condenser Tube Inspection, essential to the prevention of unwanted tube leakage and the avoidance of forced shutdown.
EMAT (Electromagnetic Acoustic Transducer) is principally a non-destructive ultrasonic testing device which has overcome many of the issues faced by conventional MUT piezo crystal probe.
EMAT is a ‘non-contact’ device and allows the generation of ultrasonic waves without the need for a couplant gel. It is possible to generate waves without the probe in physical contact with the surface.
THE EMAT PRINCIPAL – The EMAT transducer consists of a housing case with a socket, an induction coil, a protective cover, a magnetic flux concentrator and a permanent magnet. Alternating current feeds the induction coil, causing electromagnetic oscillations, which in turn induce eddy currents on the surface of the test object. The induced Eddy current interferes with the permanent magnetic field, creating ultrasonic waves directly on the surface of the test object. These waves propagate in the test object, reflecting and deflecting from the backwall and back to the EMAT coil enabling thickness measurement.
An EMAT is the only device that is able to transmit and receive transversal (shear) waves in this way (it is this phenomina that enables the EMAT probe to be applied without the use of a couplant gel). Waves transmitted by an EMAT allow much higher resolution and sensitivity than the longitudinal waves produced by piezo probes.
Advantages of EMAT
• Minimum surface preparation required
• No ultrasonic couplant (gel) is required
• Allows accurate thickness measurement through corrosive and painted coatings
• Greater accuracy as EMAT only measures the tube base material wall thickness (i.e. does not include either surface corrosion or surface oxide in the given measurement)
• Testing can be carried out by a single operative
Integrity NDT are utilising the latest advancement in EMAT probe technology ‘Electro-magnetic accoustic thickness gauge (A1270)’. This device has a newly developed innovative electro-magnetic biasing technolgy. The probes is built with a ‘pulsed’ electromagnet in place of the conventional ‘permanent magnet’. This innovative technical solution now frees the design from the need to have strong permanent magnet, this excludes the effect of strong magnetic adhesion to the test component, allowing for rapid inspection and also facilitates the option to scan the object as well as take ‘spot’ readings. A further advantage of this ‘pulsed’ magnetism, is that ferritic debris does not adhere to the probes (a disavantage when using standard EMAT probes using perminant magnets).
The advantages of TOFD & Phased Array applications are well documented, however correct method selection underpins every NDT inspection requirement and is based not only on the method capability but on many variables. By combining our NDT knowledge and expertise with plant and materials experience we are confident in providing the correct solutions.
TOFD
TOFD and its principles are well documented. Utilizing diffracted energy in the detection of flaws, the technique is less reliant on flaw orientation and morphology than standard pulse echo techniques, leading to improved sizing and probability of defect detection (POD). Additionally, computerized electronic data capture and storage, in conjunction with scanning manipulators enable rapid scanning speeds to be achieved TOFD has now been accepted as an alternative to Radiography in Pre-Service Inspections with standards such as ASME, and its use in pipe weld and thick walled pressure vessel fabrication is now common place.
Due to its’ sensitivity and sizing accuracy, TOFD is also an excellent tool for in-service material and flaw monitoring. Engineers who are monitoring root erosion, stress corrosion cracking, vessel cladding, Hydrogen Attack, Weld and Steam Chest cracks for example, are all now utilizing TOFD as part of an on-going inspection regime.
Benefits of TOFD Technology
• Based on diffraction (diffracted waves), so less reliant on weld bevel angles and flaw orientation
• Rapid weld testing/screening, circumferential and axial weld scans
• Digitally recorded data with imaging
• Accurate and reliable flaw detection and flaw sizing
• Precise defect sizing capabilities and repeatability ideal for flaw monitoring
• High Probability of Detection (POD) and low False Call Rate (FCR)
• Rapid scanning as a single beam-set gives a large area of coverage
• High level of sensitivity to all flaw types
• Wide range of thicknesses 1 – 300mm
PHASED ARRAY
Phased Array Ultrasonic Inspection (PAUT) is an advanced method of ultrasonic testing, in place of single transducer beam, phased arrays utilise multiple ultrasonic element probes which are excited under computer control to generate focussed beams of ultrasound. Phased Arrays allow real-time control of three important probe parameters: focal distance, beam angle and focal spot size. As such, phased arrays offer significant technical advantages as beams can be steered, scanned, swept and focussed electronically from a fixed probe position. This control and influence of the wave front make PAUT a very versatile method for a wide variety of applications and complex geometries.
Accepted for its high probability of detection and precise measurement capability at high speeds, two- and three-dimensional views can be generated to map and identify the size and location of detected flaws.
Benefits of Phased Array Technology
• Flexibility – adaptable to a wide range of materials and applications
• Reduced inspection times, high-speed inspection using single axis scans
• Accurate and repeatable Digitally recorded data provides precise reliable and repeatable results
• Detailed visualisation – Data display in multiple formats (A, B, C and D scan), facilitating more accurate analysis and interpretation
• Precise flaw sizing and high probability of detection
• Reduced operator dependency
• Provides alternative to Radiographic imaging
GUIDED WAVE LONG RANGE ULTRASONIC INSPECTION OF PIPELINES
Guided wave research on pipe structures is driven by the need to screen for Corrosion under Insulation (CUI) failures effectively.
Corrosion Under Insulation (CUI) is a well-known and widespread corrosion phenomena and screening for CUI by Ultrasonic Guided Waves (UGW) is now a widely accepted practice, overcoming many of the issues associated with convention method approach.
“Do you know where your higher risk CUI exposure is”?
“And do you have the needed resources applied to keeping your equipment that is susceptible to CUI safe and reliable”?
UGW inspection of pipeline has many advantages; from a single test location the system ‘guided ultrasonic waves’ can be directed in opposing direction along the pipe length testing up to several metres in length (dependant on pipe dimensions/condition), facilitating the inspection of long runs including (‘insulated’ and ‘buried’) pipeline, to screen for corrosion or cracking and corrosion-under-insulation (CUI). The inspections can be applied both to offline and operational systems (including ‘high temperature’).
For insulated pipes, only a local area of the pipe insulation is required to be removed, (along with any loose surface debris), following which the ultrasonic transducer ring can be simply fitted to the pipe and is ‘ready-for-test’. The UGW works ‘dry coupled’ and therefore requires no provision of couplant/water feed system as with standard ultrasonic applications.
Following data analysis UGW screening is backed by follow-up localised supplementary inspections for confirmation.
Integrity NDT have long-term client contracts supplying UGW inspection for both refinery system, refinery jetty and offsite pipeline systems.
Figure (a). Illustration of GWT equipment setup on pipeline with features
Figure (b). Example data obtained from GWT testing – relates to setup and features shown in Figure (b)
By utilising Synthetic Focussing, data signal responses can be positioned at ‘clock-positions’ around the pipe circumference and allows for a more accurate prediction of defect type and characterisation.
Oxide Scale Thickness Measurement
The Challenge -The formation and growth of iron oxide scale on the inner and outer surface of boiler tubes is a significant factor affecting tube life. The high temperature regions of steam boiler can cause the formation of surface ‘grown oxide’ (Magnetite) which forms progressively under long term exposure to high temperature.
The conductivity of the oxide scale is approximately 5% that of the steel, and as the grown oxide layer builds in thickness over time, it forms to acts as a thermal barrier on the surface. The resultant loss in heat transfer leads to the tube wall heating, to temperatures beyond the intended operating range. Long term exposure to raised temperatures and high internal pressure, can result in micro-cracking within the tube wall and to creep deformation and eventual failure.
Integrity NDT Solution – To determining the oxide layer thickness, Integrity NDT are using a high frequency delay line ultrasonic probe typically (Single crystal delay line compression wave probe 20MHz 0.125mm), and broadband digital UT flaw detector. The flaw detector must be capable of detecting echoes and measuring the short time interval between the peaks, representing a) the steel/oxide and b) the oxide/air boundaries.
In consideration that bore oxide thickness ≥300 microns is the threshold at which operational problems (tube conduction efficiency, overheating, spallation & blockage etc.) are likely to start manifesting themselves, oxide thickness measurements of this level and below (down to 125 – 150 microns) can be achieved ultrasonically, and avoid the need for tube cutting. Additionally, the fact that oxide thickness is proven to be proportional to tube operating temperature, gives non-destructive Oxide thickness measurements the potential to target ‘hotter’ (shorter creep life) tubes for replication across a header bank for example, if conducting creep life assessment.