In refining and chemical plants, accurately monitoring and sizing discontinuities continues to become an essential part of operations, even more so when a risk based inspection is utilised. As the need for techniques to be more accurate, reliable, effective (in time and cost) and straightforward increases, the TOFD, as a method of detecting faults in welds, has become a broadly employed tool across the industry.
It has great advantages in the speed of detection of defects, and is very sensitive and accurate in the defect sizing technique in general use. One of the important advantages of using TOFD for weld inspection is the absence of radiation. In several validation projects, TOFD has shown a proven record of a high detection rate with very high reliability in preservice and in-service inspections.
Two main groups of applications differentiated as follows:
PA allows for the detection of discontinuities that are located in difficult to observe configurations, requiring the use of special ultrasonic testing methods. PA combined with a rotating sound beam can reach almost any angle and is the best tool to determine and visualize discontinuities.
PA testing has several key benefits; the capability to examine intricate geometries; a fast and efficient speed of inspection; an improved accuracy in results; good repeatability, and the ability to visualise signs in base materials and/or welds through the use of B, C, D and S-scans (including all A-scans), allowing for data storage and providing a safer method than other NDT testing.
Internal Rotary Inspection System (IRIS)
Pipes and tubes can be tested using an ultrasonic method known as IRIS, wherein a probe is inserted into a tube flooded with water, then is slowly released while gathering data. Revealed defects include metal loss from both in and outside of the pipe wall.
Eddy Current Testing (ECT)
ECT is carried out in order to examine non ferromagnetic tubing, and is appropriate for spotting a wide range of metal discontinuities: wall loss, baffle cuts, erosion & corrosion, pitting and wear.
Magnetic Flux Leakage (MFL)
The MFL method is optimal when inspecting ferromagnetic tubes, as it gives a relatively accurate understanding of corrosion-induced metal loss in heat exchanger tubes in a short time span.
Remote Field Testing (RFT)
Two coils are utilised in RFT: the driver coil (also known as the transmit coil) and the pickup coil. A low frequency AC signal is used to excite the driver coil, and the pickup coil is given a signal which is the vector sum of a small RFT signal which comes from the tube wall, and a direct coupled signal from the diver coil.
Regular inspection of storage tanks ensures that the walls and bottom plates are sound, as tank floor plates are susceptible to corrosion from the soil beneath them, increasing the risk of hazardous chemical contamination. Using MFL allows for maximum area coverage in a short time span, so that any indication that exceeds the threshold is verified by the ultrasonic straight beam.
Used in the bottom plate of tanks.