Increasing use of risk based inspection (RBI) on chemical and refining plant places greater emphasis on the ability to accurately size and monitor pre-existing or growing in-service discontinuities. Originally developed for the Nuclear Industry, the TOFD technique is well suited to many process industry applications. In fact TOFD is an important tool for inspection of:
With TOFD scans, the transmitting and receiving probes are positioned equidistantly from the weld centre and scanned parallel with the weld. Normally a single pass is sufficient for the required inspection coverage.
During operation, ultrasound is transmitted at an angle into the weld by one probe. If the sound is obstructed by a defect, some of the energy is diffracted at its edges and detected by the receiving probe. The signals are recorded, processed with specialised software for interpretation and sizing of indications.
TOFD differs from other ultrasonic based methods in that it relies on the detection of diffracted signals rather than reflected signals (pulse-echo). The comprehensive coverage afforded by the wide beam used for TOFD inspections makes it much less dependant on probe position and defect orientation than pulse echo techniques.
Inspections are carried out using a simple frame to hold the probes or scanner with optical encoders for position information. By varying the transducer type, size, frequency, separation and number of scans the operator can "best fit" the system to the application.
The data is displayed as a composite A-scan grey scale image. Complex algorithms use the sound path timing variations to calculate the depth and cross-sectional size of any discontinuities. These are available to the operator within the TD-scan software.