TOFD, Time Off Flight Diffraction

Time of Flight Diffraction or TOFD, is one of the most promising ultrasonic techniques for the examination of welds on pressure vessels in lieu of radiography; for pipe weld quality or crack detection and also weld root erosion. TOFD is a computerized ultrasonic system able to scan, store, and evaluate indications in terms of height (through wall thickness), length and position, with a degree of accuracy and speed never achieved with other ultrasonic techniques.

Theory – TOFD consists of a separate ultrasonic transmitter & receiver. The probes are aimed at the same point in the weld volume. The entire weld is then flooded with ultrasound allowing inspection of the weld. After emission of a compressional wave from the transmitter, the first signal to arrive at the receiver is the lateral wave or OD wave which represnts the outer surface or OD. In the absence of defects, the second signal to arrive at the receiver is the L-wave backwall echo which represents the inner surface or ID. When a flaw is present, a diffracted signal is generated at the upper tip of a defect and will arrive before the signal generated at the lower tip of a defect. With a time of flight of each flight path, ultrasonic velocity and the spatial relationship of the two probes, the location and height of the defects can be accurately calculated. Gray scale imaging techniques are applied to the RF (AC) signal phase and enables weld integrity to be observed in real time.

Visual inspection is normally performed by using naked eyes. Its effectiveness may be improved with the aid of special tools. Tools include fiberscopes, borescopes, magnifying glasses and mirrors. In both cases, inspections are limited only to areas that can be directly seen by the eyes.

However, with the availability of more sophisticated equipment known as borescope, visual inspection can be extended to cover remote areas that under normal circumstances cannot be reached by naked eyes. Defects such as corrosion in boiler tube, which cannot be seen with naked eyes can easily be detected and recorded by using such equipment.

Although considered as the simplest method of NDT, such an inspection must be carried out by personnel with an adequate vision. Knowledge and experience related to components are also necessary to allow him to make correct assessment regarding the status of the components.