Time of Flight Diffraction

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:

New Weldments to record a repeatable base line image. Such data is invaluable for comparisons with subsequent inspections throughout the life of the equipment.
In-service Weldments for erosion loss at the root zone.
In-service Weldments for preferential corrosion loss at the root and heat affected zone (haz).
In-service Weldments for original build quality assessment prior to change in operating conditions of the equipment.
In-service Weldments and material for fatigue or service related cracking problems.

Brief Description

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.

Capabilities

Defect detection does not depend on orientation. Cracks not perpendicular to the measured surface, can be detected.
Determination of defect height and length.
Higher Probability of Detection (POD) improves reliability.
Inspection results are immediately available as a permanent record of the inspection.
TOFD fingerprinting, applied during construction, may reduce future in-service inspection costs.
High data collection speeds possible (250mm/second).

Limitations

Near surface defects may not be detectable due to lateral wave (dead zone).
The system is more complex than conventional ultrasonic instruments.
Harder to apply to complex geometries.
May need to be applied in conjunction with pulse-echo scans.

Preparation

Test surfaces need to be free from rust, scale, spatter and other surface contaminants that may prevent good ultrasonic coupling.

Services Required